Re: [LAD] Library for accurate time stamping using a DLL

Fons Adriaensen wrote:
> On Tue, Jan 27, 2009 at 02:07:17PM +0100, Paul Davis wrote:
>
>> gettimeofday is a non-linear, non-mononotic clock if ntp is used. it is
>> not suitable for timestamping in this way. without ntp, it will work
>> better, but it is still subject to potential non-monotonicity.
>
> But for the purpose of using or generating OSC timestamps
> gettimeofday() is the only one that can be used, since
> it is the one shared by all concerned (if they ntp).
>
> How bad is this in practice for an ntp client that
> has been running for some time (i.e. after the initial)
> adjustments) ?

If I understand things correctly, there is no *right* way to get the system
time, it depends on the context: for absolute timestamps that travel over a
network, such as OSC timetags, gettimeofday() is needed. If the timestamps are
meant to stay within a system that has a single master clock, then
clock_gettime() is the way to go.

In this regard, ffmpeg should indeed use clock_gettime() because AFAICS, the
timestamps are only used to synchronize audio and video, during the conversion
operations, within a single application, on a single computer.

But, when developing a ffmpeg component (muxer, demuxer, ..), one shouldn't call
gettimeofday() nor clock_gettime() I think. One must calls av_gettime() which
hides the way the time is retrieved, for consistency with the ffmpeg API.

So, back to Pendule, I think I shouldn't call any of these functions, but let
the user retrieve the system time as he/she wishes.

That could be done by changing pendule_cycle() (which must be called at the very
start of the process callback, to update the DLL filter) into:

void pendule_update(double system_time);

This also has the advantage that in the case of JACK, one could subtract the
value of jack_frames_since_cycle_start() from the system time passed to
pendule_update(), to get closer to the exact time the current cycle started.

Is this correct to you?

-- 
  Olivier
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