iOS 使用CoreAudio生成白噪音频数据

iOS 使用CoreAudio生成白噪音频数据（空白音频）

    /// 生成一段白噪音频数据
    /// - Parameters:
    ///   - startFrm: 开始frame
    ///   - nFrames: 持续的frame
    ///   - sampleRate: fps
    ///   - numChannels: 声道数量
    /// - Returns: 静音音频数据
    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,  // blockLength
            blockAllocator: nil,        // blockAllocator
            customBlockSource: nil,        // customBlockSource
            offsetToData: 0,          // offsetToData
            dataLength: blockSize,  // dataLength
            flags: 0,          // flags
            blockBufferOut: &block
        )
        assert(status == kCMBlockBufferNoErr)

        // we seem to get zeros from the above, but I can't find it documented. so... memset:
        status = CMBlockBufferFillDataBytes(with: 0, blockBuffer: block!, 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?

        // born ready
        status = CMAudioSampleBufferCreateReadyWithPacketDescriptions(
            allocator: kCFAllocatorDefault,
            dataBuffer: block!,      // dataBuffer
            formatDescription: formatDesc!,
            sampleCount: nFrames,    // numSamples
            presentationTimeStamp: CMTimeMake(value: startFrm, timescale: Int32(sampleRate)),    // sbufPTS
            packetDescriptions: nil,        // packetDescriptions
            sampleBufferOut: &sampleBuffer
        )
        assert(status == noErr)

        return sampleBuffer
    }