应用过滤器后保存音频文件让我很沮丧。该过滤器仅适用于 AVAudioPlayerNode，这意味着在播放器中播放期间，过滤器非常适合应用，但我该如何保存呢？我最近 3 天很挣扎，请提供任何帮助，我们将不胜感激。 这些下面的代码是我使用的，

engine = [[AVAudioEngine alloc] init];

 - (void)setupEQ
{
      NSLog(@"setupEQ");

      unitEq = [[AVAudioUnitEQ alloc] initWithNumberOfBands:12];
      unitEq.globalGain = 3.0;
      AVAudioUnitEQFilterParameters *filterParameters;
      filterParameters = unitEq.bands[0];
      filterParameters.filterType = AVAudioUnitEQFilterTypeParametric;
      filterParameters.frequency = 31;
      filterParameters.bandwidth = 1.0;
      filterParameters.gain = [[[filter_arr objectAtIndex:index_filter] objectForKey:@"filter_freq_31.5hz"] floatValue];
      filterParameters.bypass = FALSE;

      filterParameters = unitEq.bands[1];
      filterParameters.filterType = AVAudioUnitEQFilterTypeParametric;
      filterParameters.frequency = 63;
      filterParameters.bandwidth = 1.0;
      filterParameters.gain = [[[filter_arr objectAtIndex:index_filter] objectForKey:@"filter_freq_63hz"] floatValue];
      filterParameters.bypass = FALSE;

      filterParameters = unitEq.bands[2];
      filterParameters.filterType = AVAudioUnitEQFilterTypeParametric;
      filterParameters.frequency = 125;
      filterParameters.bandwidth = 1.0;
      filterParameters.gain = [[[filter_arr objectAtIndex:index_filter] objectForKey:@"filter_freq_125hz"] floatValue];
      filterParameters.bypass = FALSE;

      filterParameters = unitEq.bands[3];
      filterParameters.filterType = AVAudioUnitEQFilterTypeParametric;
      filterParameters.frequency = 250;
      filterParameters.bandwidth = 1.0;
      filterParameters.gain = [[[filter_arr objectAtIndex:index_filter] objectForKey:@"filter_freq_250hz"] floatValue];
      filterParameters.bypass = FALSE;

      filterParameters = unitEq.bands[4];
      filterParameters.filterType = AVAudioUnitEQFilterTypeParametric;
      filterParameters.frequency = 500;
      filterParameters.bandwidth = 1.0;
      filterParameters.gain = [[[filter_arr objectAtIndex:index_filter] objectForKey:@"filter_freq_500hz"] floatValue];
      filterParameters.bypass = FALSE;

      filterParameters = unitEq.bands[5];
      filterParameters.filterType = AVAudioUnitEQFilterTypeParametric;
      filterParameters.frequency = 1000;
      filterParameters.bandwidth = 1.0;
      filterParameters.gain = [[[filter_arr objectAtIndex:index_filter] objectForKey:@"filter_freq_1khz"] floatValue];
      filterParameters.bypass = FALSE;

      filterParameters = unitEq.bands[6];
      filterParameters.filterType = AVAudioUnitEQFilterTypeParametric;
      filterParameters.frequency = 2000;
      filterParameters.bandwidth = 1.0;
      filterParameters.gain = [[[filter_arr objectAtIndex:index_filter] objectForKey:@"filter_freq_2khz"] floatValue];
      filterParameters.bypass = FALSE;

      filterParameters = unitEq.bands[7];
      filterParameters.filterType = AVAudioUnitEQFilterTypeParametric;
      filterParameters.frequency = 4000;
      filterParameters.bandwidth = 1.0;
      filterParameters.gain =[[[filter_arr objectAtIndex:index_filter] objectForKey:@"filter_freq_4khz"] floatValue];
      filterParameters.bypass = FALSE;

      filterParameters = unitEq.bands[8];
      filterParameters.filterType = AVAudioUnitEQFilterTypeParametric;
      filterParameters.frequency = 8000;
      filterParameters.bandwidth = 1.0;
      filterParameters.gain = [[[filter_arr objectAtIndex:index_filter] objectForKey:@"filter_freq_8khz"] floatValue];
      filterParameters.bypass = FALSE;

      filterParameters = unitEq.bands[9];
      filterParameters.filterType = AVAudioUnitEQFilterTypeParametric;
      filterParameters.frequency = 16000;
      filterParameters.bandwidth = 1.0;
      filterParameters.gain =[[[filter_arr objectAtIndex:index_filter] objectForKey:@"filter_freq_16khz"] floatValue];
      filterParameters.bypass = FALSE;


      filterParameters = unitEq.bands[10];
      filterParameters.filterType = AVAudioUnitEQFilterTypeLowPass;
      filterParameters.frequency = 16857;
      filterParameters.bypass = FALSE;

      filterParameters = unitEq.bands[11];
      filterParameters.filterType = AVAudioUnitEQFilterTypeHighPass;
      filterParameters.frequency = 205;
      filterParameters.bypass = FALSE;

      [engine attachNode:unitEq];
}







audioFile = [[AVAudioFile alloc] initForReading:[NSURL fileURLWithPath:[NSString stringWithFormat:@"%@",[self audio_file_path:mp3FileName]]] error:&playerError];
            [self setupPlayer];
            [self setupEQ];

            AVAudioMixerNode *mixerNode = [engine mainMixerNode];
            [engine connect:audio_player_node to:unitEq format:audio_file.processingFormat];
            [engine connect:unitEq to:mixerNode format:audio_file.processingFormat];

这些是工作正常的示例。但是我怎样才能存储具有相同频率的音频文件呢？音频文件应为 mp3 格式。在苹果中，他们只将保存文件定位为 m4a。