Filter for European telephone toll pulses

[Tim Vetter <vfr@stm537.ubszh.net.CH>]

Problem:  Some European phonelines, particularly in the
German-speaking countries, send "toll pulses" intended
to trigger increments of a billing counter found on some
telephone sets.  Unfortunately, these pulses consist of
high-power (~10V RMS), high-frequency (~14kHz) tones that
tend to cause dropped connections with modems that don't
have the appropriate filtering on the front end (ie most
modems sold in North America).

Solution:  Attach a lowpass filter between the modem
and the telephone circuit.  Below is a schematic
for a simple low-pass filter using about $2 worth
of parts that should do the trick.

Caveats:

1.  It is illegal to connect unapproved electronics
to most European phone lines.  Potential fines range
up to over $5,000 in some jurisdictions, although
enforcement is thin.

2.  I offer no guarantee of the fitness of this
design, although it has worked well with my GVC
14.4 PCMCIA in both Germany and Switzerland.
Use it at your own risk.

3.  I have noticed slight degradation of voice quality
with this filter on some lines, although this is to be
expected in such a simple passive design.

4.  Newer classes of DSP modems (like the MWave) may
have the necessary filtering built into their firmware
for country-specific operation, although I have not been
able to confirm this.

              100 Ohm       100 Ohm
               ____           ____
La -------+---|____|----+----|____|---+------ a2
          |             |             |
          |             |             |
          | 100nF       | 220nF       | 100nF
        =====         =====         =====
          |             |             |
          |             |             |
          +-------------O-------------+
          |             |
          |             |
          -             |
         | |            |
         | |47 Ohm      |
         | |            |
          -             |
          |             |
          |             |
Lb -------+-------------+-------------------- b2


o means no physical connection, a cross over
+ means physical connection
La/Lb in-circuit, from line
a2/b2 out-circuit, to telephone/modem

Mounting ideas:

I soldered the parts onto a small (1cm x 2cm) bit
of perf-board and mounted the whole thing inside
a normal modular phone jack box (like the kind you
have on your wall), with two simple leads on the
output side for attaching to a local-style phone
plug or direct into the terminal box.  Serious road
warriors could probably squeeze the thing into an
even smaller housing, say putting it inline in a
modular male-female adaptor that they would want to
carry anyway.


******************************************************
Tim Vetter    vfr@stm537.ubszh.net.ch (MIME, NEXTMAIL)
              tim.vetter@ubs.com    (ASCII, alternate)
------------------------------------------------------
Forex & Fixed Income Trading Development
Union Bank of Switzerland
voice: (+41) 1/2358422   fax: (+41) 1/2355129
snail: LEIT/LITH-VFR Postfach 8021 Zurich, Switzerland
******************************************************