I was making some notes on my cassette recorder in a totally quiet
room, but when I played back the tape there was a male voice saying
"Hello, hello.... Hello, hello...." for about twenty seconds. I've
seen on rare occasions voices, sometimes music, coming from my guitar
amplifier when it was on and even from te TV set when it was actually
turned off, but this was rather freaky. How does a tape recorder do
something like that, and especially do it and only have the extraneous
voice be heard on the tape but not while it as being recorded?


A nearby radio ham could cause that.

Ham-radio operator, CB operator, police radio... any strong-enough
local source of RF can leak into the equipment and its electronics.
Power cords and speaker wires can act as antennas, making the pickup
even more sensitive.

Once inside the equipment, the RF can mix into the signals in the
active circuitry, be amplified, and you end up hearing whatever was
being transmitted. In the case of the "haunted" tape recorder, the RF
probably leaked into the microphone circuitry; the recording
electronics "detected" the audio and mixed it with what you were
saying and then recorded the combined signal onto the tape.

Under conditions of very high RF power (e.g. near an AM radio
station's tower) it's very hard to avoid this sort of bleed-in
entirely... there are stories of people hearing radio broadcasts "in
their head" as a result of RF pickup by the braces on their teeth!

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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