Create a silent audio CMSampleBufferRef

Question

How do you create a silent audio CMSampleBufferRef in Swift? I am looking to append silent CMSampleBufferRefs to an instance of AVAssetWriterInput.

Answer 1

You don't say what format you want your zeros (integer/floating point, mono/stereo, sample rate), but maybe it doesn't matter. Anyway, here's one way to create a silent CD audio style CMSampleBuffer in swift.

func createSilentAudio(startFrm: Int64, nFrames: Int, sampleRate: Float64, numChannels: UInt32) -> CMSampleBuffer? {
    let bytesPerFrame = UInt32(2 * numChannels)
    let blockSize = nFrames*Int(bytesPerFrame)

    var block: CMBlockBuffer?
    var status = CMBlockBufferCreateWithMemoryBlock(
        kCFAllocatorDefault,
        nil,
        blockSize,  // blockLength
        nil,        // blockAllocator
        nil,        // customBlockSource
        0,          // offsetToData
        blockSize,  // dataLength
        0,          // flags
        &block
    )
    assert(status == kCMBlockBufferNoErr)

    // we seem to get zeros from the above, but I can't find it documented. so... memset:
    status = CMBlockBufferFillDataBytes(0, block!, 0, blockSize)
    assert(status == kCMBlockBufferNoErr)

    var asbd = AudioStreamBasicDescription(
        mSampleRate: sampleRate,
        mFormatID: kAudioFormatLinearPCM,
        mFormatFlags: kLinearPCMFormatFlagIsSignedInteger,
        mBytesPerPacket: bytesPerFrame,
        mFramesPerPacket: 1,
        mBytesPerFrame: bytesPerFrame,
        mChannelsPerFrame: numChannels,
        mBitsPerChannel: 16,
        mReserved: 0
    )

    var formatDesc: CMAudioFormatDescription?
    status = CMAudioFormatDescriptionCreate(kCFAllocatorDefault, &asbd, 0, nil, 0, nil, nil, &formatDesc)
    assert(status == noErr)

    var sampleBuffer: CMSampleBuffer?

    // born ready
    status = CMAudioSampleBufferCreateReadyWithPacketDescriptions(
        kCFAllocatorDefault,
        block,      // dataBuffer
        formatDesc!,
        nFrames,    // numSamples
        CMTimeMake(startFrm, Int32(sampleRate)),    // sbufPTS
        nil,        // packetDescriptions
        &sampleBuffer
    )
    assert(status == noErr)

    return sampleBuffer
}

Doesn't it make you sorry you asked? Do you really need silent CMSampleBuffers? Can't you insert silence into an AVAssetWriterInput by moving the presentation time stamp forward?