some mixture of magic and weather
Thursday, January 6 2011
Badger, the first floor computer where Gretchen does most of her work, has traditionally had a pair of speakers endowed with, let us just say, poor bass response. So I decided to get Gretchen some nicer speakers. In terms of price vs. performance, the ideal seemed to be a $40 set made by Cyber Acoustics that had good reviews in Amazon.com. This set had included a subwoofer and two tiny little left and right speakers, similar to the setup I have in my laboratory on Woodchuck. So I'd ordered these, they'd arrived, and today I installed them. Since I'd embedded an FM transmitter inside one of the speakers I was replacing, I had to come up with some new place to put an FM transmitter (a cheap Chinese unit based on the BH1417F chip). I decided to make a stand-alone transmitter and house it inside the case of an old five-port 100Mb/s Gigafast ethernet switch. (I found such switches flaky, though their cases are of unusually high quality, that is, made of thick steel. Also, they've mostly been replaced in the household network with gigabit — as opposed to Gigafast — switches.)
This project ended up drawing on an unusual number of sources in my laboratory scrapyard. For audio inputs and outputs, I salvaged a the sound card connection stack from an old motherboard. I also included a salvaged toggle switch to control power; that is the single most important thing you want to be able to control with an FM transmitter, even a low-power one like this (indeed, the frequency on this particular transmitter was fixed at somewhere just beyond 107 MHz and I saw no reason to either change it or provide a means to). When you want your micropirate radio station to go dark, you want it to go dark completely an instantly.
Later, though, when I went to test the new setup, I discovered an annoying amount of 60 Hz hum in the signal from the FM transmitter. Initially I thought this was power-supply related, and I tried adding a large filter capacitor, but it did no good. It turned out that even running the transmitter from a battery didn't eliminate the hum. Indeed the hum continued when the transmitter was running from a battery with no audio inputs. Somehow the hum was being introduced to the signal after it left the antenna (a 700 millimeter wire). The only thing I could do to lessen the hum was to reorient the antenna wire. This whole hum-elimination jihad felt an aweful lot like trying to get obscure hardware to work with various Linux installations. When there are many variables interacting emergently, things start feeling like some mixture of magic and weather.
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