DSL filters help regular dial-up modem connection?

Bob Bruhns bbruhns at erols.com
Wed May 21 11:53:33 CDT 2003

-----Original Message-----
From: amradio-admin at mailman.qth.net [mailto:amradio-
admin at mailman.qth.net] On Behalf Of Jim candela
Sent: Tuesday, May 20, 2003 10:36 PM
To: amradio at mailman.qth.net
Subject: [AMRadio] Telephone line bandwidth for AM
Phone Patch


This is a little off subject, but just a little. Sometimes
when we use a phone patch on AM, we can get better results
matching phone line impedances, and maybe a little lead, or
lag to help equalize (or add pre-emphasis) to the available
bandwidth. I am sure this was studied years ago when
professional baseball games were broadcast from the
booth, and then piped for hundreds of miles down telephone
lines, and then using the remoted audio to modulate
broadcast AM transmitters.

This all occurred to me today when I made my house DSL
ready. Four lines (with a common beginning) now each have a
DSL filter at the end before going into a telephone, or
modem. I have NEVER been able to connect to the internet at
a rate higher than 28.8 Kbs. Today after installing DSL
filters (including one at the computer modem), my connection
is 48.8 kbs!!!! I just might cancel the DSL which starts
tomorrow, and keep the filters. So what technology is in use
with these filters? How do they apparently result in
improved connection speeds? Is it terminating the line in
it's characteristic impedance, or increasing the
longitudinal balance?

Remember the answers to these questions need to be discussed
in a manner that will help us AM hams have more fidelity
when we remote our stations, or use a phone patch, or
download the AMRADIO reflector's email.

Jim Candela

-----Reply Message-----
From: amradio-admin at mailman.qth.net
[mailto:amradio-admin at mailman.qth.net]On Behalf Of
COLEMANJ at sbcglobal.net
Sent: Wednesday, May 21, 2003 11:12 AM
To: amradio at mailman.qth.net
Subject: [AMRadio] Telephone line bandwidth for AM Phone

Hi Jim:

The majority of telephone equipment (answer machines,
modems, fax, ect.) has filtering in it to bypass RF.  The
bypass capacitance in these things causes the higher
frequency components of the audio to be rolled off.  This
bypassing also kills DSL signals.  The devices that you
install to make your lines DSL ready, should be called
blockers instead of filters, because they have inductors in
series with each line that will keep the higher frequency
components of any signal on the line, while stopping it from
reaching the other telephone equipment.  Obviously the
connection that is to be used for the DSL modem must not
have a blocker on it or the HF DSL signal will not reach the
DSL modem.  My house wiring was to weird to deal with so I
installed a splitter outside at the TELCO block and put one
blocker there for the house telephone equipment and ran a
separate line to the place where I wanted the DSL modem to
be.  This way I didn't have to worry about all the stuff in
the house.  You probably could have unplugged all other
telephone equipment in the house except your computer modem
and the connection rate would have been better as well.  I
don't think  that matching Z on the phone patches is very
important.  As a matter of fact I think the phone patch
should represent as high Z as posible to the phone line so
as to not load it down.  But equalization is very important
as is balance and isolation from ground to prevent common
mode hum.

Good Luck
 John, WA5BXO

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