Re: ALF AMII Clone

[Phil Tubb <googleplus@w127.com>]

On Tuesday, March 12, 2013 1:18:47 PM UTC-7, Michael J. Mahon wrote:
> Phil Tubb wrote:
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> > On Sunday, March 10, 2013 5:28:23 PM UTC-7, Steve Nickolas wrote:
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> >> On Sun, 10 Mar 2013, Phil Tubb wrote:
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> >>> The Mockingboard used the AY-3-8910 chips, which have an error in the 
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> >>> waveform they produce, resulting in anything from popping noises (with 
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> >>> slow volume changes) to inappropriate low frequencies (with fast volume 
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> >>> changes, as is typical for "envelope" control).  If you download the 
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> >>> Sept 1981 issue of Creative Computing from archive.org
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> >> I wonder if the YM-2149 fixes that.
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> >> -uso.
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> > Hard to tell from the Yamaha data sheet. From the first waveform
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> > screenprint, it appears to not be fixed.  The second waveform screenprint is illegible.
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> Changing to a bipolar output would require bipolar power, and would be
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> intrinsically incompatible with the GI chip. 
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> Changing to a +2.5v "zero" reference, so that the DC component of the
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> output would be independent of the volume would have been possible, but
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> also incompatible with the GI specs. 
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> -michael - NadaNet 3.1 and AppleCrate II: http://home.comcast.net/~mjmahon

Michael, good point.  The TI SN76489, used in ALF's 9-voice card, only has one power supply pin, so they must have used +2.5V (or some such) as "zero".  Of course it doesn't matter what voltage you use for "zero", as long as you never change what voltage it is (which the AY-3-series essentially does, every time you change the volume).