[vorbis-dev] "wrong" total time from ov_time_total

[Michael Smith]

On Friday 12 September 2003 08:31, Rodrigo Gómez wrote:
> Hello there.
>
> I'm finding that ov_time_total doesn't return the "expected" value for some
> files I'm encoding. How do I know that? Well... in the encoder, when I push
> the "record" button I store the time when I did it, and when I push the
> stop button I store also that time. I would expect that the time returned
> by ov_time_total would be some ms less than finish_time - start_time, but,
> in some cases, it is up to 5 minutes less. The strange thing: It doesn't
> seem to loose data. I mean, I can hear to the audio and hear that
> everything that it's supposed to be it's there, but the time says
> different.
>
> So, it's ov_time_total accurate? I'm having some other more serious
> problems, as I store events that are time related, and I need to seek to
> that specific times.

There may have been bugs in this function, i think, in some previous versions 
- but they resulted in completely wrong results, not slightly wrong ones. So 
you're not seeing that - and ov_time_total() will not give 'slightly wrong' 
results, ever.

>
> What I have done to "solve" this problem is get a "time factor"
> (expected_time/ov_time_total) and apply it to seek and so on, but I'm not
> happy with this, as I would like to know the cause, and not only solve an
> effect of the problem.
>
> This problems seems to be more present when saving non 44.1 Khz audio, for
> example, 8,820 Hz or so. I haven't tryed with sample rates above 44,100 Hz.
> Does something special has to be used to record audio at different sample
> rates? I'm correctly passing the sample rate and the number of channels to
> the encoder.

The most likely (I think) reason for this is that your sample clock 
(presumably on some actual audio hardware) is drifting - it's pretty common 
for them to be slightly off, but if you're seeing large differences it may be 
that your hardware is particularly bad. 

ov_time_total() relies on the sample rate - so if you've told the encoder that 
you're recording at 8820 Hz, but the _actual_ clock is, say, 8825 Hz, then 
it'll be marginally wrong. This is a hardware defect, not a software one - 
the only thing you can do is try to make your software determine the "actual 
audio clock".

Mike

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