The dbx Model 700 Digital Audio Processor was a professional audio

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/ DAC combination unit, which digitized a

stereo Stereophonic sound, or more commonly stereo, is a method of sound reproduction that recreates a multi-directional, 3-dimensional audible perspective. This is usually achieved by using two independent audio channels through a configuration ...

analog Analog or analogue may refer to: Computing and electronics * Analog signal, in which information is encoded in a continuous variable ** Analog device, an apparatus that operates on analog signals *** Analog electronics, circuits which use analo ...

audio input into a bitstream, which was then encoded and encapsulated in an analog

composite video Composite video is an analog video signal format that carries standard-definition video (typically at 525 lines or 625 lines) as a single channel. Video information is encoded on one channel, unlike the higher-quality S-Video (two channels) a ...

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.Jonathan Scull, ''Stereophile'', November 1999, Sony SCD-1 Super Audio CD/CD player, Sidebar 1: Direct Stream Digital. http://stereophile.com/digitalsourcereviews/180/index7.html

History

Unlike the many digital recording formats that would follow (e.g. DAT and

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), 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

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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," Al Fasoldt, ''The Post-Standard'', 1986, dbx 700: The best recording device you can buy. http://aroundcny.com/technofile/texts/dbx700recorder86.html and Stereophile Magazine reviewed it positively.''Stereophile'', Vol.10 No.5, August 1987. 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 14 dB more dynamic range than 44.1 kHz/16b audio, and because of its very high sample rate (644 kHz), it did not contain the same anti-aliasing filters 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 Line level is the specified strength of an audio signal used to transmit analog audio between components such as CD and DVD players, television sets, audio amplifiers, and mixing consoles. Line level sits between other levels of audio signals. ...

, balanced inputs. One popular upgrade was the addition of one or two microphone

preamps A preamplifier, also known as a preamp, is an electronic amplifier that converts a weak electrical signal into an output signal strong enough to be noise-tolerant and strong enough for further processing, or for sending to a power amplifier a ...

, 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 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

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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.

Technical specifications

* Dynamic range: 110 dB typical with A-weighted noise 20 Hz–20 kHz; >105 dB unweighted * Frequency response: 20 Hz–20 kHz, sine or pink noise, 100mV, reference record position * Total harmonic distortion: less than 0.05%, 1V input, 1 kHz *

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: less than 0.01% unweighted; 0.006% wrms * Anti-alias filtering: −3 dB at 37 kHz * Sampling rate: 644 kHz *

Bit rate In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction w ...

: 644 kbit/s * Mic preamp: adds less than 1 dB noise, 100 to 1k-ohm impedance * Max in/out levels: +24 dBm dbx Inc., ''Introduction'', dbx Model 700 Digital Audio Processor Instruction Manual.

Theory of operation

The Model 700 converted analog audio into digital data using a type of delta-sigma modulation, called "Companded Predictive Delta Modulation," or "CPDM" (both trademarked). In a traditional, single-integrated delta-sigma

, the voltage of an input signal is compared to the output of an

integrator An integrator in measurement and control applications is an element whose output signal is the time integral of its input signal. It accumulates the input quantity over a defined time to produce a representative output. Integration is an importan ...

. 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, in mathematics and digital signal processing, is the process of mapping input values from a large set (often a continuous set) to output values in a (countable) smaller set, often with a finite number of elements. Rounding and ...

. 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.Robert W. Adams, ''Technical Background'', dbx Model 700 Digital Audio Processor Instruction Manual. 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.dbx Inc., ''Appendix and Figures'', dbx Model 700 Digital Audio Processor Instruction Manual.

History

Technical specifications

Theory of operation

References

Further reading