Good question.

What Vegas does is resample on the fly to the incomming SMPTE clock.
When you chase Vegas to an external clock you will hear Vegas trying
to keep it locked up. It is very noticable when you first start as
Vegas is trying to lock up. This resample is very small - say the
differences between 44.1 and 44.025 sampling rate. There are lots of
details involved in this, but fundamentally the audio playback rate
is "profiled" and determined. Then this hardware clock is compared to
the MTC clock. The difference is then used to determine the amount to
slow down or speed up the data before it is played. This is going on
constantly. It is expensive.

There are limitations to how well this will work. When you start to
deal with multiple ports, chasing like this becomes very CPU
expensive.

The better solution is to have Vegas trigger only to SMPTE/MTC. Use a
word clock or some other digital clock that is common between the
master and the sound card. This way the sample rate is controlled in
hardware. Vegas only has to worry about when and where audio is to
start.

Does this explain it?

Peter



Dave Jacobs wrote:
>>I have a technical question about using chase mode during
>>recording and playback:
>>
>>How does it work? No really. What I am struggling to
>>understand is how can audio be played using chase mode and
>>be expected to stay in sync with an external source when
>>the clock that controls the sample rate is running
>>asynchronously with the outside world. I thought the audio
>>sample rate clock source was on the sound card itself.
>>
>>If the audio sample rate clock (located in the PC) is not
>>exactly synchronous with the external MTC source which is
>>coming from an ADAT BRC, then how can the small corrections
>>in timing be achieved (in order to correct for drift)
>>without compromising the audio quality?
>>
>>Is chase mode really controlling the timing of the audio as
>>if it were synced to an external clock?
>>
>>Dave
>>
>>P.S. Forgive me if I missed this subject in the manual. I
>>didn't see anything about this, but I've been known to miss
>>items that are in the manual before.
>>