[Asterisk-Users] Re: Audio Files, Filtering, and Formats for Asterisk

[Stephen Bosch]

Sherwood McGowan wrote:
> I listened to all the demos you showed. 
> 
> My ear discerns a little muffling and minor "slushiness" in the GSM files
> you sent, along with a much more narrow bandwidth, mainly on the high end
> side, and Allison either has a mild whistling s or slushy s sound in her
> voice or the producer didn't properly compress it to "de-ess" the recording.
> Or, I could just be rather tired. 

I don't think so. Everybody else I've played them for has made similar
observations.

> Either way, your best bet is to have the system use WAV files but also make
> available GSM and ulaw versions of the same files. This is so that the
> system can pick (and it does this automatically) the best format that
> requires the least amount of CPU power for transcoding. If I remember
> correctly, FXO/FXS cards can use straight PCM files. 
> 
> What I do to prepare a file is this:
> 
> I record in 48Khz 32 bit mode (32 bit is just higher resolution so
> processing has more to work with, 16 bit is fine since it's the end result),
> then I compress with de-essing and pop removal (maximizes volume, removes
> slushy or whistling s's and popping p's), then run through an FFT (Fast
> Fourier Transform) to bring the frequency response within the natural range
> of a telephone (if I remember correctly it's within the 300 - 4K range, I
> can check if need be).

How do you "de-ess" and remove pops? What filters do you use?

> Then I reduce the file to a 8Khz, 16 bit mono file. Then I check the audio
> again, and normalize (like compression but only raises the whole file to
> where the highest peak of audio reaches the level requested, instead of
> raising or lowering the level on a dynamic basis by using readahead of a
> couple milliseconds). I usually normalize to around -3db. 
> 
> The end result is a WAV file that sounds good over the phone. I then put the
> file on the asterisk server (or another server with sox installed) and
> convert to gsm, ulaw, and alaw (using the original WAV, not using converted
> gsm or whatever).
> 
> Hope this was helpful, and I wish you luck. If nothing else, for like $50 or
> so (depending on how many files there are) I would be willing to take a
> series of WAV files from you and perform the filtering and whatnot for you,
> and supply gsm, wav, ulaw, and alaw.
> 
> I'm also going to just try and get a series of recordings together through
> my partner's studio in Phoenix AZ, make the files available to the
> community, and take up donations for the studio and processing time. 
>  
> Talk to you soon. I'm going to cc the asterisk-users list for this, so that
> the community can benefit from the information.

Thank you, Sherwood - you've been extremely helpful! I am going to try
those steps you suggested (once I learn how to 'de-ess' and remove pops)
but I may just take you up on the filtering and processing offer.

-Stephen-