[NSRCA-discussion] RX choices

[Dean Pappas]

Well put, Jim.
I will do a short write-up soon on DSS.
Dean


Dean Pappas
Sr. Design Engineer
Kodeos Communications
111 Corporate Blvd.
South Plainfield, N.J. 07080
(908) 222-7817 phone
(908) 222-2392 fax
d.pappas at kodeos.com


-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org]On Behalf Of J.Oddino
Sent: Friday, September 15, 2006 1:58 PM
To: NSRCA Mailing List
Subject: Re: [NSRCA-discussion] RX choices


John;

A servo cannot be fast enough by definition.  Ideally it would follow the
transmitter stick position with no delay (and no overshoot).  Servos are
sometimes tested by inserting a sine wave signal at the input and measuring
the amplitude and phase delay at the output.  At very low frequency the
output will follow the input.  As the frequency of the input is increased
the out put never gets to the maximum displacement and lags the input.  You
can see this if you move your stick from side to side slowly and then as
fast as you can go.  So bottom line, fast is good.

Why expo you ask?  Expo is a means of getting better resolution of the
"Man-Machine" system.  Assuming a degree of stick gives a degree of servo
travel, one would have a hard time holding 0.1 degree.  With expo changing
the relationship to a larger stick movement to get a given servo movement it
is easier for the pilot to hold small angles of deflection.  That is he can
resolve more precise servo/control surface position.

Jim O

----- Original Message ----- 
From: "John Pavlick" <jpavlick at idseng.com>
To: "NSRCA Mailing List" <nsrca-discussion at lists.nsrca.org>
Sent: Thursday, September 14, 2006 9:49 PM
Subject: Re: [NSRCA-discussion] RX choices


> Jim,
>  The heli guys can feel the difference between the "old" Futaba 1024 PCM
and
> the new G3 - or so I'm told. One of my friends just got a 14Mz and said he
> had some interesting moments with his Rap 90 until he got used to it (the
> 2048 PCM with fast frame-rate). I'm still amazed at how we use fast
digital
> servos, then add expo to soften them up. I know there's more to it than
that
> but it seems kind of silly in a way.
>
> John Pavlick
> http://www.idseng.com
>
>
> > -----Original Message-----
> > From: nsrca-discussion-bounces at lists.nsrca.org
> > [mailto:nsrca-discussion-bounces at lists.nsrca.org]On Behalf Of J.Oddino
> > Sent: Friday, September 15, 2006 12:32 AM
> > To: NSRCA Mailing List
> > Subject: Re: [NSRCA-discussion] RX choices
> >
> >
> > 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
>
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