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Change History
Version Date Comment
0.1 28-JUL-2000 First draft
1.0 14-SEP-2000 Released To Glen
1.1 29-JAN-2002 Released To Web
05-OCT-2019: Web friendly updates
2. Introduction
This document summarises the workings of the
Radio Glen TBU prototype. The TBU was constructed after a
request from Radio Glen for a cheaper version of the standard
Sonifex dual line TBUs which sell for about £1400. The deal was
to make a dual TBU for £300 and Radio Glen would throw in their
spare 19" bay-frame in return for having nice illuminated
switches on the front.
3. General Description
The heart(s) of the unit are the ETAL P3400
line modules available from Farnell. These modules mop-up all of
the line interface functions like ring detection, physical
hybrid, on-off hook handling etc. This means that the TBU as a
whole would probably pass PSTN type approval testing if it was
subjected to it. Unfortunately these modules are no longer
manufactured and future versions will require more design effort
in the line interface area. The unit has two identical halves
for two telephone lines. It is suggested that in the Radio Glen
application, plug-in splitters are used at the phone sockets to
accommodate both the TBU and desk telephones. In this way,
either the TBU or the desk phones can be used to pick up the
lines when a call is incoming.
3.1. Operation
Each line has an illuminated PICK and DROP
switch. When the mains power is on and the phone lines are
attached, the unit should be in the idle state with both of the
green DROP lamps lit. When a call is incoming the PICK lamp on
the appropriate line will flash. The call can be answered by
pushing the flashing PICK lamp. The DROP lamp will go out and
the PICK lamp will light to show that the TBU has taken the line
off-hook and is sending audio to and from the line. The call can
progress via the mixing desk and then the line can be dropped by
hitting the DROP switch.
If the TBU needs to be placed in a technical cupboard, then it can be fully remote controlled via a 25-way male D connector on the rear. All of the front panel switch connections are available at this connector and momentarily closing the relevant contact is equivalent to a momentary push of the front panel switch. The lamp drive connections are also available on this connector and each output can provide 100mA of 12V external lamp drive. Isolated closing contact outputs are available which mimic the operation of the lamps in the front panel switches. Normally open and normally closed contacts are provided. These can be used to drive triac switching units for mains powered annunciator lamps, which may be particularly useful for ring indicators.
Hybrid crosstalk cancellation is provided by
simple analogue subtraction circuitry with an additional
phase-shift system to improve the cancellation up to broadcast
standard.
3.2. Circuits
3.2.1. TBU01-01
This sheet has the regulators and connectors to the external
switches and panel connectors. 12V is chosen as the supply
voltage as this can drive the relays and op-amps and happens to
be the point where the regulators can just remain out of
drop-out with the particular mains transformer used. This
minimises dissipation in the regulators.
3.2.2. TBU01-02
This sheet shows the P3400 line interface
modules and the relays which control the front panel lamps and
switching. A latching relay is used to control the on-hook
off-hook state.
3.2.3. TBU01-03
This sheet shows the differential amplifiers
which take the signal from the RX side of the hybrid, and the
differential amps which accept the mixer TX output for sending
to the TX side of the hybrid. The TX signal is bandpass filtered
with -3dB points of 200Hz and 3500Hz at -12dB/octave. This
avoids sending unecessarily wide audio to the telephone line and
makes cancellation of the TX crosstalk easier.
RX refers to the signal being received from the
far end of the phone line and sent to the studio mixer. TX
refers to the signal going from the studio mixer to the phone
line. Because of the imperfect nature of hybrids and phone
lines, the RX signal typically has a roughly equal amount of the
TX signal present due to crosstalk. This TX crosstalk is
cancelled by subtracting the known TX signal from the RX signal.
Measurements have shown that the TX crosstalk signal coming out
of the RX side of the hybrid is also slightly phase shifted from
the original TX signal. So an equivalent phase shift is added to
the signal that is used to cancel the TX crosstalk signal. The
cancellation amplitude and phase shift are adjustable on preset
pots. To adjust these pots, the TBU has to be put into a call.
