SCC sound output buggy?

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Por Edwin

Paragon (1182)

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21-02-2008, 14:33

In case you feel like experimenting some more. Check these two waveforms:

	db	$FC, $0C, $0C, $0C, $0C, $0C, $0C, $0C
	db	$0C, $0C, $0C, $0C, $0C, $0C, $0C, $0C
	db	$0D, $00, $00, $00, $00, $00, $00, $00
	db	$00, $00, $00, $00, $00, $00, $00, $00

	db	$FC, $0C, $0C, $0C, $0C, $0C, $0C, $0C
	db	$0C, $0C, $0C, $0C, $0C, $0C, $0C, $0C
	db	$10, $00, $00, $00, $00, $00, $00, $00
	db	$00, $00, $00, $00, $00, $00, $00, $00

I tried playing something with these waves on channel 1 (not even 4 or 5). The first one is good, the second one sounds awful. Not something that the single different byte should cause.

Por ARTRAG

Enlighted (6844)

Imagen del ARTRAG

21-02-2008, 14:37

Edwin, your theory needs at least the mix of two channels otherwise I do not see any "adder" at work.
Was you using the same waveform on two or more channels ?
If not, how do you explain that playing only one single channel gives strange results ?

Por Edwin

Paragon (1182)

Imagen del Edwin

21-02-2008, 14:46

I think you can expect adders in the volume multiplication as well. In fact, it may be the major source of the problems since I've been able to reproduce all problems with only a single active channel.

Por ARTRAG

Enlighted (6844)

Imagen del ARTRAG

21-02-2008, 14:56

In Dvik's guess, problems rise when accessing to the ram registers (any of them, waveform
table or frequency) while the SCC is reading some samples in the wavetables.
I remember that for some strange reason, when reading or writing to the ram registers while
sample 14 or 15 was being played, the output resulted corrupted.
I think that by trial and error dvik has found a timing for accessing to the SCC registers without
affecting the output.
This would go against your theory of the adder....
(Look at Leila K demo)

Por Edwin

Paragon (1182)

Imagen del Edwin

21-02-2008, 15:08

I discussed this with dvik a while back. However, as I understand it, that problem seems to occur when there is some secondary access to the SCC (like reading or writing a wave). This problem persists when there is no access to the SCC at all. For instance, when you do a "Loop: jp Loop" when the SCC slot is not selected, it will continue giving the spikes in the output.

Por hap

Paragon (2040)

Imagen del hap

22-02-2008, 12:06

In case you feel like experimenting some more. Check these two waveforms: (...) I tried playing something with these waves on channel 1 (not even 4 or 5). The first one is good, the second one sounds awful. Not something that the single different byte should cause.I can't confirm: http://home.planet.nl/~haps/crap/mrcscc.png

Por Edwin

Paragon (1182)

Imagen del Edwin

26-02-2008, 15:34

That looks pretty much as it should. Although the it is funny how unstable the constant value sections are. I must admit that I never analysed this case. I noticed in while testing my wave volume multiplier and sometimes heard distinct secondary tones in the sound.

IIRC, I did notice a big difference between the Nemesis 2 and the Flash ROM scc in this test. It appears that the SCC in the nemesis 2 cart is more heavily filtered in the DAC.

Por hap

Paragon (2040)

Imagen del hap

26-02-2008, 17:41

Although the it is funny how unstable the constant value sections are.I believe that's mostly due to constant low amplitude noise on my soundcard line-in, even if nothing's connected to it. This was just a rough measurement for the $10 thing.

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