Analog devices are important support players to the DSP. You don't speak in a digital signal. A digital signal is a language of 1s and 0s that can be processed by mathematics. We speak in real-world, analog signals. While the digital signal processor can handle signals at incredible speeds, it needs an interface to the real analog world in which we all live.
A chip called a data converter is that interface. A data converter takes a real-world signal, such as sound, light, heat or pressure, and turns it into a digital signal. That is the signal that the DSP works upon. When it has completed the signal improvement, another data converter chip converts the digital signal back to analog.
Put together, analog, mixed-signal devices and DSPs create a powerful team. While DSPs deliver the raw processing power, at either end of the DSP is an analog chip. That means as electronics go increasingly digital, the demand for analog will grow with it.
If you have Shockwave, click anywhere on the diagram below to see how a cell phone works (178K).