[Portaudio] How to obtain the time point when computer begins to record?

Ross Bencina rossb-lists at audiomulch.com
Mon Mar 21 16:36:31 EDT 2016


Hello Mengqi,

On 22/03/2016 3:20 AM, Mengqi Ren wrote:
> In my project, I need to get the computer time when the computer begins
> to record.

In my previous message I asked for your use-case. What I mean is: What 
do you intend to use the time measurement for? What is your overall 
goal? I also asked for your requirements, for example, what is your 
required time accuracy?

Please provide this information. It is difficult to provide you with a 
useful answer without it.

There is usually not one "computer time." On most platforms there are 
multiple time sources and/or multiple time APIs. Do you know the APIs 
that you intend to use as a time base? Do you need to correlate 
PortAudio timestamps with timestamps collected from other sources?

If it meets your requirements, I recommend using Pa_GetStreamTime(). 
Otherwise on OS X you should look into mach_absolute_time() and on 
Windows QueryPerformanceCounter() and/or timeGetTime().


> I'm using both Mac OS X and Windows. Which timestamp will be
> the highest accurate?What is the difference between Mac OS X and Windows?

Each audio API has its own behavior. The exact timing behavior may also 
depend on your sound card drivers.

Speaking broadly, OS X has more consistent timing, because Apple's code 
controls the scheduling of the CoreAudio callbacks, and timestamps are 
communicated throughout the API stack. On Windows, with most host APIs, 
PortAudio will regenerate timestamps when it receives the callback from 
the host. This can be subject to high jitter, since it is dependent on 
how the callback is scheduled, but once again, it depends on host API 
and drivers.

On Windows, PortAudio typically takes a time measurement and then 
subtracts the estimated latency to produce the input ADC time. On OSX, 
PA performs a similar calculation, but the timestamps are sourced from 
the driver, which is more accurate. On OS X, latency estimates are based 
on information from the driver, and are usually more accurate than on 
Windows, where the latency information is often estimated based on 
grossly incomplete information. PA never has access to information about 
all hardware latencies, so there are always limits to the accuracy of 
latency information.

In summary: callback timestamps are subject to jitter (variance between 
multiple measurements. Typically this is worse on Windows, but not 
necessarily. And further, they are subject to constant-offset error. The 
jitter can be reduced by filtering multiple time-stamp measurements. The 
usual way to deal with offset error is to do a calibrated loopback 
measurement -- but even this can't compensate for asymmetrical 
input/output latency.

As you can see, it's a big subject. As I said above, without knowing 
your requirements (and perhaps, how much time you're prepared to invest 
in getting a good time value) then it's difficult to give a more direct 
answer.

Ross.



> Thanks,
> Mengqi
>
> On Sun, Mar 20, 2016 at 8:39 PM, Ross Bencina
> <rossb-lists at audiomulch.com <mailto:rossb-lists at audiomulch.com>> wrote:
>
>     Hello Mengqi,
>
>     On 20/03/2016 7:58 AM, Mengqi Ren wrote:
>
>         I'm using PortAudio to record audio signals. In my project, I
>         need to
>         know the exact time when computer begins to record.
>
>
>     There is no such thing as "exact time". What are your requirements?
>     What is your use-case?
>
>         Would you please
>         tell me when the recordCallback function is involved and when
>         the first
>         frame of data enters the input buffer?
>
>
>     There are timestamps passed to the callback that you can use. Their
>     accuracy will depend on the platform (you didn't mention your
>     platform). At stream start time they are likely to be at their least
>     accurate. You might need to collect many timestamps and then process
>     them to estimate the start time.
>
>         Is there an API that can return
>         the computer time when microphone begins to record?
>
>
>     A combination of the callback timestamps and Pa_GetStreamTime()
>     should get you there.
>
>     The rest depends on your requirements and the platform(s) that you
>     are targeting.
>
>     This is old, but it might help:
>     http://www.rossbencina.com/static/writings/portaudio_sync_acmc2003.pdf
>
>
>     Ross.
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>
>
>
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