Television standards conversion
Encyclopedia
Television standards conversion is the process of changing one type of TV system to another. The most common is from NTSC
to PAL
or the other way around. This is done so TV programs in one nation may be viewed in a nation with a different standard. The TV video
is fed through a video standards converter
that changes the video to a different video system.
Converting between a different numbers of pixels and different frame rates in video pictures is a complex technical problem. However, the international exchange of TV programming makes standards conversion necessary and in many cases mandatory. Vastly different TV systems emerged for political and technical reasons and it is only luck that makes video programming from one nation compatible with another.
mainly with the RTF
(France) and the BBC
(UK) trying to exchange their 441 line
and 405 line programming.
The problem got worse with the introduction of PAL
, SECAM
(both 625 lines), and the French 819 line service.
Until the 1980s, standards conversion was so difficult that 24 frame/s 16 mm or 35 mm film
was the preferred medium of programming interchange.
The two TV standards are for all practical purposes, temporally and spatially incompatible with each other.
Aside from the line count being different, it is easy to see that generating 60 fields every second from a format that has only 50 fields might pose some interesting problems.
Every second, an additional 10 fields must be generated seemingly from nothing. The converter has to create new frames (from the existing input) in real time.
signal.
Teletext
signals don't need to be transferred, but the captioning data stream should be wherever it is technological possible to do so.
With HDTV broadcasting, this is less of an issue, for the most part meaning only passing the captioning datastream on to the new source material. However, DVB and ATSC have significantly different captioning datastream types.
and the Nyquist–Shannon sampling theorem
imply that conversion from one television standard to another will be easier providing:
) samples
, followed by the number of samples of the two color ("chroma") components: U/Cb
then V/Cr
, for each complete sample area.
For quality comparison, only the ratio between those values is important, so 4:4:4 could easily be called 1:1:1; however, traditionally the value for brightness is always 4, with the rest of the values scaled accordingly.
The sampling principles above apply to both digital and analog television.
This is one reason why NTSC films viewed on typical home equipment may not appear as smooth as when viewed in a cinema. The phenomenon is particularly apparent during slow, steady camera movements which appear slightly jerky when telecined.
This process is commonly referred to as telecine judder.
PAL material in which 2:2:2:2:2:2:2:2:2:2:2:3 pulldown has been applied, suffers from a similar lack of smoothness, though this effect is not usually called “telecine judder”.
In effect every 12th film frame is displayed for the duration of 3 PAL fields (60 milliseconds) – whereas the other 11 frames are all displayed for the duration of 2 PAL fields (40 milliseconds). This causes a slight “hiccup” in the video about twice a second.
Television systems converters must avoid creating telecine judder effects during the conversion process.
Avoiding this judder is of economic importance as a substantial amount of NTSC (60 Hz, technically 29.97 frame/s) resolution material that originates from film – will have this problem when convered to PAL or SECAM (both 50 Hz, 25 frame/s).
RTÉ used this method during the latter years of its use of the 405 line system.
This is not the best conversion technique but it can work if one is going from a higher resolution to a lower one – at the same frame rate. Slow phosphor
s are required on both orthicons.
The first video standards converters were analog
. That is, a special professional video camera
that used a video camera tube
would be pointed at a Cathode ray tube
video monitor
. Both the Camera and the monitor could be switched to either NTSC or PAL, to convert both ways. Robert Bosch GmbH
's Fernseh
Division made a large three rack
analog video standards converter. These were the high end converters of the 1960s and 1970s. Image Transform in Universal City, Ca used the Fernseh converter and in the 1980s made their own custom digital
converter. This was also a larger 3 rack device. As digital memory size became larger in smaller packages, converters became the size of a microwave oven
. Today one can buy a very small consumer converter for home use.
SSTV was used to transmit images from inside Apollo 7
, Apollo 8
, and Apollo 9
, as well as the Apollo 11
Lunar Module
television from the Moon
; see Apollo TV camera
.
Later Apollo missions featured color field sequential cameras that output 60-frame/s video. Each frame corresponded to one of the RGB primary colors. This method is compatible with black and white NTSC, but incompatible with color NTSC. In fact, even NTSC monochrome TV compatibility is marginal. A monochrome set could have reproduced the pictures, but the pictures would have flickered terribly. The camera color video ran at only 10 frame/s. Also, Doppler shift in the lunar signal would have caused pictures to tear and flip. For these reasons, the Apollo moon pictures required special conversion techniques.
The conversion steps were completely electromechanical, and they took place in nearly real time. First, the downlink station corrected the pictures for Doppler shift. Next, in an analog disc recorder, the downlink station recorded and replayed every video field six times. On the six-track recorder, recording and playback took place simultaneously. After the recorder, analog video processors added the missing components of the NTSC color signal: These components include...
The conversion delay lasted only some 10 seconds. Then color moon pictures left the downlink station for world distribution.
