Dbx Model 700 Digital Audio Processor
Encyclopedia
The dbx Model 700 Digital Audio Processor was a professional audio ADC
/DAC
combination unit, which digitized a stereo
analog
audio input into a bitstream, which was then encoded and encapsulated in an analog composite video
signal, for recording to tape using a VCR as a transport
. Unlike other similar pieces of equipment like the Sony PCM-F1
, the Model 700 used a technique called Companded Predictive Delta Modulation
, rather than the now-common pulse-code modulation
. At the time of its introduction in the mid-1980s the device was the first commercial product to use this method, although it had been proposed in the 1960s and prototyped in the late '70s.
and ADAT
), the Model 700 had no capability for storage on its own, and relied on an analog recording medium supplied by the user. In general, any high-quality VHS
VCR would do, although 3/4" U-matic
or Beta
decks could also have been used. If viewed on a monitor, the output stream of a Model 700 looked like analog TV "static" or noise, with slight black bars running down either side.
Early on, the machine was hailed as "the best recording device you can buy," and Stereophile Magazine
reviewed it positively. Many people liked the format because it offered more dynamic range than analog tape, but without the "hard clipping" inherent in PCM audio recorders of the time. The Model 700 had been designed from the beginning to have many 'tape-like' characteristics, including "soft saturation," and at a time when most professional and amateur recordists were used to analog tape, this was considered a significant feature. It also offered 14dB more dynamic range than 44.1kHz/16b audio, and because of its very high sample rate (644kHz), it did not contain the same anti-aliasing filter
s necessary in PCM recorders at the time, which were thought to cause undesirable harmonic interference.
The device sold for $4,600 in 1986, and that was without a video recorder on which to store the output, putting it out of the reach of all but the most wealthy home users. However, its target market was professional and studio users, and here it enjoyed relative popularity for a short amount of time as a mastering or mixdown recorder, recording the final output from a multitrack system
.
The Model 700 was available in several different versions. In its most basic incarnation, it had two line-level
, balanced
inputs. One popular upgrade was the addition of one or two microphone
preamps
, which were installed on removable cards into slots in the machine. These allowed stereo recording directly into the Model 700, bypassing a mixing console
. Since the recorder had a significantly lower noise floor
than most mixers of the same era, this method made the best use of the system's available dynamic range
. Another, much more rare accessory was the Model 700D Disc Mastering Delay. This was a device used for mastering vinyl records
, and which attached to a proprietary 25-pin digital output on the back of the Model 700 recorder. Because of the nature of vinyl records (which rotate at a constant angular velocity
but at a changing linear velocity with respect to the needle as it moves inwards), it is necessary to send the audio signal to the computer controlling the movement (pitch) of the cutting lathe. In this way, during quiet passages the pitch is decreased thus enabling more grooves per inch. The computer needs an audio signal that is exactly one rotation ahead of the actual audio fed to the cutting lathe, so that it can move the cutting head rapidly forward before a loud passage is grooved. The disc mastering delay achieved this just like the analog mastering tape decks that used an extra playback head for this purpose.
The Model 700 was developed and sold by the dbx
corporation of Newton, Massachusetts
, better known for their system of noise reduction for analog tape.
, called "Companded Predictive Delta Modulation," or "CPDM" (both trademarked). In a traditional, single-integrated delta-sigma ADC
, the voltage of an input signal is compared to the output of an integrator
. If the input signal is higher than the integrator's output, a 1 is recorded, and the integrator is given a command to increase by a certain amount. On each clock cycle, the comparison is repeated (and another 1 is recorded) until the integrator's output exceeds the input voltage, at which point a 0 is recorded and the integrator is told to decrease. In this way, the integrator attempts to follow the input signal as closely as it can. When fed a constant-voltage signal (or when the input is removed completely), the output will "idle" and produce a stream of alternating 1s and 0s. A decoder listening to this stream would produce a small sinusoidal or triangle-wave output, even though the correct output should be flat: this is a form of quantization error
.
