如何在 Swift 中创建无声音频CMSampleBufferRef？我希望将静默 CMSampleBufferRefs 附加到 AVAssetWriterInput 的实例中。
【问题讨论】：
标签： ios swift avfoundation
你没有说你想要什么格式的零（整数/浮点，单声道/立体声，采样率），但也许没关系。无论如何，这是一种在 swift 中创建无声 CD 音频样式CMSampleBuffer 的方法。
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
}
你问了这个问题，你不觉得很抱歉吗？你真的需要沉默的CMSampleBuffers 吗？您不能通过向前移动演示时间戳将静音插入AVAssetWriterInput 吗？
【讨论】：
为 XCode 10.3 更新。斯威夫特 5.0.1。
不要忘记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
}
}
【讨论】：
您需要使用CMBlockBufferCreateWithMemoryBlock() 创建一个块缓冲区。
用一堆零填充块缓冲区，然后将其传递给CMAudioSampleBufferCreateWithPacketDescriptions()。
免责声明：我实际上并没有在 Swift 中做到这一点，我尝试过，但发现自己每次都在与编译器作斗争，所以我切换到了 obj-c。 Core Media Framework 是一个低级的 C 框架，并且更容易使用而无需使用 Swifts 类型系统。我知道这不是您要购买的答案，希望它能为您指明正确的方向。
【讨论】：