From: Tomi Manninen OH2BNS (dcuptlt.csdtklq@mx.dy.fi)
Date: Tue Jun 26 2001 - 11:18:02 EEST
On Mon, 25 Jun 2001, Niall Parker wrote:
> On Mon, Jun 25, 2001 at 07:51:21PM -0700, Bob Nielsen wrote:
>
> [snip]
>
> > Thus, although you can communicate between the computer and the Baycom
> > device at 300 baud, it would still communicate with the radio at 1200
> > baud, although with a slower character rate (increased mark dead time).
> > It would not be compatible with 300 baud HF packet operation.
>
> Actually, the TCM3105 (and similar units) are dumb modems, bit rate in
> equals bit rate out. The frequency shift is 1 kHz, so the bandwidth occupied
> will be wider than the conventional HF shift of 200 kHz, but most simple
> PLL based demodulators should be able to decode it OK still.
>
> If you want to get the 200 Hz shift of a conventional HF modem, look around
> for one of the older Exar based Bell 202 modems which can be retuned for
> different frequencies and shifts by substituting resistors and/or a cap.
> ... or bypass all that and use a sound card :)
My old TNC2 clone (a Symek TNC2S) has a 300baud / 200Hz mode that is
implemented with the same TCM3105 that is used with 1200baud / 1000Hz. I
don't have the schematics of the TNC here right now but looking at the
TCM3105 data sheet I would guess it uses the CCITT V.23 600bd mode of
the TCM3105 chip. This has tx and rx tones of 1300/1700Hz and as the chip
scales the tones with the xtal frequency, dividing the regular PAL color
burst xtal by two should do the job (both shift and baud rate even if the
latter probably only affects the internal filter widths - baud rates are
in a baycom modem are generated in the PC).
-- Tomi Manninen Internet: terhi.victor@logonet.com OH2BNS AX.25: terhi.victor@logonet.com KP20ME04 Amprnet: terhi.victor@logonet.com- To unsubscribe from this list: send the line "unsubscribe linux-hams" in the body of a message to terhi.victor@logonet.com
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