IJKPLAYER源码分析-AudioTrack播放

作者：小小林熬夜学编程 | 2024-04-10 23:13:22

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1 前言

AudioTrack是Android SDK所提供的播放PCM音频的技术，与mediacodec类似，IJKPLAYER对此使用的是以native层反射到Java层的播放能力。

关于AudioTrack的官方文档，请参考AudioTrack官方文档

2 接口

2.1 pipeline

IJKFF_Pipeline结构体是对Android和iOS平台video的软硬解以及audio的播放操作的抽象，2端遵循共同的接口规范，包括各自平台的硬解和ffmpeg软解；

IJKFF_Pipeline结构体定义：


typedef struct IJKFF_Pipeline_Opaque IJKFF_Pipeline_Opaque;
typedef struct IJKFF_Pipeline IJKFF_Pipeline;
struct IJKFF_Pipeline {
    SDL_Class             *opaque_class;
    IJKFF_Pipeline_Opaque *opaque;
 
    void            (*func_destroy)             (IJKFF_Pipeline *pipeline);
    IJKFF_Pipenode *(*func_open_video_decoder)  (IJKFF_Pipeline *pipeline, FFPlayer *ffp);
    SDL_Aout       *(*func_open_audio_output)   (IJKFF_Pipeline *pipeline, FFPlayer *ffp);
    IJKFF_Pipenode *(*func_init_video_decoder)  (IJKFF_Pipeline *pipeline, FFPlayer *ffp);
    int           (*func_config_video_decoder)  (IJKFF_Pipeline *pipeline, FFPlayer *ffp);
};

IJKFF_Pipeline_Opaque结构体则是IJKFF_Pipeline结构体相关数据的封装；

展开IJKFF_Pipeline_Opaque结构体：


typedef struct IJKFF_Pipeline_Opaque {
    FFPlayer      *ffp;
    SDL_mutex     *surface_mutex;
    jobject        jsurface;
    volatile bool  is_surface_need_reconfigure;
 
    bool         (*mediacodec_select_callback)(void *opaque, ijkmp_mediacodecinfo_context *mcc);
    void          *mediacodec_select_callback_opaque;
 
    SDL_Vout      *weak_vout;
 
    float          left_volume;
    float          right_volume;
} IJKFF_Pipeline_Opaque;

创建pipeline对象调用链：

Java native_setup() => IjkMediaPlayer_native_setup（）=> ijkmp_android_create() => ffpipeline_create_from_android()

展开ijkmp_android_create方法：


IjkMediaPlayer *ijkmp_android_create(int(*msg_loop)(void*))
{
    IjkMediaPlayer *mp = ijkmp_create(msg_loop);
    if (!mp)
        goto fail;
 
    mp->ffplayer->vout = SDL_VoutAndroid_CreateForAndroidSurface();
    if (!mp->ffplayer->vout)
        goto fail;
 
    mp->ffplayer->pipeline = ffpipeline_create_from_android(mp->ffplayer);
    if (!mp->ffplayer->pipeline)
        goto fail;
 
    ffpipeline_set_vout(mp->ffplayer->pipeline, mp->ffplayer->vout);
 
    return mp;
 
fail:
    ijkmp_dec_ref_p(&mp);
    return NULL;
}

再展开ffpipeline_create_from_android方法：


IJKFF_Pipeline *ffpipeline_create_from_android(FFPlayer *ffp)
{
    ALOGD("ffpipeline_create_from_android()\n");
    IJKFF_Pipeline *pipeline = ffpipeline_alloc(&g_pipeline_class, sizeof(IJKFF_Pipeline_Opaque));
    if (!pipeline)
        return pipeline;
 
