PCM vs PPM [Archive]

Tom Ivar Helbekkmo

Apr 20, 2003, 05:44 AM

"Paul Aspinall" <paul@nospamaspy.co.uk> writes:

> Can anyone offer an explanation or a summary of the differences
> between PCM and PPM?

The basic operation of the transmitter and receiver are the same with
both systems: the transmitter goes through all the channels in order,
transmitting the position of each one, and then goes back and does it
again, over and over. The difference lies in how these values are
transmitted.

PPM stands for Pulse Period Modulation. With PPM, each channel (servo
output, that is) is transmitted as a single pulse, the length of which
tells the receiver what the deflection of that particular servo should
be right now.

PCM stands for Pulse Code Modulation. With PCM, each channel is
transmitted as a series of pulses, which can each be long or short,
signifying a bit (binary digit). 10 pulses per channel gives a
resolution of 2^10, or 1024 different positions for each servo.
Checksums are used to ensure that the data is received correctly.

How does this affect you? A PPM system doesn't know whether the data
it receives is good or bad, but a PCM system does. Thus, when you
experience a glitch, the PPM system will typically jiggle its servos
while you're out of control, while the PCM system will hold their
position, waiting for good data. Much can be said for each -- some
people prefer PPM, because they'll immediately know if if they have
interference. Others prefer PCM, because glitching won't cause
sudden, unwanted changes in control surface deflection.

A definite plus with a PCM system is that they always have a fail-safe
mode for when radio control is completely lost. This is normally set
so that the engine is stopped, limiting the amount of damage done in
the ensuing crash. Fail-safes can be added to PPM systems, though.

-tih
--
Tom Ivar Helbekkmo, Senior System Administrator, EUnet Norway
www.eunet.no T: +47-22092958 M: +47-93013940 F: +47-22092901

JDL

Apr 20, 2003, 05:44 AM

Hi Paul

If you follow the link I gave you above, it explains some of the basics
quite well, Before going and buying a radio It is always best to check what
other flyers use in your area, because it changes from area to area, and
people always say their mode is best. ( I recommend contacting a local
club).

"Tom Ivar Helbekkmo" <tih+nr@eunetnorge.no> wrote in message
news:86ista1yz6.fsf@athene.i.eunet.no...
> "Paul Aspinall" <paul@nospamaspy.co.uk> writes:
>
> > Can anyone offer an explanation or a summary of the differences
> > between PCM and PPM?
>
> The basic operation of the transmitter and receiver are the same with
> both systems: the transmitter goes through all the channels in order,
> transmitting the position of each one, and then goes back and does it
> again, over and over. The difference lies in how these values are
> transmitted.
>
> PPM stands for Pulse Period Modulation. With PPM, each channel (servo
> output, that is) is transmitted as a single pulse, the length of which
> tells the receiver what the deflection of that particular servo should
> be right now.
>
> PCM stands for Pulse Code Modulation. With PCM, each channel is
> transmitted as a series of pulses, which can each be long or short,
> signifying a bit (binary digit). 10 pulses per channel gives a
> resolution of 2^10, or 1024 different positions for each servo.
> Checksums are used to ensure that the data is received correctly.
>
> How does this affect you? A PPM system doesn't know whether the data
> it receives is good or bad, but a PCM system does. Thus, when you
> experience a glitch, the PPM system will typically jiggle its servos
> while you're out of control, while the PCM system will hold their
> position, waiting for good data. Much can be said for each -- some
> people prefer PPM, because they'll immediately know if if they have
> interference. Others prefer PCM, because glitching won't cause
> sudden, unwanted changes in control surface deflection.
>
> A definite plus with a PCM system is that they always have a fail-safe
> mode for when radio control is completely lost. This is normally set
> so that the engine is stopped, limiting the amount of damage done in
> the ensuing crash. Fail-safes can be added to PPM systems, though.
>
> -tih
> --
> Tom Ivar Helbekkmo, Senior System Administrator, EUnet Norway
> www.eunet.no T: +47-22092958 M: +47-93013940 F: +47-22092901

