Radio Glen TBU Model X

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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

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

6.5 Under Test At Home

Complete with high tech battery powered Levell oscillator.

Post Scripts

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.

2019 Update

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? :)

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