RF/GHz video link for multichannel analog video transmission

I have one idea and I need your help people. What I'm planning to do is receive several TV signals at one (high) location with good reception, demodulate them to composite video level and then use RF link in GHz range to transmit it to another location with bad reception, and use old analogue SAT receiver for reception. I have no problem doing that for one terrestrial TV signal. But the problem I have is how to multiplex several (5 or more) TV signals and send them with one RF Transmitter instead of using 5 or more separate ones. Basically, what I think is needed is to modulate every TV signal with different carrier. For example TV1 video signal with 20MHz, TV2 video signal with 30MHz, TV3 with 40MHz, then mix it all togehter to get one baseband signal and then final mixer that would transpose everything to 1 GHz or so. Remember, it has to stay analogue all the time.
I hope that I explained thoroughly what I want and that you will be able to help me with schematics, links or advice. Thank you in advance.

Re: RF/GHz video link

Hi Sinip,

What you are proposing, a multichannel analog video transmission system, is feasible the way you have described it. I don't know of any products that do what you are proposing. The real world problem being the amount of bandwidth required to impliment this analog scheme.

A typical analog video link using FM modulation is typically allocated 20 MHz allowing for some guard band. Five channels at 20 MHz each = 100 MHz of bandwidth. You probably won't find much licensed spectrum at 1 GHz allowing for that much bandwidth. Maybe up at 15 or 23 GHz.

What you are proposing is not an easy development. Since this is an analog system you will have to take care with filtering and linearity to avoid intermodulation distortion with that many carriers. Also, if you have a location with bad reception transmitting a signal with 100 MHz bandwidth is going to worsen matters substantially.

There is a lot of inexpensive single channel systems at 5.8 GHz for surviellence so that may be a way to go. These days the more conventional way of transporting multiple video signals is digitally encoding MPEG2, digitally multiplex, digitally modulate and transmit. You will find lots of vendors offering these types of digital video products, Terawave, Moseley, to name of few off the top of my head. This is professional gear so it's a bit on the expensive side. But the digital thresholds are substantially better than the multiplex analog system so you find them in popular use.

Cheers.

Re: RF/GHz video link

rfgeek said:

Hi Sinip,

What you are proposing, a multichannel analog video transmission system, is feasible the way you have described it. I don't know of any products that do what you are proposing. The real world problem being the amount of bandwidth required to impliment this analog scheme.

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Cheers.

Click to expand...

Thanks for the answer. Yes I know that nowadays it mostly goes digital but that is no go for DIY solution. Reception of GHz signal in this case would not be a problem, because I have good line of sight between those two places but second place has terrible terrestrial TV reception due to local terrain configuration and obstructions from taller buildings. And concerning needed bandwith and legal issues, well, how to put it... Not an issue. ;-) There's noone who could even figure out what's going one above 1GHz here and there is more than enough space in SAT IF range (900 to 2100MHz)... So, all I need for start would be some oscillator that could proivide 20/40/60/80 and so on MHz for first stage of composite signal FM modulation. Could you help me with that? Some sort of PLL/divider or similar.

Re: RF/GHz video link

Sinip,

Ah, ok, no worries on the spectrum. Well, I appreciate your spirit here but there is a lot that would go into a develpment like this and this forum is not the best way to go about it. The devil is in the details.

For one video signals are very sensitive to amplitude and phase balance and not tolerant of distortion. In generating the multiplexed carrier if you simply convert the video carriers with a mixing process and add them as you describe you will get an amplitude modulated signal with huge peak levels and low average power with a propensity for intermodulation distortion. A better way to go is to FM modulate each carrier then sum. The receive side is more complicated. You will have to down-convert, channelize/filter and FM demodulate.

It's a lot of work.

Re: RF/GHz video link

rfgeek said:

Sinip,

Ah, ok, no worries on the spectrum. Well, I appreciate your spirit here but there is a lot that would go into a develpment like this and this forum is not the best way to go about it. The devil is in the details.

For one video signals are very sensitive to amplitude and phase balance and not tolerant of distortion. In generating the multiplexed carrier if you simply convert the video carriers with a mixing process and add them as you describe you will get an amplitude modulated signal with huge peak levels and low average power with a propensity for intermodulation distortion. A better way to go is to FM modulate each carrier then sum. The receive side is more complicated. You will have to down-convert, channelize/filter and FM demodulate.

It's a lot of work.

Click to expand...

Yes, the devil is in the details. Been there, done that, many times...
BTW, I was planning to frequency modulate composite video signals, of course, because receiving side would be old analogue SAT receiver (FM). Thus, receiving side would not be complicated at all, because I would use ordinary TV only as monitor and use SAT receiver as tuner connected to TV via SCART cable. In this way I would have to build only the transmitter.

Re: RF/GHz video link

Ok, I have a better picture now of what you are looking for (pun intended). You may be able to use one of the standard IC video channel modulators then.
There are some parts that have been around a while. The National LM2889 and Mot/Freescale MC1374 are VHF modulators intended modulating video on CH3/CH4 but I think it can be generally programmed for anything up 100 MHz depending on the tank. The TDA5670 is a VHF/UHF modulator covers from 48 MHz to 860 MHz.

Try looking at this new part, MC44BC374, a VHF/UHF PLL modulator with most everything you need built in except a way to multiplex your 5 channels together. I believe it is a microcontroller controlled (I2C) part which complicates things. Give it a look, see if this helps you:

h**p://www.freescale.com/files/app_specific_stand_prod/doc/data_sheet/MC44BC373.pdf

Cheers.

Re: RF/GHz video link

Thanks for the links, now I have some starting point. I'll check those datasheets and see what happens after that...