A cordless phone question

[Ken]

I have two AT&T model 1475 cordless phones. (I know they are quite
old, but I like the feel of the phone even though it is a 2.4GHZ phone.)
I acquired the second one for a few dollars primarily for parts if the
first one crapped out. Both of the units work just fine in all respects.

Since they are the same model and the phone from one does NOT work on
the other base unit, I assume there is a different frequency that each
is running at. I opened up one unit and I did notice a crystal tacked
on the back of the PWB that I assume is unique for each pair.

My question: Just out of curiosity, what do the manufacturers do to
make their same model cordless phone communicate with only the base it
belongs? My guess is they change something simple like the crystal on
both the base and handset so that there is a distinction between units.
Anyone know?? Thanks.

 

[Geoffrey S. Mendelson]

My question: Just out of curiosity, what do the manufacturers do to
make their same model cordless phone communicate with only the base it
belongs? My guess is they change something simple like the crystal on
both the base and handset so that there is a distinction between units.
Anyone know?? Thanks.

The early ones used different frequencies. Then when the multiple channel units
came out (in the early 1990's) they added some sort of digital encoding to the
signal (such as a low level digital subcarrier) to identify one base station
from another.

Some of the earlier phones had switches inside to set this, later ones were
just hard coded in a ROM somewhere or with soldered jumpers on the boards.

The digital ones just include it in the digital data stream.

Generally they are designed NOT to allow to change them so that you buy a
new one when the old one dies.

The new generation of cordless phones, DECT (there are similar but not
compatible DCT phones) are IMHO best. DECT phones are wifi friendly they
operate in a different band (1.8gHz) and listen to make sure a frequency
is NOT in use by someone else before they transmit.

They also have the ability to connect multiple handsets to the same
base station and some have the ability to roam handests between multiple
base stations.

Geoff.

 

[Rich Webb]

I have two AT&T model 1475 cordless phones. (I know they are quite
old, but I like the feel of the phone even though it is a 2.4GHZ phone.)
I acquired the second one for a few dollars primarily for parts if the
first one crapped out. Both of the units work just fine in all respects.

Since they are the same model and the phone from one does NOT work on
the other base unit, I assume there is a different frequency that each
is running at. I opened up one unit and I did notice a crystal tacked
on the back of the PWB that I assume is unique for each pair.

My question: Just out of curiosity, what do the manufacturers do to
make their same model cordless phone communicate with only the base it
belongs? My guess is they change something simple like the crystal on
both the base and handset so that there is a distinction between units.
Anyone know?? Thanks.

It's almost certainly handled in the digital realm rather than a single
change in the base frequency. The handset and base are paired together
by something like both sharing the same seed value for a pseudo-random
number generator. The PRN is used as the basis of digital spectrum
spreading of the RF signal, frequency hopping or direct sequence, so
that only units that share the same sequence can inter-operate.

Some base units and handsets can be "registered" together so that the
base can operate with more than one handset. Since that capability isn't
mentioned in the 1475's manual, I'd guess that the pairing is done once,
at the factory.

*If* the pairing key in the handset is stored in an identifiable chip,
say, a serial EEPROM, then it might be possible to get at the key and
change it.

 

[Winston]

I have two AT&T model 1475 cordless phones.
(...)

My question: Just out of curiosity, what do the manufacturers do to make
their same model cordless phone communicate with only the base it
belongs?

(...)

I don't know, but I RTFM and this seems likely:

See page 36 of:
http://dl.owneriq.net/5/52d44f17-e87a-46c2-adaf-59da932f7357.pdf

Place your new-to-you base unit into a carton
and tape securely closed. Do not touch it
for about a week.

Unplug the power from your existing base unit.
Count to five slowly.

Take the battery out of the new-to-you handset,
count to five slowly and then put it back in.

Place your new-to-you handset onto your base unit
and plug the base unit power back in.

This will cause the unit to reinitialize (and
probably pair your new-to-you handset to your
base unit).

Test your old handset to see that it still works.

If this doesn't work try:
www.telephones.att.com, or call 1 800 222–3111.


--Winston

 

[Ken]

I got a couple of Unidens that interchange just fine. They do it
digitally so once you stick one in the other base, the remaaining base
and handset will not work until plugged in once. When the charge light
comes on it flickers a bit after about a half a second, that is the
handshake I would assume.

Years ago a buddy of mine bought a couple of Vtechs that did the same
thing, but he had them connected to different phone lines. That has
it's own set of advantages but mine are still hooked to the same line.
I just switch them when a battery starts dying.

However when one dies there is a loud audio noise until one of the
handsets goes on the base that was in use. Then being reinitialized of
course it hangs up, so you have to take another phone off hook to make
the transistion.

They wil also work simultaneously, they sidestep each others'
channel.

They are getting old now, the battery life is not what it once was but
I like the phones themselves. There is also a problem if I use a
certain wireless camera because it's also only 2.4 Ghz.

J

Thanks for your comments. Thanks to everyone I have several ideas now
that I did not have before.