Help with fractions re: resampling

Help with fractions re: resampling

[Christopher Paul Kalafarski]

I’m really struggling with what I thought would be relatively simple process of reading a file (with libsndfile), resampling it (using soxr), and writing it back to disk (with sndfile again). I realize this list is for neither of those libraries, but afaik soxr is extracted from SoX just with a different API, and I think my issue is in dealing with the resulting data, and not the API.

My main confusion is that if I pass in, e.g., 4096 frames at 48000, and resample to 44100, mathematically I would expect to get back 3763.2 frames. What is the proper way to handle the 0.2 frames in this case? Does the library handle that transparently? Or should I be doing some amount of math to figure out leap-frames every few cycles? No examples I’ve seen explicitly handle this, so I’m guessing it’s unnecessary, but I’m running out of other avenues to try to fix the problem I’m having, so I’ve kind of gone back to basics.

Thanks for any help.
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Re: Help with fractions re: resampling

[Erich Eckner]

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Hi,

first: I don't know the sox source code, so I have no Idea, what
sox/soxr really does, but:

Why don't you give it a different amount of frames, like 4800? So you
would get 4410 frames back and everything is fine.

greetings,
Erich

Am 06.01.2015 22:58, schrieb Christopher Paul Kalafarski:
> I’m really struggling with what I thought would be relatively simple process of reading a file (with libsndfile), resampling it (using soxr), and writing it back to disk (with sndfile again). I realize this list is for neither of those libraries, but afaik soxr is extracted from SoX just with a different API, and I think my issue is in dealing with the resulting data, and not the API.
> 
> My main confusion is that if I pass in, e.g., 4096 frames at 48000, and resample to 44100, mathematically I would expect to get back 3763.2 frames. What is the proper way to handle the 0.2 frames in this case? Does the library handle that transparently? Or should I be doing some amount of math to figure out leap-frames every few cycles? No examples I’ve seen explicitly handle this, so I’m guessing it’s unnecessary, but I’m running out of other avenues to try to fix the problem I’m having, so I’ve kind of gone back to basics.
> 
> Thanks for any help.
> ------------------------------------------------------------------------------
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Re: Help with fractions re: resampling

[Christopher Paul Kalafarski]

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Thanks for the idea. Certainly made the math easier, but I’m still having the original issue where the resampled audio is stuttering. The IO seems to be fine, because if I skip the resample, the output file matches the input file. I’m not sure how else to change how I’m configuring or executing the resample process; it seems fairly straightforward, and some of the data is getting through so I must have mismatch somewhere.


> On Jan 7, 2015, at 01:37, Erich Eckner <erich@eckner.net> wrote:
> 
> Hi,
> 
> first: I don't know the sox source code, so I have no Idea, what
> sox/soxr really does, but:
> 
> Why don't you give it a different amount of frames, like 4800? So you
> would get 4410 frames back and everything is fine.
> 
> greetings,
> Erich
> 
> Am 06.01.2015 22:58, schrieb Christopher Paul Kalafarski:
>> I’m really struggling with what I thought would be relatively simple process of reading a file (with libsndfile), resampling it (using soxr), and writing it back to disk (with sndfile again). I realize this list is for neither of those libraries, but afaik soxr is extracted from SoX just with a different API, and I think my issue is in dealing with the resulting data, and not the API.
>> 
>> My main confusion is that if I pass in, e.g., 4096 frames at 48000, and resample to 44100, mathematically I would expect to get back 3763.2 frames. What is the proper way to handle the 0.2 frames in this case? Does the library handle that transparently? Or should I be doing some amount of math to figure out leap-frames every few cycles? No examples I’ve seen explicitly handle this, so I’m guessing it’s unnecessary, but I’m running out of other avenues to try to fix the problem I’m having, so I’ve kind of gone back to basics.
>> 
>> Thanks for any help.
>> ------------------------------------------------------------------------------
>> Dive into the World of Parallel Programming! The Go Parallel Website,
>> sponsored by Intel and developed in partnership with Slashdot Media, is your
>> hub for all things parallel software development, from weekly thought
>> leadership blogs to news, videos, case studies, tutorials and more. Take a
>> look and join the conversation now. http://goparallel.sourceforge.net
>> _______________________________________________
>> Sox-users mailing list
>> Sox-users@lists.sourceforge.net
>> https://lists.sourceforge.net/lists/listinfo/sox-users
>> 
> 
> 
> ------------------------------------------------------------------------------
> Dive into the World of Parallel Programming! The Go Parallel Website,
> sponsored by Intel and developed in partnership with Slashdot Media, is your
> hub for all things parallel software development, from weekly thought
> leadership blogs to news, videos, case studies, tutorials and more. Take a
> look and join the conversation now. http://goparallel.sourceforge.net_______________________________________________
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Re: Help with fractions re: resampling

[Rob Sykes]

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     On Tuesday, 6 January 2015, 22:18, Christopher Paul Kalafarski <chris@farski.com> wrote:
   
 

 I’m really struggling with what I thought would be relatively simple process of reading a file (with libsndfile), resampling it (using soxr), and writing it back to disk (with sndfile again). I realize this list is for neither of those libraries, but afaik soxr is extracted from SoX just with a different API, and I think my issue is in dealing with the resulting data, and not the API.

My main confusion is that if I pass in, e.g., 4096 frames at 48000, and resample to 44100, mathematically I would expect to get back 3763.2 frames. What is the proper way to handle the 0.2 frames in this case?
Conceptually, the 0.2 is buffered within the resampler, so consecutive inputs of 4096 could result in 3763, 3763, 3763, 3763, 3764, 3763, ... being output. In practice though, there can be a lot more variation in the output pattern, due to internal buffering and block processing.  For this reason, it often makes sense for resampling to be output driven. E.g., if every 10ms, you need to present 441 samples to your output hardware, then every 10ms, resample as much input data as is needed to get (at least) 441 samples needed for the output. soxr supports this paradigm via the soxr_set_input_fn() and soxr_output() functions.  HTH.   

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