[NSRCA-discussion] RX choices

[J.Oddino]

PCM works better than PPM when both are working.  When you receive a good
frame of PCM it is perfect, that is, it is exactly what was encoded.  PPM is
always corrupted by the RF link.  RF noise shows up as pulse jitter in the
decoded signal.  This is usually only noticed when the signal to noise is
low as it might be at long range with bad antenna orientation.  This was
very apparent in the PPM-AM systems that proceeded PPM-FM.

The PCM system only has to decide it there is a pulse present or not.  The
PPM system must accurately measure the time between pulses.  This is easy
with nice square pulses but impossible with distorted rounded pulses.  The
net result is the PCM will work better and this is easy to demonstrate at
max range.

The Berg and Multiplex receivers that use Digital Signal Processors minimize
the jitter by essentially filtering the PPM signal.  I'm not sure of the
algorithms used but typically filtering adds a delay.  Apparently it is too
small to be detected by RC pattern pilots.  It would be interesting to know
if the heli guys can detect it.

Jim O

----- Original Message ----- 
From: "Fred Huber" <fhhuber at clearwire.com>
To: <NSRCA-discussion at lists.nsrca.org>
Sent: Thursday, September 14, 2006 7:31 PM
Subject: Re: [NSRCA-discussion] RX choices


> PCM is FM too... (well. there were a few AM PCM radios...)
>
> I don't really think there's any increase in range or  ability to avoid
> getting interference with PPM or PCM.... its more a matter of what the RX
> does when the radio signal gets "scrambled"
>
> Both use the same FM radio carrier wave.  Same power output... So the same
> interference source could scramble either.
>
> PPM will respond to the "percieved" command.
> PCM, being digital encoded, would immediately detect a "bad frame" and can
> reject it, holding the last good frame. (hinding the interferance from
> you...)
>
> A short blip of interference will be seen with the PPM as the aircraft
> jumps.
> A short blip of interference will likely be hidden from the pilot and the
> judges with PCM.
>
> PPM, by not hiding the short blips of interference will sometimes serve as
a
> warning that you need to get the plane down.  PPM your first indication
may
> be a full lockout.
>
> Advantages and disadvantages both ways...
>
>
> ----- Original Message ----- 
> From: "Wayne" <Whinkle1024 at msn.com>
> To: "NSRCA Mailing List" <nsrca-discussion at lists.nsrca.org>
> Sent: Thursday, September 14, 2006 10:28 AM
> Subject: Re: [NSRCA-discussion] RX choices
>
>
> >I have a question guys.
> >
> > I see in lots of posts guys using FM (PPM) RX's.  Why is this?
> >
> > In today's market there are so many choices of good PCM RX's I can't
> > understand why someone would actually choose to run FM. Back in the days
> > when I started RC there was no PCM and we had the choice of AM or FM. I
> > was
> > taught that my model was too important to me not to run the best link
> > possible, at the time FM was it.
> >
> > When PCM came in the price was much higher and I can see a reason to not
> > use
> > it. Today however both Futaba and JR have PCM RX's that are good for
under
> > $99, JR now even has one that PLL Synth. So why use FM?
> >
> > It is my understanding with today's PCM that noise rejection is so much
> > better with PCM that it seems along with its F/S properties both model
and
> > people are better protected. I can see the reasoning among average
modeler
> > its price why else do MPI, FMA and so on make such a profit on off brand
> > leads and servos that are in my opinion very sub par. I felt that in the
> > pattern community with available models shrinking, and the cost of
models
> > hitting larger prices that guys would trust their baby to better
> > equipment.
> > I mean a basic pattern model that is on the cheap is going to be $1000
to
> > $1500 all up. The difference in a 127 Futaba RX and say a Futaba 138DP
PCM
> > or JR 790 PCM version is about $40
> >
> > What am I missing?
> >
> > Wayne
> >
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> >
> >
> >
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