WEAK-AM

While driving home from work, I happened to be monitoring 1670, listening to a weak signal from WTDY (Madison, WI). Suddenly I picked up a “talking house” transmission. It was from a realtor (Baird and Warner) advertising a house for sale along my route. He went through the house description, and at the end, he put in a plug for the “talking house” and invited people to contact him about selling their home “the talking house way”.

That’s fairly unremarkable, but something else I noticed might be of interest here… the “woop woop woop” effect! Now, what is that? Well, as the talking house signal got weaker, it was beating with the background signal from WTDY. As the message played, it would cause this low frequency “woop woop” sound, with about a 1 second periodicity. I noticed that the “wooping” effect was strongest between words, and it would eventually die out at the end of sentences or during the quiet period before the message repeated.

Now, what exactly is this? Here’s my theory. I think there are two things going on. The power supply apparently is not well enough regulated, and the PLL is slightly unstable. On modulation peaks, the extra power pulls the supply voltage, affecting the carrier frequency. The PLL corrects it, but it overshoots, causing the “woop” effect. When there’s no modulation, the thing stabilizes. This could probably be corrected fairly easily by adding some additional regulation.

So OK, all you talking house transmitter owners, have you noticed this? I was curious about these units, but it sounds like the audio quality might not be as good as the SSTran or the Rangemaster. At least not without some work!