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Re: [Discuss-gnuradio] bell 202 modulation

From: John Ackermann N8UR
Subject: Re: [Discuss-gnuradio] bell 202 modulation
Date: Wed, 29 Mar 2006 14:44:57 -0500
User-agent: Mozilla Thunderbird 1.0 (Windows/20041206)

Having been an innocent bystander here, I think I see a classic case of people talking past each other.

Matteo, the 1200 baud modulation used in AX.25 packet radio uses BEL 202 tones, *but* the signal that's being provided to the modulator has been through an HDLC chip (or equivalent) that converts the data stream into a synchronous format that includes flags, FCS, etc., *and* uses NRZI (non-return-to-zero, inverted) modulation. That HDLC activity isn't part of the BEL 202 protocol.

What you're trying to implement isn't BEL 202, but 1200 baud AX.25, and those aren't the same things.

Bob's answers were based on the assumption you were trying to produce a BEL 202 data stream as it would appear on a telephone line. That data stream is asynchronous and doesn't include any of the AX.25 frame characteristics.

The only connection between the two is that in the ancient, early days of packet radio the first experimenters glommed onto some surplus BEL 202 modems and used those as the last stage of their cobbled-together TNCs; the use of AFSK with 1200 and 2200Hz tones is simply an artifact of that.

John N8UR

Matteo Campanella wrote:
I'm quite surprised by this response of yours, as it is not exactly
aligned with the friendly style of the list - I know that to make things
clear is more difficult than slamming doors, but I like it more.

Maybe I am referring to the BELL 202 erroneously, because I am actually
trying to implement a modem for 1200 AFSK to use on AX25 on ham vhf band -
I am not sure about it is BELL202, but I am sure it is encoded the way I
said, as I have read this on a number of papers, including those from
TAPR/ARRL Digital Communication Conferences, particularly:

PIC-et Radio:  How to Send AX.25 UI Frames Using Inexpensive PIC

by John Hansen, W2FS
State University of New York
49 Maple Avenue

3. Packet radio uses a modulation scheme called NRZI (Non-Return to Zero,
Inverted).  This means that the ones and zeros are not represented by high
and low states (or tones).  Rather, a zero is represented by a change in
tone (if it was high, it goes low, if it was low, it goes high) while a
one is represented by no change in tone.  Together with bit-stuffing, this
ensures that there will be a tone change at least every five bits, if not
more often (except for flags).  This helps the transmit and receive timing
stay in sync.i

maybe the correct name for this mod scheme is NRZI, and I wrongly called
it BELL202 as I used an MX614 in one of my implementations, that is
frequently reported as being BELL202 modem.

useless to say, I do not consider anyone an idiot - neither in the list,
nor in the whole world - that would be the very opposite of the HAM spirit
you should know very well.

yours truly
Matteo iz2eeq


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

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
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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