When one uses Nyquist subsampling
as a standards conversion technique, the horizontal and vertical resolution of the material are reduced – this is an excellent method for converting HDTV to standard definition television, but it works very poorly in reverse.
The Nyquist subsampling method of systems conversion only works for HDTV to Standard Definition Television
, so as a standards conversion technology it has a very limited use. Phase Correlation is usually preferred for HDTV to standard definition conversion.
between film (24.0 frames per second) and NTSC (approximately 29.97 frames per second).
Unlike the two other most common video formats, PAL
and SECAM
, this difference cannot be overcome by a simple speed-up, because the required 25% speed-up would be obviously noticeable.
To convert 24 frame/s film to 29.97 frame/s NTSC, a complex process called "3:2 pulldown" is utilised, in which parts of some frames are duplicated and blended. This produces irregularities in the sequence of images which some people can perceive as a jitter/stutter during slow pans of the camera. See telecine
for more details.
For viewing native PAL or SECAM material (such as European television series and some European movies) on NTSC equipment, a standards conversion has to take place. There are basically two ways to accomplish this.
(625 lines @ 25 frame/s) to NTSC
(525 lines @ 30 frame/s), the converter must eliminate 100 lines per frame. The converter must also create five frames per second.
To reduce the 625-line signal to 525, less expensive converters drop 100 lines. These converters maintain picture fidelity by evenly spacing removed lines. (For example, the system might discard every sixth line from each PAL field. After the 50th discard, this process would stop. By then the system would have passed the viewable area of the field. In the following field, the process would repeat, completing one frame.) To create the five additional frames, the converter repeats every fifth frame.
If there is little inter-frame motion, this conversion algorithm is fast, inexpensive and effective. Many inexpensive consumer television system converters have employed this technique. Yet in practise, most video features significant inter-frame motion. To reduce conversion artefacts, more modern or expensive equipment may use sophisticated techniques.
may also be required in order to produce images which can be interpolated smoothly.
Interpolation can also be used to reduce the number of scanlines in the image by averaging the colour and intensity of pixels on neighbouring lines, a technique similar to Bilinear filtering
, but applied to only one axis.
There are simple 2-line and 4 line converters. The 2-line converter creates a new line by comparing two adjacent lines, whereas a 4-line model compares 4 lines to average the 5th. Again, the greater the complexity and resulting price tag!
Interfield interpolation reduces judder, but at the expense of picture smearing. The greater the blending applied to smooth out the judder, the greater the smear caused by blending.
algorithms, and are computationally much more expensive than the simpler techniques, thus requiring more powerful hardware to be effective in real-time conversion.
Adaptive Motion algorithms capitalize on the way the human eye and brain process moving images - in particular, detail is perceived less clearly on moving objects that.
Adaptive interpolation requires that the converter analyzes multiple successive fields and to detect the amount and type of motion of different areas of the picture.
Adaptive Motion Interpolation has many variations and is commonly found in midrange converters. The quality and cost is dependent upon the accuracy in analyzing the type and amount of motion, and the selection of the most appropriate algorithm for processing the type of motion.
When panning from left to right is taking place (over say 10 fields) it is safe to assume that the 11th field will be similar or very close.
The technique is highly effective but it does require a tremendous amount of computing power. Consider a block of only 8x8 pixels. For each block, the computer has 64 possible directions and 64 pixels to be matched to the block in the next field. Also consider that the greater the motion, the further out the search must be conducted. Just to find an adjacent block in the next field would entail making a search of 9 blocks. 2 blocks out would require a search and match of 25 blocks - 3 blocks further distant and it grows to 49 etc etc.
The type of motion can exponentially compound the compute power required. Consider a rotating object, where a simple straight line motion vector is of little help in predicting where the next block should match. It can quickly be seen that the more inter frame motion introduced, the much greater the processing power required. This is the general concept of block matching. Block match converters can vary widely in price and performance depending on the attention to detail and complexity.
A weird artifact of block matching owes to the size of the block itself. If a moving object is smaller than the mosaic block, consider that it's the entire block that gets moved. In most cases, it's not an issue, but consider a thrown baseball. The ball itself has a high motion vector, but its background that makes up the rest of the block might not have any motion. The background gets transported in the moved block as well, based on the motion vector of the baseball, What you might see is the ball with a small amount of outfield or whatever, tagging along. As it's in motion, the block may be "soft" depending upon what additional techniques were used and barely noticeable unless your looking for it.
Block matching requires a staggering amount of processing horsepower, but today's microprocessors are making it a viable solution.
Phase Correlation's success lies in the fact that it is effective with coping with rapid motion and random motion. Phase Correlation doesn't easily get confused by rotating or twirling objects that confuse most other kinds of systems converters.
Phase Correlation is elegant as well as technically and conceptually complex. Its successful operation is derived by performing a Fourier Transform to each field of video.
A Fast Fourier Transform
(FFT) is an algorithm which deals with the transformation of discrete values (in this case image pixels).
When applied to a sample of finite values, a Fast Fourier Transform expresses any changes (motion) in terms of frequency components.
What is the advantage of using FFTs over simply trying to predict the motion vector on a pixel by pixel basis?
Since the result of the FFT represents only the inter-frame changes in terms of frequency distribution, there's far less data that has to be processed in order to calculate the motion vectors.
In summation: Objects in motion can be mathematically correlated to their peaks in the frequency distribution. Once the FFT is performed it becomes a computationally simple matter for the computer to track just the peaks and assign them the appropriate motion vectors. This conversion technique is both elegant and computationally involved. Sophisticated software and large amounts of processor "horsepower" are required for these complex computations.
), or digital-to-analog converter (box), is a device that receives, by means of an antenna
, a digital television
(DTV) transmission
, and converts that signal into an analog television
signal that can be received and displayed on an analog television.
These boxes cheaply convert HDTV (16:9 at 720 or 1080) to (NTSC or PAL at 4:3). Very little is known about the specific conversion technologies used by these converter boxes in the PAL and NTSC zones.
Downconversion is usually required, hence very little image quality loss is perceived by viewers at the recommended viewing distance with most TV sets.
See also (Americas)
<--> NTSC
conversion – including cross conversion (technically MPEG <--> DTV
) from the myriad of MPEG based web video formats.
Cross conversion can use any method commonly in use for TV system format conversion, but typically (in order to reduce complexity and memory use) it is left up to the codec to do the conversion.
Most modern DVDs are converted from 525 <--> 625 lines in this way, as it is very economical for most programming that originates at EDTV
resolution.
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
to PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
or the other way around. This is done so TV programs in one nation may be viewed in a nation with a different standard. The TV video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
is fed through a video standards converter
Video standards converter
Video standards converter is a video device that converts NTSC to PAL and/or PAL to NTSC.The PAL TV signals may be transcoded to or from SECAM.Video standards converter is used so TV shows can be viewed in nations with different video standards....
that changes the video to a different video system.
Converting between a different numbers of pixels and different frame rates in video pictures is a complex technical problem. However, the international exchange of TV programming makes standards conversion necessary and in many cases mandatory. Vastly different TV systems emerged for political and technical reasons and it is only luck that makes video programming from one nation compatible with another.
History
The first known case of TV systems conversion probably was in Europe a few years after World War IIWorld War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
mainly with the RTF
Radiodiffusion-Télévision Française
Radiodiffusion-Télévision Française was the French national public broadcasting organization established on 9 February 1949 to replace the post-war "Radiodiffusion Française" , which had been founded in 1945...
(France) and the BBC
BBC
The British Broadcasting Corporation is a British public service broadcaster. Its headquarters is at Broadcasting House in the City of Westminster, London. It is the largest broadcaster in the world, with about 23,000 staff...
(UK) trying to exchange their 441 line
441 lines
441 lines, or 383i if named using modern standard, is an early electronic television system. It was used with 50 interlaced frames per second in France and Germany, where it was an improvement over the previous 180 lines system...
and 405 line programming.
The problem got worse with the introduction of PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
, SECAM
SECAM
SECAM, also written SÉCAM , is an analog color television system first used in France....
(both 625 lines), and the French 819 line service.
Until the 1980s, standards conversion was so difficult that 24 frame/s 16 mm or 35 mm film
35 mm film
35 mm film is the film gauge most commonly used for chemical still photography and motion pictures. The name of the gauge refers to the width of the photographic film, which consists of strips 35 millimeters in width...
was the preferred medium of programming interchange.
Overview
Perhaps the most technical challenging conversion to make is the PAL to NTSC.- PAL is 625 lines at 50 fields/s
- NTSC is 525 lines at 59.94 fields/s (60,000/1,001 fields/s)
The two TV standards are for all practical purposes, temporally and spatially incompatible with each other.
Aside from the line count being different, it is easy to see that generating 60 fields every second from a format that has only 50 fields might pose some interesting problems.
Every second, an additional 10 fields must be generated seemingly from nothing. The converter has to create new frames (from the existing input) in real time.
Hidden signals: not always transferred
TV contains many hidden signals. One signal type that is not transferred, except on some very expensive converters, is the closed captioningClosed captioning
Closed captioning is the process of displaying text on a television, video screen or other visual display to provide additional or interpretive information to individuals who wish to access it...
signal.
Teletext
Teletext
Teletext is a television information retrieval service developed in the United Kingdom in the early 1970s. It offers a range of text-based information, typically including national, international and sporting news, weather and TV schedules...
signals don't need to be transferred, but the captioning data stream should be wherever it is technological possible to do so.
With HDTV broadcasting, this is less of an issue, for the most part meaning only passing the captioning datastream on to the new source material. However, DVB and ATSC have significantly different captioning datastream types.
