Question

I am working on a C++ program that records audio and compresses it to GSM. I am able to record audio and write the raw data to a file. But I cannot get the GSM compression to work. I am trying to use a source code for compression that I found at this website ftp://ftp.cs.cmu.edu/project/fgdata/speech-compression/GSM/gsm_13200bps.

I think my problem is when using the gsm_encode() function. After encoding and saving the compressed data to the file when you play this file it is inaudible. I know that the raw audio data is correct but the compressed audio data is incorrect.

gsm_encode() encodes an array of 160 13-bit samples (given as
gsm_signal’s, signed integral values of at least 16 bits) into
a gsm_frame of 33 bytes.

Here is my function am I sending the data into gsm_encode() incorrectly? Or is there another problem with my function? Thank you for your help 🙂

int CAudioGSM::CompressAudio(unsigned char * pRawBuffer, _int32 uiRawBufferSize, unsigned char  * pCompressedBuffer, _int32 uiCompressedBufferSize)
{
// Note: uiRawBufferSize must be a multiple of 640 (AUDIO_DMA_DESCRITOR_LEN)
if(!pRawBuffer || uiRawBufferSize == 0 || !pCompressedBuffer || uiCompressedBufferSize == 0 ||  uiRawBufferSize % AUDIO_DMA_DESCRITOR_LEN != 0)
{
    return -1; //invalid parameters
}

_int32 uiBytesCompressed = 0; // Number of bytes that have been compressed. At the end of the function this should be equal to iRawBufferSize meaning we have compressed the whole raw buffer
_int32 uiCompBuffOffset = 0; // Offset into the compressed buffer

while(uiBytesCompressed < uiRawBufferSize)
{
    if(uiCompressedBufferSize - uiCompBuffOffset < GSM_OUTPUT_SIZE || uiCompBuffOffset >= uiCompressedBufferSize)
    {
       return -2; // Compressed buffer is too small
    }

    gsm_encode(&m_GSM_EncodeStruture,(long *)pRawBuffer,m_Buffer);
    //Now we need to move the data to compressed buffer
    if(m_bFirstHalfOfBlockRecord)
    {
       //Just copy the data over
       memcpy(&pCompressedBuffer[uiCompBuffOffset],m_Buffer,GSM_OUTPUT_SIZE_FIRST_HALF);
       m_bFirstHalfOfBlockRecord = false;
       uiCompBuffOffset += GSM_OUTPUT_SIZE_FIRST_HALF;
    }
    else
    {
       memcpy(&pCompressedBuffer[uiCompBuffOffset],m_Buffer,GSM_OUTPUT_SIZE);
       m_bFirstHalfOfBlockRecord = true;
       uiCompBuffOffset += GSM_OUTPUT_SIZE;
    }

    uiBytesCompressed += AUDIO_DMA_DESCRITOR_LEN;
}

return uiCompBuffOffset; // Success, we have compressed the buffer return compressed data size
}

Answer 1

nevermind I figured it out it the pRawBuffer never gets incremented it should be

gsm_encode(&m_GSM_EncodeStruture,(long *)&pRawBuffer[uiBytesCompressed],m_Buffer);