Re: 14.4 modem with Apple IIc

[dempson@actrix.gen.nz (David Empson)]

Dave Althoff <dalloff@freenet.columbus.oh.us> wrote:

> rajagod@rajagod.es.dupont.com wrote:
> : Is it possible to hookup a 14.4 kbps or higher modem to an
> : Apple IIc?
> 
> : I have been using a 9600 baud modem and the Proterm terminal
> : emulation software so far.  But the software doesn't have 14.4
> : kbps as an option, the next highest number is 19,200.  Is there
> : some other emulation software that recognizes the newer modems?
> 
> Run the 14.4kbps modem at 19,200 baud.  It should work just fine.
> 
> I've never seen a serial port that could be set at 14,400 baud.

The PC's can, and so can the IIgs (SCC), along with most other chips
with a programmable divisor.  If it can do 7200, 9600, 19200 and 38400,
it should be able to do 14400, though might not be able to do 28800 (if
it can do 57600, then it can definitely do 28800).

Very little software would actually let you select these "unusual"
speeds, as you actually need a higher speed to be able to good
performance from an error correcting modem.

> I think that transmission speed is something the telephone people came up
> with for FAX transmissions or something...

I don't know why they ended up with 14400 bps, but it is to do with the
encoding method used, which is Trellis coding.  The data is actually
transmitted at 2400 baud with 6 bits per baud, giving 6 x 2400 = 14400
bps.  Modem modulation at 9600 bps also uses Trellis coding, but with
only 4 bits per baud.

(I don't know the technical details of any of the modem modulation
techniques faster than 2400 bps, only their names and gross features.)

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand