Unit 91, welcome to Radio Reference first off.
Yes, you will need a digital scanner to be able to listen to Austin/Travis County public safety, which you can have a look at
here. You have a number of options available to you: the GRE PSR-500(handheld) and PSR-600(mobile), the RadioShack Pro-106(handheld) and Pro-197(mobile), of which I'm personally familiar with using on this system, and you also have Uniden's 396XT(handheld) and 996XT(mobile) scanners which while I haven't used myself, I think others from the area would attest that they also get the job done. I also know that the older RadioShack Pro-96 and Pro-2096 will work too, but I can't say the same for sure of the Uniden older digital scanners 396T and 996T. The only reason I mention the older scanners is that if you're on a budget, these might be acceptable alternatives that you can pick up online.