Analog passthrough
Encyclopedia
Analog passthrough is a feature found on some digital-to-analog television converter boxes
. Boxes without analog passthrough only allow digital TV (ATSC
standard) to be viewed on older, analog-only (NTSC
standard) TVs. Those with analog passthrough allow both digital and analog television to be viewed on older TVs.
Before digital television, passthrough originally existed for VCRs (and later PVR
s and DVDRs), allowing the TV antenna or cable TV signal to pass through the VCR (with a slight insertion loss
) to the TV set automatically when the VCR was turned off, or manually with a button on the remote control
. Passthrough was turned off when the RF modulator
(typically on TV channel 3 or 4 in North America) was on, as this F connector
was originally the only way to send the VCR output to older TVs, until unmodulated
composite video
and RCA connector
s became common.
that formerly went directly to the TV) and an RF
output (which now goes directly to the TV). They may also have additional outputs. Any converter box converts the digital signal for the current digital sub-channel to an analog signal (at the reduced screen resolution of the analog standard), outputs that signal onto analog channel 3 or 4 (set by the user to avoid any conflict with local over-the-air channels) and sends that signal to the analog tuner in the TV. With a box that lacks analog passthrough, no other signals are sent to the output, so all analog stations are lost.
In the US, this primarily affects low-power and broadcast translator stations, as these are exempt from the FCC mandate
to switch
to digital broadcasting
in 2009, as well as foreign signals that will remain in analog form. A small number of TV receivers were also manufactured with built-in broadcast radio receivers; these included some small portable devices or (more rarely) sets marketed for hotel/motel use. If used with DTV converter boxes, these will need analog passthrough in order not to block incoming FM radio signals.
None of these solutions are ideal, however, requiring additional hardware (A-B boxes, splitters, or additional antennae or TVs) or manual intervention to change between digital and analog TV.
offer analog passthrough, meaning that the same output cable
which carries the converted digital
signal (on analog channel 3 or 4) while the converter is operating also retains all remaining analog signals
upon turning the converter box off.
While this typically works to some extent with the converter box on, there is often significant signal strength reduction and/or interference on the original analog channels.
Note that analog passthrough signals are only passed to the coax output
, not to any other outputs provided by the unit. While other converter outputs may still be connected and used for digital TV reception, the coaxial RF output must remain connected to pass analog TV signals.
For some VCRs (and, less often, TVs) this arrangement may cause a problem if the receiver uses the presence of active composite video
or S-video
connections to indicate that the signal should be accepted from those sources only. In such a case the two options would be to use the coax output exclusively, either when viewing analog TV only (which requires plugging and unplugging cables) or full time (with the reduced digital TV quality produced using the coax output).
The majority of TVs have a button on the unit and/or remote labeled something like "TV/VIDEO", "INPUT" or "AUX" which manually switches between coax and other sources, ensuring no problems when using both sets of connections simultaneously.
Many will want to continue to receive analog stations both before and beyond the February, 2009 transition date. In setting its requirements for coupon-eligible converter box
es (CECBs), the NTIA
permits, but does not require, analog passthrough as an available feature.
Unfortunately, only one of the initially available converter boxes included this feature, and a year before the original transition date of Feb. 17, 2009, Community Broadcasters Association
president Ron Bruno said four of 32 NTIA-certified boxes had the feature. In late March 2008, the CBA filed a lawsuit
in the U.S. Court of Appeals for the District of Columbia, seeking an injunction
to halt the sale and distribution of DTV converter box
es, charging that their failure to include analog tuners or analog passthrough violates the All-Channel Receiver Act. CBA maintained that the lack of analog support would seriously harm the LPTV and class A television stations the group represents, as it is cost-prohibitive for many or most of them to convert to digital transmissions, and the new boxes would prevent viewers from being able to watch (or even being aware of the existence of) their analog-only stations.
Responding to CBA's actions, the FCC
and NTIA urged manufacturers to include the feature voluntarily in all converter boxes, and manufacturers responded by releasing a new generation of models with the feature. Some new DVD recorder
s and personal video recorders also provide both analog and digital tuners, and therefore could perform the basic functions of a set-top box in both modes.
