4.3 音訊解碼

音訊解碼，就是將視訊檔案中的音訊部分抽離出來，生成PCM檔案，並使用Android控制元件AudioTrack進行播放。
新建，MedioPlayer專案，其目錄結構如下：

java端程式碼，MainActivity.java：

package com.lzp.decoder;

import java.io.File;

import com.lzp.decoder.view.VideoView;

import android.app.Activity;
import android.os.Bundle;
import android.os.Environment;
import
 
 android.view.Surface;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Spinner;

public class MainActivity extends Activity {

    private Player player;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        player = new
 
 Player();
    }

    public void playAudio(View btn) {
        String input = new File(Environment.getExternalStorageDirectory(), "sintel.mp4").getAbsolutePath();
        String output = new File(Environment.getExternalStorageDirectory(), "output.pcm").getAbsolutePath();
        player.sound(input, output);
    }
}
 

java端程式碼，Player.java：

package com.lzp.decoder;

import android.media.AudioFormat;
import android.media.AudioManager;
import android.media.AudioTrack;
import android.util.Log;
import android.view.Surface;

/**
 * 視訊播放的控制器
 */
public class Player {

    // 解碼視訊
    public native void render(String input, Surface surface);
    // 解碼音訊
    public native void sound(String input, String output);

    /**
     * 建立一個AudioTrac物件，用於播放
     *
     * @param nb_channels
     * @return
     */
    public AudioTrack createAudioTrack(int sampleRateInHz, int nb_channels) {
        // 固定格式的音訊碼流
        int audioFormat = AudioFormat.ENCODING_PCM_16BIT;
        Log.i("jason", "nb_channels:" + nb_channels);
        // 聲道佈局
        int channelConfig;
        if (nb_channels == 1) {
            channelConfig = android.media.AudioFormat.CHANNEL_OUT_MONO;
        } else if (nb_channels == 2) {
            channelConfig = android.media.AudioFormat.CHANNEL_OUT_STEREO;
        } else {
            channelConfig = android.media.AudioFormat.CHANNEL_OUT_STEREO;
        }

        int bufferSizeInBytes = AudioTrack.getMinBufferSize(sampleRateInHz, channelConfig, audioFormat);

        AudioTrack audioTrack = new AudioTrack(
                AudioManager.STREAM_MUSIC,
                sampleRateInHz, channelConfig,
                audioFormat,
                bufferSizeInBytes, AudioTrack.MODE_STREAM);
        // 播放
        // audioTrack.play();
        // 寫入PCM
        // audioTrack.write(audioData, offsetInBytes, sizeInBytes);
        return audioTrack;
    }

    static {
        System.loadLibrary("avutil-54");
        System.loadLibrary("swresample-1");
        System.loadLibrary("avcodec-56");
        System.loadLibrary("avformat-56");
        System.loadLibrary("swscale-3");
        System.loadLibrary("postproc-53");
        System.loadLibrary("avfilter-5");
        System.loadLibrary("avdevice-56");
        System.loadLibrary("myffmpeg");
    }
}

c端解碼程式碼，ffmpeg_audio_player.c：

#include "com_lzp_decoder_Player.h"

#include <stdlib.h>
#include <unistd.h>
#include <android/log.h>

//封裝格式
#include "libavformat/avformat.h"
//解碼
#include "libavcodec/avcodec.h"
//縮放
#include "libswscale/swscale.h"
//重取樣
#include "libswresample/swresample.h"

#define LOGI(FORMAT, ...) __android_log_print(ANDROID_LOG_INFO, "ffmpeg",FORMAT, ##__VA_ARGS__);
#define LOGE(FORMAT, ...) __android_log_print(ANDROID_LOG_ERROR, "ffmpeg",FORMAT, ##__VA_ARGS__);

#define MAX_AUDIO_FRME_SIZE 48000 * 4


JNIEXPORT void JNICALL Java_com_lzp_decoder_Player_sound
(JNIEnv *env, jobject jthiz, jstring input_jstr, jstring output_jstr){
    const char* input_cstr = (*env)->GetStringUTFChars(env, input_jstr, NULL);
    const char* output_cstr = (*env)->GetStringUTFChars(env, output_jstr, NULL);
    LOGI("%s", "sound");
    // 註冊元件
    av_register_all();
    AVFormatContext *pFormatCtx = avformat_alloc_context();
    //開啟音訊檔案
    if (avformat_open_input(&pFormatCtx, input_cstr, NULL, NULL) != 0){
        LOGI("無法開啟音訊檔案：%s\n", input_cstr);
        return;
    }
    // 獲取輸入檔案資訊
    if (avformat_find_stream_info(pFormatCtx, NULL) < 0){
        LOGI("%s", "無法獲取輸入檔案資訊");
        return;
    }
    // 獲取音訊流索引位置
    int i = 0, audio_stream_idx = -1;
    for (; i < pFormatCtx->nb_streams; i++){
        if (pFormatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_AUDIO){
            audio_stream_idx = i;
            break;
        }
    }

