 Digital to Audio analogue converter processor converter amplifier input Figure 5.48 Block diagram of a digital signal processor again ready for the next conversion. In this way a succession of samples is made. Once in a digital format the processor performs complicated mathe- matical routines upon the representation of the signal. However, to use the signal it then needs to be converted back into an analogue form where it can be amplified and passed into a loudspeaker or headphones as shown in Figure 5.48. The circuit that performs this function is not surprisingly called a digital to analogue converter, D to A converter or The advantage of digital signal processing is that once the signals are converted into a digital format they can be manipulated mathematically. This gives the advantage that all the signals can be treated far more exactly, and this enables better filtering, demodulation and general manipulation of the signal. Unfortunately it does not mean that filters can be made with infinitely steep sides because there are mathematical limitations to what can be accomplished. IF breakthrough One problem which can occur happens when signals from the antenna pass across the RF sections and enter the IF directly. Normally intermediate frequencies are chosen to fall at frequencies which are not used by high powered stations. Although this alleviates the problem to a large extent, there are sometimes situations where this is not feasible. To ensure that breakthrough is not a problem care must be taken during the design of the receiver to ensure that there is sufficient isolation between the signal input and the IF stages. In some sets special filters are included at the antenna to ensure that signals from the antenna at the intermediate frequency are removed as they enter the set. Figures for IF breakthrough are quoted in the same way as they are from image rejection. In the case of most high performance receivers it is possible to achieve figures of more than 60 dB rejection. In some cases figures of 100 dB ha