Detecting Television Recievers, Spinoff of a Beach thread Options

[jon3831]

In Shal's Utterly Boggled thread in the Beach, some interesting points were brought up about the BBC television licenses and their enforcement thereof.

Specificially, thus:

And anyone who thinks having a TV licence is weird, consider this; there are TV detector vans which have equipment in them which can detect the presence of any working TV sets. These are sent to any address with no TV licence to see if they can catch anyone operating an illegal TV set. (I'm not making this up you know)!

Which got me to wondering about the technology of such vans. How would they work? By definition, "recievers" don't transmit signals, save for very low power emissions from the internal oscillators. Now, I know IFs can be detected, but they're notoriously hard to do, easily defeated, and *very* easily spoofed.

Questions for discussion:

1) Does the BBC require any sort of cable box/decoder box/whathaveyou in order to function? Or can you just stick an antenna on the roof, hook it to a PAL-compatable television and go to town?

2) With purchase of the license, is there any sort of modification or anything that has to be done to the TV to "authenticate" the license?

I'm sure there's other considerations, but I am genuinely curious as to how the BBC can reliably (key word there, reliably) detect unauthorized television sets without actually going up and peeking in the window of a private residence.

[CJ AEGIS]

Now, I know IFs can be detected, but they're notoriously hard to do, easily defeated, and *very* easily spoofed.

ECM?

$100 Online
$150: RadioShack
Baffling the BBC with a mobile homemade ECM/ECCM suite…. Priceless!

This post has been edited by CJ AEGIS: Nov 22 2003, 04:37 PM

[LaughingVulcan]

I actually expanded on your question a little, mainly to be corrected.

Receivers do, actually, transmit signals. Anything running on electricity does, but in the case of any receiver (TV, radio, etc.) a frequency signal must be generated to the receiver. This can be done by crystals running off piezo power (a crystal AM set,) or by PLL synthesis in the case of a modern FM receiver. So, such emissions, though shielded and at extremely low power, can be detected. All receivers generate some type of intermediate frequency (I.F.) that's mixed to the receiving frequency. Also, there are some common denominators, like how a U.S. TV set's horizontal oscillator runs at 3.58 MHz. When the oscillator goes wonky (well, actually, much of the TV's operation is synchronized to the h.o. transistor so it won't run at all,) but you definitely lose your horizontal hold.

But this is off only one year at DeVry, x years of amateur radio operation, and working for my dad as a thumb-fingered assistant for several years. I'm more than willing to have a *real* engineer correct my definition.

Granted that it's all shielded, but if I had a van full of sensitive detection equipment I would bet that I could find my way past a $150 ECM device.

This post has been edited by LaughingVulcan: Nov 22 2003, 04:43 PM

[Raymond]

Yup, 'tis true.

The govt. has a list of all folks with a licence and all those without a licence.

If you have a T.V., whether you use it or not, you need a licence to avoid prosecution.

And yep, nomatteer how Orwellian it may seem, dtectoer vans prowl the streets to catch folks who have unlicenced receivers.

I have no such licence and I regulrly get mailings regarding this.
I am supposed to make a declaration saying I don't have a T.V..
I just bin 'em. Never sent one back yet.

About twice a year I have a snoop who knocks on my door and asks if I have a T.V.. I tell him I haven't and he goes away.

Hmmm ... thinking about it, I haven't heard from these folks at all this year.