Where the Model 700 differs from classical delta-sigma modulation is in its replacement of the single integrator with a complex system of comparators and high-order linear prediction filters. This was done in order to reduce the quantization error, and is accomplished in part by changing the effective "step size" of the encoder based on previously recorded information. (Thereby increasing or decreasing the slew rate
.) The system also has two analog pre-processing steps which compress the input signal in both the amplitude
and frequency domain
, in order to more closely match the abilities of the encoder. This compression is done adaptively based on previously encoded signal, and is reversed on the decoding end.
Analog-to-digital converter
An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...
/DAC
Digital-to-analog converter
In electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...
combination unit, which digitized a stereo
Stereophonic sound
The term Stereophonic, commonly called stereo, sound refers to any method of sound reproduction in which an attempt is made to create an illusion of directionality and audible perspective...
analog
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
audio input into a bitstream, which was then encoded and encapsulated in an analog 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...
signal, for recording to tape using a VCR as a transport
Transport (recording)
A transport is a device that handles a particular physical storage medium itself, and extracts or records the information to and from the medium, to an outboard set of processing electronics that the transport is connected to.A transport houses no electronics itself for encoding and decoding the...
. Unlike other similar pieces of equipment like the Sony PCM-F1
PCM adaptor
A PCM adaptor is a device used for recording digital audio in the PCM format, which in turn connects to a video cassette recorder for storage and playback of the digital audio information.-How a PCM adaptor works:...
, the Model 700 used a technique called Companded Predictive Delta Modulation
Delta-sigma modulation
Delta-sigma modulation is a method for encoding high-resolution or analog signals into lower-resolution digital signals. The conversion is done using error feedback, where the difference between the two signals is measured and used to improve the conversion...
, rather than the now-common pulse-code modulation
Pulse-code modulation
Pulse-code modulation is a method used to digitally represent sampled analog signals. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems...
. At the time of its introduction in the mid-1980s the device was the first commercial product to use this method, although it had been proposed in the 1960s and prototyped in the late '70s.
History
Unlike the many digital recording formats that would follow (e.g. DATDigital Audio Tape
Digital Audio Tape is a signal recording and playback medium developed by Sony and introduced in 1987. In appearance it is similar to a compact audio cassette, using 4 mm magnetic tape enclosed in a protective shell, but is roughly half the size at 73 mm × 54 mm × 10.5 mm. As...
and ADAT
ADAT
Alesis Digital Audio Tape or ADAT is a magnetic tape format used for the simultaneous digital recording of eight analog audio or digital audio tracks at once, onto a Super VHS tape that is used by consumer VCRs.- History :...
), the Model 700 had no capability for storage on its own, and relied on an analog recording medium supplied by the user. In general, any high-quality VHS
VHS
The Video Home System is a consumer-level analog recording videocassette standard developed by Victor Company of Japan ....
VCR would do, although 3/4" U-matic
U-matic
U-matic is an analog recording videocassette format first shown by Sony in prototype in October 1969, and introduced to the market in September 1971. It was among the first video formats to contain the videotape inside a cassette, as opposed to the various Reel-to-Reel or open-reel formats of the...
or Beta
Betamax
Betamax was a consumer-level analog videocassette magnetic tape recording format developed by Sony, released on May 10, 1975. The cassettes contain -wide videotape in a design similar to the earlier, professional wide, U-matic format...
decks could also have been used. If viewed on a monitor, the output stream of a Model 700 looked like analog TV "static" or noise, with slight black bars running down either side.
