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 AVAssetWriter

Answer 1

You need to create a block buffer using CMBlockBufferCreateWithMemoryBlock(). Fill the block buffer with a bunch of zeros and then pass it into CMAudioSampleBufferCreateWithPacketDescriptions().

Disclaimer: I haven't actually done this in Swift, I attempted it but found myself fighting the compiler at every turn so I switched to obj-c. The Core Media Framework is a low level C framework and was a lot easier to use without screwing around with Swifts type system. I know this isn't the answer you're looking for buy hopefully it will point you in the right direction.

Example

Answer 2

Updated for XCode 10.3. Swift 5.0.1. Don't forget the import CoreMedia.

import Foundation
import CoreMedia

class CMSampleBufferFactory
{
    static 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(
            allocator: kCFAllocatorDefault,
            memoryBlock: nil,
            blockLength: blockSize,
            blockAllocator: nil,
            customBlockSource: nil,
            offsetToData: 0,
            dataLength: blockSize,
            flags: 0,
            blockBufferOut: &block
        )
        assert(status == kCMBlockBufferNoErr)

        guard var eBlock = block else { return nil }

        // we seem to get zeros from the above, but I can't find it documented. so... memset:
        status = CMBlockBufferFillDataBytes(with: 0, blockBuffer: eBlock, offsetIntoDestination: 0, dataLength: 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(allocator: kCFAllocatorDefault, asbd: &asbd, layoutSize: 0, layout: nil, magicCookieSize: 0, magicCookie: nil, extensions: nil, formatDescriptionOut: &formatDesc)
        assert(status == noErr)

        var sampleBuffer: CMSampleBuffer?

        status = CMAudioSampleBufferCreateReadyWithPacketDescriptions(
            allocator: kCFAllocatorDefault,
            dataBuffer: eBlock,
            formatDescription: formatDesc!,
            sampleCount: nFrames,
            presentationTimeStamp: CMTimeMake(value: startFrm, timescale: Int32(sampleRate)),  
            packetDescriptions: nil,
            sampleBufferOut: &sampleBuffer
        )
        assert(status == noErr)
        return sampleBuffer
    }
}

Answer 3

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?