The quartz QZ1 of the preceding receivers which set the work frequency is not on the market any more. It is replaced
by a locking phase loop allowing the free choice of an unspecified frequency on the band used, either the 41, or the 72 MHz. The step between channels is 5 kHz, giving us 101 frequencies in 72 MHz and 41 in 41 MHz. In practice, the RX19 ` scans ` on the 2 frequencies chosen by the user. The first is called a Normal Frequency (Fn) and he other, a Safety Frequency (Fs). In fact, one does not take precedence over the other, except that when turning the power on, the RX19 starts by using Fn. In addition, it will use whichever signal of either Fn or Fs it will receive clearly.
To be considered as " correct ", the received signal must have a " PCM signature ", identifying the transmitter’s owner. This code is a number selected between 0 and 255, inserted in the sequence. It should be noted that we do not use PCM (Pulse Coded Modulation) for the servo’s sequence. In our humble opinion, it only brings useless and even harmful complications, the signal to be transmitted being much more complex. We remain faithful to the PPM (Pulse Position Modulation), a simple and effective method which allows easy insertion of our code. We call this new R/C technique, " PPCM " modulation. The figure above shows the principle used. The great advantage of PPCM is to remain completely compatible with existing receivers, the decoders being sensitive only to the front signal wave, they do not take any account of the position of the rear signal wave and are therefore completely unaware of the inserted code. One can thus, with the same transmitter, control a RX19 (with PPCM) or a RX17 (without PPCM)!
In a normal receiver, such as a RX17 or a RX18, the decoded signal arrives directly at the servos. If the signal is good, all is perfect. If it is bad, so be it and the servos go anywhere and the plane does aerobatics! Let us note how in one such receiver, a fault detector highlights an anomaly only after it has occurred. Obviously! In this case, the servos reacted... and made an inopportune movement... and the plane...