Why the FM quality drops down !

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In France, before the arrival of the 'Free Radios' during 1981, the FM band was under the monopoly of the Public Service. It shared 16 MHz (from 88 to 104 MHz) for the broadcast of mainly 3 channels:

At this time, the frequency map was quite simple to manage. The power of the FM transmitters (typically 10 kW RF per channel) allowed a comfortable listening without any frequency overlapping, even for the most distant countries.

After the liberalization of the Free Radios, the FM band began to fill up (with non commercial channels at the very home) and the frequency allocation became more and more difficult.
Because the FM bandwith was not infinite, it had to be quickly extended from 104 to 108 Mhz thanks to the generosity of the Army who left this part of the spectrum free sometime ago ;-)

In order to limit the frequency overlapping between the countries, the RF power of Free Radios has been limited to 500 Watts whereas the Public Service channels kept the priviledge of the higher power.

In front of such a difference, the Free Radios began to search some technical solutions allowing them to transmit with a sensation of subjective power.

Because they could not increase the transmitter RF power (which means its range), they would fill up the broadcasted message, applying it a quite identifiable style. The technical trick is to replace the standard FM stereo multiplexer, (generally a good old Telefunken coder) with a Compressor Parametric Multiplexer as known as CPM. This instrument not only multiplexes the 2 channels to the transmitter input, but also it compresses the sound. The compression level is set differently following the considered band of frequencies. This process tends to boost the signal at its highest possible level. The goal is a modulation as close as possible of 100 %...

Not only this distorts the original sound, putting the pianissimi at the same level than a fortissimo (everyone understands easily that there is nothing common between the noise made by leaves in the trees and the booming sound of a brass band...), but also some stations artificially expand the stereophony. To achieve this, a part of the left channel in reverse phasis is mixed with the right channel and vice versa.

This StereoWide process is spectacular in terms of channel separation - especially in the cars where the space is short - but it presents the drawback of making the listening quickly tiring. Indeed, the sound goes far away from the natural. Listening becomes tiring simply because the listener has to pay attention, in order to assimilate the message (which is just the opposite of what a Stereo Console usually produces).

No need to say that the use of CPM is formally condemned by this site... It is good to notice that few local stations are not especially attached to broadcast with a particular sound color and go without any CPM. Also, most of the local Radio France stations broadcast with a clean quality.

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