09 octobre 2010

Update on DSSS and video TX

This is an update to my previous measurements and comments on DSSS/FHSSS technology:
Some spektrum measurements

Recently I had problems with transmission video. It looks that some videos TX in 900 Mhz or 1.2-1.3Ghz band may generate harmonics on the 2.4Ghz band. See this thread:

So it may be very dangerous if you switch ON your RC equipment *before* the video TX: as in DSSS only 2 channels are used, you may well be unlucky and your RC may choose some channels that will be used also by the harmonics of your video TX.

To reduce this problem, you may try to switch ON your RC *after* switching ON the video TX, because then the RC will choose a free channel among those remaining free by the video TX.
But this solution is not very safe, there is another one here:
low pass video filter or here.

Overall, surely the best solution is to add this filter, but you can note that DSSS technology is not very safe in that situation.
Unless DSSS system could dynamically reallocate a new channel if the current one is detected as being saturated, FHSSS or better AFHSS has more chances to work fine in this environment, as shown here:
Review: Hitec Spectra AFHSS 2.4GHz receiver and Optima 7 module (part 2)

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