def ReadSCRMpeg1(buffer):
        """
        read SCR (timestamp) for MPEG1 at the buffer beginning (5 Bytes)
        """
        if len(buffer) < 5:
            return None

        highbit = (byte2int(buffer) >> 3) & 0x01

        low4Bytes = ((int(byte2int(buffer)) >> 1) & 0x03) << 30
        low4Bytes |= indexbytes(buffer, 1) << 22
        low4Bytes |= (indexbytes(buffer, 2) >> 1) << 15
        low4Bytes |= indexbytes(buffer, 3) << 7
        low4Bytes |= indexbytes(buffer, 4) >> 1

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