(12 September 1995)
Norman McLeod
Apart from my weekly dissertation here, I also write for various other publications, including one of the leading hi-fi mags.
Over the years I have carried out technical tests on a large number of radio tuners, from the cheap and cheerful to the exotic and expensive. Unlike other hi-fi reviewers, I make a point of measuring the frequency response, distortion and noise from the AM section of the apparatus.
The conclusion I draw from my measurements is that almost all commercially-available product is a disgrace when it comes to reproducing AM radio. The AM sections is even quite fancy tuners are token gestures with no serious engineering effort applied. And it shows.
What's galling is that the extra parts needed to make a substantial improvement in AM performance are not expensive - a few quid spent on a better IF filter and proper audio filtering with a notch at 9 kHz would make such a difference.
I have modified commercial tuners to improve their AM performance. In particular, I have opened up the frequency response from the dismal 3 kHz that is normally about the limit of effective reproduction to the 6 kHz that is actually transmitted.
The difference is like chalk and cheese. You see, there is nothing inherent in the AM broadcast process that need produce any more distortion than a typical FM path. And AM signals give more solid coverage and penetration than FM. They degrade more gracefully when the signal fades, and they can be received in locations like basement flats where FM often fails miserably.
The suggestion that current 'Gold' services should shift from AM to FM doesn't make technical sense. Those of us who were listening to pop radio in the sixties and early seventies were almost always listening on AM.
Our recollection of the music is based on hearing it through the AM medium - from the sixties pirates, RNI, or the 247-metre Radio 1. When the same tracks are heard on a compact disc latter-day remake they simply don't sound right. The whole balance has been altered, the melody is less prominent and the clinical nature of CD often reveals deficiencies in the recording or the mix which AM broadcast happily glosses over.
There are clearly formats suited to AM and formats which are not. It seems bizarre, for instance , that London has a talk service on 97.3 MHz, where stereo is a waste of time and intelligibility rather than hi- fi is the requirement, while the new country music service has to make do with AM. I also can't understand why more people aren't pressing for BBC local radio - rightly proud of its high speech content - to be shifted from FM to AM where it belongs.
What about community radio on AM? There are objections on grounds of cost and site selection, although work is being done by some companies on ways and means of engineering low cost transmission sites.
I do remember that twenty years ago pirates like Radio Jackie managed to put out useful AM signals without mortgaging their homes in the process, and radio amateurs can send a few watts on 'top band' (1.8 - 2.0MHz) within amateur budgets too. So although FM will be the preferred option for most community services I wouldn't rule it out altogether.
Any readers wishing to comment on what I have said are welcome to call me on 01 273 684 172 or send e-mail to normac@fastnet.co.uk
Copyright NJ McLeod 1995