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PICList Thread
'Help needed with digital filtering.'
1998\01\16@054447 by GERRY COX

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Hi Listers,

My project uses a PIC12C509 to monitor sensors and then outputs various
simultaneous tones.  The tones are recorded on audio tape using a compact
cassette recorder.
When the recording is finished, I want to take the recorder to a PC equipped
with a sound card and create a .WAV file from it. (or similar?)
I then need to analyse the WAV file to extract the tones and  the times at
which they occur. I need to know with some precision the time interval
between changes of any two tones.  I  imagine that I need to run various
software filters on the WAV file to do it.  Can anyone tell me the
principles of software filtering or point me to web sites that may help? I
also need to know  about the file format of a WAV file. Can anyone help
please?

Best Regards,
Gerry Cox

1998\01\16@171826 by Steve Baldwin

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> My project uses a PIC12C509 to monitor sensors and then outputs various
> simultaneous tones.  The tones are recorded on audio tape using a compact
> cassette recorder.

You may be able to save yourself some trouble if you record DTMF tones (if
that is enough) and then play it back through a DTMF decoder. You may be
able to find something on the web that will decode DTMF tones directly from
a .WAV file.

> Can anyone tell me the
> principles of software filtering or point me to web sites that may help?
I
> also need to know  about the file format of a WAV file. Can anyone help
> please?

For .WAV files; If you want lots of words, look for "Multimedia Programming
Interface and Data Specifications 1.0" by IBM and Microsoft. I think I got
it from Microsofts web site. If you want it in a simpler form, try
http://www.lightlink.com/tjweber/StripWav/Canon.html. It is a pretty simple
file format.

For learning DSP (now you're in the deep end), try
http://innet.com/~condor/wghome.html as a starting point and follow a few
links.

Steve.

======================================================
 Very funny Scotty.  Now beam down my clothes.
======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680                email: spam_OUTstevebTakeThisOuTspamkcbbs.gen.nz
New Lynn, Auckland           ph  +64 9 820-2221
New Zealand                  fax +64 9 820-1929
======================================================

1998\01\20@074232 by GERRY COX

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Sean and Listers,

Thanks for the advice and file on WAV formats.
As to the simultaneous tones,  I am trying  to record the state of various
sensors onto audio tape. The sensors give a 1 bit digital output and I need
to extract rate information from them. e.g. one of the sensors will pulse
high  for 10mS at a repetition rate of say 0 .5Hz to 100Hz, representing a
shaft rotating at 30RPM to 6000RPM.
I intended to record on tape, a tone representing the high state and another
tone for the low state. For the next shaft I would do the same with a
different pair of tones.
The tape would be replayed and an accurate  reconstruction of the rates of
acceleration and velocity of the shafts must be obtained. This will be used
to determine performance and reasons for failure  etc;

Perhaps there is a better way? I will have a look at the possibilities of
PPM modulation though.

Any Ideas are welcome.

Best Regards,
Gerry

----------
From: Sean Breheny
To: GERRY COX
Subject: Re: Help needed with digital filtering.
Date: 18 January 1998 00:15

At 10:20 AM 1/16/98 -0000, you wrote:
>Hi Listers,
>
>snip........................................
>I have attached a file which explains most of what you need to know about
>WAV file formats.
>I am just curious, why do the tones need to be simultaneous?
>
>snip........................................
>
>The basic idea behind PPM is
this:>>snip........................................
>
>The great thing abt. PPM is that it is easy to sync to, easy to decode by
>computer means, relatively easy to code, and allows multiplexing without
>messy multiple subcarriers/tones, etc.
>Good Luck,
>Sean

1998\01\20@182807 by Mike Keitz

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On Tue, 20 Jan 1998 12:21:10 -0000 GERRY COX <.....gcoxKILLspamspam@spam@DEK.COM> writes:
>The tape would be replayed and an accurate  reconstruction of the
>rates of
>acceleration and velocity of the shafts must be obtained.

Audio tape recorders don't preserve frequency and time very well,
especially recording on one machine and playing back on another.


>Perhaps there is a better way? I will have a look at the possibilities
>of
>PPM modulation though.

I think it would be best to record a digital code of the number of pulses
received on each input in a predefined interval of time controlled by the
recorder.  For example, the PIC would count for 0.1 second and record the
number ( 0 to 10) for each channel.  Using 6 bits to store the number of
pulses would take 60 bps for each channel, plus some overhead bits to
keep in sync and check for errors.

Even if the tape speed varies, exact rates and times could be recovered,
to the degree of accuracy of the clock of the PIC in the recorder.  Use
one of the modem standards and async data, and a standard PC modem could
be used to decode.  Some PC modems claim to support Bell 202 but details
are kind of uncertain.  If 300 bps is enough, use Bell 103.

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