This post comes from my old blog back in 2004. I’m reposting it here so that I don’t lose the content. The source was hand-written HTML so the formatting probably appears a bit off.
About seven years ago I wrote some code to do Mu-Law and A-Law compression. About six years ago, I decided to publish an article along with the source code for it. You can find it here. Anyway, the other day I received an email from someone who had taken it and modified it for what he was doing. In doing so, he found a piece of misinformation that has been in my article since I originally published it. Not a big deal, and I intend to rectify the issue. However, as we chatted over email, I asked him what he was using Mu-Law/A-Law compression for. Here is a clip from our email:
> So if you don’t mind me asking, what are you working on that has 13 bit
> unsigned audio input?
Sure. I am designing a system that monitors lots of radio stations to capture and
detect Emergency Alert System activations – those ugly tones and occasional
voice messages you hear on the radio. The system has some rather incredible
shortcomings for a critical system in 2005. When the emergency management
office triggers an alert they have no way of knowing whether or not radio stations
actually broadcast the alert. Sometimes the system fails – too often. So our
system listens to the stations and sends a report back to Emergency HQ. In
most cases an exception report that shows which stations did not properly
send out the alert. So if the dam is breaking or the nuke is going critical they
can try again, use the phone, send a helicopter or something.
Whoa. Code that I originally wrote to compress audio in children’s games is now being used to help monitor Emergency situations. Talk about your unintended consequences!