In early May 2008, the D.C. district court denied the CBA petition without comment, effectively telling the association that it had not exhausted all its efforts, and that there was not enough merit to take the case to the courts. The CBA responded by concentrating its lobbying efforts on the FCC, and by urging more funding for low power and Class A broadcasters to transition to digital, asking Congress to increase the number of such stations eligible for funds.
Digital television adapter
A 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...
. Boxes without analog passthrough only allow digital TV (ATSC
ATSC
ATSC standards are a set of standards developed by the Advanced Television Systems Committee for digital television transmission over terrestrial, cable, and satellite networks....
standard) to be viewed on older, analog-only (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...
standard) TVs. Those with analog passthrough allow both digital and analog television to be viewed on older TVs.
Before digital television, passthrough originally existed for VCRs (and later PVR
PVR
PVR may refer to:*Peak Vehicle Requirement, the number of public transport vehicles required to operate the highest frequency service on a route*Penn Virginia Resources, a mining company* Personal video recorder, also known as a digital video recorder...
s and DVDRs), allowing the TV antenna or cable TV signal to pass through the VCR (with a slight insertion loss
Insertion loss
In telecommunications, insertion loss is the loss of signal power resulting from the insertion of a device in a transmission line or optical fiber and is usually expressed in decibels ....
) to the TV set automatically when the VCR was turned off, or manually with a button on the remote control
Remote control
A remote control is a component of an electronics device, most commonly a television set, used for operating the television device wirelessly from a short line-of-sight distance.The remote control is usually contracted to remote...
. Passthrough was turned off when the RF modulator
RF modulator
An RF modulator is a device that takes a baseband input signal and outputs a radio frequency-modulated signal....
(typically on TV channel 3 or 4 in North America) was on, as this F connector
F connector
The F connector is a type of coaxial RF connector commonly used for "over the air" terrestrial television, cable television and universally for satellite television and cable modems, usually with RG-6/U cable or, in older installations, with RG-59/U cable. It was invented by Eric E...
was originally the only way to send the VCR output to older TVs, until unmodulated
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
composite video
Composite video
Composite video is the format of an analog television signal before it is combined with a sound signal and modulated onto an RF carrier. In contrast to component video it contains all required video information, including colors in a single line-level signal...
and RCA connector
RCA connector
An RCA connector, sometimes called a phono connector or cinch connector, is a type of electrical connector commonly used to carry audio and video signals...
s became common.
The problem
All digital-to-analog converter boxes have both an antenna input (which accepts the coaxial cableCoaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
that formerly went directly to the TV) and an RF
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
output (which now goes directly to the TV). They may also have additional outputs. Any converter box converts the digital signal for the current digital sub-channel to an analog signal (at the reduced screen resolution of the analog standard), outputs that signal onto analog channel 3 or 4 (set by the user to avoid any conflict with local over-the-air channels) and sends that signal to the analog tuner in the TV. With a box that lacks analog passthrough, no other signals are sent to the output, so all analog stations are lost.
In the US, this primarily affects low-power and broadcast translator stations, as these are exempt from the FCC mandate
DTV transition in the United States
The DTV transition in the United States was the switchover from analog to exclusively digital broadcasting of free over-the-air television programming...
to switch
Digital television transition
The digital television transition is the process in which analog television broadcasting is converted to and replaced by digital television. This primarily involves both TV stations and over-the-air viewers; however it also involves content providers like TV networks, and cable television...
to digital broadcasting
Digital television
Digital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
in 2009, as well as foreign signals that will remain in analog form. A small number of TV receivers were also manufactured with built-in broadcast radio receivers; these included some small portable devices or (more rarely) sets marketed for hotel/motel use. If used with DTV converter boxes, these will need analog passthrough in order not to block incoming FM radio signals.
Workarounds
There are several approaches used with converter boxes that lack analog passthrough:- Accept the missing analog channel reception.
- Unplug the antenna from the converter box and plug it directly into the TV when watching analog TV.
