DSP-TDD's: A new AMRAD project?

Richard Barth Richard.Barth@noaa.gov
Fri, 30 Jul 1999 15:48:54 -0400

A couple of days ago, I got a note from Ed Cummings, N3KOW, who buys up used
TDDs at hamfests, fixes them up and gives them to deaf people who can't afford
to buy new ones.  He had picked up an elderly box at Timonium and, after 
finding the HEX web site, wrote to ask me what I knew about the thing.

We talked about that for a while, then he asked whether I knew of any work
done to modify DSP-based modems to handle TDD mode as well as the various
modes.  I suggested that, since AMRAD has a group of DSP hackers and is always
looking for new and interesting projects, he might want to come to a
meeting and 
pitch the idea to the group.  

He's interested, and also knows of some people who have done this sort of
before (although for a different purpose  :-).  Before he makes the trip down 
from Philadelphia, though, we ought to make sure we have somebody (or a group
of somebodies) willing and able to take on a job like that.  Volunteers?  
Expressions of interest?

If anybody wants to step forward, we can suggest some dates and times for Ed
to come in.  He sounds like the sort of guy who ought to know more about
Mebbe the start of a Philadelphia chapter?  :-)

The latest round of email is shown below.



>Date: Fri, 30 Jul 1999 15:02:15 -0400
>To: Richard Barth <Richard.Barth@noaa.gov>
>From: e cummings <bernies@netaxs.com>
>Subject: DSP-TDD's
>At 01:40 PM 7/30/99 -0400, you wrote:
>>TDD modems have always trailed behind commercial ASCII modems by several
>>years.  TDD modems were equipped to do 300 baud ASCII back when the world
>>was moving along at 9600 or 14,400, and the first 14,400-capable TDD modems
>>I became aware of showed up when the world was moving toward 56K.  Since
>>the TDD uses FSK much like the old Bell-103, it should be quite possible to
>>make a single modem do both, but I'm not aware of anybody actually working
>>on it.  It would probably involve someone with inside connections at a
>>modem factory who could get hold of the source code used in running the DSP
>>chip.  Having that would make it a lot easier to hack the code to make it
>>handle a different set of FSK frequencies for the TDD.
>>AMRAD, the radio/computer club that sponsors the HEX, has a number of DSP
>>hackers in it and DSP has been one of the the group's interests over the
>>years.  If you could make it down to Northern Virginia you might want to
>>drop by one of our meetings and propose a project.  We're always looking
>>for something interesting to do, and that sort of thing would fit right in.
>>  Where do you live, anyway?
>Philly.  I'm acquainted with people in the hacker community who've 
>reprogrammed DSP-based modems to generate inband telephony signalling tones 
>for experimenting with various telephone switches.  The modems they used 
>were made by Zyxel, but there are several manufacturers making DSP modems 
>now.  It's apparently cheaper for them to upgrade to new protocols by 
>FLASHing a NVRAM than by changing ASIC's.
>Unfortunately, there seems to be no standardization in the DSP's used by 
>modem manufacturers, so any DSP-TDD app would probably have to be 
>modem-specific.  Still, it would be really nice for the PC-equipped 
>hearing-impaired to be able to use a single modem for both Internet dialups 
>and communicating with Baudot-only TDD users.  Such an app for a PCMCIA 
>modem could make any laptop a universal portable TDD (there are compact 
>acoustic couplers out there that plug into a modem's RJ-11 jack.)
>I ramble.  Perhaps I could take an Amtrak down to one of your meetings to 
>propose this concept.  I'm not knowledgeable enough about the technical 
>specifics to do any programming, but I can throw the idea out there.  If a 
>shareware app were put out, I'm sure it would fill a real need and spread 
>pretty rapidly in the deaf community.  It still amazes me this hasn't been 
>done yet, (to my knowledge) because it's such a natural application.  Drop 
>me a meeting schedule and directions when you get a chance.  Thanks.
>Ed, N3KOW

Richard Barth, W3HWN          * Director, Ofc of Radio Frequency Management
Phone: 301-713-1853           *     U.S. Department of Commerce / NOAA
FAX:   301-713-1861           *              Room 2246, SSMC2
Email: Richard.Barth@noaa.gov *           1325 East-West Highway
                              *           Silver Spring MD 20910