Theory behind systems conversion
Information theoryInformation theory
Information theory is a branch of applied mathematics and electrical engineering involving the quantification of information. Information theory was developed by Claude E. Shannon to find fundamental limits on signal processing operations such as compressing data and on reliably storing and...
and the Nyquist–Shannon sampling theorem
Nyquist–Shannon sampling theorem
The Nyquist–Shannon sampling theorem, after Harry Nyquist and Claude Shannon, is a fundamental result in the field of information theory, in particular telecommunications and signal processing. Sampling is the process of converting a signal into a numeric sequence...
imply that conversion from one television standard to another will be easier providing:
- one is going from a higher framerate to a lower framerate (NTSCNTSCNTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
to PALPALPAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
or SECAMSECAMSECAM, also written SÉCAM , is an analog color television system first used in France....
, for example) - one is going from a higher resolution to a lower resolution (HDTVHigh-definition televisionHigh-definition television is video that has resolution substantially higher than that of traditional television systems . HDTV has one or two million pixels per frame, roughly five times that of SD...
to NTSCNTSCNTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
) - one is not converting from one progressive source to another progressive source (interlaced PALPALPAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
and NTSCNTSCNTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
are temporally and spatially incompatible with each other) - interframe motion is limited, so as to reduce temporal or spatial judder
- signal to noise ratios in the source material are not detrimentally high
- the source material does not possess any continuous (or periodic) signal defect that inhibits translation
Sampling systems and ratios
The subsampling in a video system is usually expressed as a three part ratio. The three terms of the ratio are: the number of brightness ("luminance" "luma" or YYUV
YUV is a color space typically used as part of a color image pipeline. It encodes a color image or video taking human perception into account, allowing reduced bandwidth for chrominance components, thereby typically enabling transmission errors or compression artifacts to be more efficiently...
) samples
Sampling (signal processing)
In signal processing, sampling is the reduction of a continuous signal to a discrete signal. A common example is the conversion of a sound wave to a sequence of samples ....
, followed by the number of samples of the two color ("chroma") components: U/Cb
YUV
YUV is a color space typically used as part of a color image pipeline. It encodes a color image or video taking human perception into account, allowing reduced bandwidth for chrominance components, thereby typically enabling transmission errors or compression artifacts to be more efficiently...
then V/Cr
YUV
YUV is a color space typically used as part of a color image pipeline. It encodes a color image or video taking human perception into account, allowing reduced bandwidth for chrominance components, thereby typically enabling transmission errors or compression artifacts to be more efficiently...
, for each complete sample area.
For quality comparison, only the ratio between those values is important, so 4:4:4 could easily be called 1:1:1; however, traditionally the value for brightness is always 4, with the rest of the values scaled accordingly.
The sampling principles above apply to both digital and analog television.
Telecine judder
The “3:2 pulldown” conversion process for 24 frame/s film to television (telecine) creates a slight error in the video signal compared to the original film frames.This is one reason why NTSC films viewed on typical home equipment may not appear as smooth as when viewed in a cinema. The phenomenon is particularly apparent during slow, steady camera movements which appear slightly jerky when telecined.
This process is commonly referred to as telecine judder.
PAL material in which 2:2:2:2:2:2:2:2:2:2:2:3 pulldown has been applied, suffers from a similar lack of smoothness, though this effect is not usually called “telecine judder”.
In effect every 12th film frame is displayed for the duration of 3 PAL fields (60 milliseconds) – whereas the other 11 frames are all displayed for the duration of 2 PAL fields (40 milliseconds). This causes a slight “hiccup” in the video about twice a second.
Television systems converters must avoid creating telecine judder effects during the conversion process.
Avoiding this judder is of economic importance as a substantial amount of NTSC (60 Hz, technically 29.97 frame/s) resolution material that originates from film – will have this problem when convered to PAL or SECAM (both 50 Hz, 25 frame/s).
Orthicon to orthicon
This method was used by Ireland to convert 625 line service to 405 line service. It is perhaps the most basic television standard conversion technique.RTÉ used this method during the latter years of its use of the 405 line system.
- A standards converter was used to provide the 405 line service, but according to more than one former RTÉ engineering source the converter blew up and afterwards the 405 line service was provided by a 405 line camera pointing at a monitor!
This is not the best conversion technique but it can work if one is going from a higher resolution to a lower one – at the same frame rate. Slow phosphor
Phosphor
A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...
s are required on both orthicons.
The first video standards converters were analog
Analog television
Analog television is the analog transmission that involves the broadcasting of encoded analog audio and analog video signal: one in which the message conveyed by the broadcast signal is a function of deliberate variations in the amplitude and/or frequency of the signal...