David

Apr 20, 2003, 05:44 AM

Tom,
Your expansion of PPM should actually have been 'Pulse Postion Modulation'
although from a technical viewpoint I agree with your 'Period' definition as
this is a better description of the modulation method, nontheless PPM is
positional in the time domain hence the definition.
David

"Tom Ivar Helbekkmo" <tih+nr@eunetnorge.no> wrote in message
news:86ista1yz6.fsf@athene.i.eunet.no...
> "Paul Aspinall" <paul@nospamaspy.co.uk> writes:
>
> > Can anyone offer an explanation or a summary of the differences
> > between PCM and PPM?
>
> The basic operation of the transmitter and receiver are the same with
> both systems: the transmitter goes through all the channels in order,
> transmitting the position of each one, and then goes back and does it
> again, over and over. The difference lies in how these values are
> transmitted.
>
> PPM stands for Pulse Period Modulation. With PPM, each channel (servo
> output, that is) is transmitted as a single pulse, the length of which
> tells the receiver what the deflection of that particular servo should
> be right now.
>
> PCM stands for Pulse Code Modulation. With PCM, each channel is
> transmitted as a series of pulses, which can each be long or short,
> signifying a bit (binary digit). 10 pulses per channel gives a
> resolution of 2^10, or 1024 different positions for each servo.
> Checksums are used to ensure that the data is received correctly.
>
> How does this affect you? A PPM system doesn't know whether the data
> it receives is good or bad, but a PCM system does. Thus, when you
> experience a glitch, the PPM system will typically jiggle its servos
> while you're out of control, while the PCM system will hold their
> position, waiting for good data. Much can be said for each -- some
> people prefer PPM, because they'll immediately know if if they have
> interference. Others prefer PCM, because glitching won't cause
> sudden, unwanted changes in control surface deflection.
>
> A definite plus with a PCM system is that they always have a fail-safe
> mode for when radio control is completely lost. This is normally set
> so that the engine is stopped, limiting the amount of damage done in
> the ensuing crash. Fail-safes can be added to PPM systems, though.
>
> -tih
> --
> Tom Ivar Helbekkmo, Senior System Administrator, EUnet Norway
> www.eunet.no T: +47-22092958 M: +47-93013940 F: +47-22092901

Fred McClellan

Apr 20, 2003, 05:44 AM

On Sat, 19 Apr 2003 12:39:19 +0100, "Paul Aspinall"
<paul@nospamaspy.co.uk> wrote:

>Anyone who has read my posts knows that I am a novice at this..... so....
>
>Can anyone offer an explanation or a summary of the differences between PCM
>and PPM?
>

Rather than re-start a thread which has been reiterated _numerous
times_ in these news groups (you did cross-post your query), I will
simply point you to www.google.com and recommend that you use the
search feature to find the PCM/PPM threads. There are several threads
in the archives.

It'll be much quicker for you than waiting several days while all and
sundry trot out their favorite reasons for selecting one over the
other. Those opinions are already in the archives as well.

Paul Aspinall

Apr 20, 2003, 05:44 AM

Thanks for this info.
It is very interesting.

Now my confusion, is.... Do I use PCM or PPM??

If you are going to build an expensive heli, which would you choose??

