[移动开发]ijkplayer源码---倍速

我们看播放器都有倍速功能，倍速又是怎么实现的呢？

public void setSpeed(float speed) {
    _setPropertyFloat(FFP_PROP_FLOAT_PLAYBACK_RATE, speed);
}

public float getSpeed(float speed) {
    return _getPropertyFloat(FFP_PROP_FLOAT_PLAYBACK_RATE, .0f);
}



{ "_setPropertyFloat",      "(IF)V",                    (void *) ijkMediaPlayer_setPropertyFloat },

ffp_set_property_float(mp->ffplayer, id, value);

ffp_set_playback_rate(ffp, value);

void ffp_set_playback_rate(FFPlayer *ffp, float rate)
{
    if (!ffp)
        return;

    av_log(ffp, AV_LOG_INFO, "Playback rate: %f\n", rate);
    ffp->pf_playback_rate = rate;
    ffp->pf_playback_rate_changed = 1;
}

我们先看pf_playback_rate_changed

我们知道音频数据都来自于

static void sdl_audio_callback(void *opaque, Uint8 *stream, int len)
{

     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
    }

}

我们继续看SDL_AoutSetPlaybackRate

void SDL_AoutSetPlaybackRate(SDL_Aout *aout, float playbackRate)
{
    if (aout) {
        if (aout->func_set_playback_rate)
            aout->func_set_playback_rate(aout, playbackRate);
    }
}



SDL_Aout *SDL_AoutAndroid_CreateForAudioTrack(){
	aout->func_set_playback_rate    = func_set_playback_rate;
}


static void func_set_playback_rate(SDL_Aout *aout, float speed)
{
    if (!aout)
        return;

    SDL_Aout_Opaque *opaque = aout->opaque;
    SDL_LockMutex(opaque->wakeup_mutex);
    SDLTRACE("%s %f", __func__, (double)speed);
    opaque->speed = speed;
    opaque->speed_changed = 1;
    SDL_CondSignal(opaque->wakeup_cond);
    SDL_UnlockMutex(opaque->wakeup_mutex);
}

我们知道音频的播放是调用AudioTrack的write方法

static int aout_thread_n(JNIEnv *env, SDL_Aout *aout){

if (opaque->speed_changed) {
    opaque->speed_changed = 0;
    SDL_Android_AudioTrack_setSpeed(env, atrack, opaque->speed);
}


}

通过搜寻代码 发现最后走到的是AudioTrack的setPlaybackRate

void J4AC_android_media_AudioTrack__setSpeed(JNIEnv *env, jobject thiz, jfloat speed)
{
    if (J4A_GetSystemAndroidApiLevel(env) < 23) {
        jint sample_rate = J4AC_android_media_AudioTrack__getSampleRate(env,thiz);
        if (J4A_ExceptionCheck__throwAny(env)) {
            return;
        }
        J4AC_android_media_AudioTrack__setPlaybackRate__catchAll(env, thiz,
                                                                 (jint) (sample_rate * speed));
        return;
    }

    jobject temp = NULL;
    jobject params = J4AC_android_media_AudioTrack__getPlaybackParams(env, thiz);
    if (J4A_ExceptionCheck__throwAny(env) || !params)
        goto fail;

    temp = J4AC_android_media_PlaybackParams__setSpeed(env, params, speed);
    J4A_DeleteLocalRef__p(env, &temp);
    if (J4A_ExceptionCheck__throwAny(env))
        goto fail;

    J4A_ALOGE("%s %f", __func__, (double)speed);
    J4AC_android_media_AudioTrack__setPlaybackParams(env, thiz, params);
    if (J4A_ExceptionCheck__throwAny(env))
        goto fail;

fail:
    J4A_DeleteLocalRef__p(env, &params);
}

通过上面的代码可以看到倍速实际上就是重新设置了Audiotrack的采样率。

比如原始音频采样率是44100，speed设置为0.5。那么对Audiotrack就设置为了22050，这样就导致了原来一个时间端的数据，就会播放成两个时间段。

有一点要注意倍速最多能是多少

可以看到倍速可以变慢，但是变快最多也就是两倍。?