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Re: [Discuss-gnuradio] bell 202 modulation

From: Robert McGwier
Subject: Re: [Discuss-gnuradio] bell 202 modulation
Date: Wed, 29 Mar 2006 11:01:02 -0500
User-agent: Thunderbird 1.5 (Windows/20051201)


Normally I do not play the game this way but let me suggest that you do the following thing so we can get beyond this to give people the correct information.

Open up Google. Type in BEL-202, hit enter. Please tell me WHOSE NAME and DSP assembler code you see on the first several entries and many of the remaining entries. My code and name are not there by accident as the most referred to pages in the world on BEL-202.

The data is not Manchester encoded. It is not split level in the middle of the bit. I think your safest bet is to assume that the people who are in this group are not idiots.

100011010... is transmitted as

The Bel-202 standard allows transmission up to 1800 bps. If it were encoded biphase or Manchester, the channel would need to be > 3600Hz wide. This was done back in the ancient days before equalizers, and easy DSP so one could not mitigate for channel edges. There is no split phase data in BEL-202.

Take it or leave it.  I have left this conversation as of now.


Matteo Campanella wrote:
100011010 should become
  1        0         0           0      1    1       0        1       0

with the arbitrary decision on the first bit, that could be either space
or mark, as there is no previous state. space is 1200Hz and mark is
I have already encoded this on a DDS based on PWM on a PIC 16F628, but I
had control over REAL time there.
I have just written a block to do the differential encoding, that is, to
obtain the correct -1,1 sequence, but my doubt is how to apply this to the
freq modulator in order to obtain exactly the two tones I need for mark
and space, and the correct bit timing, as I do not have control over


AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats,
Laziness is the number one inspiration for ingenuity.  Guilty as charged!

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