Re: [linux-audio-dev] MCS: 64 bit timestamps?

New Message Reply About this list Date view Thread view Subject view Author view Other groups

Subject: Re: [linux-audio-dev] MCS: 64 bit timestamps?
From: David Olofson (audiality_AT_swipnet.se)
Date: ke loka   13 1999 - 18:57:31 EDT


On Wed, 13 Oct 1999, Eli Brandt wrote:
> Recently I did a little poking around about timebase stability --
> might be of interest. For pro gear, AES defines two classes, +/-1ppm
> and (I think it was) 10ppm. So even with the high-class gear, that's
> a few samples per minute of drift, which can't be considered minimal
> assuming we want sample-precise timing.

True. There's no such thing as two devices that don't need to be
actively kept in sync one way or another.

> Your basic uncompensated quartz crystal is speced at about 100ppm over
> its operating range. This is what most sound cards have.

Some 10 resyncs/second to stay within ±.5 samples then... (Unless I
did some miscalculation, that is. :-)

> > > ... set cycle_counter_at_last_sync_point in the normal way ...
> > > cycle_counter_at_last_sync_point +=3D (xxx - yyy) * cycles_per_sampl=
> > e.
> > >=20
> > > and we're done. whats so unclear ?
> >
> > Nothing, really... Just not sexy enough for me, I guess. ;-)
>
> And if I understand how this is used, doesn't this mean time can go
> backwards at update boundaries? If so, just fudge it so it stays flat
> for a bit instead. I really like monotonicity in my timebase --
> preserve ordering.

Yes. If we stick to sample accuracy, this is safe as long as the
resyncs are done frequently enough. But with higher timestamp
resolutions, some other solution is needed...

//David

 ·A·U·D·I·A·L·I·T·Y· P r o f e s s i o n a l L i n u x A u d i o
- - ------------------------------------------------------------- - -
    ·Rock Solid David Olofson:
    ·Low Latency www.angelfire.com/or/audiality ·Audio Hacker
    ·Plug-Ins audiality_AT_swipnet.se ·Linux Advocate
    ·Open Source ·Singer/Composer


New Message Reply About this list Date view Thread view Subject view Author view Other groups

This archive was generated by hypermail 2b28 : pe maalis 10 2000 - 07:27:59 EST