✍️ Learn how to decode stetho signal to PCM

Signal is A-law G 711 encoded with a sample rate of 4khz and each sample of 16 bit. In order to decode it in 16 bit PCM, here is an example of C algorithm:

int16_t alaw_decode(uint8_t c)
{
    int16_t  m, e;
    
    /* Restore the original
     bits                                          */
    c ^= 0x55;
    /* Extract the mantissa and the
     exponent                              */
    m = c & 0x0f;
    e = (c & 0x70) >> 4;
    /* If the exponent is greater than 0, we have to set 5th bit to
     1     */
    if (e > 0) {
        m |= 0x10;
    }
    /* Restore the full amplitude of the original
     signal                  */
    m <<= 4;
    /* And set the least significant
     bit                                  */
    m |= 0x08;
    /* Shift the mantissa according to the
     exponent                       */
    if (e > 1) {
        m <<= e-1;
    }
    /* If the most significant bit is not set, we have a negative
     number  */
    if (c < 0x80) {
        m = -m;
    }
    
    return m;
}

In Java:

object ALawCodec {

    fun decode(sample: ByteArray): List<Short> {
        return sample.map {
            var mantissa: Short
            val exponent: Short
            var valueToDecode: UByte = it.toUByte()
            valueToDecode = valueToDecode xor (0x55.toUByte())

            mantissa = (valueToDecode and (0x0f.toUByte())).toShort()
            exponent = ((valueToDecode and (0x70.toUByte())).toInt() shr 4).toShort()

            if (exponent > 0) {
                mantissa = mantissa or 0x10.toShort()
            }
            mantissa = (mantissa.toInt() shl 4).toShort()
            mantissa = mantissa or 0x08.toShort()
            if (exponent > 1) {
                mantissa = (mantissa.toInt() shl (exponent.toInt() - 1)).toShort()
            }
            if (valueToDecode < 0x80.toUByte()) {
                mantissa = (-mantissa).toShort()
            }
            mantissa
        }
    }
}