调用AudioFileGetProperty方法后，不会填充iOS AC3 AudioStreamBasicDescription属性

Question

我正在调用AudioFileGetProperty方法，以便为.ac3音频文件设置AudioStreamBasicDescription对象的属性，但结果是所有属性都为0。 这是我的代码

NSString *soundFile= [[[NSBundle mainBundle] resourcePath] stringByAppendingPathComponent:@"sample1AC3.ac3"];
            CFURLRef soundURL = CFURLCreateWithFileSystemPath(kCFAllocatorDefault, (__bridge void*)soundFile, kCFURLPOSIXPathStyle, false);
            int err;
            err = AudioFileOpenURL(soundURL, kAudioFileReadPermission, 0, &mAudioFile);
            //  
            AudioStreamBasicDescription audioFormat;
            UInt32 size1 = sizeof(audioFormat);
            //    
            err = AudioFileGetProperty(mAudioFile, kAudioFilePropertyDataFormat, &size1, &audioFormat);

这是我的日志

audioFormat AudioStreamBasicDescription {...}

mSampleRate Float64 0

mFormatID UInt32 0

mFormatFlags UInt32 0

mBytesPerPacket UInt32 0

mFramesPerPacket UInt32 0

mBytesPerFrame UInt32 0

mChannelsPerFrame UInt32 0

mBitsPerChannel UInt32 0

mReserved UInt32 0

该代码适用于.mp3，我不明白为什么它不适用于.ac3格式。