From: taylor@tix.timeplex.COM (Seth Taylor)
Newsgroups: rec.radio.amateur.misc
Subject: Phone Patches
Date: 5 Oct 1992 08:06:24 -0700
Organization: UCSD Usenet Gateway
NNTP-Posting-Host: ucsd.edu

     
 (Thomas David Kehoe) writes:
>Posted to: telecom@eecs.nwu.edu, rec.radio.amateur.misc
>
>I'm developing a product for stutterers that will process 
>speech when talking on the telephone.  To connect the 
>processor to the phone, I bought a "phone patch" for ham 
>radios, the MFJ Telepatch II, for $69.95.  It works fine, 
>etc.....

| KE4ZV writes:

|In essence, a phone patch is nothing more than a 2 wire to 4 wire
|hybrid and a protective network to meet Part 68. A standard
|telephone *already* does this. So if you connect at the handset
|cord, all you need to do is watch the balance impedances of your
|sources and loads to keep the hybrid balanced, and you have what
|you need.

|Gary KE4ZV

The network in a  500 or 2500 type set requires a 45 ohm match for
the carbon transmitter and the receiver match is 150 ohms (per ITT
Telephone Instrument Maint. Manual TIMM-2 spec. 212.01 Type 65
handset). Also, the network in the telset has a 10 dB sidetone
circuit. The effective ERL is less than optimum as compared to well
designed 2/4 wire hybrid. The ERL can approach 35 dB under ideal
matching conditions in a stand alone hybrid. However to clarify
some points:

The average speech level into the PSTN (Public Switched Telephone
Network) is -13 dBm.  Per Bellcore *Notes on the BOC Intra LATA
Networks- 1986*, TR-NPL-000275 iss. 1 April 1986, page 7-12, para.
2.61 "It has been established that the signal power during an
actual call should not exceed -13 dBm0 average over any 3 second
interval." and in para. 2.62, "Test tones for short term tests are
treated as an exception and are allowed to be at 0dBm0. Tones for
long term tests are operated at -10 dBm0".  

Any mods to a registered device (except by original equipment
manufacturer) violate part 68. Mods on telephones maybe ok for a
one time hamshack experiment, but offering for resale a modified
registered device without a disclaimer or alleging that the FCC
reg. no. of the phone is still the same after you have mods done
could lead to some legal problems.

I'd recommend that if you don't want to manufacture your own FCC
part 68 compliant unit (I don't think you want to considering
the cost to certify at an FCC approved lab), contact a company
like Tellabs in Lisle Illinois. Their largest customers include
the Regional Bell Cos and others. They have a 2/4 wire FCC part 68
NetworK Terminating assemblies with level adjustments. There are
control leads (E&M) to "draw dial tone" included. Their model 
6131B FXO plug in module when mounted in a model 1912 apparatus
case and powered  by a 48 VDC wall plug power supply would be ideal
for your use. Then any thing you connect on the four wire side
would not have be registered. Also, the ERL could be optimized.
Just make sure that the energy into the network at the 2 wire
connection to the network meets the requirements of part 68
including the limits on the average speech energy.
Seth T. KC2WE taylor@tix.timeplex.com
The opinions are my own and not those of ascom Timeplex