- Install a splitter so the antenna signal is sent to both the converter box and TV. Unfortunately, this approach will substantially reduce the signal strengthSignal strengthIn telecommunications, particularly in radio, signal strength refers to the magnitude of the electric field at a reference point that is a significant distance from the transmitting antenna. It may also be referred to as received signal level or field strength. Typically, it is expressed in...
which arrives at both the converter box and TV. - Install an amplified splitterDistribution amplifierIn electronics, a distribution amplifier or simply distribution amp, is a device that accepts a single input signal and provides this same signal to multiple isolated outputs....
in order to boost the signal strength before feeding it to converter and TV. - Install an A-B switch box to allow selection of whether the signal goes to the converter box (when watching digital TV) or directly to the TV (when watching analog TV). A-B boxes are seldom remote controlled, however.
- Use two different TVs, one with a converter box for digital TV and one without for analog TV.
- Use a splitter before the converter and a frequency-selective combiner (a VHF/UHF splitter/combiner where analog stations are UHF or a channel 3/4 "signal inserter" otherwise) to place converted signals on an unused channel.
- Use a conventional all-female splitter before the converter and a reversed splitter before the television.For pass through, which allows analog tv and fm radio (if the tv also has a radio tuner), plug a standard coaxial patch cable between the two splitters. For dtv, unplug one end of the patch between the splitters, otherwise there will be a hideous signal loss.
- For TVs with two coax antenna inputs, or one coax input and another type of composite videoComposite videoComposite video is the format of an analog television signal before it is combined with a sound signal and modulated onto an RF carrier. In contrast to component video it contains all required video information, including colors in a single line-level signal...
input which matches an output on the converter box, you can use two antennas, one going to the converter box and one going to the additional coax input on the TV, or to not require another antenna, a splitter to direct the antenna to both inputs.
None of these solutions are ideal, however, requiring additional hardware (A-B boxes, splitters, or additional antennae or TVs) or manual intervention to change between digital and analog TV.
The solution
Some converter boxesCoupon-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...
offer analog passthrough, meaning that the same output cable
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
which carries the converted digital
Digital television
Digital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
signal (on analog channel 3 or 4) while the converter is operating also retains all remaining analog signals
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...
upon turning the converter box off.
While this typically works to some extent with the converter box on, there is often significant signal strength reduction and/or interference on the original analog channels.
Note that analog passthrough signals are only passed to the coax output
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
, not to any other outputs provided by the unit. While other converter outputs may still be connected and used for digital TV reception, the coaxial RF output must remain connected to pass analog TV signals.
For some VCRs (and, less often, TVs) this arrangement may cause a problem if the receiver uses the presence of active composite video
Composite video
Composite video is the format of an analog television signal before it is combined with a sound signal and modulated onto an RF carrier. In contrast to component video it contains all required video information, including colors in a single line-level signal...
or S-video
S-Video
Separate Video, more commonly known as S-Video and Y/C, is often referred to by JVC as both an S-VHS connector and as Super Video. It is an analog video transmission scheme, in which video information is encoded on two channels: luma and chroma...
connections to indicate that the signal should be accepted from those sources only. In such a case the two options would be to use the coax output exclusively, either when viewing analog TV only (which requires plugging and unplugging cables) or full time (with the reduced digital TV quality produced using the coax output).
The majority of TVs have a button on the unit and/or remote labeled something like "TV/VIDEO", "INPUT" or "AUX" which manually switches between coax and other sources, ensuring no problems when using both sets of connections simultaneously.
United States implementation
In the US, conversion to digital TV from analog was complete by June 12, 2009. There are exceptions, however:- Class A stations, low power stations, and translator stations may continue to send analog signals with no mandatory date to convert to digital broadcasting.
