Subject: From: Date:

Re: [ECS] .WAV clarification Michael David Thu, 28 Jan 1999 22:17:38 -0500

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Hi Mark!

If I heard your examples I might be able to help.  Off hand, I would
suspect:

1) The file is playing back at one-half the sample rate.  I doubt this is
the problem, or everyone would be complaining about the messages being
reeeaaaallllllyyyyy slow.  It's been a while since I tried to get
phone-ifc-c useable, but I don't remember that being a problem.

2) An error in the Rockwell algorithm that converts from WAV to ADPCM, that
get's reversed when you go from ADPCM to WAV.

3) The frequency of your sound exceeds that of the Nyquist frequency (one
half the sample rate).  When the sampled frequency of a sound is higher than
the Nyquist frequency, the aliasing will manifest itself as a new frequency,
somewhere below the Nyquist frequency.  For example, if your original file
was a 8K tone, and you were trying to convert it to an 7.2K ADPCM, you'll
probably end up with a sound whose frequency is 4K.  A low-pass filter is
used to avoid this problem, especially with downsampling, which is what you
are doing here.  This is what I suspect the problem is with what you are
hearing, but probably doesn't have anything to do with why Rockwell's
WAV->ADPCM algorithm is so bad.

Cheers!

Michael David
michael@michaeldavid.com
----- Original Message -----
From: Mark Gilmore <omnip@usit.net>
To: <ecs-list@netbloc.com>
Sent: Friday, January 29, 1999 12:12 AM
Subject: [ECS] .WAV clarification


I have contributed to confusion regarding "WAVE" file
playback on voice modems: What I SHOULD have said was
PCM/WAVE playback, as "WAVE" files can also be in
"special" formats (such as ADPCM).

So to re-state, I am looking for a modem/utility/method/
libray which will play a PCM/WAVE file thru a voice modem,
OR convert such a file into ROCKWELL/ADPCM WITHOUT the
distortion we are experiencing.

UPDATE/FYI: When I convert a PCM/WAV to ROCKWELL/ADPCM
(using Rockwell's conversion utility), and back again
to a PCM/WAV, the 2nd PCM/WAV plays great (thru SoundBlaster).
SO I KNOW that no data is being lost or corrupted
in the conversion. When I use the Rockwell program
to play the ADPCM file thru the modem, I get the same
distortion as with ECS. So I know that I am not
playing the file "wrong".
OMT: When I convert a high-pitched test-pattern tone
to ADCPM, it plays back ONE OCTAVE lower than the original
(but plays fine if i convert back to pcm).
This can only mean one thing. Perhaps one of you will know
that that one thing is :-).
--
Mark Gilmore
Omnipotence (ECS Home Automation Software)
omnip@usit.net
http://www.usit.com/omnip

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