 kHz, i.e. 175 kHz. In view of this a total of 200 0kHz is usually allowed, enabling stations to have a small guard band and their centre frequencies on integral numbers of 100 kHz. Improvement in signal to noise ratio It has already been mentioned that FM can give a better signal to noise ratio than AM when wide bandwidths are used. The amplitude noise can be removed by limiting the signal to remove it. In fact the greater the deviation the better the noise performance. When comparing an AM signal to an FM one an improvement equal to 3D2 is obtained where D is the deviation ratio. Pre-emphasis and de-emphasis An additional improvement in signal to noise ratio can be achieved if the audio signal is pre-emphasized. To achieve this the lower level high frequency sounds are amplified to a greater degree than the lower frequency sounds before they are transmitted. Once at the receiver the signals are passed through a network with the opposite effect to restore a flat frequency response. 62 Newnes Guide to Radio and Communications Technology To achieve the pre-emphasis the signal is passed through a capacitor- resistor (CR) network. At frequencies above the cutoff frequency the signal increases in level by 6 dB per octave. Similarly at the receiver the response falls by the same amount. Both the receiver and transmitter networks must match one another. In the UK the CR time constant is chosen to be 50 us. For this the break frequency f1 is 3183 Hz. For broadcasting in North America values of 75 us with a break frequency of 2.1 kHz are used. Pre-emphasizing the audio for an FM signal is effective because the noise output from an FM system is proportional to the audio frequency. In order to reduce the level of this effect, the audio amplifier in the receiver must have a response that falls with frequency. In order to prevent the audio signal from losing the higher frequencies, the transmitter must increase the level of the higher frequencies to com- pensate. This can be achieved because the level of the high frequency s