static public function isValidAudio($filename) {
        if (!file_exists($filename))
            throw new Exception('File '.$filename.' is not present!');
        $raw = file_get_contents($filename, false, null, 0, 3);
        return ($raw == self::TAG2_SYNC || (self::FRAME_SYNC == (unpack('n*', $raw)[1] & self::FRAME_SYNC)));
    }