. That is, a special professional video camera
Professional video camera
A professional video camera is a high-end device for creating electronic moving images...
that used a video camera tube
Video camera tube
In older video cameras, before the mid to late 1980s, a video camera tube or pickup tube was used instead of a charge-coupled device for converting an optical image into an electrical signal. Several types were in use from the 1930s to the 1980s...
would be pointed at a Cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
video monitor
Video monitor
A video monitor also called a broadcast monitor, broadcast reference monitor or just reference monitor, is a display device similar to a television set, used to monitor the output of a video-generating device, such as playout from a video server, IRD, video camera, VCR, or DVD player. It may or...
. Both the Camera and the monitor could be switched to either NTSC or PAL, to convert both ways. Robert Bosch GmbH
Robert Bosch GmbH
Robert Bosch GmbH is a multinational engineering and electronics company headquartered in Gerlingen, near Stuttgart, Germany. It is the world's largest supplier of automotive components...
's Fernseh
Fernseh
The Fernseh AG television company was registered in Berlin on July 3, 1929 by John Logie Baird, Robert Bosch and other partners with an initial capital of 100,000 Reichsmark....
Division made a large three rack
19-inch rack
A 19-inch rack is a standardized frame or enclosure for mounting multiple equipment modules. Each module has a front panel that is wide, including edges or ears that protrude on each side which allow the module to be fastened to the rack frame with screws.-Overview and history:Equipment designed...
analog video standards converter. These were the high end converters of the 1960s and 1970s. Image Transform in Universal City, Ca used the Fernseh converter and in the 1980s made their own custom digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
converter. This was also a larger 3 rack device. As digital memory size became larger in smaller packages, converters became the size of a microwave oven
Microwave oven
A microwave oven is a kitchen appliance that heats food by dielectric heating, using microwave radiation to heat polarized molecules within the food...
. Today one can buy a very small consumer converter for home use.
SSTV to PAL and NTSC
The Apollo moon missions (late 1960s, early 1970s) used SSTV as opposed to normal bandwidth television; this was mostly done to save battery power. The camera used only 7 watts of power.SSTV was used to transmit images from inside Apollo 7
Apollo 7
Apollo 7 was the first manned mission in the American Apollo space program, and the first manned US space flight after a cabin fire killed the crew of what was to have been the first manned mission, AS-204 , during a launch pad test in 1967...
, Apollo 8
Apollo 8
Apollo 8, the second manned mission in the American Apollo space program, was the first human spaceflight to leave Earth orbit; the first to be captured by and escape from the gravitational field of another celestial body; and the first crewed voyage to return to Earth from another celestial...
, and Apollo 9
Apollo 9
Apollo 9, the third manned mission in the American Apollo space program, was the first flight of the Command/Service Module with the Lunar Module...
, as well as the Apollo 11
Apollo 11
In early 1969, Bill Anders accepted a job with the National Space Council effective in August 1969 and announced his retirement as an astronaut. At that point Ken Mattingly was moved from the support crew into parallel training with Anders as backup Command Module Pilot in case Apollo 11 was...
Lunar Module
Apollo Lunar Module
The Apollo Lunar Module was the lander portion of the Apollo spacecraft built for the US Apollo program by Grumman to carry a crew of two from lunar orbit to the surface and back...
television from the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
; see Apollo TV camera
Apollo TV camera
Television cameras used on the Apollo Project's missions varied in design, with image quality improving significantly with each design. A camera was carried in the Apollo Command Module...
.
- The SSTV system used in NASANASAThe National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
's early Apollo missions transferred ten frames per second with a resolution of 320 frame lines using less bandwidth than a normal TV transmission. - The early SSTV systems used by NASA differ significantly from the SSTV systems currently in use by amateur radio enthusiasts today.
- Standards conversion was necessary so that the missions could be seen by a worldwide audience in both PAL/SECAM (625 lines, 50 Hz) and NTSC (525 lines, 60 Hz) resolutions.
Later Apollo missions featured color field sequential cameras that output 60-frame/s video. Each frame corresponded to one of the RGB primary colors. This method is compatible with black and white NTSC, but incompatible with color NTSC. In fact, even NTSC monochrome TV compatibility is marginal. A monochrome set could have reproduced the pictures, but the pictures would have flickered terribly. The camera color video ran at only 10 frame/s. Also, Doppler shift in the lunar signal would have caused pictures to tear and flip. For these reasons, the Apollo moon pictures required special conversion techniques.
The conversion steps were completely electromechanical, and they took place in nearly real time. First, the downlink station corrected the pictures for Doppler shift. Next, in an analog disc recorder, the downlink station recorded and replayed every video field six times. On the six-track recorder, recording and playback took place simultaneously. After the recorder, analog video processors added the missing components of the NTSC color signal: These components include...
- The 3.58-MHz color burst
- The high-resolution monochrome signal
- The sound
- The I and Q color signals
The conversion delay lasted only some 10 seconds. Then color moon pictures left the downlink station for world distribution.
