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Re: ExtAudioFile woes
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Re: ExtAudioFile woes


  • Subject: Re: ExtAudioFile woes
  • From: John Weeks <email@hidden>
  • Date: Thu, 04 Jun 2015 11:00:33 -0700

Thanks, Christian.

> On 03 Jun 2015, at 5:21 pm, Christian Rober <email@hidden> wrote:
>
> Hi John,
>
> Without trying this out, here are some quick thoughts that may help (I apologize if you have already tried these):
>
> 1) What is value range/type of _starttime?  Is it in seconds?  Milliseconds?  CPU time?  I would make sure that it is in seconds, because a really large millisecond value may generate an offset beyond the end of a small file.

Yeah- _starttime is in seconds, and I can't get it to work even when I set the frameOffset input to literal "1".

> 2) I would use ExtAudioFileTell() as a sanity check before you call this code.  Print out the value, it should be 0 at first, if you haven't read from it already.

I did that, too. I gave zero.

> 3) Have you been reading from the file?  Just like Unix open and its corresponding file descriptor, an ExtAudioFileRead (+Async) on this file will advance the position in the file that a seek uses.  In other words, since there is no pread() equivalent in the ExtAudioFile framework, all reads seek, and you may already be at or near the end.

No, I do this just before I go into my reading loop. The reading loop works great.

> 4) In terms of the ChannelDescription issue... that is very perplexing.  I did see this comment in the documentation which confuses me:
>
> When reading, the specified layout overrides the one read from the file, if one is present in the file.
>
> Maybe the mp4 file does not have a ChannelDescription,

I think that's correct- for the mp4 file I just get a tag giving the name of a particular 5.1 format, and that name implies a channel layout.

> so it reads that description correctly, but the AIFF has a ChannelDescription, as you described by cracking the file open, so you end up inadvertently creating a bogus CD.  What is really confusing, is why would a get property overwrite anything?!?

I think you can use ExtAudioFile to convert LPCM to various formats and write the coded audio to a file. But I don't need that- I just want to read the channel layout description that's already in the file!

-John Weeks
WaveMetrics, Inc.


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References: 
 >ExtAudioFile woes (From: John Weeks <email@hidden>)
 >Re: ExtAudioFile woes (From: Christian Rober <email@hidden>)

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