ios - AVAssetReader を使用して mp4 ファイルを読み取ると、最初のオーディオ CMSampleBuffer が失われる

Question

AVAssetWriter を使用してオーディオ CMSampleBuffer を mp4 ファイルに書き込んでいますが、後で AVAssetReader を使用してそのファイルを読み取ると、データの最初のチャンクが欠落しているようです。

ライターの入力追加メソッドに渡される最初の CMSampleBuffer のデバッグの説明を次に示します (1024/44_100 のプライミング期間のアタッチメントに注意してください)。

CMSampleBuffer 0x102ea5b60 retainCount: 7 allocator: 0x1c061f840
            invalid = NO
            dataReady = YES
            makeDataReadyCallback = 0x0
            makeDataReadyRefcon = 0x0
            buffer-level attachments:
                TrimDurationAtStart    = {
            epoch = 0;
            flags = 1;
            timescale = 44100;
            value = 1024;
        }
            formatDescription = <CMAudioFormatDescription 0x281fd9720 [0x1c061f840]> {
            mediaType:'soun' 
            mediaSubType:'aac ' 
            mediaSpecific: {
                ASBD: {
                    mSampleRate: 44100.000000 
                    mFormatID: 'aac ' 
                    mFormatFlags: 0x2 
                    mBytesPerPacket: 0 
                    mFramesPerPacket: 1024 
                    mBytesPerFrame: 0 
                    mChannelsPerFrame: 2 
                    mBitsPerChannel: 0  } 
                cookie: {<CFData 0x2805f50a0 [0x1c061f840]>{length = 39, capacity = 39, bytes = 0x03808080220000000480808014401400 ... 1210068080800102}} 
                ACL: {(null)}
                FormatList Array: {
                    Index: 0 
                    ChannelLayoutTag: 0x650002 
                    ASBD: {
                    mSampleRate: 44100.000000 
                    mFormatID: 'aac ' 
                    mFormatFlags: 0x0 
                    mBytesPerPacket: 0 
                    mFramesPerPacket: 1024 
                    mBytesPerFrame: 0 
                    mChannelsPerFrame: 2 
                    mBitsPerChannel: 0  }} 
            } 
            extensions: {(null)}
        }
            sbufToTrackReadiness = 0x0
            numSamples = 1
            outputPTS = {6683542167/44100 = 151554.244, rounded}(based on cachedOutputPresentationTimeStamp)
            sampleTimingArray[1] = {
                {PTS = {6683541143/44100 = 151554.221, rounded}, DTS = {6683541143/44100 = 151554.221, rounded}, duration = {1024/44100 = 0.023}},
            }
            sampleSizeArray[1] = {
                sampleSize = 163,
            }
            dataBuffer = 0x281cc7a80

2 番目の CMSampleBuffer のデバッグの説明は次のとおりです (1088/44_100 のプライミング期間のアタッチメントに注意してください。前のトリム期間と組み合わせると、2112 の標準値が得られます)。

CMSampleBuffer 0x102e584f0 retainCount: 7 allocator: 0x1c061f840
    invalid = NO
    dataReady = YES
    makeDataReadyCallback = 0x0
    makeDataReadyRefcon = 0x0
    buffer-level attachments:
        TrimDurationAtStart    = {
    epoch = 0;
    flags = 1;
    timescale = 44100;
    value = 1088;
}
    formatDescription = <CMAudioFormatDescription 0x281fd9720 [0x1c061f840]> {
    mediaType:'soun' 
    mediaSubType:'aac ' 
    mediaSpecific: {
        ASBD: {
            mSampleRate: 44100.000000 
            mFormatID: 'aac ' 
            mFormatFlags: 0x2 
            mBytesPerPacket: 0 
            mFramesPerPacket: 1024 
            mBytesPerFrame: 0 
            mChannelsPerFrame: 2 
            mBitsPerChannel: 0  } 
        cookie: {<CFData 0x2805f50a0 [0x1c061f840]>{length = 39, capacity = 39, bytes = 0x03808080220000000480808014401400 ... 1210068080800102}} 
        ACL: {(null)}
        FormatList Array: {
            Index: 0 
            ChannelLayoutTag: 0x650002 
            ASBD: {
            mSampleRate: 44100.000000 
            mFormatID: 'aac ' 
            mFormatFlags: 0x0 
            mBytesPerPacket: 0 
            mFramesPerPacket: 1024 
            mBytesPerFrame: 0 
            mChannelsPerFrame: 2 
            mBitsPerChannel: 0  }} 
    } 
    extensions: {(null)}
}
    sbufToTrackReadiness = 0x0
    numSamples = 1
    outputPTS = {6683543255/44100 = 151554.269, rounded}(based on cachedOutputPresentationTimeStamp)
    sampleTimingArray[1] = {
        {PTS = {6683542167/44100 = 151554.244, rounded}, DTS = {6683542167/44100 = 151554.244, rounded}, duration = {1024/44100 = 0.023}},
    }
    sampleSizeArray[1] = {
        sampleSize = 179,
    }
    dataBuffer = 0x281cc4750

