Re: For the engineers on the list...

[Shawn Mamros <mamros@MIT.EDU>]

Larry wrote:
>At 4:49 PM -0700 7/21/03, Matthew Osborne wrote:
>>   I own a micro-power FM transmitter
>>built from a kit that I have hooked up to my computer
>>so I can listen to mp3s or streaming audio anywhere in
>>my apartment.  There is one problem though -- along
>>with the audio that I transmit over it, whenever I
>>have a stereo receiver set in the stereo mode, I get
>>this high pitched tone underneath the audio.
>
>Not being familiar with that unit, my best guess would be that the 
>stereo pilot is off-frequency.  As to if that can be adjusted on that 
>particular unit, I don't know.

It could also be a bad component - or maybe even a bad solder joint,
as Matthew guessed - which could cause either a ringing effect or the
generation of a spurious harmonic in the stereo generator circuit.
The tone being generated may actually not be in the audible range,
but if it were somewhere between 19 kHz and 38 kHz, that lands it
in the middle of the L-R component of an FM signal, so once it gets
processed by your receiver's stereo circuitry, it comes out as an
audible tone that's out of phase between left and right.

Years ago, we ran into that same problem at WMBR with our stereo generator
(an Orban Optimod 8000A), which was ultimately traced down to a bad
capacitor.  My EE skills are far too rusty these days to do anything
more than make a semi-educated guess, but you could start by looking at
any capacitors (or maybe even any inductors) and the solder joints
leading to them.  Good luck...

-Shawn Mamros
E-mail to: mamros@mit.edu