Müsste mon soetwas ähnliches nicht auch mit tastaturen funktionieren da diese durch ihre architektur auch wie eine antenne wirken oder ist das signal schlicht und ergreifend zu schwach ?

P.S. was ksotet die oben gzeigte apparatur ?

I had a look at various SDR-frontends designed for HAMs, but all the ones I found so far have IF bandwidths designed for PC audio cards, which is three orders of magnitude narrower than what you need for a video signal. Suitable commercially available SDR tuners with at least 20 MHz BW tend to come so far from military suppliers. The HAM-SDR community is already growing beyond the use of soundcards (e.g., GNU Radio uses a 6 MHz IF BW cable-TV tuner), and it is surely only a matter of time until they also start to play with 20–50 MHz bandwidths.

If you are interested, there is a more detailed analysis of the spectral composition of a video signal in Section 3.2 of my thesis.

You are right that in theory you can extract a video signal from an antenna signal using merely periodic averaging. A periodic (85 Hz) video signal will have a comb spectrum where all spectral energy is located exclusively at multiples of 85 Hz (assuming an infinitely long periodic signal), and the entire video-baseband spectrum is repeated throughout the spectrum at intervals that correspond to the pixel clock frequency. The periodic averaging is nothing but a comb filter that attenuates all frequencies in the spectrum that are not a multiple of 85 Hz, thereby eliminating all other noise sources.

I tried several times to connect an AD converter directly to an antenna amplifier for periodic averaging, but I found that in practice you get very bad results this way. You always need some form of analog preselection before you can digitize an antenna signal. There is simply way too much energy across the entire ADC input band for averaging to become effective. The frequency selectivity of a wideband receiver does in practice offer a dramatic improvement of the achievable signal quality.

One thing you could do is go for a Software Defined Radio design. There are a couple of interesting articals up on the web, and Eric Youngblood actually sells a quite decent SDR kit, that he described in the articals available from,

http://www.flex-radio.com/articles.htm

Whilst away for a few days with the family and nothing much better to do whilst out walking I had an idle thought or two one of which was,

IF you assume the video signal is actually sampled at the video dot clock rate, then by the process of sampaling the signal energy is spread at multiples of the dot clock all across the radio spectrum (one of which is what you tune into effectivly). However insted of using a conventional receiver why not use a SDR without front end preselection, and drive the sampaling system at the same rate as the dot clock (ie a high speed A-D clocked at the dot clock rate). The result would be to reverse the effect of the dot clock.

Now on the very loose assumption that it would behave like a channel bank receiver with a common IF, then it would effectivly corelate all the multiples of the video signal giving you (a limited) gain of the wanted video signal whilst other (interfering) signals would benifit less.

You might well find that instead of needing a wideband receiver all you need is a high speed A-D without analog signal filtering (ie your current board), pre-faced by a high quality amplifier.

As I said just an idle though whilst my 4 year old son was creating mayhem amongst the wildlife in the countryside where we were walking.

At the moment, the only board of this type that I have is still with the project sponsor, but a second one is already ordered. Simply connecting the existing setup to a photomultiplier should already give a visible signal, but in order to really read text, I will have to add a digital deconvolution filter. There is plenty of space left on the FPGA, so I don’t expect any big problems. However, such deconvolution filters work best if they are applied to a very low-noise signal, therefore I’ll also need periodic-averaging in the COVISP.

A minor problem may be that this board was really designed for IF signal processing. The A/D converters are connected via transformers that will filter out frequencies below 1 MHz. The lack of DC coupling should not affect text readability, but it may affect the overall image quality.

Time permitting, I might also build my own wide-band receiver at some point, possibly as a little daughter card to go onto the FPGA development board. I could also reimplement the project on a smaller and cheaper FPGA. Both things combined might lead to a very powerful eavesdropping demonstration for less than £1000 worth of ingredients. There is a bit more on this written up in a recent paper “Eavesdropping attacks on computer displays”.

Thanks for the component suggestion, I’ll certainly have a look at the data sheet.