First off, latency doesn't change over time. The hardware only reports
the one figure. You don't re-fetch all the time.
Second, it looks like you are measuring time intervals based on when
your IOProc is called. As I said, this is not correct. The time when
your IOProc is invoked has nothing really to do with the latency. The
time stamp that matters is the output time stamp your IOProc is
provided. That is the time stamp that tells you when the data your are
providing is going to hit the hardware.
Finally, a word on style. Your IOProc is making blocking calls to the
HAL and it is calling NSLog which allocates memory and blocks in fun
and unexpected ways. You absolutely cannot be making these calls from
inside your IOProc. You also cannot be making calls to any ObjC or CF
objects from inside your IOProc. Doing any of these will eventually
cause glitching.
On Jun 22, 2009, at 1:24 PM, Ravneet Kaur wrote:
Hello Jeff
Thanks for the reply. I added following code in my Callback Proc as
shown below
The frame latency is always constant and the output is 30. While the
audio stream latency is always zero. I was expecting to see these
numbers correspond to hardware drift reported by external testing
device(1ms - 200 ms).
Do you think I am doing something wrong over here? I can send you
the whole source code if needed.Do you recommend Audio Queues for
better time synchronization?
Best Regards
Ravneet
On Tue, Jun 23, 2009 at 1:32 AM, Jeff Moore <email@hidden> wrote:
You should get no differences in terms of IO latency when using
AUHAL versus when using the HAL directly. AUHAL does not induce any
additional latency at all. So, I'm not sure I follow what you mean
when you say that "the Callback method was having an offset which
varied between 1ms-200 ms".
At any rate, the output time stamp that you get in your Render
callback or your IOProc is the time at which the audio stack will be
finished getting the first sample frame of the data to the hardware.
So to get the time at which the first sample frame will hit the
speaker, all you need to do is add the device's latency. The latency
figure can be gotten from adding together the HAL with the
properties, kAudioDevicePropertyLatency and
kAudioStreamPropertyLatency for the device and stream combination
you are interested in.
--
Jeff Moore
Core Audio
Apple
On Jun 22, 2009, at 11:17 AM, Ravneet Kaur wrote:
Hello All,
I am working on CoreAudio API to play an audio file and get exact
time of audio playback. I have used following structure
A) Get Components
ComponentDescription desc;
err = OpenAComponent(comp, theOutputUnit); //gains access to the
services provided by the component
B) Get The Audio Unit
verify_noerr(AudioUnitInitialize(*theOutputUnit));
C)Set Output Unit Properties
//Set the stream format of the output to match the input
result = AudioUnitSetProperty
(*theUnit
,kAudioUnitProperty_StreamFormat
,kAudioUnitScope_Input,theInputBus,theDesc,size);
result = AudioUnitSetParameter(*theUnit,
kAudioUnitParameterUnit_LinearGain, kAudioUnitScope_Output, 0,
(Float32)volume, 0);
D) Read File Information
FSPathMakeRef ((const UInt8 *)fileName, fileRef, 0); //Obtain
filesystem reference to the file
err = AudioFileOpen(fileRef, fsRdPerm,0,fileID); //Obtain
AudioFileID
E)Make Converter
err = AudioConverterNew( inASBD,outASBD , conv);
F)Setup Callback
//Sets the callback for the Audio Unit
err = AudioUnitSetProperty (*theOutputUnit,
kAudioUnitProperty_SetRenderCallback, kAudioUnitScope_Input,
0,renderCallback, sizeof(AURenderCallbackStruct));
G)Start the Audio Unit
Now the time I get in the Callback method was having an offset which
varied between 1ms-200 ms in worst cases. So I decided to get more
low level and binded callback method with HAL layer. I binded a
simple Proc with my default device
I used MTCoreAudio as I have limited familiarity with HAL layer and
its c libraries.
myDevice = [MTCoreAudioDevice defaultOutputDevice];
[myDevice setIOProc:mySimpleIOProc withClientData:nil];
[myDevice deviceStart];
Now as soon as I start my Audio Unit, my callback method gets called
and then my HAL device Proc "mySimpleIOProc" (as shown above) gets
called. I was assuming that mySimpleIOProc will have better timing
accuracy in terms of inNow or if I take current time inside this
proc.
Can someone please help me in finding the exact playback time? I
read that HAL does provide synchronization capability but could not
find any samples for the same. The sample in CoreAudio SDK is too
difficult for a beginner like me.
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--
Jeff Moore
Core Audio
Apple
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