AMPLIFIER & LOUDSPEAKER No. 12A
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. MAKING A CALL 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. ANSWERING A CALL 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. 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.
Diagrams - N4446 and SA10224.
Drawing - 94494.
Sales Brochure - PH2870.
Harmony loudspeaking unit
The Harmony Loudspeaking Unit (Amplifier & Loudspeaker
No. 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.
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.
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:-
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:-
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.
Last revised: April 26, 2023