linuxaudio.org

On Sat, Sep 26, 2009 at 06:28:12AM +0200, Florian Faber wrote:

quoted text
> Fons.

>

> > Not really. Suppose you have to do this:

> >

> > 1. copy an audio track from a video recording,

> > 2. do some work on it,

> > 3. copy the result back to the video recording.

> >

> > Your (nominal) sample rate is 48k, the video is

> > exactly 100s long when played at the exact frame

> > rate.

> >

> > If the the clock used by the video equipment and your

> > sample clock are not coherent (not derived from the

> > same source), there will be a relative error between

> > them, both in steps 1 and 3.

>

> If the source is analogue video (and analogue audio), you derive your

> word clock from the VBI/video lines or SMPTE, if you have this information.

>

> If the source is digital, well, there is no problem, since you work on a

> fixed number of samples.

>

> So please tell me how this can ever happen in real life?

Deriving the word clock from the timecode or video

frequencies, or ensuring both audio and video clocks

derive from the same source, is of course the real

solution to this problem. But it does assume you have

the hardware to do this.
Slaving a sample clock to SMPTE could be done

relatively easily in software provided you can

control the sample clock in software in small

steps. It doesn't even require decoding the SMPTE,

just extracting the bit clock is enough. This is

why I asked you about this feature in the RME

drivers some time ago.
Ciao,
--

FA
Io lo dico sempre: l'Italia è troppo stretta e lunga.
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