Mansfield radio station hit by 'winker' song hijacker

Discuss UK-based Radio outside the South East of England
e.g. Birmingham, Bristol, Leeds & Manchester
'Legal' radio topics often go here too.
Giggles

Re: Mansfield radio station hit by 'winker' song hijacker

Post by Giggles » Sat Aug 05, 2017 11:50 pm

16alpha wrote:
Thu Aug 03, 2017 6:31 pm
You are wrong Albert,the synth for the VHF was a commercialy built unit,if I recall it was a Taylor unit,however I can check cos I have it.
Albert wrong! This isnt gonna go well! :)

Albert H
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Re: Mansfield radio station hit by 'winker' song hijacker

Post by Albert H » Sun Aug 06, 2017 3:27 am

16alpha wrote:
Fri Aug 04, 2017 3:45 pm
What I was referring to was the hijacking of a VHF tx on the I.O.W didn't know about the tele thing,and there is a security tone with the audio link to the VHF transmitters in the high KHz area.,I would be somewhat sceptical about a 25hz tone as to remove it would be exceedingly difficult particularly back in the 70s or 80s.
There were two control; tones used - the 25Hz was the original, and was removed from the audio path at the receiving end by a critically tuned notch filter (a twin-T type) based on a 4558 op-amp, followed by the 70Hz high-pass filter that was used to reduce hum modulation. There was a further 100 Hz notch to get rid of the first harmonic of the mains, though that was usually jumpered out. I can draw the circuits from memory, since they were an early project for me at Motspur Park! The rejection of the 25Hz was of the order of -80dB, so it was in the noise floor of the modulator. The 25Hz was detected with an NE567 tone decoder IC, and it took several seconds for the output to switch since the bandwidth was set to be very critical to prevent "chatter" due to programme content.

The other control tone sometimes used was at 57kHz (this was pre-RDS) and was derived from the 19kHz stereo pilot with a crude CMOS PLL. This was another Motspur Park project, and also used the NE567 tone decoder IC at the receiving end. The 25Hz and 57kHz control tones were ANDed together, so both had to be present to keep the rig on the air. There were also RSSI triggers for when RBR was being used (rather than the PO lines), which gave rise to some interesting problems with BBC transmitters repeating French, Belgian, Dutch and even Spanish stations during "lift" conditions! This was a fairly frequent problem in the summer months when the PO lines were taken out by lightning and the high pressure gave us good continental reception - the Re-Broadcast Receivers would happily feed Tacolneston with BRT from Belgium, for example.

Other approaches were used too - the 19kHz pilot was sent from the studio end on some of the the local stations, and this was used to switch the output rig.

The introduction of PCM digital distribution removed the need for any of the control-tone trickery. The system that was developed used 16-bit words, with 13 bits used for the PCM audio and the other three bits used for control purposes, In some instances, it was possible to change the mode of operation of the transmitter - mono / stereo, high or low power - and some of the medium wave transmitters used reduced power at night to reduce interference to co-channel continental stations. The transmitter sites sometimes included rudimentary telemetry in their outputs, and most sites had telephone auto-dialler alarm systems with pre-recorded tape messages (some of which were hilarious, since they were recorded by the engineers). In the event of failures or unauthorised entry to the site, the auto-dialler would call a number at Broadcasting House in London and report the issue.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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