Does this mean I have won, do you suppose?

~~~~~ Raymond ~~~~~ ®

[jon3831]

(Reply to LaughingVulcan)

You're absolutely right, of course. I said "save for very low power emissions from the internal oscillators." in an attempt to simplify, so we wouldn't have to get into the mechanics of how the recievers work. I should've known better than to think that'd pass muster.

As far as my background, I've been an amateur radio operator for... gosh, 8 years now, active in VHF/UHF, T-hunting, packet, and a dabbling of FM repeaters... I'm been building microwave gear for about a year and a half, and I'm just getting started with ATV.

As to specific points...

All receivers generate some type of intermediate frequency (I.F.) that's mixed to the receiving frequency.

Precisely my point. How does the BBC know a television IF from a broadcast band radio IF from a BFO from a clock oscillator from a computer from a...

Fact is, there's a *lot* of RF out there to be recieved, if you're looking for it, especially in this day and age of consumer electronics out the wazoo. Heck, I can go downstairs, tune around the 80 meter (3.5-4 MHz) and hear all sorts of birdies and spurious emissions and all sorts of wonderful stuff like that.

As to direction finding it, it's simple enough to do, provided you know what you're looking for, and can seperate it from all the other hash out there.

That said, it's still relatively easy to defeat. Metal TV cabinets, proper grounding, filtering... Easy to do without extensive training. And as Chadee pointed out in the other thread, it's relatively easy to get spoofed the other way, so that the BBC thinks there's something there when there isn't.

[Delvo]

Another problem they're going to have is tuners built into devices that are already shielded anyway. For example, you can probably bet that this detection system was set up with cathode-ray tubes in mind (in fact, I first figured they'd be looking for emissions from the CRT itself), which aren't shielded; set a big unshielded speaker next to a CRT while operating both, and watch the pretty acid-trip colors. But do LCD, DLP, plasma, and (so far, still experimental) LED displays contain electronics that would need to be shielded so they don't interfere with something else? If so, then the shielding that was meant for the display technology would also block whatever tuner was also inside the case with it.

But what really made me think of this is something that I already know is shielded: computers. You can tune TV signals with a special card inserted in your computer's PCI or AGP slot, and use your computer's monitor as the TV screen (or put TV in a little window while you use the rest of the screen to work on other stuff). Also, TVs are coming to have more computer parts in them, the same parts that make it necessary to shield computers, so TVs will therefor be more and more shielded in the future...

[LaughingVulcan]

Well, what they apparently detect is not the I.F., which as you know is a fixed oscillation, but rather the bleed from the mixer output, which as you know must produce signal at the channel frequency plus the I.F. frequency (and it's harmonics.) So they listen for signal at 39.5 MHz over the channel frequency (and I also learned that apparently the 'Beeb' operates on fixed frequencies across England - i.e. channel '2' BBC content is broadcast on the same frequency consistently. So they can tell if you're monitoring a BBC freq.

No, I'm not quite that good or psychic - got the information from here, a protestors site. I especially liked their conclusion for what happens if all their sensitive detection stuff gets bolloxed-up.

Despite all of the expensive gadgets developed by BBC R&D detection of TV Licence evasion depends heavily on an address based system [database called "Lassy"], so if you're not on their supposedly exhaustive list, they nip round, listen really carefully at the door and bust you if they hear Anne Robinson's voice.

Anyway, the site above has lots of interesting information about the subject.

*edited because the site won't seem to let me post a string of morse the way I'd like to. *

This post has been edited by LaughingVulcan: Nov 22 2003, 09:11 PM

[WildChildCait]

Hi,

I personally believe they would have trouble detecting something 14 stories high (which is where I live) or through brick walls. - most signals are direct line of site, at least, I seem to remember someting about that in my radio course.

That said, I do believe they do have means of detecting signal as in this flat building at least, the tv signal does come out of a socket in the wall, meaning it has to be monitorable.

Cheers
Chaddee

[Christopher]

I personally believe they would have trouble detecting something 14 stories high (which is where I live) or through brick walls.

Well, the TV signal has to pass through brick walls to get to the TV, so I wouldn't think a brick wall would pose a problem.

[Orpheus]

"I wouldn't think a brick wall would pose a problem."

Oh, the trouble those words have caused in my life!

[Delvo]

Well, the TV signal has to pass through brick walls to get to the TV, so I wouldn't think a brick wall would pose a problem.

It does weaken the signal. The difference is in the signal you start with. A TV broadcast is an intentionally powerful and heavily modulated signal coming from a tower, so it isn't necessarily always weakened by walls enough for it to be noticed, although it can be. We're talking about weak static-like signals fitzing off of small, low-voltage electronics as a side effect.

[usmarox]

also learned that apparently the 'Beeb' operates on fixed frequencies across England - i.e. channel '2' BBC content is broadcast on the same frequency consistently. So they can tell if you're monitoring a BBC freq.

Not so, I'm afraid. Any given channel can be pretty much anywhere in the UHF spectrum, depending on the local spectrum planning requirements. So BBC2 could be anywhere from channel 21 up to channel 68 (my list of enhanced priority sites only shows a range of 25 to 64, but that's far from exhaustive). Also you have to take the precision offset into account, so for any given UHF channel, there are three possible frequencies. So that's at least 141 places for BBC2 to sit on analogue alone.