how do tv licence detectors know if you have a colour or black and white tv?

[joko]

the signal fromthe station is the same, it just the piece of machine that receives it that turn it black and white.
so how can they tell?

thanks

[joko]

i am just curious as to how they can tell, as a tv gives out no signals it receives them. (i think)

does anyone know?

[Ethel]

Televisions -and radio receivers for that matter- are so called super heterodyne receivers. Incoming high frequency signals [the TV channels] are mixed with the TV's internal oscillator [local oscillator] to produce a lower, fixed-frequency signal [the intermediate frequency of 39.5Mhz] that is used for further processing [audio/video]. Although great care is taken to shield the local oscillator from the mixer, some of this signal leaks back up the aerial/cable. This signal is transmitted for a short distance, but far enough to be picked up by the detector vans.

From this site:
http://www.tvlicensing.biz/detection/index.htm

[joko]

thanks for your replies but my question was how do they know if you have a black and white or colour set?

does anyone know?

[Ethel]

From the signal sent out by your television and picked up by the detector.

[SteveD]

Who has a black and white TV set these days? Do they still exist apart from small ones for security cameras?