he increased audio bandwidth means that noise can often be perceived. Even then it is considerably better than that obtained using an AM system. It is particularly 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 that is generated in the receiver. The process of increasing the treble signals is called pre-emphasis, and reducing them 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 Europe and Australia the time constant is 50 us whereas in North America it is 75 us. Slight modifications are required for the circuitry for receivers supplied to different parts of the world, although it is perfectly possible to receive stations with passable quality even when the time constants are incorrect. The difference would be easily noticed on a hi-fi receiver, although on a small portable radio the difference might not be very noticeable. Stereo In line with the improved quality available on the VHF FM broadcasts, it is possible to transmit stereo. However, this has to be accomplished in such a way that ordinary mono radios can still receive the transmissions without any degradation in performance. A stereo signal consists of two channels that can be labelled L and R (Left and Right), one channel for each speaker. The ordinary mono signal 208 Newnes Guide to Radio and Communications Technology (a) Pre-emphasis at the transmitter (b) De-emphasis at the receiver (reduces hiss) (c) Overall response from transmitter input to receiver output Figure 7.1 Pre-emphasis and de-emphasis of a signal consists of the summation of the two channels, i.e. L + R, and this can be transmitted i