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• Does the Remote I/O audio unit set the number of channels in the buffer ?

  25 novembre 2013, par awfulcode

  I'm using kxmovie (it's a ffmpeg-based video player) as a base for an app and I'm trying to figure out how the RemoteI/O unit works on iOS when the only thing connected to a device is headphones and we're playing a track with more than 2 channels (say a surround 6 track channel). It seems like it is going with the output channel setting and the buffer only has 2 channels. Is this because of Core Audio's pull structure ? And if so, what's happening to the other channels in the track ? Are they being downmixed or simply ignored ?

  The code for the render callback connected to the remoteio unit is here :

  - (BOOL) renderFrames: (UInt32) numFrames
  
                 ioData: (AudioBufferList *) ioData
  
  {
  
      NSLog(@"Number of channels in buffer: %lu",ioData->mNumberBuffers);
  
  
  
      for (int iBuffer=0; iBuffer < ioData->mNumberBuffers; ++iBuffer) {
  
          memset(ioData->mBuffers[iBuffer].mData, 0, ioData->mBuffers[iBuffer].mDataByteSize);
  
      }
  
  
  
  
  
      if (_playing && _outputBlock ) {
  
  
  
          // Collect data to render from the callbacks
  
          _outputBlock(_outData, numFrames, _numOutputChannels);
  
  
  
          // Put the rendered data into the output buffer
  
          if (_numBytesPerSample == 4) // then we've already got floats
  
          {
  
              float zero = 0.0;
  
  
  
              for (int iBuffer=0; iBuffer < ioData->mNumberBuffers; ++iBuffer) {
  
  
  
                  int thisNumChannels = ioData->mBuffers[iBuffer].mNumberChannels;
  
  
  
                  for (int iChannel = 0; iChannel < thisNumChannels; ++iChannel) {
  
                      vDSP_vsadd(_outData+iChannel, _numOutputChannels, &zero, (float *)ioData->mBuffers[iBuffer].mData, thisNumChannels, numFrames);
  
                  }
  
              }
  
          }
  
          else if (_numBytesPerSample == 2) // then we need to convert SInt16 -> Float (and also scale)
  
          {
  
              float scale = (float)INT16_MAX;
  
              vDSP_vsmul(_outData, 1, &scale, _outData, 1, numFrames*_numOutputChannels);
  
  
  
              for (int iBuffer=0; iBuffer < ioData->mNumberBuffers; ++iBuffer) {
  
  
  
                  int thisNumChannels = ioData->mBuffers[iBuffer].mNumberChannels;
  
  
  
                  for (int iChannel = 0; iChannel < thisNumChannels; ++iChannel) {
  
                      vDSP_vfix16(_outData+iChannel, _numOutputChannels, (SInt16 *)ioData->mBuffers[iBuffer].mData+iChannel, thisNumChannels, numFrames);
  
                  }
  
              }
  
  
  
          }        
  
      }
  
  
  
      return noErr;
  
  }

  Thanks !

  edit : Here's the code for the ASBD (_ouputFormat). It's getting its values straight from the remoteio. You can also check the whole method file here.

  if (checkError(AudioUnitGetProperty(_audioUnit,
  
                                      kAudioUnitProperty_StreamFormat,
  
                                      kAudioUnitScope_Input,
  
                                      0,
  
                                      &_outputFormat,
  
                                      &size),
  
                 "Couldn't get the hardware output stream format"))
  
      return NO;
  
  
  
  
  
  _outputFormat.mSampleRate = _samplingRate;
  
  if (checkError(AudioUnitSetProperty(_audioUnit,
  
                                      kAudioUnitProperty_StreamFormat,
  
                                      kAudioUnitScope_Input,
  
                                      0,
  
                                      &_outputFormat,
  
                                      size),
  
                 "Couldn't set the hardware output stream format")) {
  
  
  
      // just warning
  
  }
  
  
  
  _numBytesPerSample = _outputFormat.mBitsPerChannel / 8;
  
  _numOutputChannels = _outputFormat.mChannelsPerFrame;
  
  
  
  NSLog(@"Current output bytes per sample: %ld", _numBytesPerSample);
  
  NSLog(@"Current output num channels: %ld", _numOutputChannels);
  
  
  
  // Slap a render callback on the unit
  
  AURenderCallbackStruct callbackStruct;
  
  callbackStruct.inputProc = renderCallback;
  
  callbackStruct.inputProcRefCon = (__bridge void *)(self);
  
  
  
  if (checkError(AudioUnitSetProperty(_audioUnit,
  
                                      kAudioUnitProperty_SetRenderCallback,
  
                                      kAudioUnitScope_Input,
  
                                      0,
  
                                      &callbackStruct,
  
                                      sizeof(callbackStruct)),
  
                 "Couldn't set the render callback on the audio unit"))
  
      return NO;

• libx264 performances on iOS

  8 mai 2012, par Gilad

  I was wondering whether someone has some experience using libx264 on iPhone.

  How does it perform ? What framerate can I expect ? Will it work for simultaneous encoding & decoding (video call) or the CPU can't handle it ? I'm mainly interested with iPhone 4 and above (iPhone4/iPad 2)

  Are there any precompiled universal binaries I can use ?

• How to convert from .avi container to .mp4

  9 novembre 2014, par Manuel Selva

  I have an .avi file containing the following streams :

  Stream #0.0: Video: mpeg4 (Advanced Simple Profile), yuv420p, 608x256 [PAR 1:1 DAR 19:8], 23.98 tbn, 23.98 tbc
  
  
  
  Stream #0.1: Audio: mp3, 48000 Hz, stereo, s16p, 128 kb/s

  I want to play this movie on an Iphone supposed to support mpeg4 video streams as stated in a previous question here. For now, I don’t care about the audio stream (mp3 doesn’t seem to be supported by the Iphone)

  As a consequence I am looking for a way to just convert the "container" format of my file from .avi to .mp4 required by the Iphone. Is it doable, and if yes how should I do that ?