CAPTIONS -------- Fig. 1a-d. Modem schematic ; split into 4 parts for legibility. Fig 2a. Frequency response and phase delay of a typical NBFM receiver. Note that the frequency axis is normalised. f=1.0 corresponds to 9600 Hz. Fig 2b. Impulse response corresponding to fig 2a. Horizontal axis time "ticks" are at intervals of 1 bit (1/9600 sec), and has been shifted some 125 us so as to centre the peak. An isolated, unequalised bit would emerge from the receiver shaped like this. A stream of bits would be the sum of many of these. Bad features such as significant non-zero values at T = -2, +1 and +2 bits would give rise to substantial inter-bit interference and a very poor eye, making error free communication impossible. The purpose of equalisation is to eliminate this phenomenon. Fig 2c. Equalised Transmit bit. If the transmitter sends an isolated bit shaped like this, the receiver will give an output like fig 2e. This is the transmit waveform for one isolated bit. If this corresponds to a binary "1", a binary "0" is a negative pulse like this. Fig 2d. Spectrum of the transmitted audio pulse of fig 2c. Fig 2e. Receiver output response to an isolated bit which has been properly equalised. Note that generally zero crossings at the bit ticks (excepting T=0) are zero. This is the "Nyquist Pulse". In fact the equalisation is not perfect (e.g. at T=4). This is due to the fairly extreme radio response chosen for illustrative purposes. Fig 2f. The convolution of many bits of fig 2e superimposed looks like this, an "EYE" diagram showing a few hundred bits over 3 bit periods. You would see this on an oscilloscope. The data detector samples this waveform at the widest part of the eye. See how as a result of equalisation the eye is wide open, giving maximum noise immunity. Vertical spread at the sample point is mainly due to the aberration at T = 4 in fig 2e. Note also the considerable spread in zero crossing instants typical of narrow bandwidth systems, which lead to the need for careful clock recovery circuit design.