Early on, the machine was hailed as "the best recording device you can buy," and Stereophile Magazine
Stereophile
Stereophile is a monthly magazine that focuses on high end audio equipment, such as loudspeakers and amplifiers, and audio-related news, such as online audio streaming. It was founded in 1962 by J. Gordon Holt....
reviewed it positively. Many people liked the format because it offered more dynamic range than analog tape, but without the "hard clipping" inherent in PCM audio recorders of the time. The Model 700 had been designed from the beginning to have many 'tape-like' characteristics, including "soft saturation," and at a time when most professional and amateur recordists were used to analog tape, this was considered a significant feature. It also offered 14dB more dynamic range than 44.1kHz/16b audio, and because of its very high sample rate (644kHz), it did not contain the same anti-aliasing filter
Anti-aliasing filter
An anti-aliasing filter is a filter used before a signal sampler, to restrict the bandwidth of a signal to approximately satisfy the sampling theorem....
s necessary in PCM recorders at the time, which were thought to cause undesirable harmonic interference.
The device sold for $4,600 in 1986, and that was without a video recorder on which to store the output, putting it out of the reach of all but the most wealthy home users. However, its target market was professional and studio users, and here it enjoyed relative popularity for a short amount of time as a mastering or mixdown recorder, recording the final output from a multitrack system
Multitrack recording
Multitrack recording is a method of sound recording that allows for the separate recording of multiple sound sources to create a cohesive whole...
.
The Model 700 was available in several different versions. In its most basic incarnation, it had two line-level
Line level
Line level is a term used to denote the strength of an audio signal used to transmit analog sound between audio components such as CD and DVD players, TVs, audio amplifiers, and mixing consoles, and sometimes MP3 players....
, balanced
Balanced audio
Balanced audio is a method of interconnecting audio equipment using impedance-balanced lines. This type of connection is very important in sound recording and production because it allows for the use of long cables while reducing susceptibility to external noise.Balanced connections use...
inputs. One popular upgrade was the addition of one or two microphone
Microphone
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
preamps
Preamplifier
A preamplifier is an electronic amplifier that prepares a small electrical signal for further amplification or processing. A preamplifier is often placed close to the sensor to reduce the effects of noise and interference. It is used to boost the signal strength to drive the cable to the main...
, which were installed on removable cards into slots in the machine. These allowed stereo recording directly into the Model 700, bypassing a mixing console
Mixing console
In professional audio, a mixing console, or audio mixer, also called a sound board, mixing desk, or mixer is an electronic device for combining , routing, and changing the level, timbre and/or dynamics of audio signals. A mixer can mix analog or digital signals, depending on the type of mixer...
. Since the recorder had a significantly lower noise floor
Noise floor
In signal theory, the noise floor is the measure of the signal created from the sum of all the noise sources and unwanted signals within a measurement system, where the noise is defined as any signal other than the one being monitored....
than most mixers of the same era, this method made the best use of the system's available dynamic range
Dynamic range
Dynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
. Another, much more rare accessory was the Model 700D Disc Mastering Delay. This was a device used for mastering vinyl records
Gramophone record
A gramophone record, commonly known as a phonograph record , vinyl record , or colloquially, a record, is an analog sound storage medium consisting of a flat disc with an inscribed, modulated spiral groove...
, and which attached to a proprietary 25-pin digital output on the back of the Model 700 recorder. Because of the nature of vinyl records (which rotate at a constant angular velocity
Angular velocity
In physics, the angular velocity is a vector quantity which specifies the angular speed of an object and the axis about which the object is rotating. The SI unit of angular velocity is radians per second, although it may be measured in other units such as degrees per second, revolutions per...
but at a changing linear velocity with respect to the needle as it moves inwards), it is necessary to send the audio signal to the computer controlling the movement (pitch) of the cutting lathe. In this way, during quiet passages the pitch is decreased thus enabling more grooves per inch. The computer needs an audio signal that is exactly one rotation ahead of the actual audio fed to the cutting lathe, so that it can move the cutting head rapidly forward before a loud passage is grooved. The disc mastering delay achieved this just like the analog mastering tape decks that used an extra playback head for this purpose.
