[ 音频篇 ] 29 - 调试智能音箱中音频通路的回采(Ref信号)

// multi 插件完成两个声卡数据的叠加合并，相当于plughw:3,0 是6ch的音频数据，而plughw:1,0是2ch的音频数据，而通过arecord -Dsub_input 是8ch的音频数据。而bindings参数可以控制通道数据的调整。
pcm.sub_input {
    type multi
    slaves.a.pcm "plughw:3,0"
    slaves.a.channels 6
    slaves.b.pcm "plughw:1,0"
    slaves.b.channels 2
    bindings.0.slave a
    bindings.0.channel 0
    bindings.1.slave a
    bindings.1.channel 1
    bindings.2.slave a
    bindings.2.channel 2
    bindings.3.slave a
    bindings.3.channel 3
    bindings.4.slave a
    bindings.4.channel 4
    bindings.5.slave a
    bindings.5.channel 5
    bindings.6.slave b
    bindings.6.channel 0
    bindings.7.slave b
    bindings.7.channel 1
}

# arecord -l 
card 1: MapleTree [MapleTree], device 0: Playback ad82584f-0 []
  Subdevices: 0/1
  Subdevice #0: subdevice #0
card 1: MapleTree [MapleTree], device 1: Capture ES7210 4CH ADC 0-1 []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 2: Dummy [Dummy], device 0: DSP PCM [DSP PCM]
  Subdevices: 0/1
  Subdevice #0: subdevice #0

# aplay -l 
card 1: MapleTree [MapleTree], device 0: Playback ad82584f-0 []
  Subdevices: 1/1
  Subdevice #0: subdevice #0
# 设置系统音量
amixer -c 1 set Master 180 
# 一边播放一边录音
aplay -Dplughw:1,0  1ksine-48k-4s.wav | 
arecord  -D hw:1,0 -c 2 -r 48000 -f S32_LE -d 4 ref_48k_S32.wav

snd_pcm_t *Open(std::string name, uint32_t rate, uint8_t channels,snd_pcm_format_t format,snd_pcm_stream_t streamType){
    uint32_t pcm;
    //snd_pcm_format_t format_;
    //uint8_t channels_;
    snd_pcm_t * handle_;
    snd_pcm_hw_params_t * hwParams_;
    snd_pcm_sw_params_t * swParams_;
    //snd_pcm_uframes_t buffer_size_ = BUFFER_SIZE;
    if( (pcm = snd_pcm_open(&handle_, name.c_str(), streamType, 0)) < 0)
    {
        std::cout << "ERROR! Cannot open " << name << std::endl;
        return nullptr;
    }
    snd_pcm_hw_params_alloca(&hwParams_);
    snd_pcm_sw_params_alloca(&swParams_);
    snd_pcm_sw_params_current(handle_, swParams_);
    snd_pcm_hw_params_any(handle_, hwParams_);

    if( (pcm = snd_pcm_hw_params_set_access(handle_, hwParams_, SND_PCM_ACCESS_RW_INTERLEAVED)) < 0)
    {
        std::cout << "ERROR! Cannot set interleaved mode" << std::endl;
        return nullptr;
    }

    //format_ = format;
    if( (pcm = snd_pcm_hw_params_set_format(handle_, hwParams_, format)) < 0)
    {
        std::cout << "ERROR! Cannot set format" << std::endl;
        return nullptr;
    }

    //channels_ = channels;
    if( (pcm = snd_pcm_hw_params_set_channels(handle_, hwParams_, channels)) < 0)
    {
        std::cout << "ERROR! Cannot set number of channels" << std::endl;
        return nullptr;
    }

    if( (pcm = snd_pcm_hw_params_set_rate_near(handle_, hwParams_, &rate , 0)) < 0)
    {
        std::cout << "ERROR! Cannot set format" << std::endl;
        return nullptr;
    }

    /*  // 延迟的主要原因
    if( (pcm = snd_pcm_hw_params_set_buffer_size_near( handle_,hwParams_, &buffer_size_ )) < 0)
    {
        std::cout << "ERROR! Cannot set buffer" << std::endl;
        return nullptr;
    }
    */
    int dir;
    snd_pcm_uframes_t frames = (SAMPLE_RATE * PERIOD_TIME_MS)/ MS_IN_ONE_SECOND;
    snd_pcm_hw_params_set_period_size_near(handle_, hwParams_, &frames, &dir);
    if( (pcm  = snd_pcm_hw_params(handle_, hwParams_)) < 0)
    {
        std::cout << "ERROR! Cannot set hw parameters" << std::endl;
        return nullptr;
    }
    //frame_size_  = (format_ == SND_PCM_FORMAT_S32_LE ? 4 : 2) * channels_;
    return handle_;
}

# alsa-lib-1.1.3/src/pcm/pcm.c
5652 /**
5653  * \brief Restrict a configuration space to have buffer size nearest to a target
5654  * \param pcm PCM handle
5655  * \param params Configuration space
5656  * \param val approximate target buffer size in frames / returned chosen approximate target buffer size in frames
5657  * \return 0 otherwise a negative error code if configuration space is empty
5658  */
5659 #ifndef DOXYGEN
5660 int INTERNAL(snd_pcm_hw_params_set_buffer_size_near)(snd_pcm_t *pcm, snd_pcm_hw_params_t *params, snd_pcm_uframes_t *val)
5661 #else
5662 int snd_pcm_hw_params_set_buffer_size_near(snd_pcm_t *pcm, snd_pcm_hw_params_t *params, snd_pcm_uframes_t *val)
5663 #endif
5664 {
5665 >---unsigned int _val = *val;
5666 >---int err = snd_pcm_hw_param_set_near(pcm, params, SND_PCM_HW_PARAM_BUFFER_SIZE, &_val, NULL);
5667 >---if (err >= 0)
5668 >--->---*val = _val;
5669 >---return err;
5670 }