在Linux上签署16位ALSA PCM数据到U8转换

时间:2015-02-16 02:04:33

标签: c++ c linux audio alsa

我正在尝试将16位ALSA PCM样本转换为无符号8位PCM样本,以便在Linux上进行无线传输。接收机正在成功播放传输的数据,录制的语音在那里可识别,但质量很差,噪音很大。我已经尝试了两端的ALSA混音器来调整流,但它似乎没有那么好。我相信我将样本转换为8位PCM有问题,但它只是一个简单的转换,所以我不确定是什么错误。有没有人有任何建议或我的转换代码有任何问题?感谢。

转换代码:

            // This byte array needs to be the packet size we wish to send
            QByteArray prepareToSend;
            prepareToSend.clear();

            // Keep reading from ALSA until we fill one full frame
            int frames = 1;
            while ( prepareToSend.size() < TARGET_TX_BUFFER_SIZE ) {

                // Create a ByteArray
                QByteArray readBytes;
                readBytes.resize(size);

                // Read with ALSA
                short sample[1]; // Data is signed 16-bit
                int rc = snd_pcm_readi(m_PlaybackHandle, sample, frames);
                if (rc == -EPIPE) {
                    /* EPIPE means overrun */
                    fprintf(stderr, "Overrun occurred\n");
                    snd_pcm_prepare(m_PlaybackHandle);
                } else if (rc < 0) {
                    fprintf(stderr,
                            "Error from read: %s\n",
                            snd_strerror(rc));
                } else if (rc != (int)frames) {
                    fprintf(stderr, "Short read, read %d frames\n", rc);
                }
                else {
                    // Copy bytes to the prepare to send buffer
                    //qDebug() << "Bytes for sample buffer: " << sizeof(sample);
                    prepareToSend.append((qint16)(sample[0]) >> 8); // signed 16-bit becomes u8
                }

            }

ALSA配置:

        // Setup parameters
        int size;
        snd_pcm_t *m_PlaybackHandle;
        snd_pcm_hw_params_t *m_HwParams;
        char *buffer;

        qDebug() << "Desire to Transmit Data - Setting up ALSA Now....";

        // Error handling
        int err;

        // Device to Write to
        const char *snd_device_in = "hw:1,0";

        if ((err = snd_pcm_open (&m_PlaybackHandle, snd_device_in, SND_PCM_STREAM_CAPTURE, 0)) < 0) {
            fprintf (stderr, "Cannot open audio device %s (%s)\n",
                     snd_device_in,
                     snd_strerror (err));
            exit (1);
        }

        /* Allocate a hardware parameters object. */
        snd_pcm_hw_params_alloca(&m_HwParams);

        if ((err = snd_pcm_hw_params_malloc (&m_HwParams)) < 0) {
            fprintf (stderr, "Cannot allocate hardware parameter structure (%s)\n",
                     snd_strerror (err));
            exit (1);
        }

        if ((err = snd_pcm_hw_params_any (m_PlaybackHandle, m_HwParams)) < 0) {
            fprintf (stderr, "Cannot initialize hardware parameter structure (%s)\n",
                     snd_strerror (err));
            exit (1);
        }

        if ((err = snd_pcm_hw_params_set_access (m_PlaybackHandle, m_HwParams, SND_PCM_ACCESS_RW_INTERLEAVED)) < 0) {
            fprintf (stderr, "Cannot set access type (%s)\n",
                     snd_strerror (err));
            exit (1);
        }

        if ((err = snd_pcm_hw_params_set_format(m_PlaybackHandle, m_HwParams, SND_PCM_FORMAT_S16)) < 0) { // Has to be 16 bit
            fprintf (stderr, "Cannot set sample format (%s)\n",
                     snd_strerror (err));
            exit (1);

        }

        uint sample_rate = 8000;
        if ((err = snd_pcm_hw_params_set_rate (m_PlaybackHandle, m_HwParams, sample_rate, 0)) < 0) { // 8 KHz
            fprintf (stderr, "Cannot set sample rate (%s)\n",
                     snd_strerror (err));
            exit (1);
        }

        if ((err = snd_pcm_hw_params_set_channels (m_PlaybackHandle, m_HwParams, 1)) < 0) { // 1 Channel Mono
            fprintf (stderr, "Cannot set channel count (%s)\n",
                     snd_strerror (err));
            exit (1);
        }

        /*
        Frames: samples x channels (i.e: stereo frames are composed of two samples, mono frames are composed of 1 sample,...)
        Period: Number of samples tranferred after which the device acknowledges the transfer to the apllication (usually via an interrupt).
        */

        /* Submit params to device */
        if ((err = snd_pcm_hw_params(m_PlaybackHandle, m_HwParams)) < 0) {
            fprintf (stderr, "Cannot set parameters (%s)\n",
                     snd_strerror (err));
            exit (1);
        }

        /* Free the Struct */
        snd_pcm_hw_params_free(m_HwParams);

        // Flush handle prepare for record
        snd_pcm_drop(m_PlaybackHandle);

        if ((err = snd_pcm_prepare (m_PlaybackHandle)) < 0) {
            fprintf (stderr, "cannot prepare audio interface for use (%s)\n",
                     snd_strerror (err));
            exit (1);
        }

        qDebug() << "Done Setting up ALSA....";

        // Prepare the device
        if ((err = snd_pcm_prepare (m_PlaybackHandle)) < 0) {
            fprintf (stderr, "cannot prepare audio interface for use (%s)\n",
                     snd_strerror (err));
            exit (1);
        }

2 个答案:

答案 0 :(得分:1)

(qint16)(sample[0]) >> 8会将带符号的线性16位PCM转换为带符号的线性8位PCM。如果你想要无符号线性8位,那么它将是((quint16)sample[0] ^ 0x8000) >> 8

虽然16位PCM几乎总是在线性刻度上,但8位PCM更常见于对数刻度(μ律或A律),并且查找表通常用于转换。如果您确实需要线性8位,那么您可能需要先调整增益,使峰值为0 dBFS,并使用音频压缩来减小动态范围,使其适合8位。

答案 1 :(得分:1)

如果您使用plughw:1,0代替hw:1,0,则可以告诉设备您需要SND_PCM_FORMAT_U8,并自动转换样本。 (这也适用于μ-Law和A-Law。)