Re: [hatari-devel] Sound sync patch to play with

[Nicolas Pomarède <npomarede@xxxxxxxxxxxx>]

On 19/05/2012 05:26, David Savinkoff wrote:
> On Tuesday 15 May 2012 13:41, Eero Tamminen wrote:
> > Hi,
> >
> > On maanantai 14 toukokuu 2012, David Savinkoff wrote:
> > > I believe the latency is caused when a sound card does not consume
> > > the samples as fast as the computer provides them. This causes the
> > > buffers to eventually fill to the maximum before samples are discarded.
> > >
> > > A slow cpu will result in no latency, whereas a fast cpu could cause
> > > latency. My sound card is slower than the computer, and your sound
> > > card is faster (no sound card is the same speed as the computer).
> >
> > Isn't Hatari sound sample speed controlled by the selected sound frequency?
> Hi,
>
> It may be possible to make the sound card to use the 44100 Hz sample rate,
> while throttling YM2149 sampling with 44000 to 44200 Hz in a control loop.
> > I.e. isn't this a user error of selecting higher output HZ from Hatari
> > options than what the sound card supports?  And shouldn't Hatari audio
> > part reject / correct such a choice?
> The problem is that the sound FIFO is not controlled at a given level,
> therefore drifts to either overflowing or underflowing. The host
> computer puts samples into the FIFO at what it believes to be
> approximately 44100 Hz, and the sound card takes samples from
> the FIFO at what it believes to be approximately 44100 Hz, where
> Host sample rate != Sound card sample rate, and both are subject
> to independent frequency variations (temperature, age, tolerance).

I agree on this 2 different clocks (cpu and sound card),  but then I 
would assume this is the work of the audio callback function to trigger 
when the current sound buffer is played.

If a PC has such drift and the audio driver is correct, then I think the 
OS audio callback should be able to request sample a little sooner or a 
little earlier, which should have the same effect as your patch. Your 
patch adjust the video part to match the audio, a proper audio driver 
(which is the bridge between the audio hardware and the cpu) should IMO 
detect this itself by changing slightly the audio callback frequency.

Also, even if clocks are different between cpu and audio, we're just 
talking about 44.1 kHz most of the time, I'm rather surprised some sound 
hardwares are not able to keep this frequency with a unnoticable 
precision (especially since most sound cards are wired on the 
motherboard, so getting a proper clock common with the cpu should be 
possible).

No "offence" to your PC, but maybe you're using a crappy sound card or 
the audio driver was not correctly coded ?

Still, as your patch is not too intrusive and can be disabled, I will 
add it ; I'm quite skeptical it will be of use to lots of people, but 
who knows :)


Nicolas