Good illustrations of DSP in the earlier answers. Want to add a little more to your audio/video question. Although the audio processing field may appear saturated at a naieve glance, yet, to me, there is a huge renaissance in the field, when you consider, 'speech recognition' and 'speech processing'. DSP community has worked for more than 40 years on these problems. But it is now, that they are being integrateed in everyday products. This brings new challenges however. Imagine someone commanding a Siri to find something while working on an oil rig, or close to an airport. There are tons of noisy environments, where speech processing and recognition lags. We can imagine a future where robots respond to speech commands. Would need new signal processing paradigms and algorithms. Indeed, you can even start today, and instead of targetting an audio processing software, target a speech recognition engine. One with a limited dictionary lets say. That can recognize your custom commands better. I bet you can improve on the commercially available ones. Then try putting the commands in the dictionary that may be useful. For example commands for controlling IoT components. Link it to an IoT sensor/actuator. There.. You have a modern signal processing system, not available off the shelf, able to perform useful stuff, and can even be marketed if found robust enough.