Chas, the term "modem" is a contraction of "modulator"
and "demodulator"; it purpose is the bidirectional conversion of
digital signals to analog signals. There are many different kinds of
modems, employing different modulation techniques to achieve
different speeds and error rates over different transmission media. A
modem used for internet dialup access, for example, is optimized for
use with telephone lines and uses industry-standard signalling
conventions so it can interoperate with any other telephone modem.
The RTTY modem in in a TNC (Terminal Node Controller) uses the baudot
code and RTTY mark and space tones and should be optimized for
transmission and reception over HF. Circuits that modulate and
demodulate PSK, FSK, Pactor, Amtor, or Olivia are all referred to as
modems, but their implementations are radically different. Multi-mode
TNCs typically employ a microprocessor and perhaps a programmable DSP
circuit; software running on this one set of hardware components can
thus implement multiple protocols -- but only one protocol is
typically running at any one time.
The modem in your PC is most likely a telephone modem; it may also
have facsimile capabilities.
The Icom 756 definitely does not include a modem; I doubt that the
736 or 746 do either. More recent Icom transceivers like the 7800 and
756 Pro 3 include a dedicated RTTY modem; to my knowledge, this modem
cannot be used to encode or decode any other protocol.
You mentioned the Icom CI-V bus in an earlier post. This is the means
by which an application running on a PC can control an Icom
transceiver -- read or write its frequency or mode, etc. It is not a
modem -- its a simple serial protocol using open-collector TTL levels
(a binary 0 is represented by 0 volts, and a binary 1 is represented
by 5 volts). To use this with a PC serial port, a level converter to
RS-232 levels is required.
73,
Dave, AA6YQ
--- In digitalradio@yahoogroups.com, <[EMAIL PROTECTED]> wrote:
>
> On Mon, 28 Aug 2006 00:17:06 -0700, Chris Jewell
> <[EMAIL PROTECTED]> wrote:
> >Suppose you're using your sound card as a modem to receive Pactor I
> >data. Your sound card takes care of turning tones from the
receiver
> >into 1s and 0s. There's no problem there.
>
> actually, there is a modem in the 736.
> why can't that handle the packet checking tasks?
> it IS an FSK system but should be available, using HRD to run it,
to handle
> any modem duties in the txcvr. Also, I THINK, that there is a
high speed
> modem built into my mobo as well as 10/100 etherlink.
>
> I am just trying to figure out why I have to add another piece of
hardware to
> the system. we KNOW that the latest, deep pocket ICOMs like the
756P3 are
> fully set up with an internal "TNC", or so I have been assured by
some of
> those elmers/gurus (the ones who do not agree with each other <G>)
> The reason I am here is to try to find out what is true and what is
bushwa and
> to simplify the shack to as great an extent as possible. My
experience is
> that the more hardware you add, the greater the load on output of
final
> product. that applies to anything with a pretty limited source of
power, be
> it computing, motive or whatever.
>
> I truly do appreciate all the information I am getting here.
>
> Again, do I HAVE TO HAVE a TNC with an IC736, 746 or 756?
>
> thanks
> 73/chas
> --
> K5DAM Houston EL29fu AAR6TU
> http://tinyurl.com/df55x (BPL Presentation)
>
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