AMP & LS No. 12A

E5 F1012
Issue 1, Mar 1981

The Amplifier and Loudspeaker No. 12A, sales name HARMONY, is a loudspeaking telephone attachment which can be added to existing handset telephone installations, (except Internal Telephone System) to provide full, voice-switched, loudspeaking telephone facilities. Colours: dark brown, light brown and mid grey.

The circuitry of HARMONY is based on that of the DORIC (Amplifier and Loudspeaker unit No. 11A), which it will eventually replace. The case has been styled to be compatible with the Herald call connect system and the Ambassador telephone range.

The HARMONY incorporates a line connect relay which operates when the "on" button is pressed, this connects the unit across the line pair and disconnects the telephone instrument. The relay has an additional break unit which is brought out onto terminals T9 and T10 for use on C wire signalling PMBX extensions.

Controls on the unit consist of:-

1. Drum Wheel Volume Control To adjust the incoming speech level.

2. A red light emitting diode (LED) indicator, which is illuminated when the HARMONY is switched on.

3. Three Press Button Keys Their function from left to right is as follows:-
  • Mute Key - Marked M Depression of this key will mute the microphone such that the distant party cannot hear conversation taking place in the vicinity of the HARMONY. The key must be held depressed throughout the required duration.
  • On Key Depression of this key operates the line-connect relay which connects HARMONY to the line.
  • Off Key Depression of this key releases the line-connect play and restores the line to the telephone instrument.

The operating instructions for HARMONY are similar to those of DORIC except when used on C wire signalling PMBX extensions and are as follows:-

                 METHOD 1                        METHOD 2

Used when connected to:-		Used when  connected to:-
Plan extensions Nos. 2         		Plan extensions Nos. 1,
2A and 105/107 (main)          		1A, 4 and 105/107 (extn).
Keymasters 3 and 4.           		Exclusive and shared
                                        service exchange lines.
                                        Two wire and C wire
                                        signalling PBX extensions.
Pick up the telephone          		Pick up the telephone
handset and call the           		handset and call the
desired numbers                		desired number.
Press the "on" key. The		     	Press the "on" key. The
indicator will light   and     		indicator will light and
remain illuminated.            		remain illuminated.
The telephone set           		The telephone handset
will be disconnected but       		will be disconnected and
must not be replaced on        		may, if desired, be
its cradle.                    		replaced on its cradle.
Adjust the loudspeaker         		Adjust the loudspeaker
volume to the level you        		volume to the level you
prefer.                        		prefer.

The call must be answered      		Press the "on" key. The
normally by handset and        		indicator will light and
then transferred to the        		remain illuminated
unit.              			Adjust the loudspeaker volume 
					to the level you prefer.

the telephone dial or press button unit must be allowed to finish pulsing out
to line before the HARMONY "on" button is pressed.                          

           		METHOD 1                        METHOD 2
TRANSFERRING FROM    Press the "on" key. The       Press the  "on" key. The
HANDSET TO UNIT      indicator will light and      indicator  will light and
                     remain illuminated.           remain illuminated.
                     The telephone handset         The telephone handset may,
                     will be disconnected but      if desired, be replaced
                     must not be replaced on       on its cradle.
                     its cradle.                   Adjust the loudspeaker
                     Adjust the loudspeaker        volume to the level you
                     volume to the level you       prefer.

TRANSFERRING FROM    Press the "off" key.          Lift the telephone handset
UNIT TO HANDSET      Continue your conversation    from its cradle and then
                     using the telephone           press the "off" key.
                     handset. Replacing the        Continue your conversation
                     handset will terminate        using the telephone handset.
                     the call.                     Replacing the handset will
                                                   terminate the call.

The unit uses voice switching techniques to prevent acoustic feedback or howl when the apparatus is in use. The housing is designed to provide acoustic isolation between microphone, (which is an electret type situated behind the front edge of the unit) and loudspeaker, (situated under the grill to the rear of the unit). The unit possesses omni-directional characteristics, and functions without the user having to be situated close to the microphone.

The unit employs circuitry which virtually eliminates speech clipping in normal use and also has a degree of tolerance to constant background noise without switching permanently to the 'Send' mode. However, to achieve good performance the usual requirements for room acoustic conditions still apply.

Power for the HARMONY is provided by a transformer unit 17A which transforms mains voltage to 18v a.c.

