Bulletins from the Pacific Packet Radio Society - page 157
hf (particularly on 10 meters when the path is good). I plan to present a paper on this modem at the March 19 packet conference.
So, on to the purpose of this message. In order. for a synchronous demod to attain lock, it needs to sample both mark and space frequencies, preferably more than once each. This is true not only for the above-described modem but also for other synchronous modems. A synchronous modem buys us 3 dB, nothing to fritter away for satellite or long-haul terrestrial vhf/uhf packet paths.
Ideally, it would be good to have a pre-frame sync of 2 octets of reversals. Probably not as good, but possibly good enough, would be the transmission of 2 flags before the start of the frame. Unacceptable possibilities (for synchronous modems to work) would be (a) the binary "1" idle (marking) condition or (b) nothing at all.
I checked the Intel 8273 data sheets and found that sixteen line transitions are guaranteed with the chip in the Pre-Frame Sync Mode. Also, I note that this mode is enabled in the Vancouver TIP.
With the information readily available, I was unable to determine whether pre-frame sync (reversals) was possible with the Zilog 8530 or the WD-1933. Terry Fox, WB4JFI made a couple of calls to Zilog and was told that it is possible to produce with liberal amounts of chicken blood (of the software type).
Now, I'm finally coming to the point. What can you tell me about the possibility of implementing the 16-transition type of pre-frame sync on the WD-1933 which you are using? Have you implemented it and how? I would appreciate your inputs as soon as possible.
I would like to be able to write pre-frame sync into the Level 1 document. Eric Scace, K3NA advises that this subject belongs to level 1 (not 2) because it deals with extra-frame signals which are there to make the modems work.
Bob and I realize that this demod is not the simplest one that could be used to copy the satellite. We are also working on a simplified PLL-type suboptimal MSK demod and will let you know more about this one when it is further along.
73, Paul, W4RI
FILE QST.83.01.31.0.N6ECT
To All Interested Packeteers
From: Curtis, N6ECT
Date: January 31, 1983Subject: Year Report on CALLS LOGGED 1982
Click for Original Graphic Image of this page.
