A question I have is:
What frequency does a TV screen give off when the screen is say all white.
If I could tune a reciever to low enough frequencies at what frequency would
I pick up the TV screen.
The reason I need to know this is I want to know if I can produce different
frequencies using a tv screen as a special type of oscillator.
I would just display a certain image on the screen and produce an oscillator
frequency with the screen.
I don't know if anyone has tried anything like this before.
This is very important for new research I am working on.
Thanks for any input.

 

The scan frequencies don't change with colour constant.

I think you need to learn more about how TV works.

http://en.wikipedia.org/wiki/Television

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maybe you're thinking of video level difference from baseline to solid white?

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Paul Obrien

There was a spy technique in the old CGA display days of syncing an oscillator in a receiver to the line frequency then reading the received blips caused by the on/off of the colour gun as an image. Rooms were screened to prevent this from happening. So it is an old technique, I have seen a utility for a Casio watch which flashed the screen to send data to the watch, again very old.

 

I know the screen always scans at the same frequency but if one were to say
light up every second pixel on the display wouldn't the readings from a
distance be half the scan frequency?
And that was my main question. What is the scan frequency of a modern
colour tv set? Is it still about 15,000 hz?

 

OK a better explanation of what I'm looking for,
and my understanding of how a tv works.
Each of the 3 electron guns sends out pulses of electrons at a certain frequency, and then those pulses are bent by electro-magnets so they strike
the screen at a certain point(screen pixel). Those pulses of electrons are also regulated in intensity to give you different levels of brightness for each pixel.

Now there is a fixed frequency at which those pulses for each gun are produced. I need to know what that frequency is.
If those pulses were alternated between full brightness and least brightness,
then from a distance the pulse frequency would seem to be half what it really is. And by using longer full and least brightnessess, one should be able to get
1/2 1/4 1/8 ...etc. the pulse frequency, observed from a distance.

The reason I need to know this is, I am researching the effects
these frequencies might have on an individual standing in the line of fire.

Thanks for the help so far. Harold.

 

Your understanding is faulty, the electron guns don't send out pulses of electrons - they send out a continuous stream of electrons, which is modulated by the brightness at any particular point.

You are also incorrect thinking about 'pixels', CRT's don't have pixels, they are a completely analogue device.

However, they do have a resolution, because of the coloured phosphor dots on the ins1de of the screen and the shadowmask.

For a 625 line PAL TV the line scan frequency is 15,625Hz, NTSC 525 is slower, but I've no idea what it is?.

But as your entire premise is flawed I suspect your 'research' is not looking good?.

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"NTSC 525 is slower, but I've no idea what it is?."
60 * 525 / 2 = ?

 

Canada uses the NTSC standard, same as the US does. Given that here's a neat link that explains a fair amount: http://www.ntsc-tv.com/ntsc-index-02.htm

NTSC Basic Parameters
line/field = 525hz / 60hz
horizontal frequency = 15,734.2637 hz
vertical frequency = 60 Hz
color sub carrier freq. = 3.579545 MHz
video bandwidth = 4.2 MHz
audio carrier = 4.5 MHz (FM)
video frames = 30/sec.

As for using the term "pixels" on a CRT, while technically incorrect, it has become commonplace to refer to individual phosphor dots as pixels as CRT technology advanced over time.

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The information I found gives the vertical rate as 59.94Hz.

 

Thar seems rather strange?, it's identical to the mains frequency of the country in question, so 50Hz in Europe and 60Hz in America.

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I agree, thar does seem strange, but apparently the (monochrome) frame frequency was originally 60Hz, with a corresponding line frequency of 15750 Hz, but now it's as stated above. The first Google hit for NTSC line frequency has the full story.

 

Yes, the North American black and white TV image was sync'd to the 60Hz mains, but the colour NTSC vertical signal is a little slower.

If you modulated every even "pixel" on and every odd "pixel" off, then the frequency would be about 2.0MHz. The interlace messes up what would happen if you make the top half of the screen white and the bottom half black. Then you would have a low frequency of about 120Hz.

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Fsc=3579545 Hz. Everything else is based on this.
The field rate is Fsc/227.5/262.5 ~ 59.94 Hz
There are 227.5 cycles of subcarrier per line, so that subcarrier (of constant phase) would make a fine (hopefully unobtrusive) checkerboard on a black and white TV, and/or so a comb filter could do a halfway decent job of filtering subcarrier from luminance (and vice-versa).
As we all know, alternate fields are interlaced to make a frame, hence the 262.5 lines per field.

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Ron

 

Yes, the North American black and white TV image was sync'd to the 60Hz mains, but the colour NTSC vertical signal is a little slower.

That's not correct. Televisions have always had sync separators to pull out the vertical and horizontal sync pulses and the TV synchs on those, not the power line frequency. Too many areas had their own power generation facilities that were not connected to the grid, especially in the earlier days of television, and a picture would never sync to the transmitted image if that statement were true. And battery-operated TVs would really have problems, wouldn't they?

Dean

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