Beefchicken Industries

Buried Signals

Listening to Evan Doorbell’s telephone recordings, the frequency inverted carrier crosstalk always fascinated me. So I thought I’d try to recover one of the original adjacent channel signals from one of his recordings. After an initial trial, I shared the results with Evan Doorbell, and he suggested a better source recording:

http://download.evan-doorbell.com/production/Greenville1xxp2.flac

The crosstalk starts at around 21:00.

I took a sample of this, and ran the following with Audacity’s Nyquist Prompt plugin:

(lowpass8 (mult 2 (highpass8 s 3750) (hzosc 4000)) 2000)

This high-passes the input to select the high-frequency crosstalk, modulates it with a 4 kHz carrier, and then low-pass filters the result to remove the high-frequency component that is now the adjacent channel cross-talk from our original channel.

Most of these numbers were picked by trial and error, except the 4kHz frequency; this was selected based on the typical carrier channel spacing of 4KHz. It is also the frequency at which the least amount of ‘frogging’ occurs.

Here’s what the input sounds like:

source.flac

And here’s the output:

filtered.flac

My trial run produced a result where the accent of the speaker was apparent, but most of the words were intelligible. This one is much clearer, and of a much more sombre subject matter.