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In this section we have tried to present a more detailed description of
the technical facilities at Broadcasting House in 1932 than we have given
on the earlier pages. Our main source of information is "A Technical Description
of Broadcasting House" published by the BBC at the time. Some extra details
have been gleaned from Edward Pawley's "BBC Engineering 1922-1972"
(BBC, 1972), the 1933 BBC Handbook and F. C. Brooker's 1942 Engineering
Division Training Manual. The drawings have been reconstructed from originals
in the Technical Description book. Some of the photos from the main part
of the site reappear here, where it seems appropriate.
 It
has not been possible to work out how every part of the system operated.
In some areas there is a total lack of information, in others we find
contradictions. But hopefully we have presented here as accurate a description
as is possible today, at least until more detailed contemporary sources
can be found.
We'll begin with the technical heart of the building - the Control Room.
The photo, left, was taken from a position near the bottom of the
general layout drawing, below.
It's important to remember that this really was a 'control' room. Many
of the studios had just a single microphone the output of which was wired
directly to the control room. In these, and indeed in most other cases,
the control of level was entirely done at the control positions in CR.
Studios were not equipped with programme meters. Those studios with more
than one microphone had a simple passive mixer installed in a 'listening
room' which also mixed in the output from gramophones where these were
provided. If there was a 'silence' or 'announcing room' then the output
from the mixer was fed through the announcer's desk where he could switch
(not cross-fade, it seems) between the studio microphone and his microphone
for announcements. The output of this desk was then fed to CR, and was
there routed through a control position.
There were a further two types of mixer, one for music programmes, the
other for drama. These were located remotely from the studios, and allowed
the operators to monitor the programme on loud-speakers - not possible
in the Control Room, where headphones were the order of the day.
Just two types of microphones were commonly in use,
the first BBC ribbon microphone not being introduced until 1935. There
was the Marconi-Reisz carbon microphone and the bomb-shaped Western Electric
condenser microphone, which, because of its low output, contained a preamplifier
within its housing.  The outputs from the studios were nominally minus
68dBu, and were wired to the control room in lead-covered cable. This
photo shows the mic cables running in vertical chases formed in the outside
walls of the studio tower.
Apart from the preamplifier built into the condenser
microphones and mains-powered loudspeaker amplifiers, the Control Room
housed all the amplifiers necessary to the broadcast chain. There seems
to have been three main reasons for this:
- The system had been 'tried and tested' at Savoy Hill
- The amplifiers were only switched on when needed as the valves used had a limited life
- All the amplifiers were powered from storage batteries as mains powering wasn't considered
reliable.
Powering from storage batteries was without doubt the major reason since the amplifiers required:
- 6 volts for the valve heaters
- 300 volts for the high tension (HT) supply
- 3 grid-bias supplies of 2, 12 and 24 volts
- 4 volt filament supply for the 'A' amplifiers only which were indirectly
heated to reduce valve noise and to minimise interference caused by
switching other amplifiers fed from the same battery.
Distributing that lot all round the building would not have been sensible. Even so 300 volts
and 8 volts had to be provided from CR to studios for the preamplifier of the condenser microphones.
The 8-volt supply was also needed for polarising the Marconi-Reisz carbon microphones.
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