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Re: [Discuss-gnuradio] bell 202 modulation

From: Matteo Campanella
Subject: Re: [Discuss-gnuradio] bell 202 modulation
Date: Wed, 29 Mar 2006 22:07:21 +0200 (CEST)
User-agent: SquirrelMail/1.4.4

yep, my fault to use the wrong name for it :-)
I am indeed working on the bits to symbol (nrzi) block right now, and
later on I am willing to do the HDLC block with flag, FCS and bit stuffing
in order to put up the following chain:


right now i am stuck at modulation, that is, to find a method to use the
frequency modulator to do that at baseband, as suggested from Eric...

73, Matteo iz2eeq

> 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.
> 73,
> 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
>> Microprocessors
>> 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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