    IJKFF_Pipeline_Opaque *opaque = pipeline->opaque;
    opaque->ffp                   = ffp;
    opaque->surface_mutex         = SDL_CreateMutex();
    opaque->left_volume           = 1.0f;
    opaque->right_volume          = 1.0f;
    if (!opaque->surface_mutex) {
        ALOGE("ffpipeline-android:create SDL_CreateMutex failed\n");
        goto fail;
    }
 
    pipeline->func_destroy              = func_destroy;
    pipeline->func_open_video_decoder   = func_open_video_decoder;
    pipeline->func_open_audio_output    = func_open_audio_output;
    pipeline->func_init_video_decoder   = func_init_video_decoder;
    pipeline->func_config_video_decoder = func_config_video_decoder;
 
    return pipeline;
fail:
    ffpipeline_free_p(&pipeline);
    return NULL;
}

2.2 SDL_Aout

该结构体抽象了Android和iOS端对声音硬件的所有操作，2端几乎遵循共同的接口规范；


struct SDL_Aout {
    SDL_mutex *mutex;
    double     minimal_latency_seconds;
 
    SDL_Class       *opaque_class;
    SDL_Aout_Opaque *opaque;
    void (*free_l)(SDL_Aout *vout);
    int (*open_audio)(SDL_Aout *aout, const SDL_AudioSpec *desired, SDL_AudioSpec *obtained);
    void (*pause_audio)(SDL_Aout *aout, int pause_on);
    void (*flush_audio)(SDL_Aout *aout);
    void (*set_volume)(SDL_Aout *aout, float left, float right);
    void (*close_audio)(SDL_Aout *aout);
 
    double (*func_get_latency_seconds)(SDL_Aout *aout);
    void   (*func_set_default_latency_seconds)(SDL_Aout *aout, double latency);
 
    // optional
    void   (*func_set_playback_rate)(SDL_Aout *aout, float playbackRate);
    void   (*func_set_playback_volume)(SDL_Aout *aout, float playbackVolume);
    int    (*func_get_audio_persecond_callbacks)(SDL_Aout *aout);
 
    // Android only
    int    (*func_get_audio_session_id)(SDL_Aout *aout);
};

2.3 SDL_Aout_Opaque

该结构体是SDL_Aout结构的内部数据成员，封装了对音频操作相关的参数、数据与mutex和cond等；


typedef struct SDL_Aout_Opaque {
    SDL_cond *wakeup_cond;
    SDL_mutex *wakeup_mutex;
 
    SDL_AudioSpec spec;
    SDL_Android_AudioTrack* atrack;
    uint8_t *buffer;
    int buffer_size;
 
    volatile bool need_flush;
    volatile bool pause_on;
    volatile bool abort_request;
 
    volatile bool need_set_volume;
    volatile float left_volume;
    volatile float right_volume;
 
    SDL_Thread *audio_tid;
    SDL_Thread _audio_tid;
 
    int audio_session_id;
 
    volatile float speed;
    volatile bool speed_changed;
} SDL_Aout_Opaque;

3 创建SDL_Aout

ffp_prepare_async_l() => ffpipeline_open_audio_output() => func_open_audio_output() => SDL_AoutAndroid_CreateForAudioTrack()

最后，走到此处创建SDL_Aout对象：


SDL_Aout *SDL_AoutAndroid_CreateForAudioTrack()
{
    SDL_Aout *aout = SDL_Aout_CreateInternal(sizeof(SDL_Aout_Opaque));
    if (!aout)
        return NULL;
 
    SDL_Aout_Opaque *opaque = aout->opaque;
    opaque->wakeup_cond  = SDL_CreateCond();
    opaque->wakeup_mutex = SDL_CreateMutex();
    opaque->speed        = 1.0f;
 
    aout->opaque_class = &g_audiotrack_class;
    aout->free_l       = aout_free_l;
    aout->open_audio   = aout_open_audio;
    aout->pause_audio  = aout_pause_audio;
    aout->flush_audio  = aout_flush_audio;
    aout->set_volume   = aout_set_volume;
    aout->close_audio  = aout_close_audio;
    aout->func_get_audio_session_id = aout_get_audio_session_id;
    aout->func_set_playback_rate    = func_set_playback_rate;
 
    return aout;
}

4 open_audio

主要做以下事情：

打开audio，其实是分配一个jobject类型的AudioTrack对象，并把音频源的参数传递给它，后续对AudioTrack的操作均使用该对象；
值得一提的是，IJKPLAYER对音频源的channel和pcm格式以及采样率有限制，比如只允许播放单双通道、16bit或8bit的pcm格式、采样率也必须在4000~48000之间；
保存AudioTrack所能播放的音频参数在is->audio_tgt中，后续音频参数变更重采样之用；
开启audio_thread线程异步处理对AudioTrack的操作；
设置AudioTrack的缺省时延，用于音视频同步时纠正音频的时钟；


