AY Crudentials: 8 Bit Samples

Twilighte · Post by **Twilighte** » Sat Jan 21, 2006 2:19 pm

The hardware of the Ay-3-8912 can only handle up to 4 bit resolution but may handle up to 8 bit through the mixing of the 3 available channels.

A phenomenon exists that means that when the volume of channel A is set to one value, and the volume of channel B is set to another, the effective single output will not be one or the other, but a fraction higher.

Using this phenomenon it is possible to build up a virtual 8 bit resolution register using a combination of volumes accross the 3 available channels.

However, though 8 bit the speed is greatly reduced because all three volume registers must be sent information for every sample Value.
This reduces the quality of the sample and in my experience, 4 bit played at 10khz is better than 8 bit played at 5Khz.

Euphoric · Post by **Euphoric** » Mon Jan 23, 2006 11:59 am

Hello Jon, are you sure of that ? I would have imagined that two channels would produce a louder sound that a single one (more decibels)...
How have you get this value of "between 6 and 7" ? By hearing the two cases one after the other ? Or with an oscilloscope ? If you are right, I might have to change how Euphoric mixes the channels...

Cheers,

Fabrice

Twilighte · Post by **Twilighte** » Sat Jan 28, 2006 9:15 pm

Apologies Fabrice, the values where very much guessed (They were given more as an example than proven values). Suffice to say i have corrected the passage at the top.

JamesD · Post by **JamesD** » Mon May 25, 2009 11:19 pm

I think this provides a good explanation of the logarithmic nature of the AY PSG volume.

http://map.grauw.nl/articles/psg_sample.php#8bitsamples

I created a fully commented version of the Z80 assembly if anyone is interested.

JamesD · Post by **JamesD** » Mon May 25, 2009 11:26 pm

Another MSX topic on PCM audio playback.

http://www.msx.org/forumtopic5686.html