The way these cars work is called OOK transmission (on-off keyed transmission). The controller chip makes digital pulse codes, that actually turn the transmitter on/off at something like 13-17 times a second (don't know the exact numbers)-but it varies on what buttons you press.
The receiver picks up the "chopped" radio signal code, and the chip in the car decodes the pulses and triggers it's outputs (FWD-REV...) according to what "pulse code" it's getting.
Hold the transmitter real close to an FM radio and you should hear different beeping and buzzing sounds for each button pressed. Kinds sounds like an old 56k modem dialing up
A 27Mhz transmitter is actually sending 2 signals at once, first, the 27Mhz radio signal itself-- (the carrier), and second, the pulse codes (on/off breaks in that signal)-- the modulation.
If you could see the radio signal from the 27Mhz, it would be like a wavy line with chunks cut out of it (the pulse breaks)
/\/\/\/\/\/ /\/\/\/\/\/ /\/\/\/\/\/ /\/\/\/\/\/
The 57Mhz would look the same, except the wavy parts would be much more compressed (about twice as many /\/\/\/ in the same space)
It would look more like.... ||||||||| |||||||||| ||||||||| |||||||||
--well you get the idea, waves are so close that they look more like up/down lines.
The only difference you might notice is the range you get with different controllers, some have those annoying "cripple capacitors that drop the signal power to almost nothing, but usually it's buildings and appliances the mess with the radio signal, some frequencies just seem to make it, while some don't.
I tried a 27Mhz and a 49Mhz outside and they were almost the same, but in my basement the 27's have less range...
If you have a baby monitor running then the 49Mhz is going to have real problems, living close to a schoolbus dispatch CB radio tower will mess up the 27's bigtime
Linear Mode
