What does AVEncoderAudioQualityKey do?

According to the scanty docs, the AVEncoderAudioQualityKeys are --

Keys that specify sample rate conversion quality, used for the AVSampleRateConverterAudioQualityKey property.

enum {
   AVAu开发者_如何学运维dioQualityMin       = 0,
   AVAudioQualityLow       = 0x20,
   AVAudioQualityMedium    = 0x40,
   AVAudioQualityHigh      = 0x60,
   AVAudioQualityMax       = 0x7F
};
typedef NSInteger AVAudioQuality;

Testing with AVAudioRecorder, there is virtually no difference in resulting audio file size or quality for a 2 minute recording recorded with AVAudioQualityMin versus one recorded with AVAudioQualityMax. The test used an 8KHz sample rate using kAudioFormatMPEG4AAC.

Can someone enlighten me as to the theoretical and practical purpose of this key?

These keys are used for specifying quality in sample rate conversions. Audio file format, which specifies the sample rate and bit depth, determines your file size. According to the Apple docs they are:

Keys that specify sample rate conversion quality, used for the AVSampleRateConverterAudioQualityKey property.

So, these quality keys only determine how well you want to re-sample audio into another format.

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