◆ PARALLAX

Has anyone else noticed that certain older CRTs emit a sub-audible pressure sensation when left on a blank blue screen? Before the comedians pile in: I am not talking about the 15.7 kHz flyback whine, nor the ordinary 50/60 Hz hum. I mean a slow pulsing phenomenon that seems to sit around 19.83 Hz according to my very crude microphone + Sound Forge setup.

The number is interesting because 19.83 Hz is close to one of the predicted resonances in some Schumann harmonic models, depending on ground conductivity and ionospheric height. It is also uncomfortably close to frequencies discussed in older Soviet-era material on visual entrainment, although good luck finding a clean translation now that all those FTP archives are vanishing.

My working hypothesis: late 80s / early 90s CRTs with certain degaussing circuits can act as accidental receivers for ELF modulation riding on the power grid. Not a transmitter, not exactly. More like a bell that rings when the right invisible hand touches it. If the blank raster is blue, the effect seems stronger. I do not yet know if this is due to phosphor persistence, the blue gun, or simply because blue forces the visual cortex into a more vulnerable idle state.

Protocol so far:

1. Use a pre-1994 CRT.
2. Set desktop background to solid blue, no icons.
3. Leave room lights off but door open.
4. Sit 6-8 feet back.
5. Do not stare directly. Peripheral vision only.
6. Record microphone input near the case.

Results: after 11-17 minutes I consistently get a sensation of being watched from behind the left shoulder. My wife gets nausea. My brother got nothing, but he also claimed he could not hear the flyback, so his nervous system may simply be coarse.

This may be nothing more than poorly shielded equipment. But then why does the pattern appear only between midnight and 3 AM, and why did it stop for three nights after the local substation fire?

First thing I would do is make the boring cases as identical as possible. CRT on with blue raster, CRT on with white raster, CRT off with a blue card taped over the glass, same chair, same mic position, same time window.

Also try a different room if the cable will reach. If it follows the set, interesting. If it follows the corner of the house, less interesting but still useful.

Degauss is not supposed to be doing anything 11 minutes later unless something is wrong. Service manual before seance.

Log the boring household stuff too -- fridge starting, dimmer on the same circuit, little wall transformer warming up, anything with a cheap iron core. White raster, blue raster, no raster. Then I might start believing the blue part.

At 19.83 Hz I would be very suspicious of the recording chain before the room. Some sound cards make little hills down there from DC blocking, fan wobble through the desk, or a ground loop between PC and whatever is feeding the CRT.

Record the mic with the monitor unplugged, then record with the mic cable plugged in but the mic wrapped and still. Sound Forge will draw a very convincing ghost if the input is the thing breathing.

New here, so apologies if too basic, but could your wife or brother swap the inputs without telling you? Like blue screen, white screen, blue card, set off, all written on cards and shuffled.

If the watched-from-behind-left-shoulder feeling picks the blue raster card better than chance, that is stronger than everyone staring at the set waiting for it.

Attached Sound Forge grab from tonight. It is a narrow rise centered just under 20 Hz, not a clean spike, more like a little mesa from about 19.6 to 20.1 with the top moving by a few hundredths over the run.

The white raster trace is flatter but not dead. CRT off gives a low wandering lump lower down, 16-ish, which I had been ignoring. I will repeat with the blue card because that is an ugly good control.

Substation fire stopping it for three nights is interesting in the same way a street going quiet after roadworks is interesting. It puts a pin on a map, it does not prove what was making the noise.

If you can note the exact nights and the midnight to 3 AM window against your own recordings, that is the bit I would keep. Hums get mapped before they get named.

Here come the clipboard men demanding the phenomenon perform tricks under fluorescent light. Always the same purity test -- if the signal dislikes your stale little boxes, the signal is blamed.

Still, monk, do the cards if you must. Just do not let them define the thing out of existence before it has spoken.