VHF noise

[kellymarie]

I have been re listening to some old recordings myself and a friend made of various VHF FM broadcast stations some of these go back to 1974 my recordings are from the 80s one curiouse thing I notice in these recordings there is a lot of electrical interference evident some from unsupressed appliances some from car ignition systems. I've noticed that modern receivers don't seem to suffer from this what do they do now to stop interference from affecting FM reception? I realise noise levels of impulse interference are a lot less now but even so there is still some about

[John_BS]

I think there are a couple of factors, both working to improve matters.
1. More modern FM receivers with IC limiters etc offer much improved AM rejection over discrete valve/transistor IF's.

2. The EMC directive effectively prohibits gross spectral pullution from electrical devices: its introduction certainly raised awareness of such matters and there's not much excuse now for causing real harm.

It is a matter of fact that if you read right back to the 1920's (through to the 50's and later) there was always a preoccupation with ignition interference. It must have been a significant issue. The BBC, when conducting early research on FM (late 40's /early fifties), set up an experiment from Wrotham to compare wideband AM with FM (mono of course). FM was chosen with interference rejection as a major factor.
John

[G6Tanuki]

Better limiting in the IF stages is part of the story: the coming of ICs specially designed for FM reception, which included multi-stage high gain limiting-amplifiers - made the elimination of impulse interference a lot easier than it was in the days of AF117s and the like.

Also, the early receivers using tuned-circuits in the IF stages could suffer with passband-ripple which led to group-delay issues leading to AM-interference susceptibility: later ceramic 'block' filters could have this designed-out more easily.

[kellymarie]

Thanks for your replys I think I understand now its a perfect example of improvements in technology really improving things at a basic level. I checked band 2 in AM mode there's not much impulse noise just a few random clicks which is good its definitely a LOT better than it was years ago

[Graham G3ZVT]

There used to be a lot of AM stations, police, fire brigade, and general PMR above about 100MHz and below the Light Programme on the VHF/FM band.

It was the mark of a good tuner, one with lots of IF limiting and a well balanced discriminator, that these signals produced little or no audio particularly when the signals were strong.

[Restoration73]

If the later recordings are from a Stereo FM transmission (which uses a form of AM to
derive the two channels) then the signal to noise may be inferior to a Mono transmission.

There was a brief version of Dolby FM noise reduction, but this did not become popular
because of the expense of incorporation in new sets or retrofitting to old.

[kellymarie]

Hi Rambo yes I remember the police around 100 Megacycles my dad used to listen to them. I also remember trying to tune a radio to radio 2 for an elderly relative and hearing some strange tones and once a female voice adking someone when they wanted collecting she called him sir so I'm guessing it was a business radio system I was hearing. The recordings I have were a short lived attempt to hear the chart show in stereo on FM reception was very hissy I gave up after a week or 2 and went back to sticking to radio 1 on mediumwave

[rontech]

I remember our first TV in Sep 1956 ( EKCO 17" ) gave very crisp pictures but seemed quite susceptible to car ignition interference. Showers of 1/4" white dots running across the screen.

[John_BS]

the signal to noise may be inferior to a Mono transmission.

Indeed: theoretically it's c. 20dB (rounded) , but this is only valid when the audio s/n is limted by RF signal level. Tt some point the tuner reaches backstop s/n ratio, where performanc eis limted by ether LO phase noise or the s/n ratio of the demodulator/decoder etc. This will normally happen at around 3mV RF input level for a decent tuner.

Regarding white-spot ignition interference: this was the reason the 625 line TV systems were specified with inverted modulation; you then got black dots, which are much less visible!

John

[kellymarie]

My nan lived on a main road near Fareham she had a 405 line TV on an indoor areal and I remember sometimes the picture was all but obliterated by white spots on the picture and that oh so annoying crackling whine on the sound. Great improvements were obvious when she got a UHF TV. Incidents I also remember a sort of rolling bars effect when helicopters from the navy base flew over not sure why that happened