Δ DIGITAL AT DBX
above: The dbx 700 digital processor uses delta (Δ) modulation,
not PCM.
As I write this, the 72nd Audio Engineering Society Convention will be getting underway in a few days. Thus, it will be "autumn in Anaheim," as the convention holds forth at the Disneyland Convention Center. This will be the first of the "new era" AES conventions. Up to now, there has been an AES convention in an overseas location in March, a May convention in Los Angeles, and-"autumn in New York"--a fall convention at the Waldorf. Under a new AES policy, there is one overseas convention and one convention alternating between the East and West Coasts in this country each year. This 72nd AES Convention in Anaheim will be followed by the 73rd in Amsterdam in March 1983, and the 74th at the Waldorf in New York in late October 1983. Obviously, under this setup there will be an interval of two years between conventions on the West Coast and the East Coast. Considering the rapid developments in audio technology, some feel this is too long a period. Time will tell.
As I have noted before, digital audio is "hot" these days, and you can be certain there will be many new developments in digital audio at the AES Convention. We can expect second- and even third-generation PCM digital recorders. There are sure to be various models of the Compact DAD (digital audio disc), especially since the DAD was officially launched in Tokyo late in September.
Among all the PCM digital recorders at this convention will be a surprise package from dbx. This enterprising company, heretofore associated with noise-reduction equipment, will be unveiling their new Model 700 digital audio processor. The real eyebrow-raiser here is that instead of the ubiquitous PCM (pulse-code modulation) used in current digital processors, the dbx 700 uses delta modulation.
In delta modulation, the numbers produced by the system's A/D (analog-to-digital) converter represent differences between successive sampled voltages, rather than a PCM adaptor's instantaneous voltage of the input signal at each point of time. Delta modulation is not new; about 10 years ago I reviewed the Delta T digital delay system in Audio. (Incidentally, Barry Blesser, one of our foremost digital experts and outgoing President of the AES, was a co-developer of this system.) I believe the manufacturing company evolved into Lexicon, well-known for their PCM digital delay systems.
There are some inherent disadvantages in delta modulation which up to now have precluded its use in digital audio recording. Dynamic range was limited, noise modulation problems ("breathing") were present, and the noise floor exhibited distinct tonal characteristics. On the plus side, delta modulation has inherently less sensitivity to bit errors. Its anti-aliasing filters are simple, easy to build and have small phase shifts, as compared to the complex filters and large phase shifts of PCM equipment.
Some years ago, delta modulation was improved by what was known as Adaptive Delta Modulation (ADM). In the words of Robert Adams, Senior Project Engineer of dbx, "In ADM, the digital number produced by the A/D converter is allowed to represent varying differences (step sizes) between successive samples. When the input signal changes quickly, the step size becomes larger, producing a digital output which tracks the input. When the input signal changes slowly, the step size is adjusted ('adapted') to be smaller, to more accurately reproduce the waveforms. By adjusting the step size to suit the input signal, ADM extends the dynamic range of delta modulation." This ADM is a definite improvement over conventional delta modulation, but still is not good enough for serious recording.
Stimulated by the possibility of a low-cost, professional digital processor, Mr. Adams and his team of engineers set out to resolve the problems of ADM. They succeeded in the development of two innovative circuits which essentially give their delta modulation processor performance equal to that of a 16-bit PCM processor. Mr. Adams describes the dbx Companded Predictive Delta Modulation (CPDM) circuits thusly: "The analog-to-digital conversion process in the dbx Model 700 differs from that used in ADM in two important respects. First, in ADM, step size is varied to follow the signal.
The dbx converter uses a precision compander in which the signal itself is varied with a voltage-controlled amplifier to avoid overloading a fixed delta modulator. Second, the dbx delta modulator uses a 'linear-predictive filter,' which relies on the recent history of the audio signal to predict its future.
These two differences result in substantial improvements in performance." The use of a fixed step-size, non-adaptive delta modulator is particularly important. For example, "dither noise" (a white noise added to the signal in order to mask quantization noise) cannot be used in ADM. However, in the dbx 700, dither noise can be added at the input of the fixed delta modulator to eliminate the noise floor anomalies (audible tonal effects) that plague ADM. In addition, the use of the linear predictive filter can increase the dynamic range of a fixed delta modulator by more than 10 dB, which is enough to eliminate all possibility (so it is claimed) of audible noise modulation.
The dbx Model 700 is meant to be used with professional U-matic helical-scan video recorders, although con sumer VHS and Beta video recorders can be used as well. Because of the video format, the dbx 700 is furnished with a memory that has 16K bits of RAM (random access memory) storage for wow and flutter absorption, data interleaving and de-interleaving, and video requirements. As Robert Adams points out, the A/D converter produces a steady stream of bits during record. Since the video format has several areas where data cannot be recorded, the memory is asked to store the data bits during these times. Of the 16K memory, 8K is assigned for data interleaving (time scrambling) and 4K for storing data during the video-synch intervals.
During playback, the memory must supply the D/A converter with a steady stream of data while receiving the data from the VCR. Since the VCR has wow and flutter, this causes variations in the bit rate sent to the memory. The memory absorbs these variations with the last 4K bits of storage, and the result is a claim of very low flutter (less than 0.01%) in the decoded signal.
According to Adams, his error-correction system works "by adding one extra parity bit for every three data bits.
The parity bits are mathematically derived from the data bits so that any bit-errors on playback will produce a unique error pattern in the received parity bits. This error pattern is decoded to find exactly which bits are in error, and the offending bits are then corrected. This correction circuit works in conjunction with the memory interleaving in such a way that a long burst error is presented to it as a series of short errors separated by good data." The dbx Model 700's error-correction circuit is claimed to be so good that it will completely correct dropout or burst errors up to 1,024 bits long, even if they occur as frequently as 240 times per second. An LED flashes when a bit is being corrected, which also gives the user some information about the videotape quality.
The dbx 700 is a two-channel system, with frequency response claimed to be 10 Hz to 20 kHz, ± 0.5 dB. Dynamic range is quoted at a breathtaking 110 dB! Total harmonic distortion is said to be less than 0.03% at 1 kHz, 1 volt rms, with wow and flutter less than 0.01% unweighted. Sampling rate is quoted in a rather odd fashion as "700k bits/sec." Some quick work on a calculator will show that this would be roughly equivalent to 16 bits at 44.056k/S in the PCM world.
The new dbx unit is rack-mountable and measures 5 1/4 in. H x 19 in. W x 11 1/2 in. D.
It weighs in at 20 pounds, and construction is totally modular.
There is a metering system of two columns of 30 LEDs switchable for three separate functions. A microphone preamp is optional. The front panel has input, output, and microphone preamp modules with appropriate controls. The price is "less than $5,000" which certainly qualifies it as the lowest priced professional digital processor on the market. Needless to say, the 700 is an exciting development.
It must be noted that dbx has said nothing about editing facilities for this unit. There is some talk about a fixed-head transport on which "razor-blade" editing would be possible. A consumer model of the processor is said to be in the works, and I would hazard a guess that it will cost between $1,000 and $1,500. Here, the lack of editing facilities would place it in the same class as other consumer digital processors on the market, so this would not be a drawback.
In any case, the dbx 700 digital processor breaks some new ground with its delta modulation, and it will be interesting to see the reactions.
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(Audio magazine, Jan. 1983; Bert Whyte )
Also see:
Dr. Thomas Stockham on the Future of Digital Recording (Feb. 1980)
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