🚀 go-pugleaf

In article <1990Jan31.034508.1304@cec1.wustl.edu> sjs@wucs1.wustl.edu (Steven J. Sadoff) writes:
>In article <29559@dhw68k.cts.com> of comp.binaries.mac, Juri Munkki writes:
>>	1) Start with Old=128
>>	2) Read a byte into New
>>	3) Write Old-New
>>	4) Old=New
>>	5) Go to step 2 if there is more data
>
>This implementation of delta modulation is NOT FULLY REVERSIBLE.
>Step #3 is flawed.  It does not handle overflow
>(i.e., subtraction of two 8 bit numbers can yield a 9 bit number).
>If you do not want corrupted sounds, DeltaSound 1.0 should only be used
>on backup copies of your sounds.

I appreciate that you contacted me by mail, but you didn't tell me you
posted your warning. I noticed this article by pure chance.

Your description of the flaw is flawed. Always try out an example before
saying that something doesn't work. I assume you thought that the case
where we have 0 and 255 (or anything with a difference >127) doesn't work.
Let's try out this particular case:

	Old=0
	New=255
	Old-New=-255 which is truncated into 8 bits -> 1

Now when the process is reversed, we discover that subtracting 1 from 0
leads to -1, which is again truncated into 8 bits -> 255.

The algorithm works because of wrap-around.

I'm usually quite careful when posting anything. In this case I tried
converting stuffit-archives. Stuffit keeps a checksum on everything, so
even the slightest change would have triggered an alert. Always doubt
anything you see on the net: in this case it's ok that my work was
doubted, but without proof, I hope most people had doubts about the
warning.

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|     Juri Munkki jmunkki@hut.fi  jmunkki@fingate.bitnet        I Want   Ne   |
|     Helsinki University of Technology Computing Centre        My Own   XT   |
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