獲取M4A文件的原始樣本數據繪製波形

Question

我使用AudioToolbox訪問與下面的代碼M4A等音頻文件：

UInt32 packetsToRead = 1; //Does it makes difference? 

void *buffer = malloc(maxPacketSize * packetsToRead); 

for (UInt64 packetIndex = 0; packetIndex < packetCount; packetIndex++) 
{ 
    ioNumberOfPackets = packetsToRead; 
    ioNumberOfBytes = maxPacketSize * ioNumberOfPackets; 

    AudioFileReadPacketData(audioFile, NO, &ioNumbersOfBytes, NULL, packetIndex, &ioNumberOFPackets, buffer); 
    for (UInt32 batchPacketIndex = 0; batchPacketIndex < ioNumberOfPackets; batchPacketIndex++) 
    { 
    //What to do here to get amplitude value? How to get sample value? 
    } 
    packetIndex+=ioNumberOfPackets; 
} 

我的音頻格式是： AppleM4A，8000赫茲，16位，4096幀每個數據包

Answer 1

解決方案是使用擴展音頻文件服務。您只需設置客戶端格式和PCM之間的轉換。得到正確的方式在Audio Processing: Playing with volume level。

Answer 2

要獲得波形數據，您可能首先需要將壓縮音頻文件轉換爲原始PCM樣本，如在WAV文件內或其他非壓縮音頻格式中找到。嘗試AVAssetReader，et.al.