Nyquist subsampling
This conversion technique may become popular with manufacturers of HDTV --> NTSC and HDTV --> PAL converter boxes for the ongoing global conversion to HDTV.- Multiple Nyquist subsampling was used by the defunct MUSE HDTV system that was used in Japan.
- MUSE chipsets that can be used for systems conversion do exist, or can be revised for the needs of HDTV --> Analog TV converter boxes.
How it works
In a typical image transmission setup, all stationary images are transmitted at full resolution. Moving pictures possess a lower resolution visually, based on complexity of interframe image content.When one uses Nyquist subsampling
Downsampling
In signal processing, downsampling is the process of reducing the sampling rate of a signal. This is usually done to reduce the data rate or the size of the data....
as a standards conversion technique, the horizontal and vertical resolution of the material are reduced – this is an excellent method for converting HDTV to standard definition television, but it works very poorly in reverse.
- As the horizontal and vertical content change from frame to frame, moving images will be blurred (in a manner similar to using 16 mm movie film for HDTV projection).
- In fact, whole-camera pans would result in a loss of 50% of the horizontal resolution.
The Nyquist subsampling method of systems conversion only works for HDTV to Standard Definition Television
Standard-definition television
Sorete-definition television is a television system that uses a resolution that is not considered to be either enhanced-definition television or high-definition television . The term is usually used in reference to digital television, in particular when broadcasting at the same resolution as...
, so as a standards conversion technology it has a very limited use. Phase Correlation is usually preferred for HDTV to standard definition conversion.
Framerate conversion
There is a large difference in frame rateFrame rate
Frame rate is the frequency at which an imaging device produces unique consecutive images called frames. The term applies equally well to computer graphics, video cameras, film cameras, and motion capture systems...
between film (24.0 frames per second) and NTSC (approximately 29.97 frames per second).
Unlike the two other most common video formats, PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
and SECAM
SECAM
SECAM, also written SÉCAM , is an analog color television system first used in France....
, this difference cannot be overcome by a simple speed-up, because the required 25% speed-up would be obviously noticeable.
To convert 24 frame/s film to 29.97 frame/s NTSC, a complex process called "3:2 pulldown" is utilised, in which parts of some frames are duplicated and blended. This produces irregularities in the sequence of images which some people can perceive as a jitter/stutter during slow pans of the camera. See telecine
Telecine
Telecine is transferring motion picture film into video and is performed in a color suite. The term is also used to refer to the equipment used in the post-production process....
for more details.
For viewing native PAL or SECAM material (such as European television series and some European movies) on NTSC equipment, a standards conversion has to take place. There are basically two ways to accomplish this.
- The framerate can be slowed from 25 to 23.976 frames per second (a slowdown of about 4%) to subsequently apply 3:2 pulldown.
- InterpolationInterpolationIn the mathematical field of numerical analysis, interpolation is a method of constructing new data points within the range of a discrete set of known data points....
of the contents of adjacent framesFilm frameIn filmmaking, video production, animation, and related fields, a film frame or video frame is one of the many still images which compose the complete moving picture...
in order to produce new intermediate frames; this introduces artifactsArtifact (error)In natural science and signal processing, an artifact is any error in the perception or representation of any visual or aural information introduced by the involved equipment or technique....
, and even the most modestly trained of eyes can quickly spot video that has been converted between formats.
Linear interpolation
When converting PALPAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
(625 lines @ 25 frame/s) to NTSC
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
(525 lines @ 30 frame/s), the converter must eliminate 100 lines per frame. The converter must also create five frames per second.
To reduce the 625-line signal to 525, less expensive converters drop 100 lines. These converters maintain picture fidelity by evenly spacing removed lines. (For example, the system might discard every sixth line from each PAL field. After the 50th discard, this process would stop. By then the system would have passed the viewable area of the field. In the following field, the process would repeat, completing one frame.) To create the five additional frames, the converter repeats every fifth frame.
If there is little inter-frame motion, this conversion algorithm is fast, inexpensive and effective. Many inexpensive consumer television system converters have employed this technique. Yet in practise, most video features significant inter-frame motion. To reduce conversion artefacts, more modern or expensive equipment may use sophisticated techniques.
Doubler
The most basic and literal way to double lines is to repeat each scanline, though the results of this are generally very crude. Linear interpolation use digital interpolation to recreate the missing lines in an interlaced signal, and the resulting quality depends on the technique used. Generally the bob version of linear deinterlacer will only interpolate within a single field, rather than merging information from adjacent fields, to preserve the smoothness of motion, resulting in a frame rate equal to the field rate (i.e. a 60i signal would be converted to 60p.) The former technique in moving areas and the latter in static areas, which improves overall sharpness.Interfield interpolation
Interfield Interpolation is a technique in which new frames are created by blending adjacent frames, rather than repeating a single frame. This is more complex and computationally expensive than linear interpolation, because it requires the interpolator to have knowledge of the preceding and the following frames to produce an intermediate blended frame. DeinterlacingDeinterlacing
Deinterlacing is the process of converting interlaced video, such as common analog television signals or 1080i format HDTV signals, into a non-interlaced form....
may also be required in order to produce images which can be interpolated smoothly.
