I can hear you, just barely hear you
April 10, 2006 4:36 PM   Subscribe

I used to have one of those old VCRs where you had to tune each channel in by hand. It had a set of tiny little dials that let you scan the entire TV spectrum (and beyond?). When the cable wire was plugged in, I could tune in some bizzare things in the audio: rhythmic clicking, low pulses, and most interestingly, other people's phone calls. I assume I was picking up cordless phones, but how did this work? Did the cable system act as a giant antenna all over town? Has this phenomenon ever been documented anywhere?
posted by stopgap to Technology (6 answers total)
 
Old TV sets with UHF could, when tuned past channel 60 or so, pick up old technology cellphones because the unused high number UHF channels and cellphones were in the same frequency space. I think they stopped making TV's tune UHF channels past 60 by law in the late 80's because of the cellphone thing, although I may be wrong.
posted by cropshy at 5:38 PM on April 10, 2006


was that a tmbg reference?
posted by potch at 7:19 PM on April 10, 2006



From the Amateur Radio Relay League (ARRL)...

UHF Band Ch.14 - 83 are on 70 - 1002 Mhz

Advanced Mobile Phone Service (AMPS) cell towers: 869.040 to 893.970 MHz range. Cell phones transmit in the 824.040 to 848.970 MHz.

The audio portions of the TV signal are FM, just like your FM car radio and the old AMPS system was FM, too, so the circuits for recovering both audio types were the same.

The old VCRs you mention used something called varactor tuning, and the little controls you saw were adjustments to tune ANY UHF channel to ANY UHF frequency you wanted, manually. Since they covered a wide range, it was easy to tune right through the frequency range occupied by the AMPS phones, and thus, pick up the intelligence (which is what we radio type call the voice or picture or data on a radio channel).

AMPS is one of the original cell phone technologies, which most people call 'analog' these days. The conversations were broadcast in an easily recoverable form, unlike more modern phones using technology called CDMA and other schemes. These are digital in nature and exceedingly difficult to intecept, though it is technically possible.

You could also listen in on AMPS using a variety of scanners, some of which you can still get and which tune the channels directly. Usually, only 1/2 the conversation shows up. The towers broadcast to the phones on a separate frequency than the phones use to broadcast to the towers. You can pick up what are called 'images' of signals, too and some scanners are designed to intentionally do this.

Anyway, more info than you wanted, probably.
posted by FauxScot at 7:52 PM on April 10, 2006


Awesome, thanks for the responses. And, potch, yes.
posted by stopgap at 8:31 PM on April 10, 2006


I've got one of those scanners [and now the jackboots will kick down my front door momentarily, as they were made illegal in another one of those slippery slope maneuvers by Congress about a decade ago]. It's collecting dust in a drawer, and recently I took it out and fired it up to see if I could pick up SuitSat. Which failed pretty much right away, so now all I could do with is what I used to do with it, which was scanning the analog cell band for peeks into other people's banal lives. Alas, these days there is virtually nothing going on in the analog band, or so my 15 minutes at peak evening time tell me. Certainly, if you get out of the city and find someplace that doesn't have digital cell coverage, then you'll hear something, but all the exciting conversations are back in the city :)
posted by intermod at 8:42 PM on April 10, 2006


grrr suitsat. same here with the scanner + homemade dipole to try to pull it in. it didnt actually fail outright, but it was not transmitting with nearly the power they thought it would.
posted by joeblough at 1:06 AM on April 11, 2006


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