Question

I have a Multichannel Mixer audio unit playing back audio files in an iOS app, and I need to figure out how to update the app’s UI and perform a reset when the render callback hits the end of the longest audio file (which is set up to run on bus 0). As my code below shows I am trying to use KVO to achieve this (using the boolean variable tapesUnderway – the AutoreleasePool is necessary as this Objective-C code is running outside of its normal domain, see http://www.cocoabuilder.com/archive/cocoa/57412-nscfnumber-no-pool-in-place-just-leaking.html).

static OSStatus tapesRenderInput(void *inRefCon, AudioUnitRenderActionFlags *ioActionFlags, const AudioTimeStamp *inTimeStamp, UInt32 inBusNumber, UInt32 inNumberFrames, AudioBufferList *ioData)
{
    SoundBufferPtr sndbuf = (SoundBufferPtr)inRefCon;

    UInt32 bufferFrames = sndbuf[inBusNumber].numFrames;
    AudioUnitSampleType *in = sndbuf[inBusNumber].data; 

    // These mBuffers are the output buffers and are empty; these two lines are just  setting the references to them (via outA and outB)
    AudioUnitSampleType *outA = (AudioUnitSampleType *)ioData->mBuffers[0].mData;
    AudioUnitSampleType *outB = (AudioUnitSampleType *)ioData->mBuffers[1].mData;

    UInt32 sample = sndbuf[inBusNumber].sampleNum;


    // --------------------------------------------------------------
    // Set the start time here
    if(inBusNumber == 0 && !tapesFirstRenderPast)
    {
        printf("Tapes first render past\n");

        tapesStartSample = inTimeStamp->mSampleTime;
        tapesFirstRenderPast = YES;                     // MAKE SURE TO RESET THIS ON SONG RESTART
        firstPauseSample = tapesStartSample;
    }

    // --------------------------------------------------------------
    // Now process the samples
     for(UInt32 i = 0; i < inNumberFrames; ++i)
     {
         if(inBusNumber == 0)
         {
            // ------------------------------------------------------
            // Bus 0 is the backing track, and is always playing back

            outA[i] = in[sample++];
            outB[i] = in[sample++];     // For stereo set desc.SetAUCanonical to (2, true) and increment samples in both output calls

            lastSample = inTimeStamp->mSampleTime + (Float64)i;     // Set the last played sample in order to compensate for pauses


            // ------------------------------------------------------
            // Use this logic to mark end of tune
            if(sample >= (bufferFrames * 2) && !tapesEndPast)
            {
                // USE KVO TO NOTIFY METHOD OF VALUE CHANGE

                NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
                FuturesEPMedia *futuresMedia = [FuturesEPMedia sharedFuturesEPMedia];
                NSNumber *boolNo = [[NSNumber alloc] initWithBool: NO];
                [futuresMedia setValue: boolNo forKey: @"tapesUnderway"];
                [boolNo release];
                [pool release];

                tapesEndPast = YES;
            }
        }
        else
        {
            // ------------------------------------------------------
            // The other buses are the open sections, and are synched through the tapesSectionsTimes array

            Float64 sectionTime = tapesSectionTimes[inBusNumber] * kGraphSampleRate;        // Section time in samples
            Float64 currentSample = inTimeStamp->mSampleTime + (Float64)i;

            if(!isPaused && !playFirstRenderPast)
            {
                pauseGap += currentSample - firstPauseSample;
                playFirstRenderPast = YES;
                pauseFirstRenderPast = NO;
            }


            if(currentSample > (tapesStartSample + sectionTime + pauseGap) && sample < (bufferFrames * 2))
            {
                outA[i] = in[sample++];
                outB[i] = in[sample++];
            }
            else
            {
                outA[i] = 0;
                outB[i] = 0;
            }
        }
    }

   sndbuf[inBusNumber].sampleNum = sample;

   return noErr;
}

At the moment when this variable is changed it triggers a method in self, but this leads to an unacceptable delay (20-30 seconds) when executed from this render callback (I am thinking because it is Objective-C code running in the high priority audio thread?). How do I effectively trigger such a change without the delay? (The trigger will change a pause button to a play button and call a reset method to prepare for the next play.)

Thanks

Answer 1

Yes. Don’t use objc code in the render thread since its high priority. If you store state in memory (ptr or struct) and then get a timer in the main thread to poll (check) the value(s) in memory. The timer need not be anywhere near as fast as the render thread and will be very accurate.