sired channel is known as (+2.4 kHz) will suffer attenuations of 3 dB and 6 dB adjacent channel interference. approximately with respect to the carrier. (See the table Adjacent channel interference causes signals from ad- in Figure 2.1-30.) The new spectrum of the signal is shown jacent channels to be heard in the desired channel. It is in Figure 2.1-31(b). particularly bad when strong adjacent channel signals are Comparison of Figures 2.1.31 (a) and (b) shows clearly present relative to the desired station. What does this that amplitude distortion of the sidebands has occurred mean in practice? It means that you will obtain interference but what does this mean in practice? If the transmitted from the unwanted station. In a broadcast receiver, you signal had been music, there would have been an ampli- often hear signals from both channels simultaneously. tude reduction in high notes. If the transmitted signal had Broadcasting authorities minimise adjacent channel been speech, the speaker's voice would sound less natural. interference by forbidding other transmitters situated From the above discussion, it should be noted that for near the desired station to broadcast on an adjacent good quality reproduction the selectivity curve of channel. Stations geographically distant from the desired Fig. 2.1-31 (a) Transmitted and (b) distorted spectrum. CHAPTER 2. 1 Basic features of radio communication systems An alternative way of specifying signal-to-noise ratios is to quote the ratio in decibels. This is defined by Actual response curve Equation 2.1.24: Ideal response curve signal-to-noise ratio = 5(dB) signal power 10 log10 noise power (2.1.24) Actual response curve A strong signal relative to the noise at the receiver input is essential for good reception. In practice, we re- quire an S/N of 10-20 dB to distinguish speech, an S/N Log frequency (kHz) of 30 dB to hear speech clearly, and an S/N of 40 dB or better for good television pictures. Fig. 2.1-32 A comparison between the ideal and practical selectivity curve. No