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The end is near for analogue radio... radio gets very high

Like all technologies, AM radio had advantages and some problems. Today we have a brief look at the 1930s technology we still use today: Frequency Modulation on the Very High Frequency band.

Like all technologies, AM radio had advantages and some problem
published on UK Free TV

One interesting characteristic of radio signal that are transmitted in the Medium Waveband is that the signal is carried in two ways, both of which are useful.

One is called a "ground wave", which means the signal follows the curve of the Earth. This means that the receiver and transmitter do not need to have a line-of-sight. So it is possible for a medium wave transmitter can serve a radio set 500 miles or more away. For example the old BBC World Service transmitter at Orfordness on 1296 kHz at 1.9MW could serve Belgium, the Netherlands, Luxembourg, north-east France and north-west Germany.

Another feature of Medium Waves that, at night, they also make "sky waves". After dark, the signals reflect off ionosphere (if the mid-layer ionization is strong) like a mirror. So, for example, Orfordness could be received in much of Europe at night. For this reason, AM radio station have to lower their power output after dark. Long Wave radio signals do not reflect.

The benefit to broadcasters of operating a low number of medium wave transmitter sites is offset against the need to guard for interference from other broadcasters long distances away. Even low-power stations daytime stations can become the source of interference many times their usual coverage area.

To this end, there has to be international co-ordination of frequencies and power output levels for Medium Wave and Long Wave. The agreements put stations at 9 kHz apart from each other. Between 531 kHz and 1611 kHz there are 118 "slots", 75 of which are used in some way in the UK. Some of these are for national "single frequency networks", such as Absolute Radio 1215 kHz and sometimes for a single fill-in station such as BBC Radio Scotland (AM) 585 kHz at 1.2kW at Dumfries.



The BBC gets high

The limitations of Medium Wave transmission didn't just limit the number of stations that could be broadcast. Spacing stations 9kHz also means that the there was a limit to the quality of sound, as human hearing ranges up to 2kHz, and the signals using AM were mono. Whilst the 9kHz range is fine for listening to a single human voice (Voice Fundamentals - Human Speech Frequency), it isn't very good for the dynamicfrequency range of music.

Frequency Modulation was patented in 1933, but the BBC only started using the system for broadcasting radio in 1955. However this time the Very High Frequency range was used for transmission (from 88.0 to 94.6 MHz).

The FM provides a much better bandwidth - the mono signal goes up to 15 kHz. The system has also been enhanced by the addition of by two stereo "difference" signals, both 15 kHz wide, but this addition is susceptible to noise and multipath distortion and FM radios will flip back to mono when the signal is weak.

The benefit to the quality and reliability of an FM radio signal are clear, but the broadcaster has to provide a large network of lower power transmitters. For example, Radio 1 FM has 214 FM transmitters, whereas TalkSport, using the old Radio 1 MW network, has 29 AM transmitters.



However, the smaller wavelength of higher frequency transmitters means that FM transmitters can be mounted onto existing transmitter masts (such as those used for UHF television or mobile phone masts) and multiple services can be provided from the same location.

However, FM does have drawbacks. The initial design of the networks assumed that a rooftop aerial and home listening would occur. Most masts were changed to "mixed polarization" to allow for mobile and in-car listening.

FM took off when commercial radio started in the UK, and now most analogue listening is to VHF.

The benefits and drawbacks of FM

FM radio is seen by some as a "gold standard" of broadcasting.

It is certainly true that compared to AM, with it "spooky sounds".



The CD is a good measure reference: with two-channel 16-bit PCM encoding at a 44.1 kHz sampling rate per channel, and thus a dynamic frequency range of 22.1kHz matching the best human ears - see Frequency Range of Human Hearing.

FM/VHF is limited to 15 kHz, and the stereo signal is provided in two subcarriers, rather than as two independent signals (known as "joint stereo"). If the FM signal is weak the receiver will switch to the mono signal.

The addition of the RDS system FM transmission and to car radios means that modern in-car tuners can, using a second tuner, test all the possible reception frequencies for the channel being listen to and automatically switch to the best. This prevents frequent re-tuning whilst driving.

However, on main drawback remains: there are a limited number of national stations you can put out on FM. Radio 2 uses 88.1 to 90.2, which is 22 "slots", Radio 3 from 90.2 to 92.6, Radio 4 FM from 92.5 to 96.1 (as well as some of 103.5 to 104.9), Radio 1 from 97.1-99.8, Classic FM from 99.9 to 101.9, with a host of local stations using 96.2-97.0 and 102-107.9MHz.

You can explore the frequencies for yourself here:

List of all analogue radio frequencies - ukfree.tv

In the modern world, this limited choice has found solutions in digital radio.

More tomorrow in Part 3, where we look at the development of satellite, Freeview, online and DAB radios.

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Comments
Thursday, 29 August 2013
Briantist
sentiment_very_satisfiedOwner

10:51 PM

Wikipedia says "Notionally, therefore, Baldock [meaning I think Babcock International Group] still retains the right to transmit from Orfordness on the other frequency (1296 kHz)." Orfordness transmitting station - Wikipedia, the free encyclopedia

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Briantist's 38,844 posts GB
Saturday, 31 August 2013
Tracy
8:27 PM
Ipswich

"Babcock International Group still retains the right to transmit from Orfordness on the other frequency (1296 kHz)."
Yes they still do. Someone I know who works for them was test signaling from the site in late July 2013.