Interpolation can also be used to reduce the number of scanlines in the image by averaging the colour and intensity of pixels on neighbouring lines, a technique similar to Bilinear filtering
Bilinear filtering
Bilinear filtering is a texture filtering method used to smooth textures when displayed larger or smaller than they actually are.Most of the time, when drawing a textured shape on the screen, the texture is not displayed exactly as it is stored, without any distortion...
, but applied to only one axis.
There are simple 2-line and 4 line converters. The 2-line converter creates a new line by comparing two adjacent lines, whereas a 4-line model compares 4 lines to average the 5th. Again, the greater the complexity and resulting price tag!
Interfield interpolation reduces judder, but at the expense of picture smearing. The greater the blending applied to smooth out the judder, the greater the smear caused by blending.
Adaptive motion interpolation
Some more advanced techniques measure the nature and degree of inter-frame motion in the source, and use adaptive algorithms to blend the image based on the results. Some such techniques are known as motion compensationMotion compensation
Motion compensation is an algorithmic technique employed in the encoding of video data for video compression, for example in the generation of MPEG-2 files. Motion compensation describes a picture in terms of the transformation of a reference picture to the current picture. The reference picture...
algorithms, and are computationally much more expensive than the simpler techniques, thus requiring more powerful hardware to be effective in real-time conversion.
Adaptive Motion algorithms capitalize on the way the human eye and brain process moving images - in particular, detail is perceived less clearly on moving objects that.
Adaptive interpolation requires that the converter analyzes multiple successive fields and to detect the amount and type of motion of different areas of the picture.
- Where little motion is detected, the converter can use linear interpolation.
- When greater motion is detected, the converter can switch to an inter-field technique which sacrifices detail for smoother motion.
Adaptive Motion Interpolation has many variations and is commonly found in midrange converters. The quality and cost is dependent upon the accuracy in analyzing the type and amount of motion, and the selection of the most appropriate algorithm for processing the type of motion.
Adaptive motion interpolation + block matching
Block matching involves dividing the image into mosaic blocks - say perhaps for the sake of explanation, 8x8 pixels. The blocks are then stored in memory. The next field read out is also divided up into the same number and size of mosaic blocks. The converter's computer then goes to work and starts matching up blocks. The blocks that stayed in the same relative position (read: there was no motion in this part of the image) receive relatively little processing.- For each block that changed, the converter searches in every direction through its memory, looking for a match to find out where the "block" went (if there's motion, the block obviously had to have gone somewhere..).
- The search starts at the immediate surrounding blocks (assuming little motion).
- If a match isn't found, then it searches further and further out until it finds a match.
- When the matching block is found, the converter then knows how far the block moved and in which direction.
- This data is then stored as a motion vector for this block.
- Since interframe motion is often predictable owing to Newton's laws of motion in the real world, the motion vector can then be used to calculate where the block will probably be in the next field.
- The Newtonian method saves a lot of search and processing time.
When panning from left to right is taking place (over say 10 fields) it is safe to assume that the 11th field will be similar or very close.
- Block matching can be seen as the "cutting and pasting" of image blocks.
The technique is highly effective but it does require a tremendous amount of computing power. Consider a block of only 8x8 pixels. For each block, the computer has 64 possible directions and 64 pixels to be matched to the block in the next field. Also consider that the greater the motion, the further out the search must be conducted. Just to find an adjacent block in the next field would entail making a search of 9 blocks. 2 blocks out would require a search and match of 25 blocks - 3 blocks further distant and it grows to 49 etc etc.
The type of motion can exponentially compound the compute power required. Consider a rotating object, where a simple straight line motion vector is of little help in predicting where the next block should match. It can quickly be seen that the more inter frame motion introduced, the much greater the processing power required. This is the general concept of block matching. Block match converters can vary widely in price and performance depending on the attention to detail and complexity.
A weird artifact of block matching owes to the size of the block itself. If a moving object is smaller than the mosaic block, consider that it's the entire block that gets moved. In most cases, it's not an issue, but consider a thrown baseball. The ball itself has a high motion vector, but its background that makes up the rest of the block might not have any motion. The background gets transported in the moved block as well, based on the motion vector of the baseball, What you might see is the ball with a small amount of outfield or whatever, tagging along. As it's in motion, the block may be "soft" depending upon what additional techniques were used and barely noticeable unless your looking for it.
Block matching requires a staggering amount of processing horsepower, but today's microprocessors are making it a viable solution.
