AY sounds

[Pere Serrat]

I have a source file that contains three sound definitions. The data begins like that:

-------------------------------------------------------

; Sounds.

fx1 defb 128+15 ; volume and mixer.

     defb 31         ; white noise.

     defw 1000     ; tone register.

-------------------------------------------------------

Three are 10 blocks of 4 bytes and then a defb 255 as an end of sound tag.

I need to convert these bytes to be used in the 6809 engine for CoCo and Dragon computers.
We are using an external cartridge that has a SN76489 chip that emulates well the

AY-3-891x at music level, but it is very different when coming to sounds/effects

So, I need to understand what these four bytes are intended for ...

1) I assume the first one is the volume of the noise channel and in this case it is maximum value (15)

2) Next value determines the 'noise' frequency according to this fomula:

f = 1.78MHz/(16 x 31) here it will be 3588.71 --- but it could also be
f = 0.89MHz/(16 x 31) = 1794.35

So I need to know what frequency is used to feed the AY-3-891x inside the ZX-spectrum 128.
I have not found that info in Inet

3) Last word 1000d = $3e8 seems to be the factor to create a sound in a music channel, giving:

f = 1.78MHz/(16*1000) = 111.25Hz or 55.62 using 0.89MHz
. Again the main frequency being very important.

This seems to be a mix of a white noise sound and a low frequency square wave out of two different channels

Could anyone, please, confirm I am not wrong and tell me the right frequency value I should be using for the other sound values?



Thanks beforehand

pere

[Pere Serrat]

It seems that the frequency could be 1,7734MHz for ZX-Spectrum 128K
Anyway it would be good if someone could confirm this point.
cheers
pere

[Jonathan Cauldwell]

Hi Pere,

I can confirm that the clock frequency of the AY chip is 1.7734MHz and the basic unit of tone is the frequency divided by 16, so that's 110.83KHz.

The first byte is the volume with mixer bits d6 and d7 to determine whether tone and/or white noise is played.
The second byte is the white noise frequency. The period is taken by counting down the 5 low bits every sound clock period divided by 16, so values 0 to 31 only.
Bytes 3 and 4 are the tone frequency. As these are all 12 bit values frequencies from 27Hz to 110KHz are possible.

I know you're liaising with some of the other AGD developers but if you need any help from me just drop me a PM and I'll give you my email address, or send a friend request on Facebook.