The dbx Model 700 Digital Audio Processor was a professional audio ADC/ DAC combination unit, which digitized a

stereo Stereophonic sound, commonly shortened to 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 configurat ...

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

composite video Composite video, also known as CVBS (composite video baseband signal or color, video, blanking and sync), is an analog video format that combines image information—such as brightness (luminance), color (chrominance), and synchronization, int ...

signal, for recording to tape using a VCR as a

transport Transport (in British English) or transportation (in American English) is the intentional Motion, movement of humans, animals, and cargo, goods from one location to another. Mode of transport, Modes of transport include aviation, air, land tr ...

. 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 Pulse-code modulation (PCM) is a method used to digitally represent analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio applications. In a PCM stream, the amplitud ...

. 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 In digital recording, an audio signal, audio or video signal is converted into a stream of discrete numbers representing the changes over time in air pressure for audio, or Color, chroma and luminance values for video. This number stream is s ...

formats that would follow (e.g. DAT and

ADAT Alesis Digital Audio Tape, commonly referred to as ADAT, is a magnetic tape format used for the Sound recording and reproduction, recording of eight digital audio tracks onto the same S-VHS tape used by consumer VCRs, and the basis of a serie ...

), 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 (Video Home System) is a discontinued standard for consumer-level analog video recording on tape cassettes, introduced in 1976 by JVC. It was the dominant home video format throughout the tape media period of the 1980s and 1990s. Ma ...

VCR would do, although 3/4" U-matic or

Beta Beta (, ; uppercase , lowercase , or cursive ; or ) is the second letter of the Greek alphabet. In the system of Greek numerals, it has a value of 2. In Ancient Greek, beta represented the voiced bilabial plosive . In Modern Greek, it represe ...

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 sound between audio components such as CD and DVD players, television sets, audio amplifiers, and mixing consoles. Generally, line level signals sit in the middl ...

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

microphone A microphone, colloquially called a mic (), or mike, is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and publi ...

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 A mixing console or mixing desk is an electronic device for Audio mixing (recorded music), mixing audio signals, used in sound recording and reproduction and sound reinforcement systems. Inputs to the console include microphones, signals fro ...

. Since the recorder had a significantly lower

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 noise is defined as any signal other than the one being monitored. In radio com ...

than most mixers of the same era, this method made the best use of the system's available

dynamic range Dynamics (from Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' " power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics), the study of forces and their effect on motion Brands and ent ...

. 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

angular velocity In physics, angular velocity (symbol or \vec, the lowercase Greek letter omega), also known as the angular frequency vector,(UP1) is a pseudovector representation of how the angular position or orientation of an object changes with time, i ...

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 or DBX may refer to: * dbx (debugger), a Unix source-level debugger * dbx (company), a professional audio recording equipment company ** dbx (noise reduction), a noise reduction system invented by dbx, Inc. * .dbx, the file extension for Micros ...

corporation of

Newton, Massachusetts Newton is a city in Middlesex County, Massachusetts, United States. It is located roughly west of Downtown Boston, and comprises a patchwork of thirteen villages. The city borders Boston to the northeast and southeast (via the neighborhoods of ...

, better known for their system of noise reduction for analog tape.

Technical specifications

Dynamic range Dynamics (from Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' " power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics), the study of forces and their effect on motion Brands and ent ...

: 110 dB typical with

A-weighted A-weighting is a form of frequency weighting and the most commonly used of a family of curves defined in the International standard IEC 61672:2003 and various national standards relating to the measurement of sound pressure level. A-weightin ...

noise 20 Hz–20 kHz; >105 dB unweighted *

Frequency response In signal processing and electronics, the frequency response of a system is the quantitative measure of the magnitude and Phase (waves), phase of the output as a function of input frequency. The frequency response is widely used in the design and ...

: 20 Hz–20 kHz, sine or pink noise, 100mV, reference record position *

Total harmonic distortion The total harmonic distortion (THD or THDi) is a measurement of the harmonic distortion present in a signal and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. Distortion facto ...

: less than 0.05%, 1V input, 1 kHz * Wow and flutter: less than 0.01% unweighted; 0.006% wrms * Anti-alias filtering: −3 dB at 37 kHz *

Sampling rate In signal processing, sampling is the reduction of a continuous-time signal to a discrete-time signal. A common example is the conversion of a sound wave to a sequence of "samples". A sample is a value of the signal at a point in time and/or s ...

: 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 ...

: 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 Delta-sigma (ΔΣ; or sigma-delta, ΣΔ) modulation is an oversampling method for encoding signals into low bit depth digital signals at a very high sample-frequency as part of the process of delta-sigma analog-to-digital converters (A ...

, 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 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 In electronics and electromagnetics, slew rate is defined as the change of voltage or current, or any other electrical or electromagnetic quantity, per unit of time. Expressed in SI units, the unit of measurement is given as the change per seco ...

.) The system also has two analog pre-processing steps which compress the input signal in both the

amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...

and

frequency domain In mathematics, physics, electronics, control systems engineering, and statistics, the frequency domain refers to the analysis of mathematical functions or signals with respect to frequency (and possibly phase), rather than time, as in time ser ...

, 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