Decoded CELT audio to WAV

Question

I need to extract voice data from a CS:GO game server. The engine gives me fragments of compressed audio stream, which I then decompress via celt library (IVoiceCodec::Decompress) the same way as it is done in this source code: https://github.com/perilouswithadollarsign/cstrike15_src/blob/f82112a2388b841d72cb62ca48ab1846dfcc11c8/engine/audio/private/voice.cpp

The interface doesn't provide any information about decoded data, but from the source code I can guess that it should be single channel 16bit/22050KHz PCM audio

static std::vector buffer;

char decompressedData[MAX_UNCOMPRESSED_VOICE_BUFFER_SIZE];  
const int decompressedBytes = g_pCodec->Decompress(data.data(),
                                                   data.size(),
                                                   decompressedData,
                                                   MAX_UNCOMPRESSED_VOICE_BUFFER_SIZE);

if(decompressedBytes > 0)
    buffer.insert(buffer.end(),
                  decompressedData,
                  decompressedData + decompressedBytes);
        
if(buffer.size() > 50000)
{
    static int fileIndex = 0;
    const std::string fileName("wav/test_" + std::to_string(clientIndex) + "_" + std::to_string(fileIndex) + ".wav");
    const bool result = WriteWaveFile(fileName.c_str(),
                    buffer.data(),
                    buffer.size(),
                    16, // Bits per sample
                    1, // Channels
                    22050 // Samples per sec
                    );
                    
    buffer.clear();
        
    ++fileIndex;
}

The decoder works without errors and outputs as it seams to me correct data (I can make out voices after decoding). I then save the fragments without any changes as single channel 16bit/22050KHz PCM WAV files. Technically I prepend 42 bytes of WAV header to the data. And after that I merge the files. Here is an example of what I get: https://drive.google.com/file/d/16iSC-GXzzl5wwYsvAQN1Hkk93LuSyRYG/view?usp=share_link The pitch sounds correct, but the playback rate is too fast and I can hear noise.

Here is my code for writing wav files (I also got it from the engine source code):

void WriteDWord(FileHandle_t fp, unsigned long val) 
{
    g_pFileSystem->Write(&val, 4, fp);
}

void WriteWord(FileHandle_t fp, unsigned short val) 
{
    g_pFileSystem->Write(&val, 2, fp);
}

bool WriteWaveFile(
    const char *pFilename, 
    const char *pData, 
    int nBytes, 
    int wBitsPerSample, 
    int nChannels, 
    int nSamplesPerSec)
{
    FileHandle_t fp = g_pFileSystem->Open(pFilename, "wb");
    if(!fp)
        return false;

    // Write the RIFF chunk.
    g_pFileSystem->Write("RIFF", 4, fp);
    WriteDWord(fp, 0);
    g_pFileSystem->Write("WAVE", 4, fp);


    // Write the FORMAT chunk.
    g_pFileSystem->Write("fmt ", 4, fp);

    WriteDWord(fp, 0x10);
    WriteWord(fp, WAVE_FORMAT_PCM);
    WriteWord(fp, (unsigned short)nChannels);   
    WriteDWord(fp, (unsigned long)nSamplesPerSec);
    WriteDWord(fp, (unsigned long)((wBitsPerSample / 8) * nChannels * nSamplesPerSec));
    WriteWord(fp, (unsigned short)((wBitsPerSample / 8) * nChannels));
    WriteWord(fp, (unsigned long)wBitsPerSample);

    // Write the DATA chunk.
    g_pFileSystem->Write("data", 4, fp);
    WriteDWord(fp, (unsigned long)nBytes);
    g_pFileSystem->Write(pData, nBytes, fp);

    // Go back and write the length of the riff file.
    unsigned long dwVal = g_pFileSystem->Tell(fp) - 8;
    g_pFileSystem->Seek(fp, 4, FILESYSTEM_SEEK_HEAD);
    WriteDWord(fp, dwVal);

    g_pFileSystem->Close(fp);
    return true;
}

I tried changing the sample rate and the number of bits per sample but still get the wrong result. What can be wrong with the wav files?

Decoded CELT audio to WAV

Answers (1)

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