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Tracy's 9 posts GB
Sunday, 1 September 2013
M
MikeG
sentiment_satisfiedBronze

8:39 AM

I don't think any government would want to alienate most of the population by doing away with analogue FM broadcasting. They've been talking about it for years but always delayed making the decision. For a start there are estimated to be over 9 MILLION car radios that would need replacing, most hi-fi systems in homes and most mobile phones that have FM radio incorporated. It would be a mammoth task and extremely unpopular. If any government wanted to loose an election it would be a sure way of doing it. Yes, they'd like us to switch over to DAB but it's expensive and most people do not see the advantage; it's been very slow on the uptake and I can't personally see it really taking off in the next few years. If it were to happen the listening figures would plummet as most would not bother to upgrade and would give up listening to radio and just listen to their recorded music on the various equipment used today. The other point being that it's not suitable for car radios as the signal would be constantly dropping out instead of the lesser problem of dropping to a noisy or mono reception as it does sometimes with FM today.
Mike (Retired radio and Electronics Engineer).

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MikeG's 31 posts GB
D
Doug
12:51 PM

DAB is not the answer - system is ridiculously power hungry sets eat batteries - less than 8 hours from a new set is just not on! And quality is so poor compared to FM.
Hope Government leave our FM alone and scrap the expensive DAB campaign - they must know that even their propaganda will do nothing to improve DAB audio quality no matter how much they push it!
Leave our FM alone!

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Doug's 1 post GB
D
david
sentiment_satisfiedBronze

5:11 PM

dab is not the way fm sounds far better. and write off am if cheap drm radio become available iwill give mw and lw anew life drm plus could be used on the fm bands test are being done in mainland europe

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david's 63 posts GB
Monday, 2 September 2013
R
Richard Davis
sentiment_satisfiedBronze

6:28 PM

Hi, Brian -
Can I, with all respect, correct an error in the terminology you use? At one point you say "Whilst the 9kHz range is fine for listening to a single human voice, it isn't very good for the dynamic range of music." In another place you talk about "...a dynamic range of 22.1kHz matching the best human ears." You've used the wrong term here - "dynamic range" means the volume range of the signal, and is measured in decibels! What you actually mean in these cases is the audio frequency response or audio frequency range.




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Richard Davis's 26 posts GB
Briantist
sentiment_very_satisfiedOwner

7:01 PM

Richard Davis: Sorry, yes, I meant "Hearing range", rather than dynamic range.

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Briantist's 38,844 posts GB
MikeP
sentiment_very_satisfiedPlatinum

8:17 PM

It is perhaps worth bearing in mind that the hearing range of the human ear is not all that matters. Listening to a pipe organ through either an analogue or digital recording medium reduces the overall range of frequencies conveyed to the ear. A typical CD will limit the reproduced frequency range to approximately half the sampling rate, commonly 44.1 KHz so the limit becomes about 22 KHz. A very good analogue recording may reproduce frequencies up to around 30 KHz on good equipment. A 'live' performance of the pipe organ contains fundamental frequencies above 100 KHz (measured with a spectrum analyser) plus several 'overtones'. The ear cannot hear the actual sounds at that end of the audio spectrum - but the listener can detect the 'shaping' effect on the sound heard as a difference in tonal perception. Often, when listening to pipe organ music on any digital medium I miss the 'breathiness' typical of a pipe organ that is not reproduced by most audio equipment, hence I prefer live performances - my personal choice.

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MikeP's 3,056 posts GB
Briantist
sentiment_very_satisfiedOwner

9:58 PM

MikeP: You are correct, the limit frequency for digital record is known as the Nyquist limit. It's half the sample rate, so it's 22.05kHz for a standard CD.

I'm interested to know what recording equipment you are using that will store and reproduce up to 30kHz.

It is argued (and I believe I have seen demonstrated to my satisfaction) that there is no need to record frequencies above 22kHz as the recording device will be able to record the harmonics that will be within the hearing range of humans and provide the listening experience.

Of course, when you are at a live event, be it an organ or sound system, you will be experiencing a lot of hearing by bone conduction - Bone conduction - Wikipedia, the free encyclopedia - but this tends to enhance bass frequencies, rather than the top end.

I spent a lot of childhood listening to reporductions on QUAD ESL electrostatic speakers, as my godfather was employed there.

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Briantist's 38,844 posts GB
Tuesday, 3 September 2013
MikeP
sentiment_very_satisfiedPlatinum

10:25 PM

Briantist:
I have sung with choirs and choral societies since the age of 11 (57 years now!) and there is a distinct difference in the audible experience of a live concert and anything recorded. The best recording system I am aware of, and heard, is a direct cut Deutsche Gramaphon vinyl recording where the groove was cut directly into the surface of the disc, rather than being pressed as with a 'standard' vinyl record. Of course you had to use really good quality HiFi equipment to get the full effect on playback. The discs are very expensive so quite unusual and rather unique. They are no longer made, sadly for HiFi buffs.
The point is that whilst the human ear does not hear fundementals above around 30 KHz for young people, and substantially less as we get older, the experience of the tonal qualities of the sound are shaped by the overtones that we do not directly hear. Imagine you see a square wave on an oscilloscope, you can't hear the 'attack' but can see it on the waveform, the square wave being made up of numerous upper harmonics. An audio signal can have a similar 'attack' front but the ear cannot pick up the harmonics that cause the sharpness but the sensation of the suddenness of the start is perceived if the overtones or harmonics are present, but if they are not there (as they are above the Nyquist frequency for the reproduction system) then there is some loss of fidelity. Many tests have been made on the hearing and perception of musicians as compared to non-musicians, the results are interesting in that many people do not appear to hear the same as a musician does - or so it is said.
For my part, I can tell the difference between performing with live music and with recorded music, I personally prefer live instruments and dislike many CDs because of the lack of 'presence' in the reproduced sound.
(My background is with electronics, including radio, TV, Video, Audio recording systems, Satellite systems and communications electronics, backed up with higher degrees in electronics and physics. Plus experience with EDA systems when working in an international software company).

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MikeP's 3,056 posts GB
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