    // 设置缺省时延，若有func_set_default_latency_seconds回调则通过回调更新，没有则设置变量minimal_latency_seconds的值
    SDL_AoutSetDefaultLatencySeconds(ffp->aout, ((double)(2 * spec.size)) / audio_hw_params->bytes_per_sec);

完整调用链：

read_thread() => stream_component_open() => audio_open() => SDL_AoutOpenAudio() => aout_open_audio() => aout_open_audio_n()

我们主要来看看aout_open_audio_n函数：

将音频源的采样参数告知给AudioTrack，分配一个jobject类型的AudioTrack对象，后续对AudioTrack的操作均是由此对象发起；
按getMinBufferSize() * 2分配一个用于缓存PCM数据的buffer，一定大于256byte；
开启一个audio_thread线程，用以异步执行对AudioTrack的操作，诸如setVolume() / pause() / flush() / close_audio() / setPlaybackRate()等；
将audio硬件PCM播放的参数在全局is->audio_tgt变量中，后续参数变更重采样用；
设置播放的初始音量；


static int aout_open_audio_n(JNIEnv *env, SDL_Aout *aout, const SDL_AudioSpec *desired, SDL_AudioSpec *obtained)
{
    assert(desired);
    SDL_Aout_Opaque *opaque = aout->opaque;
 
    opaque->spec = *desired;
    // 将pcm采样参数告知audiotrack
    opaque->atrack = SDL_Android_AudioTrack_new_from_sdl_spec(env, desired);
    if (!opaque->atrack) {
        ALOGE("aout_open_audio_n: failed to new AudioTrcak()");
        return -1;
    }
 
    opaque->buffer_size = SDL_Android_AudioTrack_get_min_buffer_size(opaque->atrack);
    if (opaque->buffer_size <= 0) {
        ALOGE("aout_open_audio_n: failed to getMinBufferSize()");
        SDL_Android_AudioTrack_free(env, opaque->atrack);
        opaque->atrack = NULL;
        return -1;
    }
 
    opaque->buffer = malloc(opaque->buffer_size);
    if (!opaque->buffer) {
        ALOGE("aout_open_audio_n: failed to allocate buffer");
        SDL_Android_AudioTrack_free(env, opaque->atrack);
        opaque->atrack = NULL;
        return -1;
    }
 
    if (obtained) {
        SDL_Android_AudioTrack_get_target_spec(opaque->atrack, obtained);
        SDLTRACE("audio target format fmt:0x%x, channel:0x%x", (int)obtained->format, (int)obtained->channels);
    }
 
    opaque->audio_session_id = SDL_Android_AudioTrack_getAudioSessionId(env, opaque->atrack);
    ALOGI("audio_session_id = %d\n", opaque->audio_session_id);
 
    opaque->pause_on = 1;
    opaque->abort_request = 0;
    opaque->audio_tid = SDL_CreateThreadEx(&opaque->_audio_tid, aout_thread, aout, "ff_aout_android");
    if (!opaque->audio_tid) {
        ALOGE("aout_open_audio_n: failed to create audio thread");
        SDL_Android_AudioTrack_free(env, opaque->atrack);
        opaque->atrack = NULL;
        return -1;
    }
 
    return 0;
}

此外，这里介绍一下getMinBufferSize函数：

getMinBufferSize会综合考虑硬件情况（诸如是否支持采样率，硬件本身的延迟情况等）后，得出一个最小缓冲区的大小。一般我们分配的缓冲大小会是它的整数倍。

5 audio_thread

对Android SDK的AudioTrack异步执行操作，如pause()/play()/setVolume()/flush()/setSpped()；
通过sdl_audio_callback回调copy固定256byte的PCM数据，然后喂给AudioTrack播放；

