Bulletins from the Pacific Packet Radio Society - page 156

Just in case that you haven't come across this short article, W4UCH/2, using a TRS-80 Model 1, has created a software 8273. You can find this article on page 53 of the February 1983 issue of QST; titled "PACKET RADIO - A SOFTWARE APPROACH.

73, -CAS-


To: Den Connors, KD2S, Lyle Johnson, WA7GXD Hank Magnuski, KA6M

Info: Doug Lockhart, VE7APU, John Spraggs, VE7ADE

Subject: Pre-Frame Sync

Despite outward appearances, I am making progress with the Level 1 document for AX.25 which will include those physical-layer requirements needed for Phase III-B packets. Den saw part of it during his latest trip to Washington. I've run into a slight snag and would appreciate your early inputs.

As you know, HDLC itself, per ISO 3309-1976(E), begins the frame with a flag. Para 3.8 gives only this guidance: "Interframe fill shall be accomplished by transmitting either contiguous flags or a minimum of seven contiguous "1" bits or a combination of both. A selection of the inter-frame time fill methods depends on systems requirement."

Also as you are aware, I am working with Bob Watson on the design of a modem which we are now calling "PAM" for Packet Adaptive Modem. It is a multi-speed modem with a constant frequency shift of 600 Hz. The speeds will be 75, 150, 300, 600, and 1200 bauds, remotely selectable via three lines. It started out life as an hf packet modem whose speed could be software controlled to adjust to changing ionospheric circuit conditions. The modulator design is finished. The (MF-10 switched-capacitor) demodulator design is well along, and that portion which operates at speeds up to 600 bauds is complete for optimal demodulation of fading hf packets. When we tested the demod, its performance was slightly degraded at 1200 bauds, owing to the usual problem of filtering out carrier components which are close to the signaling rate. We were planning to leave the hf modem at that then start working on a separate optimal (PSK-type) MSK demodulator, preferably one which is simple and easily reproducible. Then, in the examination of design alternatives, Bob was reluctant to give up the neat MF-10 mark and space filters. So, he is now designing an optimal, synchronous MSK demodulator which is an add-on to the original MF-10 design. So what we'll end up is a modem which is an optimal limiterless type for hf 600-Hz-shift FSK at 75, 150, 300 and 600 bauds and an optimal MSK demod for 1200 bauds. The 1200-baud speed is primarily intended for satellite work but could also be used for terrestrial packets and

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