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From: williams@kirk.DEC (John Williams 223-3402)
Newsgroups: net.analog
Subject: Synthesizer Recommendations
Message-ID: <3613@decwrl.UUCP>
Date: Wed, 14-Aug-85 10:37:59 EDT
Article-I.D.: decwrl.3613
Posted: Wed Aug 14 10:37:59 1985
Date-Received: Mon, 19-Aug-85 06:48:27 EDT
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	To answer some of your questions:

	Companding DAC's are neither linear nor logrithmic. This makes
computation a major pain in the neck. Companding DAC's have an
exponent and a mantissa. A true logrithmic converter would be the
ultimate choice for modulation techniques, whereas a linear DAC would
be best if you wanted to add alot of waveforms together. The advantages
of a companding DAC would not be significant inside a closed box, the
major advantage being noise immunity, and hopefully you could control
this fairly well inside the box.

	The best realistic S/N ratio you should ever want is 95 dB. This
is the limit for the human ear. This works out to 18 bits. 12 bits will
yield about 70 dB, which is probably good enough for alot of applications.
You can use a notch filter to improve this by setting it at the sampling
frequency. There is very little you can do about the lower harmonics that
are generated by the quantization error.

	I would seriously look into MIDI. Any piece of equipment that is
worth anything will have this interface, because:

	1) It allows digital recording

	2) It allows synchronized playback

	3) It allows modularity of different components.

	4) It's *REAL* cheap to impliment.


	Some Suggestions:

1) Set the sampling rate at something that fits nicely with the temperment.
Most modern keyboards are set at equal temperament, which is a compromise
of harmony for transposability.

2) Do some research on the MIDI standard and include it.

3) Use lookup tables for all your functions except the really simple ones.
This will probably mean a rather large ROM. Reconstruction just doesn't
cut it at those speeds unless you have a CRAY 1.

4) Play test it. Get some professional musicians to try it.

				Good Luck,
						John Williams