ios - 音频AAC解码

上一回说到音频AAC编码，这回我们把编码后的文件解码，解码的文件就是上一章录制的音频。

在iOS下进行音频解码及播放的大体流程如下：
1、打开 AAC 文件。
2、获取音频格式信息。如通道数，采样率等。
3、从 AAC 文件中取出一帧 AAC 数据。
4、使用 AudioToolbox 解码 AAC 数据包。
5、将解码后的 PCM 数据送给 AudioUnit 播放声音。
6、重复 3-5 步，直到整个 AAC 文件被读完。

AACDecoder.m

- (void)customAudioConfig {
    //NSURL *url = [[NSBundle mainBundle] URLForResource:@"abc" withExtension:@"aac"];

    NSFileManager *manager = [NSFileManager defaultManager];
    NSURL *path = [[manager URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] firstObject];
    NSURL *url = [path URLByAppendingPathComponent:@"abc.aac"];

    // 打开 AAC 文件。
    OSStatus status = AudioFileOpenURL((__bridge CFURLRef)url, kAudioFileReadPermission, 0, &audioFileID); //Open an existing audio file specified by a URL.
    if (status != noErr) {
        NSLog(@"打开文件失败 %@", url);
        return ;
    }
    uint32_t size = sizeof(audioStreamBasicDescrpition);
    status = AudioFileGetProperty(audioFileID, kAudioFilePropertyDataFormat, &size, &audioStreamBasicDescrpition); // Gets the value of an audio file property.
    NSAssert(status == noErr, @"error");

    status = AudioQueueNewOutput(&audioStreamBasicDescrpition, bufferReady, (__bridge void * _Nullable)(self), NULL, NULL, 0, &audioQueue); // Creates a new playback audio queue object.
    NSAssert(status == noErr, @"error");

    if (audioStreamBasicDescrpition.mBytesPerPacket == 0 || audioStreamBasicDescrpition.mFramesPerPacket == 0) {
        uint32_t maxSize;
        size = sizeof(maxSize);
        AudioFileGetProperty(audioFileID, kAudioFilePropertyPacketSizeUpperBound, &size, &maxSize); // The theoretical maximum packet size in the file.
        if (maxSize > CONST_BUFFER_SIZE) {
            maxSize = CONST_BUFFER_SIZE;
        }
        packetNums = CONST_BUFFER_SIZE / maxSize;
        audioStreamPacketDescrption = malloc(sizeof(AudioStreamPacketDescription) * packetNums);
    }
    else {
        packetNums = CONST_BUFFER_SIZE / audioStreamBasicDescrpition.mBytesPerPacket;
        audioStreamPacketDescrption = nil;
    }

    char cookies[100];
    memset(cookies, 0, sizeof(cookies));
    // 这里的100 有问题
    AudioFileGetProperty(audioFileID, kAudioFilePropertyMagicCookieData, &size, cookies); // Some file types require that a magic cookie be provided before packets can be written to an audio file.
    if (size > 0) {
        AudioQueueSetProperty(audioQueue, kAudioQueueProperty_MagicCookie, cookies, size); // Sets an audio queue property value.
    }

    readedPacket = 0;
    // 循环执行 3-5步，直到文件结束。
    for (int i = 0; i < CONST_BUFFER_COUNT; ++i) {
        AudioQueueAllocateBuffer(audioQueue, CONST_BUFFER_SIZE, &audioBuffers[i]); // Asks an audio queue object to allocate an audio queue buffer.
        if ([self fillBuffer:audioBuffers[i]]) {
            // full
            break;
        }
        NSLog(@"buffer%d full", i);
    }
}

void bufferReady(void *inUserData,AudioQueueRef inAQ,
                 AudioQueueBufferRef buffer){
    NSLog(@"refresh buffer");
    AACDecoder* player = (__bridge AACDecoder *)inUserData;
    if (!player) {
        NSLog(@"player nil");
        return ;
    }
    if ([player fillBuffer:buffer]) {
        NSLog(@"play end");
    }

}

- (bool)fillBuffer:(AudioQueueBufferRef)buffer {
    bool full = NO;
    uint32_t bytes = 0, packets = (uint32_t)packetNums;
    OSStatus status = AudioFileReadPackets(audioFileID, NO, &bytes, audioStreamPacketDescrption, readedPacket, &packets, buffer->mAudioData); // Reads packets of audio data from an audio file.

    NSAssert(status == noErr, ([NSString stringWithFormat:@"error status %d", status]) );
    if (packets > 0) {
        buffer->mAudioDataByteSize = bytes;
        AudioQueueEnqueueBuffer(audioQueue, buffer, packets, audioStreamPacketDescrption);
        readedPacket += packets;
    }
    else {
        AudioQueueStop(audioQueue, NO);
        full = YES;
    }

    return full;
}

- (void)play {
    AudioQueueSetParameter(audioQueue, kAudioQueueParam_Volume, 1.0); // Sets a playback audio queue parameter value.
    AudioQueueStart(audioQueue, NULL); // Begins playing or recording audio.
}

ViewController.m

- (void)decoderPlay{
    // 播放
    self.player = [[AACDecoder alloc] init];
    [self.player play];
}

到这里就把AAC编码的文件给解码播放了。

了解音频AAC编码

Demo下载地址