flyhi

My Feedback: (28)

Can someone please explain to me what this neg and pos shift stuff is? I use plain old hitec/futaba and don't understand the shift business. Are they interchangeable? if not what do you have to do to make a receiver with a different shift work with hitec/futaba, or can that even be done? thanks.

Big_Bird

My Feedback: (1)

FLYHI, when frequency modulation (FM) is being used is being used in the transmitter with pulse position modulation (PPM) coming from the transmitter encoder to the transmitter output RF module, the carrier RF frequency is shifted to follow the input from the encoder module. Thus, this is the FM modulation which carries the position information to the receiver. A positive frequency shift (low going high) will be from carrier center frequency + the information frequency shift (as generated by the encoder (PPM))and on another transmitter with high going low modulation, the frequency shift will be from carrier center frequency - the information frequency shift.

Some people don't understand the relationship of FM to PPM and PCM. A transmitter in PCM mode still uses FM modulated by the PCM encoder and the frequency shift will be the same direction no matter which mode the the transmitter is in.

The receiver, in its demodulation section, must be able to operate with the same frequency shift direction as the transmitter. Most receivers are fixed in this regard. However, FMA (WWW.FMADIRECT.COM) has a receiver that detects either frequency shift from the transmitter and I quote from their web site: "The new Magnum 8 is a full-featured, full-range, 8 channel, dual conversion receiver. Featuring automatic universal operation with any standard FM transmitter, the Magnum 8 automatically detects a change in positive or negative shift without the necessity of jumpers or dip switches. "

It would take a pretty high tech individual to change the shift direction on a receiver or transmitter. I don't worry about things like this because the receiver and transmitter will be compatable or they won't work at all with each other. At that point you get another receiver that will work.

Hope this helps,

Ken

Rodney

For the USA 72 MHZ units, Hitec and Futaba are compatable (negative shift) and Airtronics and JR are compatable (positive shift). I am not sure but I believe all 6 meter units use positive shift regardless of the manufacturer.

flyhi

My Feedback: (28)

Thanks so much for the info, fellows. to keep my head from hurting so much when I try to think, I'll just stick to what I know and hitec/futaba instead of trying to see if a jr will work

hebertjj

Flyhi,

I think the info given is right on and as noted, the main thing is that Hitec and Futaba are compatible, and JR and Airtronics are compatible. I don't know either about the 6 meter stuff although I can't understand why it would be different from the 75 Mhz stuff.

BUT, there are some exceptions. Hitec makes receivers for JR/AT transmitters (Pos shift) and also receivers for JR/AT transmitters using old Airtronics servo plugs (reversed + and - pins) so don't assume anything if you're getting used equipment or swapmeet stuff, read the label on the Rx! Hitec, GWS, and others make RXs that are switchable between Pos and Neg shift also. And serveral of the higher feaure model transmitters can switch between pos and neg shift output.

One of the problems with the term "FM" is that that modulation technique is more commonly referred to as Phase Shift Keying among radio people. It's not the same as the FM on your car radio. The FM-PSK modulation is used to transmit data to the receiver and that data is either in PPM form or digital form. PPM is Pulse Position Modulation and sends a string of pulses, the width of which controls the servo positions. Digital sends a string of pulses that are interpreted as ones and zeroes, according to pulse width, and decoded by the Rx into servo positions.

That was probably more info than you wanted, but just take what you need.

Howzat,
Joel