Core Audio Ring Buffer Data comes out blank

Question

I am working off a demo from the book "Learning Core Audio: A Hands-On Guide to Audio Programming for Mac and iOS." Chapter 8 shows how to set up a simple AudioUnit graph to play through from the AUHAL input unit to an output unit. This setup doesn't actually connect the audio units; instead, both units use a callback and pass audio data through an instance of CARingBuffer. I'm coding for MacOS 10.15.6, and using code directly from the publisher here. Here's a picture of how it works:

The code builds and runs, but I get no audio. Note that later, after introducing a speech synthesis unit, I do get playback, so I know the basics are working.

InputRenderProc asks the AUHAL unit for input and stores it in the ring buffer.

    MyAUGraphPlayer *player = (MyAUGraphPlayer*) inRefCon;
    
    // have we ever logged input timing? (for offset calculation)
    if (player->firstInputSampleTime < 0.0) {
        player->firstInputSampleTime = inTimeStamp->mSampleTime;
        if ((player->firstOutputSampleTime > -1.0) &&
            (player->inToOutSampleTimeOffset < 0.0)) {
            player->inToOutSampleTimeOffset = player->firstInputSampleTime - player->firstOutputSampleTime;
        }
    }
    
    // render into our buffer
    OSStatus inputProcErr = noErr;
    inputProcErr = AudioUnitRender(player->inputUnit,
                                   ioActionFlags,
                                   inTimeStamp,
                                   inBusNumber,
                                   inNumberFrames,
                                   player->inputBuffer);

    if (! inputProcErr) {
        inputProcErr = player->ringBuffer->Store(player->inputBuffer,
                                                 inNumberFrames,
                                                 inTimeStamp->mSampleTime);
        
        UInt32 sz = sizeof(player->inputBuffer);
        printf ("stored %d frames at time %f (%d bytes)\n", inNumberFrames, inTimeStamp->mSampleTime, sz);
                        
        for (int i = 0; i < player->inputBuffer->mNumberBuffers; i++ ){
            //printf("stored audio string[%d]: %s\n", i, player->inputBuffer->mBuffers[i].mData);
        }
                 
                
    }

If I uncomment the printf statement, I see what looks like audio data being stored.

stored audio string[1]: #P'\274a\353\273\336^\274x\205 \2741\330B\2747'\274\371\361U\274\346\274\274}\212C\274\334\365%\274\261\367\273\340\307/\274E
stored 512 frames at time 134610.000000 (8 bytes)

However, when I fetch from the ring buffer in the GraphRenderCallback like this...

    MyAUGraphPlayer *player = (MyAUGraphPlayer*) inRefCon;
    
    // have we ever logged output timing? (for offset calculation)
    if (player->firstOutputSampleTime < 0.0) {
        player->firstOutputSampleTime = inTimeStamp->mSampleTime;
        if ((player->firstInputSampleTime > -1.0) &&
            (player->inToOutSampleTimeOffset < 0.0)) {
            player->inToOutSampleTimeOffset = player->firstInputSampleTime - player->firstOutputSampleTime;
        }
    }
    
    
    // copy samples out of ring buffer
    OSStatus outputProcErr = noErr;
    // new CARingBuffer doesn't take bool 4th arg
    outputProcErr = player->ringBuffer->Fetch(ioData,
                                              inNumberFrames,
                                              inTimeStamp->mSampleTime + player->inToOutSampleTimeOffset);
    

I get nothing (I know I can't expect proper null-terminated string output, but I thought I'd see something).

fetched 512 frames at time 160776.000000
fetched audio string[0, size 2048]: xx
fetched audio string[1, size 2048]: xx
fetched 512 frames at time 161288.000000
fetched audio string[0, size 2048]: xx
fetched audio string[1, size 2048]: xx

This is not a permission problem; I have other non-AudioUnit code that can get mic input. In addition, I created a plist that makes this app prompt for mic access every time, so I know that is working. I cannot understand why data goes into this ring buffer, but never comes out.

Answer 1

These days you need to declare that you want to use the microphone, providing an explanation string. This wasn't the case in 2012 when Learning Core Audio was published.

In short, you now need to:

1. add an NSMicrophoneUsageDescription string to your Info.plist
2. add sandboxing capability and enable Audio Input

The sample code you're using is a command line tool, so adding an Info.plist to it in Xcode isn't as simple as with a .app package. Also the code does not seem to work if you run it from Xcode. In my case it has to be run for Terminal.app. This may be due to the fact that my Terminal has microphone permissions (viewable in System Preferences > Security & Privacy > Microphone). You can and probably should explicitly request microphone access from the user (yourself in this case!) by using requestAccessForMediaType on an AVCaptureDevice. That's right, AVFoundation code in a Core Audio tutorial, what's the world coming to.

There are more details on the above steps in this answer

p.s. I think the person who thought capturing zeroes instead of returning an error was a good idea is probably good friends with whoever invented returning HTTP 200 with an error code in the body.