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From: eta@ic.Berkeley.EDU (Eric T. Anderson)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Simultaneously played digitized sounds
Keywords: divide
Message-ID: <25974@pasteur.Berkeley.EDU>
Date: 6 Jul 90 01:02:41 GMT
References: <1032@swan.ulowell.edu> <36690003@hpindwa.HP.COM>
Sender: news@pasteur.Berkeley.EDU
Reply-To: eta@ic.Berkeley.EDU.UUCP (Eric T. Anderson)
Organization: U.C. Berkeley EECS CAD Group
Lines: 36
X-Local-Date: 5 Jul 90 18:02:41 PDT

In article <36690003@hpindwa.HP.COM> krisl@hpindwa.HP.COM (Kris Livingston) writes:

>
>...  The trick to avoid the
>nasty distortion is to pre-divide the values in each of the waveforms by 4
>(the number of voices being combined).  The advantage to this is that the
>only processing your VBL task needs to do is addition.  The disadvantage is
>that the volume (loudness) of the overall sound is reduced.  You could
>experiment with dividing the waveform data by 2 (instead of 4) and then
>check for overflow.  (If sum > 255 then sum = 255).  I know, it's crude,
>but I've used it and it works--just don't plan on using the CPU for much
>else while this is going on.  :-)
>

Hey -- be careful.  You know you're throwing away bits for no reason
if you pre-divide.  How about add first and then post-divide?  On a
Mac, doing byte-addition is just as quick as 16-bit addition (except
if you're adding constants). Right?  Why add to your distortion
(especially for soft sounds where the lower bits make up for a lot of
the sound level)?

Let me point out another possibility that you might consider:

You want four sounds to play, so you add them up.  Map a log function
onto the continuum of 1024 possible values you can get by adding.  Map
it linearly for low sound levels (so it approaches a ramp at small
values) and slope it down for higher sound levels as you approach
1024.  It seems to me that this might be a good idea and it only costs
you a 1K-byte table.  (add + lookup = cheap, right?)

Yeah, okay -- I haven't tried this, I'm just speculating.

Best Wishes

-Eric Anderson
eta@ic.berkeley.edu