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

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
    }
}