将音频从单声道转换为立体声，以便在AVPlayer（iOS）中使用

Question

我有一个AVPlayer播放来自单声道AVAsset的音频，并通过音频处理点击进行一些处理。 如何在到达水龙头之前将此资产转换为立体声？第二个频道可以为空或第一个频道的副本（它会在水龙头中手动填充）。

我已经尝试在水龙头内将单声道转换为立体声，但是当我们进入水龙头时，显然我们无法控制ASBD或AudioBufferList结构。我也完成了离线转换，但这会带来很大的障碍（可能很慢，不适合网络流媒体）。

以下是可以与任何单声道音频文件一起使用的准系统（但完整）代码。你会看到，当它到达处理水龙头时，只有一个通道可用而不是所需的两个通道。要使用代码，只需将下面的MediaPlayer和TapProcessor类添加到空白的单一视图应用程序中，使用以下ViewController代码代替默认代码，然后添加您自己的单声道音频文件到您的项目。谢谢你的阅读。

MediaPlayer.h

#import <Foundation/Foundation.h>

@interface MediaPlayer : NSObject

@end

MediaPlayer.m

#import "MediaPlayer.h"
#import "TapProcessor.h"
#import <AVFoundation/AVFoundation.h>

@interface MediaPlayer()

@property (nonatomic, strong) AVAsset *asset;
@property (nonatomic, strong) AVPlayer *player;
@property (nonatomic, strong) TapProcessor *audioTapProcessor;

@end


@implementation MediaPlayer

- (id)init {
    if (self = [super init]){
        NSString *path = [[NSBundle mainBundle] pathForResource:@"MonoSource"
                                                         ofType:@"mp3"];
        [self loadFileWithPath:path];
    }
    return self;
}

-(void)loadFileWithPath:(NSString*)path{
    NSURL *fileURL = [NSURL fileURLWithPath:path];
    NSDictionary *options = [NSDictionary dictionaryWithObject:[NSNumber numberWithBool:YES]
                                                        forKey:AVURLAssetPreferPreciseDurationAndTimingKey];
    self.asset = [AVURLAsset URLAssetWithURL:fileURL options:options];
    [self.asset loadValuesAsynchronouslyForKeys:@[@"tracks"] completionHandler:^{
        dispatch_async(dispatch_get_main_queue(), ^{
            AVKeyValueStatus status = [self.asset statusOfValueForKey:@"tracks" error:nil];
            switch (status) {
                case AVKeyValueStatusLoaded:
                    [self setupPlayer];
                    break;
                default:
                    break;
            }
        });
    }];
}

- (void) setupPlayer{

    AVPlayerItem *item = [AVPlayerItem playerItemWithAsset:self.asset];

    AVAssetTrack *audioTrack = [[self.asset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
    [self printInfoForTrack:audioTrack];
    TapProcessor *newProcessor = [[TapProcessor alloc] initWithTrack:audioTrack];

    AVAudioMix *audioMix = [newProcessor audioMix];
    item.audioMix = audioMix;

    self.player = [AVPlayer playerWithPlayerItem:item];

    [self.player play];
}

-(void) printInfoForTrack:(AVAssetTrack*)track{
    CMAudioFormatDescriptionRef item = (__bridge CMAudioFormatDescriptionRef)[track.formatDescriptions objectAtIndex:0];
    const AudioStreamBasicDescription* desc = CMAudioFormatDescriptionGetStreamBasicDescription(item);
    NSLog(@"Number of track channels: %d", desc->mChannelsPerFrame);
}


@end

TapProcessor.h

#import <Foundation/Foundation.h>
#import <AVFoundation/AVFoundation.h>

@interface TapProcessor : NSObject

- (id)initWithTrack:(AVAssetTrack *)track;

@property (readonly, nonatomic) AVAssetTrack *track;
@property (readonly, nonatomic) AVAudioMix *audioMix;

@end

TapProcessor.m

#import "TapProcessor.h"

// TAP CALLBACKS
static void tap_InitCallback(MTAudioProcessingTapRef tap,
                             void *clientInfo,
                             void **tapStorageOut){
}

static void tap_FinalizeCallback(MTAudioProcessingTapRef tap){
}

static void tap_PrepareCallback(MTAudioProcessingTapRef tap,
                                CMItemCount maxFrames,
                                const AudioStreamBasicDescription *processingFormat){
    NSLog(@"Number of tap channels: %d", processingFormat->mChannelsPerFrame);
}

static void tap_UnprepareCallback(MTAudioProcessingTapRef tap){
}

static void tap_ProcessCallback(MTAudioProcessingTapRef tap,
                                CMItemCount numberFrames,
                                MTAudioProcessingTapFlags flags,
                                AudioBufferList *bufferListInOut,
                                CMItemCount *numberFramesOut,
                                MTAudioProcessingTapFlags *flagsOut){
    MTAudioProcessingTapGetSourceAudio(tap, numberFrames, bufferListInOut, NULL, NULL, NULL);
    *numberFramesOut = numberFrames;
}


@implementation TapProcessor

- (id)initWithTrack:(AVAssetTrack *)track{
    self = [super init];
    if (self){
        _track = track;
    }
    return self;
}

@synthesize audioMix = _audioMix;
- (AVAudioMix *)audioMix {
    if (!_audioMix){
        AVMutableAudioMix *audioMix = [AVMutableAudioMix audioMix];
        if (audioMix){
            AVMutableAudioMixInputParameters *audioMixInputParameters = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:self.track];
            if (audioMixInputParameters) {
                MTAudioProcessingTapCallbacks callbacks;
                callbacks.version = kMTAudioProcessingTapCallbacksVersion_0;
                callbacks.clientInfo = (__bridge void *)self,
                callbacks.init = tap_InitCallback;
                callbacks.finalize = tap_FinalizeCallback;
                callbacks.prepare = tap_PrepareCallback;
                callbacks.unprepare = tap_UnprepareCallback;
                callbacks.process = tap_ProcessCallback;

                MTAudioProcessingTapRef audioProcessingTap;
                if (noErr == MTAudioProcessingTapCreate(kCFAllocatorDefault,
                                                        &callbacks,
                                                        kMTAudioProcessingTapCreationFlag_PreEffects,
                                                        &audioProcessingTap)){
                    audioMixInputParameters.audioTapProcessor = audioProcessingTap;
                    CFRelease(audioProcessingTap);
                    audioMix.inputParameters = @[audioMixInputParameters];
                    _audioMix = audioMix;
                }
            }
        }
    }
    return _audioMix;
}

@end

ViewController.h

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController


@end

ViewController.m

#import "ViewController.h"
#import "MediaPlayer.h"

@interface ViewController ()

@property (nonatomic,strong) MediaPlayer *mediaPlayer;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.mediaPlayer = [[MediaPlayer alloc] init];
}

@end