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SubjectYM2612 Envelope Generator new Reply to this message
Posted bybcs
Posted on05/16/06 08:31 PM



Hi,

I'm trying to convert the envelope generator's attack/decay rates, the 6-bit values, into actual time. The only documentation of the YM2612 that I know of
(http://www.smspower.org/maxim/docs/ym2612/index.html) doesn't talk about this at all. Some manuals for related chips (OPL3, OPM) give tables, but they are different from one another and the different clocks probably mean that the tables don't apply.

Does anyone know exactly what the chip does with these rate values, and where the emulator writers know that from?
Also, if someone knows where I can get the YM2612 manual, I would appreciate it (although the exact wait mechanism isn't likely to be described since it isn't in other Yamaha FM chips' manuals).

Thanks,
Benjamin





SubjectRe: YM2612 Envelope Generator new Reply to this message
Posted bysmf
Posted on05/31/06 03:50 PM



I would imagine the chip loads the values ( or the result from a lookup table ) into a counter and decrements the counter on each clock. Previous implementations may have been based on documentation, guess work or timings from a real chip.

smf





SubjectRe: YM2612 Envelope Generator new Reply to this message
Posted bybcs
Posted on06/02/06 08:38 PM



Thanks for the answer. Acutally it's more complicated than that, because time periods depend more or less exponentially on the 6-bit rate values. And because for higher rate values, the time periods decrease.

The similar chip YM2413's manual gives a table:
http://www.smspower.org/maxim/docs/ym2413am/index.html#page13

The time is exactly divided by 2 every time RM increases, but it's difficult to devise a precise formula, and in any case I'm not even sure that during this period the volume decreases/increases linearly.

This could look like a detail that the ear wouldn't discriminate, but it's not because of the nature of FM synthesis (the resulting sound is not heard directly but is used to modulate another sinusoid). Very small time differences make big timbre differences in the final sound.





SubjectRe: YM2612 Envelope Generator Reply to this message
Posted by_Stef_
Posted on07/10/06 06:14 AM



Hi,

I searched for a good technical document for age, i finally used the MAME core constantes to write my own core. Another way is to do hours and hours of tests with a genesis flash card ;)

- Stef

> Hi,
>
> I'm trying to convert the envelope generator's attack/decay rates, the 6-bit
> values, into actual time. The only documentation of the YM2612 that I know of
> (http://www.smspower.org/maxim/docs/ym2612/index.html) doesn't talk about this
> at all. Some manuals for related chips (OPL3, OPM) give tables, but they are
> different from one another and the different clocks probably mean that the
> tables don't apply.
>
> Does anyone know exactly what the chip does with these rate values, and where
> the emulator writers know that from?
> Also, if someone knows where I can get the YM2612 manual, I would appreciate it
> (although the exact wait mechanism isn't likely to be described since it isn't
> in other Yamaha FM chips' manuals).
>
> Thanks,
> Benjamin
>



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