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One of the problems with the high quality VHF FM transmissions is that the increased audio bandwidth means that background noise can often be perceived.
As VHF FM is intended to provide high quality audio transmissions, background noise needs to be reduced as far as possible.
One method of reducing he background noise is to use a scheme called pre-emphasis.
The pre-emphasis and de-emphasis idea can be used on VHF FM because the background noise is more noticeable towards the treble end of the audio spectrum, where it can be heard as a background hiss.
To overcome this it is possible to increase the level of the treble frequencies at the transmitter. At the receiver they are correspondingly attenuated to restore the balance. This also has the effect of reducing the treble background hiss which is generated in the receiver.
The process of increasing the treble signals is called pre-emphasis, and reducing the in the receiver is called de-emphasis. The rate of pre-emphasis and de-emphasis is expressed as a time constant. It is the time constant of the capacitor-resistor network used to give the required level of change. In the UK, Europe and Australia the time constant is 50 µs whereas in North America it is 75 µs.
VHF FM receivers have the de-emphasis circuitry - consisting of a simple CR network built into the circuitry immediately after the FM demodulator. In this way it is not incorporated into an amplifier that may be used for other audio sources that would not require the de-emphasis applied to VHF FM transmissions.