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:
message_source->hdlc->nrzi->modulation->duc 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 >> >> >> >>>Matteo: >>> >>>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 >>>x10010101... >>> >>>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. >>> >>>Bob >>> >>> >>> >>> >>> >>> >>>Matteo Campanella wrote: >>> >>>>100011010 should become >>>> 1 0 0 0 1 1 0 1 0 >>>>space-space,mark-mark,space-space,mark-mark-mark-mark,space-space-mark,space >>>> >>>>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 >>>>2200Hz. >>>>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 >>>>time... >>>> >>>>MC >>>> >>>> >>> >>> >>>-- >>>AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats, >>>NJQRP/AMQRP, QRP ARCI, QCWA, FRC. ARRL SDR Wrk Grp Chairman >>>Laziness is the number one inspiration for ingenuity. Guilty as >>> charged! >>> >>> >>> >> >> >> >> >> >> _______________________________________________ >> Discuss-gnuradio mailing list >> [email protected] >> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio >> >> > > > _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