The receiver of the phone that is dialled up may be simply left
on a cushion to block the mouthpiece. A 1KHz sine is sent to the
TX from the mixer and the amount of this sine on the RX is
measured with an oscilloscope. The amplitude pot is adjusted
first, to minimise the level of the crosstalk signal. Then the
phase pot is adjusted to minimise the crosstalk signal. There is
not much interaction between the two pots but it may be of
benefit to readjust the amplitude pot and then the phase pot
again.
This adjustment should ideally be done when the
TBU is connected to the actual line to be used in the field. In
practice however, the difference between phone lines has a
minimal impact and no adjustment should be needed.
3.2.4. TBU01-04
The RX signal from the cancellation circuitry
is passed on to a bandpass filter which rolls off at 12dB/octave
with -3dB fo at 200Hz and 3500Hz. This provides for the full
available telephone bandwidth while providing some cut-off of
hum and high frequency noise that may be present. A differential
driver output provides the balanced signal for the studio mixer.
This output can drive an ordinary high impedance balanced input
but will struggle if a true 600R balanced load is applied. The
DG444 analogue switch cuts off the RX signal to the mixer when
the TBU is on-hook. This cut-off of the RX audio prevents a
feedback path from TX to RX which can occur when in the on-hook
state. The switch drive signal is effectively 12V logic and
comes from the latching relay which controls the hook state.
4. Remote Control Pin Assignments
Remote Connector Pin Assignments | |
Pin | Function |
1 | line1 pick annunciator pole |
2 | line1 pick annunciator normally open |
3 | line1 pick annunciator nomally closed |
4 | line1 1 drop annunciator pole |
5 | line1 drop annunciator normally open |
6 | line1 drop annunciator normally closed |
7 | line1 remote pick switch (closing contact to GND) |
8 | line1 remote pick lamp (100mA max to GND) |
9 | line1 remote drop switch (closing contact to GND) |
10 | line1 remote drop lamp (100mA max to GND) |
11 | GND |
12 | GND |
13 | not connected |
14 | line 2 pick annunciator pole |
15 | line 2 pick annunciator normally open |
16 | line 2 pick annunciator nomally closed |
17 | line 2 drop annunciator pole |
18 | line 2 drop annunciator normally open |
19 | line 2 drop annunciator normally closed |
20 | line 2 remote pick switch (closing contact to GND) |
21 | line 2 remote pick lamp (100mA max to GND) |
22 | line 2 remote drop switch (closing contact to GND) |
23 | line 2 remote drop lamp (100mA max to GND) |
24 | GND |
25 | GND |
5. Schematics
5.1. Connectors and PSU
tbu01-01.sch
5.2. ETAL Telephone Line Interface Modules tbu01-02.sch
5.3. Hybrid Crosstalk Cancellation tbu01-03.sch
5.4. Filters and Audio I/O tbu01-04.sch
6. Pictures
6.1. Twin Line
Telebalance Unit Rear Panel Picture
6.2. Twin Line Telebalance Front Panel With Illuminated Push Switches
6.3. Telebalance Circuit Boards Insides
6.4. Installed In Radio Glen Studio 1
Complete with high tech battery powered Levell oscillator.
I recently saw a circuit in
Electronics Design News which showed a rather cunning way to
improve the crosstalk cancellation despite variations in the
line impedance. It used a ladder network in front of the hybrid
which was similar to an RF isolator used to isolate RF PAs from
changing antenna impedances. The trade off is that you need to
drive more signal into the isolator to overcome the losses in
the resistor network, but power is not in short supply in a
mains powered device so this could be a very useful trick.
The ETAL modules are long since gone
and had a nasty habit of failing for no reason after some years
in service. This should not stop you from being a bit
brave and adapting a cheap desk 'phone bought from Argos though,
as long as all the audio is electrically isolated using decent
telecoms grade isolating transformers and telecoms relays for
hook control and ring signal detection. Unless you have
£4000 already, in which case just buy one. Or you could
modify a mobile phone, which no-doubt has its own interesting
elephant traps to look out for. But I wouldn't know
anything about that, right? :)