Phase correlation
Phase Correlation is perhaps the most computationally complex of the general algorithms.Phase Correlation's success lies in the fact that it is effective with coping with rapid motion and random motion. Phase Correlation doesn't easily get confused by rotating or twirling objects that confuse most other kinds of systems converters.
Phase Correlation is elegant as well as technically and conceptually complex. Its successful operation is derived by performing a Fourier Transform to each field of video.
A Fast Fourier Transform
Fast Fourier transform
A fast Fourier transform is an efficient algorithm to compute the discrete Fourier transform and its inverse. "The FFT has been called the most important numerical algorithm of our lifetime ." There are many distinct FFT algorithms involving a wide range of mathematics, from simple...
(FFT) is an algorithm which deals with the transformation of discrete values (in this case image pixels).
When applied to a sample of finite values, a Fast Fourier Transform expresses any changes (motion) in terms of frequency components.
What is the advantage of using FFTs over simply trying to predict the motion vector on a pixel by pixel basis?
- Mathematically, it's far easier and faster to recognize and process frequency signatures from which very accurate motion vectors can then be calculated.
- Rather than having to measure where every pixel goes from frame to frame the FFT rather results in representing just the changes from one frame to the next.
Since the result of the FFT represents only the inter-frame changes in terms of frequency distribution, there's far less data that has to be processed in order to calculate the motion vectors.
- Unlike other motion vector calculating methods, the FFT technique is not easily fooled by objects that have rotational or spiraling motions.
- What results from the FFT is a three dimensional frequency distribution represented mathematically by peaks in a three dimensional wave pattern.
- The 3rd dimension in this coordinate system represents subsequent fields of video.
In summation: Objects in motion can be mathematically correlated to their peaks in the frequency distribution. Once the FFT is performed it becomes a computationally simple matter for the computer to track just the peaks and assign them the appropriate motion vectors. This conversion technique is both elegant and computationally involved. Sophisticated software and large amounts of processor "horsepower" are required for these complex computations.
DTV to analog converters for consumers
A digital television adapter, (CECBCoupon-eligible converter box
A coupon-eligible converter box was a digital television adapter that met eligibility specifications for subsidy "coupons" from the United States government...
), or digital-to-analog converter (box), is a device that receives, by means of an antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
, a digital television
Digital television
Digital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
(DTV) transmission
Transmission (telecommunications)
Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
, and converts that signal into an analog television
Analog television
Analog television is the analog transmission that involves the broadcasting of encoded analog audio and analog video signal: one in which the message conveyed by the broadcast signal is a function of deliberate variations in the amplitude and/or frequency of the signal...
signal that can be received and displayed on an analog television.
These boxes cheaply convert HDTV (16:9 at 720 or 1080) to (NTSC or PAL at 4:3). Very little is known about the specific conversion technologies used by these converter boxes in the PAL and NTSC zones.
Downconversion is usually required, hence very little image quality loss is perceived by viewers at the recommended viewing distance with most TV sets.
See also (Americas)
- ATSC tunerATSC tunerAn ATSC tuner, often called an ATSC receiver or HDTV tuner is a type of television tuner that allows reception of digital television television channels transmitted by television stations in North America, parts of Central America and South Korea that use ATSC standards...
- Digital televisionDigital televisionDigital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
- Digital television adapterDigital television adapterA digital television adapter , or digital-to-analog converter [set-top box], or commonly known as a converter box, it is a television tuner that receives a digital television transmission, and converts the digital signal into an analog signal that can be received and displayed on an analog...
- DTV transition in the United StatesDTV transition in the United StatesThe DTV transition in the United States was the switchover from analog to exclusively digital broadcasting of free over-the-air television programming...
- Set-top boxSet-top boxA set-top box or set-top unit is an information appliance device that generally contains a tuner and connects to a television set and an external source of signal, turning the signal into content which is then displayed on the television screen or other display device.-History:Before the...
Offline conversion
A lot of cross format television conversion is done offline. There are several DVD packages that offer offline PALPAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
<--> NTSC
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
conversion – including cross conversion (technically MPEG <--> DTV
Digital television
Digital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
) from the myriad of MPEG based web video formats.
Cross conversion can use any method commonly in use for TV system format conversion, but typically (in order to reduce complexity and memory use) it is left up to the codec to do the conversion.
Most modern DVDs are converted from 525 <--> 625 lines in this way, as it is very economical for most programming that originates at EDTV
Enhanced-definition television
Enhanced-definition television, or extended-definition television, is a United States Consumer Electronics Association marketing shorthand term for certain digital television formats and devices...
resolution.
See also
- Reverse Standards ConversionReverse Standards ConversionReverse Standards Conversion or RSC is a process developed by a team led by James Insell at the BBC for the restoration of video recordings which have already been converted between different video standards using early conversion techniques....
- IEEE papers on systems conversion
- AES/EBU papers on systems conversions
- http://www.hawestv.com/moon_cam/moonctel.htm