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From: legrady@ug.cs.dal.ca (Tom Legrady)
Newsgroups: comp.dsp
Subject: Re: 1 Bit D to A Revisited
Summary: Delta Modulation, Adaptive Delta Modulation discussed
Keywords: Delta Modulation
Message-ID: <1989Nov22.162629.11325@ug.cs.dal.ca>
Date: 22 Nov 89 16:26:29 GMT
References: <8911201702.AA03844@en.ecn.purdue.edu>
Reply-To: legrady@ug.cs.dal.ca.UUCP (Tom Legrady)
Distribution: na
Organization: Math, Stats & CS, Dalhousie University, Halifax, NS, Canada
Lines: 47

This general family of conversions is known as Delta Modulation. As Paul Zawada
says, you use a single bit to indicate whether the output should be incremented
or decremented.  The nice thing about the method is the simplicity of the
conversions:

	On A->D, if the analog signal is greater (less than) the reference, 
	then output a 1 (0) and increment (decrement) the value of the 
	reference before the next sampling.

	On D->A, if the input is 1 (0), then increment (decrement) the 
	counter and send the result to the DAC.

If the signal has not changed at all, the result is least significant bit
jitter as the reference signal goes just above, then just below the actual
input.

As you can see, this method eliminates successive aproximation loops and other
complications. For the method to be successful, it is essential that the 
change in the signal be very small between samples. Normally this means
that the sampling rate is much higher than with SA or flash conversion
techniques.

An ordinary converter at 44 KHz could change from one extreme value to the
other from one sample to the next.  This occurs only on the edge of Nyquists
limit, and the presence of such changes makes it likely that signals over the
limit could occur. Usually, signals change in an orderly manner.  If that
converter wasa 16 bit SA converter, the A->D was actually operating at about
 .7Mhz.  A DM converter at .7Mhz could, in the same interval, only change by
 16 samples. For a full-scale triangle wave input, the maximum frequency which
 can be followed is 10 Hz. 

Usually, the situation is much better, but this inefficiency is one of the
reasons for the developement of Adaptive Delta Modulation techniqes.
Essentially, these are two-bit converters: one bit says whether the value
should be increased or decreased, and the other says whether the size of the
increment should be increased or decreased.

Thus while the basic DM degenerates to a ramp, the ADM degenerates to something
closer to successive approximation.  With these variations, the joints benefits
of (relatively) simple one stage converters and decreased data storage 
requirements are obtained.  If you want to process the music, as in a
synthesizer or effects box, you're better off with parallel techniques. If your
application calls for the storage of data in compressed form, and especially
if your medium is serial to start with ADM type methods are ideal.  That's
why it was used in (I think it was the ..) DBX 700, an Industrial quality
digital audio system which stored stereo audio of very high quality on a
consumer Beta or VHS video tape, disguised as a video signal.