Contrary to common belief, the Vex system is highly R/C compatible (though they like to claim the system is proprietary and give no info out). The servos and motors have what most people would call male (Bare pin) connectors instead of the more normal R/C standard of what most people would call female (pins plug into them), but in the R/C community are called male. They call the ones with pins (and a shroud) female. Do a gender swap and they plug in to regular R/C receivers and operate just fine. This change was made, I believe, not to make them "incompatible", but to make them easy to plug in to breadboards.
As noted in my earlier post, the easiest solution is to pick up a regular R/C radio receiver that works with it, and there are several. The microreceiver from
http://www.microbotparts.com works with the Vex receiver crystals, and he has a 6 channel available for, I believe, $29.95. It is not on his web site yet, and you will have to contact him directly to get it. I have 4 so far. They are probably less than 1/4 the size of the Vex RX. Comparison pictures of the 4ch and 6ch with a standard sized servo and an MTroniks micro ESC are at:
http://picasaweb.google.com/wrenow/ReceiverTest
Note: Not all receivers are compatible. These, and the ones mentioned earlier, are ones that are.
By the way, I picked up quite a few on E-Bay as well, but look at the shipping costs. My case of 6 from All Electronics had $7 shipping, or only a little over a buck a radio. Shipping for 5 from another source was over $9 per radio, and I have seen up to $20 for shipping on them from E-Bay. My last E-bay batch was 5 radios for about $57 all in, shipping and all.
There are some gotchas: They come in any frequency you want, as long as you only want Ch89. The TX modules are proprietary. Period.
But, on the other hand, 4 channel frequency kits are relatively cheap (if you get a radio, get at least Kit A, so you have 5 possible frequencies).
Why have I stocked up? Got a club full of ships, including 9 club ships operational, 4 operational personal ships, etc. The format and layout is great for our purposes, and the price is incredible. Plus, the radio makes a good donor if you want the gimbals, etc. for some other project.
I have done a review of the TX and the receivers for our newsletter. If there is interest, I could edit it a bit to relevantize it and post it here.
I have not tried decoding the PPM stream with a Picaxe, but keep in mind one thing, the timing. What a decoder does is just strip the pulses out, in order, to the various servo pins. As such, the pulse frame of 20ms is not an issue. However, if you try to read the pulse stream, fiddle with it, and then output the pulses, you might have trouble doing so within the frame unless you output the pulses in parallel. Even then, it might be "tight."
Or strip the pulses like a decoder. But, then, you wouldn't need a Picaxe, just a decoder chip/circuit. For a more complete discussion of R/C Decoder theory and circuits, see
http://homepages.which.net/~paul.hills/Circuits/RxDecoder/RxDecoder.html
Cheers,
Wreno