def ReadSCRMpeg2(buffer):
        """
        read SCR (timestamp) for MPEG2 at the buffer beginning (6 Bytes)
        """
        if len(buffer) < 6:
            return None

        highbit = (byte2int(buffer) & 0x20) >> 5

        low4Bytes = ((int(byte2int(buffer)) & 0x18) >> 3) << 30
        low4Bytes |= (byte2int(buffer) & 0x03) << 28
        low4Bytes |= indexbytes(buffer, 1) << 20
        low4Bytes |= (indexbytes(buffer, 2) & 0xF8) << 12
        low4Bytes |= (indexbytes(buffer, 2) & 0x03) << 13
        low4Bytes |= indexbytes(buffer, 3) << 5
        low4Bytes |= (indexbytes(buffer, 4)) >> 3

        sys_clock_ref = (indexbytes(buffer, 4) & 0x3) << 7
        sys_clock_ref |= (indexbytes(buffer, 5) >> 1)

        return (int(highbit * (1 << 16) * (1 << 16)) + low4Bytes) / 90000