5.1 执行操作

所有对AudioTrack的操作，都是在此线程里异步执行：

值得一提的是，若播放器处于pause状态时，该线程会一直条件等待opaque->pause_on非false(也即可播状态)或程序退出，线程空转；


static int aout_thread_n(JNIEnv *env, SDL_Aout *aout)
{
    SDL_Aout_Opaque *opaque = aout->opaque;
    SDL_Android_AudioTrack *atrack = opaque->atrack;
    SDL_AudioCallback audio_cblk = opaque->spec.callback;
    void *userdata = opaque->spec.userdata;
    uint8_t *buffer = opaque->buffer;
    // 单次喂给AudioTrack的PCM的bytes,不宜喂得太少,也不宜太多,单次应能播一会儿,5ms
    int copy_size = 256;
 
    assert(atrack);
    assert(buffer);
 
    SDL_SetThreadPriority(SDL_THREAD_PRIORITY_HIGH);
 
    if (!opaque->abort_request && !opaque->pause_on)
        SDL_Android_AudioTrack_play(env, atrack);
 
    while (!opaque->abort_request) {
        SDL_LockMutex(opaque->wakeup_mutex);
        if (!opaque->abort_request && opaque->pause_on) {
            SDL_Android_AudioTrack_pause(env, atrack);
            // 若暂停了，当前线程一直在此条件等待播放
            while (!opaque->abort_request && opaque->pause_on) {
                SDL_CondWaitTimeout(opaque->wakeup_cond, opaque->wakeup_mutex, 1000);
            }
            if (!opaque->abort_request && !opaque->pause_on) {
                if (opaque->need_flush) {
                    opaque->need_flush = 0;
                    SDL_Android_AudioTrack_flush(env, atrack);
                }
                SDL_Android_AudioTrack_play(env, atrack);
            }
        }
        if (opaque->need_flush) {
            opaque->need_flush = 0;
            SDL_Android_AudioTrack_flush(env, atrack);
        }
        if (opaque->need_set_volume) {
            opaque->need_set_volume = 0;
            SDL_Android_AudioTrack_set_volume(env, atrack, opaque->left_volume, opaque->right_volume);
        }
        if (opaque->speed_changed) {
            opaque->speed_changed = 0;
            SDL_Android_AudioTrack_setSpeed(env, atrack, opaque->speed);
        }
        SDL_UnlockMutex(opaque->wakeup_mutex);
 
        // copy解码后的pcm数据，每次固定256byte
        audio_cblk(userdata, buffer, copy_size);
        if (opaque->need_flush) {
            SDL_Android_AudioTrack_flush(env, atrack);
            opaque->need_flush = false;
        }
 
        if (opaque->need_flush) {
            opaque->need_flush = 0;
            SDL_Android_AudioTrack_flush(env, atrack);
        } else {
            // 将pcm数据喂给AudioTrack播放
            int written = SDL_Android_AudioTrack_write(env, atrack, buffer, copy_size);
            if (written != copy_size) {
                ALOGW("AudioTrack: not all data copied %d/%d", (int)written, (int)copy_size);
            }
        }
 
        // TODO: 1 if callback return -1 or 0
    }
 
    SDL_Android_AudioTrack_free(env, atrack);
    return 0;
}

5.2 sdl_audio_callback

audio_thread通过callback的方式从解码后的音频队列FrameQueue拷贝走固定长度256byte的pcm数据；


/* prepare a new audio buffer */
static void sdl_audio_callback(void *opaque, Uint8 *stream, int len)
{
    FFPlayer *ffp = opaque;
    VideoState *is = ffp->is;
    int audio_size, len1;
    if (!ffp || !is) {
        memset(stream, 0, len);
        return;
    }
 
    ffp->audio_callback_time = av_gettime_relative();
 
    if (ffp->pf_playback_rate_changed) {
        ffp->pf_playback_rate_changed = 0;
#if defined(__ANDROID__)
        if (!ffp->soundtouch_enable) {
            SDL_AoutSetPlaybackRate(ffp->aout, ffp->pf_playback_rate);
        }
#else
        SDL_AoutSetPlaybackRate(ffp->aout, ffp->pf_playback_rate);
#endif
    }
    if (ffp->pf_playback_volume_changed) {
        ffp->pf_playback_volume_changed = 0;
        SDL_AoutSetPlaybackVolume(ffp->aout, ffp->pf_playback_volume);
    }
 