The Model 700 was developed and sold by the dbx
Dbx, Inc.
dbx, Inc. is an American producer of professional audio recording equipment. It was founded by David E. Blackmer in 1971. The original company goal was: "To get closer to the realism of a live performance." Its early products were based on the concept of using decibel expansion which gave the...
corporation of Newton, Massachusetts
Newton, Massachusetts
Newton is a city in Middlesex County, Massachusetts, United States bordered to the east by Boston. According to the 2010 U.S. Census, the population of Newton was 85,146, making it the eleventh largest city in the state.-Villages:...
, better known for their system of noise reduction for analog tape.
Technical specifications
- Dynamic Range: 110dB typical with A-weighted noise 20Hz-20kHz; >105dB unweighted
- Frequency Response: 20Hz-20kHz, sine or pink noise, 100mV, reference record position
- THD: less than 0.05%, 1V input, 1 kHz
- Wow and flutter: less than 0.01% unweighted; 0.006% wrms
- Anti-aliasing filters: -3dB at 37 kHz
- Sampling Rate: 644 kHz
- Bit Rate: 644 kbit/s
- Mic Pre: adds less than 1dB noise, 100 to 1k-ohm impedance
- Max In/Out Levels: +24dBm
Theory of operation
The Model 700 converted analog audio into digital data using a type of delta-sigma modulationDelta-sigma modulation
Delta-sigma modulation is a method for encoding high-resolution or analog signals into lower-resolution digital signals. The conversion is done using error feedback, where the difference between the two signals is measured and used to improve the conversion...
, called "Companded Predictive Delta Modulation," or "CPDM" (both trademarked). In a traditional, single-integrated delta-sigma ADC
Analog-to-digital converter
An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...
, the voltage of an input signal is compared to the output of an integrator
Integrator
An integrator is a device to perform the mathematical operation known as integration, a fundamental operation in calculus.The integration function is often part of engineering, physics, mechanical, chemical and scientific calculations....
. If the input signal is higher than the integrator's output, a 1 is recorded, and the integrator is given a command to increase by a certain amount. On each clock cycle, the comparison is repeated (and another 1 is recorded) until the integrator's output exceeds the input voltage, at which point a 0 is recorded and the integrator is told to decrease. In this way, the integrator attempts to follow the input signal as closely as it can. When fed a constant-voltage signal (or when the input is removed completely), the output will "idle" and produce a stream of alternating 1s and 0s. A decoder listening to this stream would produce a small sinusoidal or triangle-wave output, even though the correct output should be flat: this is a form of quantization error
Quantization error
In analog-to-digital conversion, the difference between the actual analog value and quantized digital value is called quantization error or quantization distortion. This error is either due to rounding or truncation...
.
Where the Model 700 differs from classical delta-sigma modulation is in its replacement of the single integrator with a complex system of comparators and high-order linear prediction filters. This was done in order to reduce the quantization error, and is accomplished in part by changing the effective "step size" of the encoder based on previously recorded information. (Thereby increasing or decreasing the slew rate
Slew rate
In electronics, the slew rate represents the maximum rate of change of a signal at any point in a circuit.Limitations in slew rate capability can give rise to non linear effects in electronic amplifiers...
.) The system also has two analog pre-processing steps which compress the input signal in both the amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
and frequency domain
Frequency domain
In electronics, control systems engineering, and statistics, frequency domain is a term used to describe the domain for analysis of mathematical functions or signals with respect to frequency, rather than time....
, in order to more closely match the abilities of the encoder. This compression is done adaptively based on previously encoded signal, and is reversed on the decoding end.
Further reading
- http://www.allaudios.org/detail-7102664.html Archive of a discussion of the dbx 700 by several apparently knowledgeable individuals.
- http://lists.bostonradio.org/pipermail/boston-radio-interest/2004-September/003920.html WGBH-FM Boston used the dbx 700 as its studio to transmitter link.