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From: daniel@hexagon.pkmab.se (Daniel Deimert)
Newsgroups: comp.sys.atari.st
Subject: Four voices on a 3 voice soundchip (was: Re: Demos and Games are not the Same)
Keywords: programming, sound
Message-ID: <191@hexagon.pkmab.se>
Date: 30 Jun 90 15:01:18 GMT
References: <1990Jun27.201016.27871@wam.umd.edu> <190@hexagon.pkmab.se> <815@ehviea.ine.philips.nl>
Reply-To: daniel@hexagon.pkmab.se (Daniel Deimert)
Organization: Hexagon Computer Club, Tech College of OErebro, Sweden
Lines: 30

In article <815@ehviea.ine.philips.nl> leo@ehviea.UUCP (Leo de Wit) writes:
>
>Uh, how do you do 4-channel sound on a 3-channel sound chip? Yes, I
>understand you can quickly change pitches on a channel to simulate a
>second channel, but I think this will sound awful. So how do you do it?

It's even possible to use just _one_ of the soundchip channels, if you
don't care much about "hi-fi" :-)

Seriosly spoken, the basic principle could be described like this:

  a) all sounds are samples -- not "beep"-style (ym/gi soundchip, that is)
  b) play four sounds at the same time, by simply adding them together

  The hard work is to calculate when to play what...  When you
whish to change the frequency of the sound, just play it slower or
faster. This effect can easily be achieved by using tables with true
or false values, which of course are pre-calculated to save time.

  I think there are a few commersial programs on the market that makes
use of this technique.  [ One is from Tommysoft {Germany}, the other
program I know of is from Worksoft {Sweden}.  (Actually, a Swedish
demo-progammer from the more or less known group "SYNC" is the author) ]

Hope this helped you out,
daniel

-- 
Daniel Deimert			   "Only use unneccessary force if neccessary!"
daniel@hexagon.pkmab.se			 ...!sunic!kullmar!pkmab!hexagon!daniel