ここで、AVAssetReader を使用してオーディオ トラックを読み取ると、最初に取得する CMSampleBuffer は次のようになります。

CMSampleBuffer 0x102ed7b20 retainCount: 7 allocator: 0x1c061f840
    invalid = NO
    dataReady = YES
    makeDataReadyCallback = 0x0
    makeDataReadyRefcon = 0x0
    buffer-level attachments:
        EmptyMedia(P) = true
    formatDescription = (null)
    sbufToTrackReadiness = 0x0
    numSamples = 0
    outputPTS = {0/1 = 0.000}(based on outputPresentationTimeStamp)
    sampleTimingArray[1] = {
        {PTS = {0/1 = 0.000}, DTS = {INVALID}, duration = {0/1 = 0.000}},
    }
    dataBuffer = 0x0

次のものには、1088/44_100 のプライミング情報が含まれています。

CMSampleBuffer 0x10318bc00 retainCount: 7 allocator: 0x1c061f840
    invalid = NO
    dataReady = YES
    makeDataReadyCallback = 0x0
    makeDataReadyRefcon = 0x0
    buffer-level attachments:
        FillDiscontinuitiesWithSilence(P) = true
        GradualDecoderRefresh(P) = 1
        TrimDurationAtStart(P) = {
    epoch = 0;
    flags = 1;
    timescale = 44100;
    value = 1088;
}
        IsGradualDecoderRefreshAuthoritative(P) = false
    formatDescription = <CMAudioFormatDescription 0x281fdcaa0 [0x1c061f840]> {
    mediaType:'soun' 
    mediaSubType:'aac ' 
    mediaSpecific: {
        ASBD: {
            mSampleRate: 44100.000000 
            mFormatID: 'aac ' 
            mFormatFlags: 0x0 
            mBytesPerPacket: 0 
            mFramesPerPacket: 1024 
            mBytesPerFrame: 0 
            mChannelsPerFrame: 2 
            mBitsPerChannel: 0  } 
        cookie: {<CFData 0x2805f3800 [0x1c061f840]>{length = 39, capacity = 39, bytes = 0x03808080220000000480808014401400 ... 1210068080800102}} 
        ACL: {Stereo (L R)}
        FormatList Array: {
            Index: 0 
            ChannelLayoutTag: 0x650002 
            ASBD: {
            mSampleRate: 44100.000000 
            mFormatID: 'aac ' 
            mFormatFlags: 0x0 
            mBytesPerPacket: 0 
            mFramesPerPacket: 1024 
            mBytesPerFrame: 0 
            mChannelsPerFrame: 2 
            mBitsPerChannel: 0  }} 
    } 
    extensions: {{
    VerbatimISOSampleEntry = {length = 87, bytes = 0x00000057 6d703461 00000000 00000001 ... 12100680 80800102 };
}}
}
    sbufToTrackReadiness = 0x0
    numSamples = 43
    outputPTS = {83/600 = 0.138}(based on outputPresentationTimeStamp)
    sampleTimingArray[1] = {
        {PTS = {1024/44100 = 0.023}, DTS = {1024/44100 = 0.023}, duration = {1024/44100 = 0.023}},
    }
    sampleSizeArray[43] = {
        sampleSize = 179,
        sampleSize = 173,
        sampleSize = 178,
        sampleSize = 172,
        sampleSize = 172,
        sampleSize = 159,
        sampleSize = 180,
        sampleSize = 200,
        sampleSize = 187,
        sampleSize = 189,
        sampleSize = 206,
        sampleSize = 192,
        sampleSize = 195,
        sampleSize = 186,
        sampleSize = 183,
        sampleSize = 189,
        sampleSize = 211,
        sampleSize = 198,
        sampleSize = 204,
        sampleSize = 211,
        sampleSize = 204,
        sampleSize = 202,
        sampleSize = 218,
        sampleSize = 210,
        sampleSize = 206,
        sampleSize = 207,
        sampleSize = 221,
        sampleSize = 219,
        sampleSize = 236,
        sampleSize = 219,
        sampleSize = 227,
        sampleSize = 225,
        sampleSize = 225,
        sampleSize = 229,
        sampleSize = 225,
        sampleSize = 236,
        sampleSize = 233,
        sampleSize = 231,
        sampleSize = 249,
        sampleSize = 234,
        sampleSize = 250,
        sampleSize = 249,
        sampleSize = 259,
    }
    dataBuffer = 0x281cde370

入力追加メソッドはtrue、原則としてすべてのサンプル バッファが追加されたことを意味しますが、何らかの理由でデータの最初のチャンクをスキップします。ここで私が間違っていることはありますか？

次のコードを使用してファイルを読み取ります。

let asset = AVAsset(url: fileURL)
guard let assetReader = try? AVAssetReader(asset: asset) else {
    return
}

asset.loadValuesAsynchronously(forKeys: ["tracks"]) { in
    guard let audioTrack = asset.tracks(withMediaType: .audio).first else { return }
    let audioOutput = AVAssetReaderTrackOutput(track: audioTrack, outputSettings: nil)
    assetReader.startReading()

    while assetReader.status == .reading {
        if let sampleBuffer = audioOutput.copyNextSampleBuffer() {
            // do something
        }
    }
}