On Wed, 04 Feb 2015 18:35:17 GMT, Paul Ratcliffe
<ab...@orac12.clara34.co56.uk78> wrote:
>>>Some stations use in band signalling, but using sub-audible tones
>>
>> Sub audible tones?, do tell never heard of those in Broadcast systems.
>
>Something like 20Hz in the S signal if my memory serves (increasingly
>unlikely with time).
I thought I vaguely remembered some details of a system used in the
early days of the ILR stations, so I looked it up.
There's an article in IBA Technical Review No 5 that explains it. To
save an extra programme circuit that would only occasionally be
required, the transmitter could be queried about its operating
parameters via an in-band signal that would be sent to it via the
programme path, and its reply would be sent via the transmission.
The query signal would inevitably be transmitted too of course, so for
a query from the studio centre, there would be two burst of data, the
query and its reply, about 15 seconds apart, although the transmitter
could also issue a signal spontaneously in the case of an alarm of
some sort, in which case there would only be one.
For FM transmitters, the carrier signal was at 14kHz, and for AM it
was 4.7kHz. The level for FM was -34dBm, though I couldn't find any
mention of what it was for AM. Otherwise the signalling system was the
same for both; a burst of about half a second contained an eight bit
word by phase modulating the carrier +/- 36 deg, and for the duration
of the burst an appropriate steep notch filter was inserted in the
audio feed. The intention was that nobody would notice, though I
recall hearing them quite clearly, even in the car. I suspect the
notch filter did most of the damage. I still find it astonishing that
a system like this could ever have been proposed, never mind accepted,
by people working in any business related to broadcast audio.
I think they must do it differently nowadays.
Rod.