19 February 1994
Norman McLeod
Those of you who noted my absence last week may like to know that I had no time to write my column as usual as I was in the process of putting a local RSL on the air.
Freedom FM is our local dance music station, and the challenge was to build a mixer chain from a bare shell to full operation in five days. I would have liked a fortnight so that I had as long to de-bug the system as I had to build it, but this was not how it turned out. The last soldered joint was made at 11- 15pm, and the station was on the air at midnight.
Needless to say there were some problems to sort out, involving the uniquely nerve-wracking process of making modifications to a desk that is live on the air. But I enjoy a challenge like this, and something that struck me straight away was the difficulty of processing modern dance music given the constraints of the FM broadcast system.
Let's just be clear what the restraints are. On FM broadcasts you are allowed at most +/- 75 kHz deviation independent of modulating frequency. But all FM receivers have significant treble cut, known as de-emphasis. The response of an FM radio is about 3 dB down at 3 kHz, and more than 10 dB down above 10 kHz. So a flat, unequalised signal, passed through a limiter into the transmitter, will sound muffled and murky when received.
It is necessary to provide pre- emphasis (treble boost) to ensure that the response of the system is flat across the audio band. But immediately we have a problem. If we put the pre-emphasis before the limiter, the limiter will be receiving a highly treble-boosted signal, and will produce strange effects.
So if instead we put the pre- emphasis after the limiter and just before the transmitter input, we will not get funny effects. But the limiter will be unable to control the maximum deviation at the top end of the spectrum with the pre- emphasis applied. If set for 75 kHz deviation at mid-band, the maximum possible deviation at 15 kHz would be about 14 dB more than that. In other words, there would be nothing to stop the boosted treble from overdeviating on treble energy to a large extent, causing sibilance and splatter.
In practice the problem is less serious than it sounds - on most material the excess is rarely more than 6dB. But still there is a problem. If the drive is backed off so that no over-deviation occurs, the audio level will be far too low in the rest of the spectrum.
The answer is some sort of automatic pre-emphasis, which controls the treble boost separately from the overall level in such a way that were there to be a risk of overdeviation, the HF energy will be backed off momentarily and then immediately restored when the HF transient has passed. Done well, this process can be almost inaudible and is standard practice in all professional broadcast operations.
It is possible to go further and introduce a circuit which can boost pre-emphasis as well as cut it to even out the difference between bright and dull material arriving at the transmitter. More next week.
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