The Transformer Unit 17A is a plug top type transformer for use in 13 amp 3 pin mains socket outlets and incorporates a 100mA input fuse.

Fuses for transformer units 17A are:-
20mm HRC ceramic tube type fuse, 100mA

Harmony loudspeaking unit

The Harmony Loudspeaking Unit (Amplifier & Loudspeaker 12A) is an add-on module giving loudspeaking telephone facilities when used with a handset telephone. It can be used alongside the associated telephone, or mechanically linked to other units in the BT house style - such as the Ambassador telephone and the Herald call-connect system terminals.

Developed from Doric
Harmony is derived from the Loudspeaking Telephone (LST) 8 (Accord) and its operation in loudspeaking mode is identical. One of the development aims was to obtain a better performance than the LST4. Also it was decided that it would be more cost-effective to use an existing design. if one could be found with suitable performance - rather than developing new loudspeaking circuitry. To this end, a licence was negotiated with Northern Telecom of Canada to use the design of their ‘Companion’ unit; a limited number of which were already marketed by BT as the ‘Doric’. So development effort was concentrated in re-engineering the circuit to fit into an Ambassador style case.

The circuitry is such that only two adjustments are needed during production. One ensures that the output voltage of the power supply is set to 1 2V, and the other is to balance the send and receive switching levels. It is not intended that these adjustments should be carried out in the field.

Improved operation
To prevent feed back from the loudspeaker to the microphone it is necessary, using present technology, to use voice-switching techniques to allow either the LST user to speak to the distant party or the distant party to speak to the LST user - but not both at the same time.
The direction of transmission is decided by a circuit which continuously monitors the signals received from the microphone and the telephone line. Priority is usually given to the loudest signal if both parties are speaking simultaneously, but after the direction of transmission is selected, a very loud signal in the other direction is required to interrupt the original party, except during pauses in speech. This feature is known as ‘switching hysteresis’ and is deliberately provided to prevent the unit from continually switching between channels under marginal conditions.

There are two circuit problems with voice-switching LST's. The first is the time taken to decide the direction of transmission and to switch the unit to that direction. This can result in the first part of a word being lost - known as clipping.
The second is that room noise tends to cause the unit to assume that the noise is a wanted signal and switches into its send direction. The distant party then has to speak very loudly in order to overcome the noise level - due to the hysteresis described - and the LST reverts to the send direction during pauses in the distant party’s speech, In extreme cases, the distant end user will be unable to overcome the noise at all.

Harmony attempts to improve these characteristics by using a technique known as ‘soft-switching’. The on-off channel switching is carried out by variable attenuators rather than electronic switches as in the LST4. The unit is designed so that there is always 40 dB total attenuation between the microphone and loudspeaker to prevent acoustic feedback. This attenuation is divided between the send and receive channels as required thus enabling the unit to have at least three states of voice switching:-

  • Send - where all the attenuation is in the receive channel
  • Receive - where all the attenuation is in the send channel
  • Centre - where the attenuation is equally divided between the send and receive channels

When neither party is speaking, the unit will be in the centre state. This helps to speed up the unit switching when one of the parties speaks as it is already halfway between the send and receive states. As neither channel is attenuated completely, the first part of the sentence is audible, although at reduced volume.

The room noise problem is improved by special circuitry which discriminates between speech and noise and performs two functions:-

  • If noise is present together with wanted speech at the LST microphone, the unit switches to send, but inserts an extra 10 dB attenuation in the send channel. This reduces the levels of both the noise and the speech sent to line. This may appear to be a disadvantage, but as a user tends to speak loudly in the presence of noise, compensating for the attenuation, the overall effect is to reduce the level of noise sent to line. thus maintaining the level of speech.
  • The second feature is that the unit ignores any room noise if there is no speech signal present - so overcoming the second problem previously described.

During the development of Harmony it was felt desirable to avoid the normal BT power units which were large and usually required fixing to a wall. The transformer unit 17A - a plug-mounted transformer providing 20 volts ac nominal at about 250mA was selected. Connexion to the power unit is by screw terminals which are shrouded for safety on the underside. Protection is provided by a standard replaceable mains fuse and a non-replaceable thermal fuse which operates if the internal temperature rises above 125 degrees C.

Taken from Maintenance News - No. 21 - Autumn/Winter 1982.
By Tim Keats BT Merlin/SE1 .1.2

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