- Cross-border stations, many of which are significantly viewed in the US, will continue to broadcast in analog format until 2011 from CanadaCanadaCanada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, and 2021 from MexicoMexicoThe United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
. Portions of FloridaFloridaFlorida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...
are also adjacent to CubaCubaThe Republic of Cuba is an island nation in the Caribbean. The nation of Cuba consists of the main island of Cuba, the Isla de la Juventud, and several archipelagos. Havana is the largest city in Cuba and the country's capital. Santiago de Cuba is the second largest city...
n stations or to ZNS-TVZNS-TVZNS is a national radio and TV broadcaster operated by the state-owned Broadcasting Corporation of The Bahamas....
from The BahamasThe BahamasThe Bahamas , officially the Commonwealth of the Bahamas, is a nation consisting of 29 islands, 661 cays, and 2,387 islets . It is located in the Atlantic Ocean north of Cuba and Hispaniola , northwest of the Turks and Caicos Islands, and southeast of the United States...
.
Many will want to continue to receive analog stations both before and beyond the February, 2009 transition date. In setting its requirements for coupon-eligible converter box
Coupon-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...
es (CECBs), the NTIA
National Telecommunications and Information Administration
The National Telecommunications and Information Administration is an agency of the United States Department of Commerce that serves as the President's principal adviser on telecommunications policies pertaining to the United States' economic and technological advancement and to regulation of the...
permits, but does not require, analog passthrough as an available feature.
Unfortunately, only one of the initially available converter boxes included this feature, and a year before the original transition date of Feb. 17, 2009, Community Broadcasters Association
Community Broadcasters Association
The Community Broadcasters Association was a trade organization representing low-power broadcasting interests, including LPTV and Class A television stations, in the United States of America...
president Ron Bruno said four of 32 NTIA-certified boxes had the feature. In late March 2008, the CBA filed a lawsuit
Lawsuit
A lawsuit or "suit in law" is a civil action brought in a court of law in which a plaintiff, a party who claims to have incurred loss as a result of a defendant's actions, demands a legal or equitable remedy. The defendant is required to respond to the plaintiff's complaint...
in the U.S. Court of Appeals for the District of Columbia, seeking an injunction
Injunction
An injunction is an equitable remedy in the form of a court order that requires a party to do or refrain from doing certain acts. A party that fails to comply with an injunction faces criminal or civil penalties and may have to pay damages or accept sanctions...
to halt the sale and distribution of DTV converter box
Set-top box
A 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...
es, charging that their failure to include analog tuners or analog passthrough violates the All-Channel Receiver Act. CBA maintained that the lack of analog support would seriously harm the LPTV and class A television stations the group represents, as it is cost-prohibitive for many or most of them to convert to digital transmissions, and the new boxes would prevent viewers from being able to watch (or even being aware of the existence of) their analog-only stations.
Responding to CBA's actions, the FCC
Federal Communications Commission
The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...
and NTIA urged manufacturers to include the feature voluntarily in all converter boxes, and manufacturers responded by releasing a new generation of models with the feature. Some new DVD recorder
DVD recorder
A DVD recorder , is an optical disc recorder that uses Optical disc recording technologies to digitally record analog signal or digital signals onto blank writable DVD media...
s and personal video recorders also provide both analog and digital tuners, and therefore could perform the basic functions of a set-top box in both modes.
In early May 2008, the D.C. district court denied the CBA petition without comment, effectively telling the association that it had not exhausted all its efforts, and that there was not enough merit to take the case to the courts. The CBA responded by concentrating its lobbying efforts on the FCC, and by urging more funding for low power and Class A broadcasters to transition to digital, asking Congress to increase the number of such stations eligible for funds.
See also
- 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...
and coupon-eligible converter boxCoupon-eligible converter boxA coupon-eligible converter box was a digital television adapter that met eligibility specifications for subsidy "coupons" from the United States government... - All-Channel Receiver Act
- Composite videoComposite videoComposite video is the format of an analog television signal before it is combined with a sound signal and modulated onto an RF carrier. In contrast to component video it contains all required video information, including colors in a single line-level signal...
and S-videoS-VideoSeparate Video, more commonly known as S-Video and Y/C, is often referred to by JVC as both an S-VHS connector and as Super Video. It is an analog video transmission scheme, in which video information is encoded on two channels: luma and chroma...