    // 循环是确保copy走len字节的pcm数据
    while (len > 0) {
        if (is->audio_buf_index >= is->audio_buf_size) {
           audio_size = audio_decode_frame(ffp);
           if (audio_size < 0) {
                /* if error, just output silence */
               is->audio_buf = NULL;
               is->audio_buf_size = SDL_AUDIO_MIN_BUFFER_SIZE / is->audio_tgt.frame_size * is->audio_tgt.frame_size;
           } else {
               if (is->show_mode != SHOW_MODE_VIDEO)
                   update_sample_display(is, (int16_t *)is->audio_buf, audio_size);
               is->audio_buf_size = audio_size;
           }
           is->audio_buf_index = 0;
        }
        if (is->auddec.pkt_serial != is->audioq.serial) {
            is->audio_buf_index = is->audio_buf_size;
            // 静音播放
            memset(stream, 0, len);
            // flush掉seek前后的pcm数据
            SDL_AoutFlushAudio(ffp->aout);
            break;
        }
        len1 = is->audio_buf_size - is->audio_buf_index;
        if (len1 > len)
            len1 = len;
        if (!is->muted && is->audio_buf && is->audio_volume == SDL_MIX_MAXVOLUME)
            // 在此copy走pcm数据
            memcpy(stream, (uint8_t *)is->audio_buf + is->audio_buf_index, len1);
        else {
            memset(stream, 0, len1);
            if (!is->muted && is->audio_buf)
                SDL_MixAudio(stream, (uint8_t *)is->audio_buf + is->audio_buf_index, len1, is->audio_volume);
        }
        len -= len1;
        stream += len1;
        is->audio_buf_index += len1;
    }
    is->audio_write_buf_size = is->audio_buf_size - is->audio_buf_index;
    /* Let's assume the audio driver that is used by SDL has two periods. */
    if (!isnan(is->audio_clock)) {
        // 计算Audio参考时钟时应将硬件里的PCM样本缓存考虑进去(opensl es and audiounit),以及is->audio_write_buf_size
        set_clock_at(&is->audclk, is->audio_clock - (double)(is->audio_write_buf_size) / is->audio_tgt.bytes_per_sec - SDL_AoutGetLatencySeconds(ffp->aout), is->audio_clock_serial, ffp->audio_callback_time / 1000000.0);
        sync_clock_to_slave(&is->extclk, &is->audclk);
    }
    if (!ffp->first_audio_frame_rendered) {
        ffp->first_audio_frame_rendered = 1;
        ffp_notify_msg1(ffp, FFP_MSG_AUDIO_RENDERING_START);
    }
    if (is->latest_audio_seek_load_serial == is->audio_clock_serial) {
        int latest_audio_seek_load_serial = __atomic_exchange_n(&(is->latest_audio_seek_load_serial), -1, memory_order_seq_cst);
        if (latest_audio_seek_load_serial == is->audio_clock_serial) {
            if (ffp->av_sync_type == AV_SYNC_AUDIO_MASTER) {
                ffp_notify_msg2(ffp, FFP_MSG_AUDIO_SEEK_RENDERING_START, 1);
            } else {
                ffp_notify_msg2(ffp, FFP_MSG_AUDIO_SEEK_RENDERING_START, 0);
            }
        }
    }
    if (ffp->render_wait_start && !ffp->start_on_prepared && is->pause_req) {
        while (is->pause_req && !is->abort_request) {
            SDL_Delay(20);
        }
    }
}

5.3 喂PCM数据


        ......
 
        // 从FrameQueue队列里取走256个byte的pcm数据
        audio_cblk(userdata, buffer, copy_size);
        if (opaque->need_flush) {
            SDL_Android_AudioTrack_flush(env, atrack);
            opaque->need_flush = false;
        }
 
        if (opaque->need_flush) {
            opaque->need_flush = 0;
            SDL_Android_AudioTrack_flush(env, atrack);
        } else {
            // 将从FrameQueue队列里copy过来的pcm数据喂给AudioTrack播放
            int written = SDL_Android_AudioTrack_write(env, atrack, buffer, copy_size);
            if (written != copy_size) {
                ALOGW("AudioTrack: not all data copied %d/%d", (int)written, (int)copy_size);
            }
        }
 
        ......

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