Create a silent audio CMSampleBufferRef - ios

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?

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 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.
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