Demonstrating 2.4 ghz video downlinks at shows

[Bryce Allcorn - BritFlight.co.uk]

I thought the problem of interference would be more due to the bandwidth size of a FM signal from a camera transmitted in the Ghz range being vast enough to 'blank-out' a specific signal within that (transmitted) range.

[Clive Matthews]

Brian, your math is good. I was confusing EU and UK output regs. I meant directivity, although I suspect some of these cheap 2.4Ghz links splatter a bit as well.

 

With highly directional antennae, the system would of course be more efficient but would then require an extra hand to point!

 

My enthusiasm for 'enthusiast' equipment of this type is somewhat tempered by the amount of effort involved in a successful link being maintained from a moving object. I guess that it all depends on expectation and I sometimes wonder what people expect in terms of quality and reliability. 

[Erfolg]

I have tried to follow this thread, with difficulty, as a consequence of my lack of technical understanding.

 

I get the impression there are two positions. One that much amateur equipment produces a significant potential for interference, and as a consequence is not as efficient as it could be, but works well in most circumstances. The second is that good equipment generates little potential for interference, is more efficient, and is directionally sensitive.

 

I guess as a potentially ordinary user, I am often less interested in efficiency (other than anorak interest) if there is little apparent cost to me. However I am very keenly interested in a system that works well, in the manner I would use it and at a financial cost that I can afford. An example of this, is modern computers, where the code used is no longer optimised as a priority, uses a lot of computing power (memory etc.) and is generally inefficient compared with the optimum. However as a user, works well.

 

So is the trend likely to be a system which cases my 2.4 a problem?

 

Erfolg

[Clive Matthews]

The 2.4Ghz video downlink equipment on the market is cheap and plentiful. It is also efficient and well designed. The problem is in the application. Awareness of this issue and it's implications will result in problem free operation.

 

'Broadcast quality' (whatever that is these days!!)  video, requires a great deal of bandwidth (analogue) or smart encoding (digital) to transmit. To maintain this quality, professional systems employ a directional antenna system to ensure that the signal is received at maximum gain and minimum interference from outside sources. To give another analogy, a light shower spread over a square mile will have less impact on a person than if all that water was channelled down a pipe onto his head.

 

Most of the systems I have seen on the market have antennae designed to spread rather than focus. Many users then modify the antenna to achieve focus.  The problem then is ensuring that the antennae are pointing at each other at all times. For the 'lower quality' video required there may be no need to do this but it does improve the stream quality.

 

Interference both to and from is a problem for all these systems, regardless of cost and design. If you think to the last time you watched a motor race, the on car camera is rarely 'solid' all the way round the track. In order for these to co-exist with oncar telemetry and radio systems requires a great deal of careful positioning and shielding. The cheaper consumer units tend to be made of plastic and are not well shielded and will cause interference as well as being susceptible. 

 

Erflogs question hints at a hope for systems operating on other frequencies. They are available but at a cost (either money, licence or law). They wouldn't guarantee trouble free operation either.  5.8Ghz equipment is readily available, but not legal in this application. I've no doubt that it is used and there will be information on the net concerning it's useability.

[Erfolg]

My fear is that many other uses apparently use the 2.4 spectrum. I get the impression, that our transmitter power is low relative to some other transmitting devices. I worry that the apparent advantages for us of using 2.4 are wiped out by some  other device turning up in our operating environment, which can or does interfere with either our Rx or drowns out our Tx signal.

[Bryce Allcorn - BritFlight.co.uk]

They tend to be limited to the same transmission output (10mW) so it's not the power that is the problem.

 

A really simplistic way of looking at it is to imagine the 2.4G range as the alphabet.  so for arguments sake there is a bandwidth of 26 letters.

 

Our 2.4G radio transmitter transmits on say letter  E and had a bandwidth of one letter - it can frequency hop to any other letter in the alphabet.  But the video camera transmitter has a much larger bandwidth of say 5 letters R, S, T, U and V but is fixed (doesn't hop).  My understanding is that while the camera transmission can't take control of the anything within the planes Rx because all it is doing is sending a FM signal (the image) so can't affect the commands from the RC Tx it could possibly swamp the signal.

 

I need to see it happen, so far I've run this with a Spektrum DX6I , a Planet 5, a Esky 2.4G transmitter and a Radiologic T6EAP with no problems.

Edited By Bryce Allcorn - BritFlight.co.uk on 26/03/2010 12:17:35

[Chris Bott - Moderator]

Bryce I believe that Spektrum transmitters choose two free channels at switch on and then never change. If these channels were close together AND the Video system was switched on afterwards, then there is scope for it to swamp both channels??