    // 獲取解碼器
    AVCodecContext *codecCtx = pFormatCtx->streams[audio_stream_idx]->codec;
    AVCodec *codec = avcodec_find_decoder(codecCtx->codec_id);
    if (codec == NULL){
        LOGI("%s", "無法獲取解碼器");
        return;
    }
    // 開啟解碼器
    if (avcodec_open2(codecCtx, codec, NULL) < 0){
        LOGI("%s", "無法開啟解碼器");
        return;
    }
    // 壓縮資料
    AVPacket *packet = (AVPacket *)av_malloc(sizeof(AVPacket));
    // 解壓縮資料
    AVFrame *frame = av_frame_alloc();
    // frame->16bit 44100 PCM 統一音訊取樣格式與取樣率
    SwrContext *swrCtx = swr_alloc();

    // 重取樣設定引數-------------start
    // 輸入的取樣格式
    enum AVSampleFormat in_sample_fmt = codecCtx->sample_fmt;
    // 輸出取樣格式16bit PCM
    enum AVSampleFormat out_sample_fmt = AV_SAMPLE_FMT_S16;
    // 輸入取樣率
    int in_sample_rate = codecCtx->sample_rate;
    // 輸出取樣率
    int out_sample_rate = in_sample_rate;
    // 獲取輸入的聲道佈局
    // 根據聲道個數獲取預設的聲道佈局（2個聲道，預設立體聲stereo）
    // av_get_default_channel_layout(codecCtx->channels);
    uint64_t in_ch_layout = codecCtx->channel_layout;
    // 輸出的聲道佈局（立體聲）
    uint64_t out_ch_layout = AV_CH_LAYOUT_STEREO;

    swr_alloc_set_opts(swrCtx,
        out_ch_layout, out_sample_fmt, out_sample_rate,
        in_ch_layout, in_sample_fmt, in_sample_rate,
        0, NULL);
    swr_init(swrCtx);

    // 輸出的聲道個數
    int out_channel_nb = av_get_channel_layout_nb_channels(out_ch_layout);

    // 重取樣設定引數-------------end

    // JNI begin------------------
    // JasonPlayer
    jclass player_class = (*env)->GetObjectClass(env, jthiz);

    // AudioTrack物件
    jmethodID create_audio_track_mid = (*env)->GetMethodID(env, player_class, "createAudioTrack", "(II)Landroid/media/AudioTrack;");
    jobject audio_track = (*env)->CallObjectMethod(env, jthiz, create_audio_track_mid, out_sample_rate, out_channel_nb);

    // 呼叫AudioTrack.play方法
    jclass audio_track_class = (*env)->GetObjectClass(env, audio_track);
    jmethodID audio_track_play_mid = (*env)->GetMethodID(env, audio_track_class, "play", "()V");
    (*env)->CallVoidMethod(env, audio_track, audio_track_play_mid);

    // AudioTrack.write
    jmethodID audio_track_write_mid = (*env)->GetMethodID(env, audio_track_class, "write", "([BII)I");

    // JNI end------------------
    FILE *fp_pcm = fopen(output_cstr, "wb");

    // 16bit 44100 PCM 資料
    uint8_t *out_buffer = (uint8_t *)av_malloc(MAX_AUDIO_FRME_SIZE);

    int got_frame = 0, index = 0, ret;
    // 不斷讀取壓縮資料
    while (av_read_frame(pFormatCtx, packet) >= 0){
        // 解碼音訊型別的Packet
        if (packet->stream_index == audio_stream_idx){
            // 解碼
            ret = avcodec_decode_audio4(codecCtx, frame, &got_frame, packet);

            if (ret < 0){
                LOGI("%s", "解碼完成");
            }
            // 解碼一幀成功
            if (got_frame > 0){
                LOGI("解碼：%d", index++);
                swr_convert(swrCtx, &out_buffer, MAX_AUDIO_FRME_SIZE, (const uint8_t **)frame->data, frame->nb_samples);
                // 獲取sample的size
                int out_buffer_size = av_samples_get_buffer_size(NULL, out_channel_nb,
                    frame->nb_samples, out_sample_fmt, 1);
                fwrite(out_buffer, 1, out_buffer_size, fp_pcm);

                // out_buffer緩衝區資料，轉成byte陣列
                jbyteArray audio_sample_array = (*env)->NewByteArray(env, out_buffer_size);
                jbyte* sample_bytep = (*env)->GetByteArrayElements(env, audio_sample_array, NULL);
                // out_buffer的資料複製到sampe_bytep
                memcpy(sample_bytep, out_buffer, out_buffer_size);
                // 同步
                (*env)->ReleaseByteArrayElements(env, audio_sample_array, sample_bytep, 0);

                // AudioTrack.write PCM資料
                (*env)->CallIntMethod(env, audio_track, audio_track_write_mid,
                    audio_sample_array, 0, out_buffer_size);
                // 釋放區域性引用
                (*env)->DeleteLocalRef(env, audio_sample_array);
                usleep(1000 * 16);
            }
        }

        av_free_packet(packet);
    }

    av_frame_free(&frame);
    av_free(out_buffer);

    swr_free(&swrCtx);
    avcodec_close(codecCtx);
    avformat_close_input(&pFormatCtx);

    (*env)->ReleaseStringUTFChars(env, input_jstr, input_cstr);
    (*env)->ReleaseStringUTFChars(env, output_jstr, output_cstr);
}

執行結果日誌：