Create a silent audio CMSampleBufferRef

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?

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
    }
}