Re: iPhone RemoteIO timeline strange behavior
Re: iPhone RemoteIO timeline strange behavior
- Subject: Re: iPhone RemoteIO timeline strange behavior
- From: Jeff Moore <email@hidden>
- Date: Thu, 19 Nov 2009 09:28:23 -0800
Your math around the sample time looks wrong to me, but perhaps there isn't enough context here to judge.
If you are looking for the number of samples since an anchor point, you'd simply subtract the current time in samples from your anchor time in samples. I don't understand why you are also multiplying by the sample rate here. If you wanted to convert that into a duration in seconds, you'd actually want to divide by the sample rate, not multiply.
The calculation for converting mach_absolute_time to nano seconds seems correct, though.
So, perhaps you ought to explain how you are deriving this calculation and what it is for, because it doesn't seem to me to be measuring elapsed time in samples.
On Nov 19, 2009, at 9:09 AM, Zachary Kulis wrote:
> I am trying to synchronize input (recorded) and output audio samples
> using the RemoteIO timeline. Unfortunately, I am getting inconsistent
> timestamp information between the mSampleTime field and the mHostTime
> fields in the AudioTimeStamp struct.
>
> My first question is: does the mSampleTime provided in the input and
> output callbacks refer to the same audio timeline? Below, I outline the
> code I am using to produce the issue...
>
> Since the RemoteIO input callback is always invoked before the output
> callback, I simply set sampleTime0 equal to the first input timestamp
> (mSampleTime). I do the same thing for hostTime0 using the first host
> timestamp. For each subsequent callback, I compute elapsed times using
> these initial timestamps. This code is shown below (assuming 16kHz
> sampling):
>
> sampleTimeElapsed = (sampleTimeCurrent - sampleTime0)*16000
> hostTimeElapsed = AudioUtils::elapsedTime(hostTimeCurrent - hostTime0),
> where elapsedTime() is defined below (I know that this function works
> correctly).
>
> Float64 AudioUtils::elapsedTime(UInt64 elapsed) {
> mach_timebase_info_data_t info;
> mach_timebase_info(&info);
>
> elapsed *= info.numer;
> elapsed /= info.denom;
>
> return (elapsed*1e-9);
> }
>
> The problem is that the results are inconsistent. On the iPhone (2.2.1),
> the elapsed host and sample times for the input are roughly equal, but
> drift apart as time proceeds (to be expected). However, on the
> simulator, the elapsed input timestamps (sample vs host) do not agree at
> all. Also, for both platforms, the elapsed output timestamps (sample vs
> host) do not agree.
>
> I am only trying to measure the delay between the input and output. I am
> currently quite confused as to the cause of these inconsistencies. Any
> insight would be greatly appreciated.
--
Jeff Moore
Core Audio
Apple
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