ALSA音频编程中period的帧大小应设置为多少？

我是音频编程新手，参考《Linux Journal - Introduction to Sound Programming with ALSA》和网络资料开发，需求是播放已读取的WAV文件所有样本。

当前实现是打开默认播放设备，按音频文件格式设置参数，把period设为包含所有样本，调用一次snd_pcm_writei完成播放，之后调用snd_pcm_drain和snd_pcm_close退出，能正常运行，但不确定这种方式是否合规。

尝试设置更小的period，通过循环连续播放样本时，出现输出不同步、失真或播放一段时间后产生噪音的问题。

当前可行代码 #

int audio_play(unsigned int frequency, int bits_per_sample, int channels_count, char *samples, long samples_count) {

  snd_pcm_t *handle;
  snd_pcm_uframes_t frames;
  int dir;
  int write_result;
  snd_pcm_hw_params_t *params;

  if (snd_pcm_open(&handle, "default", SND_PCM_STREAM_PLAYBACK, 0) < 0) {printf("\naudio_play failed: snd_pcm_open failed.\n"); return 2;}

  snd_pcm_hw_params_alloca(&params);
  snd_pcm_hw_params_any(handle, params);

  snd_pcm_hw_params_set_access(handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);

  if (bits_per_sample == 8)
    snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_U8);
  else if (bits_per_sample == 16)
    snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16);
  else if (bits_per_sample == 24)
    snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S24);
  else if (bits_per_sample == 32)
    snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S32);

  if (channels_count == 1)
    snd_pcm_hw_params_set_channels(handle, params, 1);
  else if (channels_count == 2)
    snd_pcm_hw_params_set_channels(handle, params, 2);

  snd_pcm_hw_params_set_rate(handle, params, frequency, 0);

  frames = samples_count;
  snd_pcm_hw_params_set_period_size(handle, params, frames, 0);

  if (snd_pcm_hw_params(handle, params) < 0) {printf("\naudio_play: snd_pcm_hw_params_failed.\n"); return 1;}

  snd_pcm_writei(handle, samples, frames);

  snd_pcm_drain(handle);
  snd_pcm_close(handle);

  return 0;

}

尝试的错误实现代码 #

int audio_play(unsigned int frequency, int bits_per_sample, int channels_count, char *samples, long samples_count) {

  snd_pcm_t *handle;
  snd_pcm_uframes_t frames;
  int dir;
  int write_result;
  snd_pcm_hw_params_t *params;

  if (snd_pcm_open(&handle, "default", SND_PCM_STREAM_PLAYBACK, 0) < 0) {printf("\naudio_play failed: snd_pcm_open failed.\n"); return 2;}

  snd_pcm_hw_params_alloca(&params);
  snd_pcm_hw_params_any(handle, params);

  snd_pcm_hw_params_set_access(handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);

  if (bits_per_sample == 8)
    snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_U8);
  else if (bits_per_sample == 16)
    snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16);
  else if (bits_per_sample == 24)
    snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S24);
  else if (bits_per_sample == 32)
    snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S32);

  if (channels_count == 1)
    snd_pcm_hw_params_set_channels(handle, params, 1);
  else if (channels_count == 2)
    snd_pcm_hw_params_set_channels(handle, params, 2);

  snd_pcm_hw_params_set_rate(handle, params, frequency, 0);

  frames = samples_count / (bits_per_sample / 8 * channels_count);
  snd_pcm_hw_params_set_period_size(handle, params, frames, 0);

  if (snd_pcm_hw_params(handle, params) < 0) {printf("\naudio_play: snd_pcm_hw_params_failed.\n"); return 1;}

  while (samples_count > 0) {
    snd_pcm_writei(handle, samples, frames);
    samples_count -= frames;
    samples += frames;
  }

  snd_pcm_drain(handle);
  snd_pcm_close(handle);

  return 0;
}

测试所用音频文件参数 #

音频文件1 #

file type: RIFF
file format: WAVE
block format: fmt
data block id: data 
channels count: 2
frequency: 44100
bytes per sec: 176400
bytes per block: 4
bits per sample: 16
samples count: 262094

音频文件2 #

file type: RIFF
file format: WAVE 
block format: fmt
data block id: data
channels count: 1
frequency: 22257
bytes per sec: 22257
bytes per block: 1
bits per sample: 8
samples count: 91240

