A SDR radio can send and receive analog voice, digital voice, data, or all three, depending on the software. All modern public-safety and cell-phone radios are at least partly software defined, meaning they use software-controlled digital electronic circuits to do most of their internal signal processing; and they do it with better performance and at lower cost than their analog predecessors. SDR's offer advantages to the public-safety community. They do not require the expert tuning and adjustment of analog radios, and most of their signal processing is done with digital electronics. This switch to digital is important, because digital parts and circuits are usually much less expensive than analog and because an analog system can usually do only a few tasks; whereas, a digital system can do many tasks, depending on the software it uses. A cognitive radio is essentially a SDR operating under the control of an intelligent software package called a "cognitive engine." The cognitive engine does many things an intelligent human operator would do, thus freeing operators to do their primary tasks. One application for cognitive radios is dynamic frequency sharing. This is a technique by which radios find and use open frequencies ("white space" in the radio spectrum) or in which radios share channels on the basis of a priority system. In principle, this would give police access to more spectrum when needed and allow sharing of public-safety channels with others during idle times. Other potential applications for CR in public safety are in improving interoperability or in automatic adjustment of radio performance.