Re: Using AVPlayer to play MPMediaItem and add iPod EQ to the output
Re: Using AVPlayer to play MPMediaItem and add iPod EQ to the output
- Subject: Re: Using AVPlayer to play MPMediaItem and add iPod EQ to the output
- From: Kevin Dixon <email@hidden>
- Date: Tue, 31 Jul 2012 14:52:52 -0700
Thanks Heinrich, I hadn't known about that property before. Now that I
am setting it to YES, I actually see a different behavior in
decoding-- those songs which I expect to be gapless are indeed playing
that way (I suppose removing the priming/remainder samples?)
-Kevin
On Tue, Jul 31, 2012 at 1:06 PM, Heinrich Fink <email@hidden> wrote:
> hi kevin,
>
>> This works well enough, but I've noticed that some tracks that play as
>> gapless under the main iOS Music App, or even in Winamp do not come
>> out gapless through this API. The decoded MP3s (from AVAssetReader)
>> may have 500-2000 samples of silence at the beginning or end, so I'm
>> curious about this as well.
>
> I believe what you describe are the priming/remainder samples which are specific to some audio codecs. AAC, for example, could have something like 2112 priming sample and 134 remainder samples. I think there are similar numbers for the mp3 codec.
>
> I have also recently build an audio scheduling system with an AUGraph that uses an AVAssetReader to access audio samples. I use an AVAssetReaderTrackOutput that is configured to return decoded non-interleaved float audio data. I ran into a problem that is similar to yours: When I read the AVURLAsset’s property “duration” (I have set ’AVURLAssetPreferPreciseDurationAndTimingKey’ set to ‘YES’): the returned property shows the full number of samples (priming samples + valid samples + remainder samples). However, I believe the correct number should be the ‘valid’ samples only as this is also the amount of samples returned by the AVAssetReaderTrackOutput. The ExtAudioFile API, for example, explicitly handles priming/remainder samples (checkout the property kExtAudioFileProperty_PacketTable described here http://developer.apple.com/library/ios/#documentation/MusicAudio/Reference/ExtendedAudioFileServicesReference/Reference/reference.html#//apple_ref/doc/c_ref/kExtAudioFileProperty_PacketTable)
>
> I have filed a bug about the AVURLAsset.duration not returning the correct amount of actual decodable audio samples. Does anyone else have experience with AVFoundation and priming/remainder samples?
>
> best regards,
>
> Heinrich Fink
>
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