[Max Ripple]

The mention of ultrasonic signals in relation to remote control,conjures up reminiscence of the early tv remote controls that utilised that system,and images of channel changes etc, when you rattled your key's!
But of course,it means that the frequencies employed were in the ultrasonic waveband,and some critters(eg bat's)can hear up to 150khz!!!
Bang&Olufsen developed an ultrasonic remote control system during the 'seventies,that was,and still is,for fans of vintage equipment,quite robust.
The system employ's piezo elements to transmit and receive the acoustic carrier,which is modulated by the keystroke data.
The infra red system(only the light beam is at ir frequency of course) was developed as remote controls became more complex.As with digital sampling,the higher the carrier frequency of the composite transmission,the more robust the operation.
Again,with B&O systems they employ 455kh rather then the common 38khz for the carrier frequency,although I understand that they employ Bluetooth rf technology in their current products remote control systems,as do also Sky.
There seem to be remote control tester's that analyse the output of vehicle remote control keyfob's,but not for common or garden versions?
Thanks again for your interest.
Nick