iOS音频转换为mp3格式后时长不准问题。

背景：网上好多都没说到解决音频获取时长的要点，索性自己再总结一下。

1.需求：项目中服务器需要上传mp3格式的音频文件，所以本地需要先把.caf格式转为mp3格式上传。

2.问题：从服务器返回的音频url地址获取音频时长不准，且每次可能不一样。

3.分析可能有原因：

1.音频转为mp3时参数设置问题，如采样通道数、采样率等。

2.[AVURLAsset URLAssetWithURL:[NSURL URLWithString:url] options:nil];中option参数没有设置,网上大多这里都是设为nil。

4.测试正常的音频录音参数

- (NSDictionary *)recordingSettings
{
    NSMutableDictionary *recordSetting =[NSMutableDictionary dictionaryWithCapacity:10];
    [recordSetting setObject:[NSNumber numberWithInt: kAudioFormatLinearPCM] forKey: AVFormatIDKey];
    //2 采样率
    [recordSetting setObject:[NSNumber numberWithFloat:11025.0] forKey: AVSampleRateKey];
    //3 通道的数目
    [recordSetting setObject:[NSNumber numberWithInt:2]forKey:AVNumberOfChannelsKey];
    //4 采样位数  默认 16
    [recordSetting setObject:[NSNumber numberWithInt:16] forKey:AVLinearPCMBitDepthKey];
    //5音频质量,采样质量
    [recordSetting setValue:[NSNumber numberWithInt:AVAudioQualityMin] forKey:AVEncoderAudioQualityKey];
     return recordSetting;
}

5.测试正常的音频mp3转换方式

//CAF转换mp3的lame方法
+ (NSString *)audioCAFtoMP3:(NSString *)wavPath {
    
    NSString *cafFilePath = wavPath;
    
    NSString *mp3FilePath = [NSString stringWithFormat:@"%@.mp3",[NSString stringWithFormat:@"%@%@",[cafFilePath substringToIndex:cafFilePath.length - 4],[self getTimestamp]]];
    
    @try {
        int read, write;
        
        FILE *pcm = fopen([cafFilePath cStringUsingEncoding:1], "rb");  //source 被转换的音频文件位置
        fseek(pcm, 4*1024, SEEK_CUR);                                   //skip file header
        FILE *mp3 = fopen([mp3FilePath cStringUsingEncoding:1], "wb");  //output 输出生成的Mp3文件位置
        
        const int PCM_SIZE = 8192;
        const int MP3_SIZE = 8192;
        short int pcm_buffer[PCM_SIZE*2];
        unsigned char mp3_buffer[MP3_SIZE];
        
        lame_t lame = lame_init();
        lame_set_num_channels(lame,2);//通道数跟原音频参数设置一致
        lame_set_in_samplerate(lame, 11025.0);//采样率跟原音频参数设置一致
        lame_set_VBR(lame, vbr_default);
        lame_init_params(lame);
        do {
            read = fread(pcm_buffer, 2*sizeof(short int), PCM_SIZE, pcm);
            if (read == 0)
                write = lame_encode_flush(lame, mp3_buffer, MP3_SIZE);
            else
                write = lame_encode_buffer_interleaved(lame, pcm_buffer, read, mp3_buffer, MP3_SIZE);
            
            fwrite(mp3_buffer, write, 1, mp3);
            
        } while (read != 0);
        
        lame_close(lame);
        fclose(mp3);
        fclose(pcm);
    }
    @catch (NSException *exception) {
        NSLog(@"%@",[exception description]);
    }
    @finally {
        [YWCommonUtils deleteFileWithPath:cafFilePath];
        return mp3FilePath;
    }
}

6.测试正常的音频时间长度设置

+ (NSTimeInterval)AudioDurationFromUrl:(NSString *)url {
    //只有这个方法获取时间是准确的 audioPlayer.duration获得的时间不准
    AVURLAsset* audioAsset = nil;
    NSDictionary *dic = @{AVURLAssetPreferPreciseDurationAndTimingKey:@(YES)};
    if ([url hasPrefix:@"http://"]) {
        audioAsset =[AVURLAsset URLAssetWithURL:[NSURL URLWithString:url] options:dic];
    }else {//播放本机录制的文件
        audioAsset =[AVURLAsset URLAssetWithURL:[NSURL fileURLWithPath:url] options:dic];
    }
    CMTime audioDuration = audioAsset.duration;
    float audioDurationSeconds =CMTimeGetSeconds(audioDuration);
    return audioDurationSeconds;
}