Digital Siginal Processor aka DSP which are used to convert analog to digital and vice versa. It is same like we use the graphic cards in our computers but in case of routers need to use DSP. The DSP is required because router has to do do the routing but when the voice comes in picture the load becomes double, to reduce the load DSP are used. In the old days, cisco used to sell NM-HDV cards which carry 3 or 5 DSP to convert analog to voice signals. But now a days ISR router comes with inbuit DSP processor.