问题解答 #

一、设置超大Period的潜在问题 #

直接把period设为整个音频的样本数，虽然能运行，但存在不少隐患：

• 内存压力：音频设备需要一次性分配足够内存容纳所有样本，对于大文件（比如几十分钟的音频），可能导致内存分配失败，或占用过多系统资源。
• 设备兼容性：部分音频硬件或驱动对period size有最大限制，超过限制会导致snd_pcm_hw_params调用失败，无法初始化设备。
• 响应性差：如果播放过程中需要暂停、停止或调整音频，这种方式无法及时响应，必须等整个音频播放完成才能处理。

二、正确的循环播放实现 #

你的错误实现有三个核心问题：单位不统一、未处理snd_pcm_writei返回值、指针偏移错误。修正后的代码如下：

int audio_play(unsigned int frequency, int bits_per_sample, int channels_count, char *samples, long samples_bytes) {
    snd_pcm_t *handle;
    snd_pcm_uframes_t period_frames = 4096; // 选择设备兼容的常见period大小
    snd_pcm_uframes_t frames_to_write;
    int dir;
    int write_result;
    snd_pcm_hw_params_t *params;
    int bytes_per_frame = (bits_per_sample / 8) * channels_count;

    if (snd_pcm_open(&handle, "default", SND_PCM_STREAM_PLAYBACK, 0) < 0) {
        printf("\naudio_play failed: snd_pcm_open failed.\n");
        return 2;
    }

    snd_pcm_hw_params_alloca(&params);
    snd_pcm_hw_params_any(handle, params);

    snd_pcm_hw_params_set_access(handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);

    // 设置采样格式
    switch (bits_per_sample) {
        case 8:
            snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_U8);
            break;
        case 16:
            snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16);
            break;
        case 24:
            snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S24);
            break;
        case 32:
            snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S32);
            break;
        default:
            printf("\naudio_play: unsupported bits per sample.\n");
            snd_pcm_close(handle);
            return 3;
    }

    // 设置通道数
    if (channels_count != 1 && channels_count != 2) {
        printf("\naudio_play: unsupported channels count.\n");
        snd_pcm_close(handle);
        return 4;
    }
    snd_pcm_hw_params_set_channels(handle, params, channels_count);

    // 设置采样率
    if (snd_pcm_hw_params_set_rate(handle, params, frequency, 0) < 0) {
        printf("\naudio_play: failed to set sample rate.\n");
        snd_pcm_close(handle);
        return 5;
    }

    // 让ALSA自动调整到设备支持的period大小
    if (snd_pcm_hw_params_set_period_size_near(handle, params, &period_frames, &dir) < 0) {
        printf("\naudio_play: failed to set period size.\n");
        snd_pcm_close(handle);
        return 6;
    }

    if (snd_pcm_hw_params(handle, params) < 0) {
        printf("\naudio_play: snd_pcm_hw_params_failed.\n");
        snd_pcm_close(handle);
        return 1;
    }

    long remaining_bytes = samples_bytes;
    char *current_samples = samples;

    while (remaining_bytes > 0) {
        // 计算本次可写入的最大帧数
        frames_to_write = period_frames;
        long max_frames = remaining_bytes / bytes_per_frame;
        if (frames_to_write > max_frames) {
            frames_to_write = max_frames;
        }

        write_result = snd_pcm_writei(handle, current_samples, frames_to_write);
        if (write_result < 0) {
            // 处理underrun错误，尝试恢复设备
            if (write_result == -EPIPE) {
                printf("\naudio_play: underrun occurred, attempting recovery.\n");
                snd_pcm_prepare(handle);
                continue;
            } else {
                printf("\naudio_play: snd_pcm_writei failed with error: %d\n", write_result);
                snd_pcm_close(handle);
                return 7;
            }
        }

        // 更新剩余字节数和当前数据指针
        remaining_bytes -= write_result * bytes_per_frame;
        current_samples += write_result * bytes_per_frame;
    }

    snd_pcm_drain(handle);
    snd_pcm_close(handle);

    return 0;
}

关键修正点说明

1. 使用snd_pcm_hw_params_set_period_size_near：让ALSA自动选择设备支持的最接近目标值的period大小，避免设置不支持的period导致初始化失败。
2. 单位统一：严格区分字节数和帧数，所有计算保持单位一致，指针偏移按实际写入的字节数计算。
3. 处理返回值：捕获-EPIPE（underrun）错误，调用snd_pcm_prepare恢复设备；确保所有音频数据都被正确写入。
4. 合理的period大小：选择4096、2048这类常见值，平衡内存占用和播放稳定性。

内容的提问来源于stack exchange，提问作者Dominik