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Asked: 2026-06-16T05:26:29+00:00

Here is some code that sets the Audio Capture Buffer Size (in secs) union

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Here is some code that sets the Audio Capture Buffer Size (in secs)

union 
{
    OSStatus propertyResult;
    char a[4];
} u;

union
{
    UInt32 UI32sessionCat;
    char a[4];
} usc;

UInt32 UI32sessionCatSize = 4;
Float32 F32realIOBufferDuration = 0.0;
Float32 F32requestedIOBufferDuration = 0.5;
UInt32 F32datasize = 4;

u.propertyResult = AudioSessionGetProperty (  kAudioSessionProperty_PreferredHardwareIOBufferDuration , &F32datasize, &F32realIOBufferDuration );
NSLog(@"Error Get IO Duration Time %ld %lx %c%c%c%c",u.propertyResult,u.propertyResult,u.a[3],u.a[2],u.a[1],u.a[0]);
NSLog(@"IO Buffer Duration is %f",F32realIOBufferDuration);

u.propertyResult = AudioSessionSetProperty (  kAudioSessionProperty_PreferredHardwareIOBufferDuration ,sizeof(F32requestedIOBufferDuration) , &F32requestedIOBufferDuration );
NSLog(@"Error Set IO Duration Time %ld %lx %c%c%c%c",u.propertyResult,u.propertyResult,u.a[3],u.a[2],u.a[1],u.a[0]);

u.propertyResult = AudioSessionGetProperty (  kAudioSessionProperty_PreferredHardwareIOBufferDuration , &F32datasize, &F32realIOBufferDuration );
NSLog(@"Error Get IO Duration Time %ld %lx %c%c%c%c",u.propertyResult,u.propertyResult,u.a[3],u.a[2],u.a[1],u.a[0]);
NSLog(@"IO Buffer Duration is %f",F32realIOBufferDuration);

u.propertyResult = AudioSessionGetProperty (  kAudioSessionProperty_AudioCategory, &UI32sessionCatSize, &usc.UI32sessionCat );
NSLog(@"Error Get Property %ld %lx %c%c%c%c",u.propertyResult,u.propertyResult,u.a[3],u.a[2],u.a[1],u.a[0]);
NSLog(@"Category %ld %lx %c%c%c%c",usc.UI32sessionCat,usc.UI32sessionCat,usc.a[3],usc.a[2],usc.a[1],usc.a[0]);

Which seems to be working / setting the duration…

2012-01-25 12:28:01.275 recordonlytest[1623:707] Error Get IO Duration Time 0 0 
2012-01-25 12:28:01.276 recordonlytest[1623:707] IO Buffer Duration is 0.023000
2012-01-25 12:28:01.278 recordonlytest[1623:707] Error Set IO Duration Time 0 0 
2012-01-25 12:28:01.280 recordonlytest[1623:707] Error Get IO Duration Time 0 0 
2012-01-25 12:28:01.281 recordonlytest[1623:707] IO Buffer Duration is 0.500000
2012-01-25 12:28:01.283 recordonlytest[1623:707] Error Get Property 0 0 
2012-01-25 12:28:01.284 recordonlytest[1623:707] Category 1919247201 72656361 reca

You can see the get properties is indeed returning 0.50000

But the callbacks are still happening at the .023000 speed

2012-01-25 12:28:04.035 recordonlytest[1623:707] In Capture Output
2012-01-25 12:28:04.058 recordonlytest[1623:707] In Capture Output
2012-01-25 12:28:04.081 recordonlytest[1623:707] In Capture Output
2012-01-25 12:28:04.104 recordonlytest[1623:707] In Capture Output
2012-01-25 12:28:04.128 recordonlytest[1623:707] In Capture Output
2012-01-25 12:28:04.151 recordonlytest[1623:707] In Capture Output
2012-01-25 12:28:04.174 recordonlytest[1623:707] In Capture Output
2012-01-25 12:28:04.197 recordonlytest[1623:707] In Capture Output
2012-01-25 12:28:04.220 recordonlytest[1623:707] In Capture Output
2012-01-25 12:28:04.244 recordonlytest[1623:707] In Capture Output

This is getting set 

 [self getRequestedSampleRate];  // routine to set the duration

 NSError* activationError = nil;

 [[AVAudioSession sharedInstance] setActive: YES error: &activationError];

 audioCaptureDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeAudio]; 
 captureSession = [[AVCaptureSession alloc] init];

So is there a different sequence, etc that needs to be done, or is IOS just ignoring this request?

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1 Answer

  1. Editorial Team

    From here:

    While it is safe to set Audio Session properties such as audio category (kAudioSessionProperty_AudioCategory) or the property listener regardless of the sessions activation state, it is generally better to make preference requests such as preferred hardware buffer duration (kAudioSessionProperty_PreferredHardwareIOBufferDuration) or preferred hardware sample rate (kAudioSessionProperty_PreferredHardwareSampleRate) when the Audio Session is NOT active.

    You may be missing the:

    // * Activate the Audio Session before asking for the “Current” properties *
    AudioSessionSetActive(true);

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