whats the fuss about FM?

[Graham.]

Ambient noise related volume would be good, but the the cars I have owned recently seem to use the speedo.

[Graham.]

Fine sets these Ferguson's. Did Previn steal that slogan from you?

[Chris Holmes]

Hi All,

I have just been given an HMV 1251 AC 7 Valve Superhet AM FM set. By appointment to The Late King no less!

I have so far invented what I call an ?IPhone Dock? by way of its Gram input. But can?t get anything on FM via it?s internal aerial. I?ve not tried an external yet.

It got me wondering, what was the USP of FM back in the 50s? In the 70s at least in London, there was stereo, but was mono FM a more stable signal? Or was it a question of what stations were available?

Or were the government going to phase out AM in a few years the same way they were going to phase out FM ?shortly? about a decade ago?

As my learned colleague Jim Butterworth only has older sets and these are AM only, I assume it was new fangled around 1955??

[Mark Carver]

The USP of FM was Hi-Fi quality, although until the poor quality GPO landlines to the transmitters were replaced with PCM links in the early

70s, the further you were from London, the worse the quality.

Ironically the technical quality of 70s/80s FM broadcasts surpassed what we have today on FM. That's progress apparantly

[The Natural Philosopher]

It was down to one thing and one thing only. Quality

A good FM signal and aerial nets you around 60dB S/N and 18kHz bandwidth compared with AM at 50dB and at best 4Khz bandwidth,

When I were a lad in the 50s and 1960s there were just three channels. The Home Service, The Light Programme and the Third Programme.

[Tim Lamb]

And Radio Luxembourg on 208 was it?

>

[Brian Gaff (Sofa]

FM there is less Interference from AM impulse noise as the signal saturates the receiver as all you want is to detect the frequency deviation. That means of course that mono is very good and the bandwidth is much wider, around 75Khz rather than the 9Khz of AM. True High quality. Unfortunately, in order to make stereo, the advantages tended to be lost on weaker signals. In the way it is done you modulate the main FM bandwidth with mono, but send a weaker AM difference signal via an am carrier piggy backed on this so to speak, its still FM but the detection of the AM is a secondary process, and is quite noisy. Due to this you need more than ten times the signal to get the same signal to noise on stereo, also the hiss and other interference tends to be out of phase on each channel. This is due to the matrix used to rebuild the stereo from Mono and the difference signal. Purist will no doubt be along to tell me the carrier is suppressed and that a pilot tone above hearing at about 19Khz is used to turn on and off the stereo decoder, which seems never to happen these days.

Brian

[Brian Gaff (Sofa]

Yes its all down to optimod and its various spin offs, At one point Capital radio experimented with Dolby B to reduce the his, but in the end as take up of the standard on FM was slow they went down the compression route just like everyone else and the result is a fixed level ear hurting mess we all get now on any radio station and delivery system. Unfortunately with just a few exceptions dynamic range on CDs and DVDs has also been compromised in favour of punchy bass and hardly any dynamic range at all. The missed chance of Digital is that you could easily have sent the uncompressed signals via it and done any processing in the radio itself during the decoding and have it under the listeners control, so portable and car use you would up the compression and for home use turn it off completely. It was not done this way however, for whatever reason, probably laziness and so as not to have to generate a clean feed ad an am fm one. Brian

[Nick Odell]

<snip>

<snip>

Personal experience suggests mono FM sounds better when the signal strength isn't too good. I have one or two wireless sets with switchable mono/stereo FM and they sound much clearer when receiving a weaker signal if the stereo decoder is switched off.

Nick

[Brian Gaff (Sofa]

Unfortunately, now there are so many fm stations on the band its actually hard to pick them up without interference. In my view there are probably five times more stations on than can be accommodated under flat conditions. particularly in London. Also of course DAB and streaming systems has some awful examples of low bit rate and poor lossy compression in use generally making listening a chore rather than a pleasure. Either the whole world is deaf or nobody has ever heard good quality audio, whether it be the spoken word or music.

Brian

[Brian Gaff (Sofa]

Yes radio Buxom Girl as we called it could only broadcast after dark as it needed the ionosphere reflection to work, but the fading in and out was pretty dire It was the only commercial radio we had at the time.

Then cam the pirate ships of course. Brian

[Brian Gaff (Sofa]

Philips used to make a set that had a sliding stereo separation gauged on the signal strength. It was used ion early car stereos. There was a circuit for it published as I recall. Sadly people found the ever changing stereo image width a bit off-putting!

Brian

[Tim Streater]

And the cops. We used to listen to the Met transmitting to pleece cars. You couldn't hear the cars themselves except rarely when MP would organise a car to car but it was interesting nonetheless.

[Mark Carver]

That's right. It's because the stereo difference channels are quadrature modulated on a 38kHz suppressed carrier, within the 'baseband' FM signal. FM has a triangular noise spectrum, so the higher the modulating frequency, the more it is affected by noise.

Switching to mono quenches the difference channels, and you end up with mono, but with reduced noise.

In recent decades receivers have gently and gradually  suppressed the difference component as the signal deteriorates, so you get a graceful blend back to mono as the signal becomes weaker

[Mark Carver]

    Some bedtime reading for you. The BBC's appraisal in 1963 of the Zenith-GE system (now used worldwide) for FM Stereo broadcasting

[charles]

and Radio Luxemburg in the evenings ;-)

[Paul]

I don't think you'll be getting stereo. Just the one speaker in the cabinet.

Paul

[Nick Odell]

Interesting emphasis there on the problems generated by automobile ignition systems. I imagine that got better over the intervening years but I wonder whether all-electric vehicles have improved things even further or whether they are now regarded by serious listeners as mobile Powerline Adaptors on wheels?

Nick

[Clive Arthur]

IME the fibreglass body of a hired Reliant Robin made the radio unusable.

[Pamela]

Your reference to ten times the signal might explain why my Quad FM2 tuner back in the 70s was so terrible at reception. I don't think I ever listened to a full radio programme on it the whole time I had it. It would seem to drift off tune and become noisy and hissy and I blamed the design but maybe it was just the signal varying and causing those strange effects.