JuanRodriguez
Elite Veteran
Location: Rochester, New York
My Posts This: Topic Forum | Yes, no problem.......Your servos will work just fine with PCM........
Here is the best explanation describing the difference between ppm and pcm that I have seen in recent times as written by RR member dkshema. Just to avoid any confusion, folks oftentimes use the term "FM" vs "PCM" when they should be saying "PPM" vs "PCM" as both modulate on a "FM" signal........ The writeup should answer your other questions....
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| FM/PPM systems are analog, the signal is a pulse width that varies smoothly between about 1 msec to 2 msec with a neutral width of about 1.5 msec.
There is no digitization for an FM/PPM system, there are no discrete "steps". The encoded stick signal simply changes the transmitted frequency slightly as you move the stick. The width of each channel's pulse is what varies as you move the stick, that causes the transmitted frequency to vary slightly, the carrier is modulated by varying its frequency. The receiver is designed to recover the pulse-widths by processing the time-varying incoming frequency signals. The processing is analog, there are no discrete steps.
When you switch to PCM, the signal, while still being transmitted using frequency modulation, conveys totally different data. Instead of simply varying the transmitter output frequency to represent a varying pulse width, the transmitter digitizes (samples) the stick positions. If you've wondered what "PCM512", "PCM1024" or "PCM2048" means, it means that the stick position can be represented as one of 512, 1024, or 2048 individual, discrete positions, or steps.
The transmitter uses an analog to digital converter to convert the stick position into a number value.
A PCM 512 system can only represent the entire stick movement in one of 512 values. A PCM 1024 system can only represent the stick position in one of 1024 values. And a PCM 2048 system can only represent the stick position in one of 2048 values. This would be the "resolution" of the system. If overall stick movement is 100 degrees, the PCM 512 system chops that 100 degree movement into 512 pieces. The PCM 1024 system chops the 100 degree movement into 1024 pieces, and the PCM 2048 system chops it into 2048 pieces. The pieces get finer and finer.
But in all of those systems, the positions get values of 0, 1, 2, 3, 4, --- 511 (for a 512 bit system), --- 1023 for a 1024 bit system, and 2047 for a 2048 bit system.
As you move the stick in a PCM encoded system, there is a small range of movement that is encoded as a "0", another small range of movement encoded as a "1"....up to the upper limit. But, there are no "halves" or "quarters" or other fractions in between the integers 0,1,2,3, ... up through the upper limit of the system.
The digitization results in the stick being sensed as moving in discrete steps, not a smooth transition from full up to down....
The digitized values of each stick and switch position is assembled into an encoded serial bit stream as binary "ones" and "zeroes". Some other information regarding the validity of the encoded bit stream is thrown in, and the serial "words" are transmitted. The PCM receiver instead of simply seeing an encoded pulse width and recovering it (as an FM/PPM system would do)sees the varying incoming frequency signal as a series of "ones" and "zeros". The processor in the receiver assembles the incoming serial bit streams, looks at the encoded validity values, and decides if the latest "frame" of data is good or bad. If good, it's passed on to the decoder section where the discrete number values are run through a digital to analog conversion, to turn those encoded ones and zeroes back into a pulse whose width represents the encoded bit position that was assembled back in the transmitter. If the received frame is "bad" (or corrupted) it's NOT decoded, but if enough "bad" frames in sequence are received, the Failsafe function is triggered.
Since the digitization can only make discrete values at the sticks, the receiver can only reproduce the pulse widths from full "down" stick to full "up" stick as discrete values, or steps. It can't make a pulse-width that is "between" position 256 and 257, for example (where the analog system can, and does).
When you switch from FM/PPM to PCM, you may gain some signal processing ability to trigger a failsafe function, but you lose the fine resolution of an analog system. As a result, your servos don't move smoothly from stop to stop, they actually move in repeatable, discrete steps.
The analog system can have essentially infinite resolution of stick position, the digitized system can't.
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