Re: [hatari-devel] Questions on Falcon DMA sound

[Christian Zietz <czietz@xxxxxxx>]

Roger Burrows schrieb:

> Quite possibly the documentation is just wrong.  However, in TOS4, there are
> manipulations of 'mystery bits' that are not documented in the development
> notes or the Compendium.  Specifically:
> 	. bit 3 of $ff8937 is toggled
> 	. bit 2 of $ff8938 is toggled
> 	. bit 3 of $ff8938 is cleared (only)
> [...]
> But meanwhile I would be interested to hear comments from the smart hardware
> people about those mystery bits.

There is this collection of technical documents that among other things
covers the Falcon. Unfortunately, just the document that would contain
these registers is damaged, so that only the text version (without
formatting) survives, which is a bit hard to interpret.

There, I can see:

> xxFF8936 DAC and Record Control (RW)
>
> D15D8
> D7D0
>
> 000000
> rr
> 0000
> r
> p
> e
> a
>
>     rr - Record Channel Select
>          00 - Record tracks 1 and 2
>          01 - Record tracks 1 thru 4
>          10 - Record tracks 1 thru 6
>          11 - Record tracks 1 thru 8 ( ie. all )
>
>     r  - Global Sound Reset
>          1  - Reset Sound Subsection
>          ( Not self clearing )
>
>     p  - Input Select1
>          0  - CODEC ADC
>          1  - PSG
>
>     e  - Matrix output to CODEC enable2
>          1  - Enable matrix data output to the CODEC
>
>     a  - Alternate data output to CODEC enable1,2
>          1  - Enable PSG data output to CODEC

Hence bit 3 of $FF8937 should be "Global Sound Reset". And...

> xxFF8938 AUX A Control Field (RW)
>
> D15D8
> D7D0
>
> L16-L19
> expn
> mute
> mux
> left gain
> right gain
>
>     L16-L19 - Left sample 4 least significant bits
>
>     expn  - Expand
>
>     mute  - Mute
>        1 - Mute output
>
>     mux   - Input Mux1
>      00 - Channel 1 (microphone)
>      11 - Channel 2 (PSG input)
>
>     left gain , right gain
>
>     0000 - 0 dB gain for ADC
>        .
>             .     ( 1.5dB increments )
>        .
>          1111 - 22.5 dB gain for ADC

Thus, bit 3 of $FF8938 would be "Expand" (whatever that is) and bit 2
would be "mute". No idea why one would toggle "mute", though. Also, I
don't know if everything is actually implemented in hardware or some
bits were only planned for future revisions of the Falcon that never
came to be.

Regards
Christian
--
Christian Zietz  -  CHZ-Soft  -  czietz@xxxxxxx
WWW: https://www.chzsoft.de/
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