Thanks

"Tom Ivar Helbekkmo" <tih+nr@eunetnorge.no> wrote in message
news:86ista1yz6.fsf@athene.i.eunet.no...
> "Paul Aspinall" <paul@nospamaspy.co.uk> writes:
>
> > Can anyone offer an explanation or a summary of the differences
> > between PCM and PPM?
>
> The basic operation of the transmitter and receiver are the same with
> both systems: the transmitter goes through all the channels in order,
> transmitting the position of each one, and then goes back and does it
> again, over and over. The difference lies in how these values are
> transmitted.
>
> PPM stands for Pulse Period Modulation. With PPM, each channel (servo
> output, that is) is transmitted as a single pulse, the length of which
> tells the receiver what the deflection of that particular servo should
> be right now.
>
> PCM stands for Pulse Code Modulation. With PCM, each channel is
> transmitted as a series of pulses, which can each be long or short,
> signifying a bit (binary digit). 10 pulses per channel gives a
> resolution of 2^10, or 1024 different positions for each servo.
> Checksums are used to ensure that the data is received correctly.
>
> How does this affect you? A PPM system doesn't know whether the data
> it receives is good or bad, but a PCM system does. Thus, when you
> experience a glitch, the PPM system will typically jiggle its servos
> while you're out of control, while the PCM system will hold their
> position, waiting for good data. Much can be said for each -- some
> people prefer PPM, because they'll immediately know if if they have
> interference. Others prefer PCM, because glitching won't cause
> sudden, unwanted changes in control surface deflection.
>
> A definite plus with a PCM system is that they always have a fail-safe
> mode for when radio control is completely lost. This is normally set
> so that the engine is stopped, limiting the amount of damage done in
> the ensuing crash. Fail-safes can be added to PPM systems, though.
>
> -tih
> --
> Tom Ivar Helbekkmo, Senior System Administrator, EUnet Norway
> www.eunet.no T: +47-22092958 M: +47-93013940 F: +47-22092901

Chuck W

Apr 20, 2003, 05:44 AM

Use PCM if you can afford it.

"Paul Aspinall" <paul@nospamaspy.co.uk> wrote in message
news:9tcoa.694$H65.248@news-binary.blueyonder.co.uk...
> Thanks for this info.
> It is very interesting.
>
>
> Now my confusion, is.... Do I use PCM or PPM??
>
> If you are going to build an expensive heli, which would you choose??
>
> Thanks
>
>
>
> "Tom Ivar Helbekkmo" <tih+nr@eunetnorge.no> wrote in message
> news:86ista1yz6.fsf@athene.i.eunet.no...
> > "Paul Aspinall" <paul@nospamaspy.co.uk> writes:
> >
> > > Can anyone offer an explanation or a summary of the differences
> > > between PCM and PPM?
> >
> > The basic operation of the transmitter and receiver are the same with
> > both systems: the transmitter goes through all the channels in order,
> > transmitting the position of each one, and then goes back and does it
> > again, over and over. The difference lies in how these values are
> > transmitted.
> >
> > PPM stands for Pulse Period Modulation. With PPM, each channel (servo
> > output, that is) is transmitted as a single pulse, the length of which
> > tells the receiver what the deflection of that particular servo should
> > be right now.
> >
> > PCM stands for Pulse Code Modulation. With PCM, each channel is
> > transmitted as a series of pulses, which can each be long or short,
> > signifying a bit (binary digit). 10 pulses per channel gives a
> > resolution of 2^10, or 1024 different positions for each servo.
> > Checksums are used to ensure that the data is received correctly.
> >
> > How does this affect you? A PPM system doesn't know whether the data
> > it receives is good or bad, but a PCM system does. Thus, when you
> > experience a glitch, the PPM system will typically jiggle its servos
> > while you're out of control, while the PCM system will hold their
> > position, waiting for good data. Much can be said for each -- some
> > people prefer PPM, because they'll immediately know if if they have
> > interference. Others prefer PCM, because glitching won't cause
> > sudden, unwanted changes in control surface deflection.
> >
> > A definite plus with a PCM system is that they always have a fail-safe
> > mode for when radio control is completely lost. This is normally set
> > so that the engine is stopped, limiting the amount of damage done in
> > the ensuing crash. Fail-safes can be added to PPM systems, though.
> >
> > -tih
> > --
> > Tom Ivar Helbekkmo, Senior System Administrator, EUnet Norway
> > www.eunet.no T: +47-22092958 M: +47-93013940 F: +47-22092901
>
>

Fred McClellan

Apr 20, 2003, 05:44 AM

On Sat, 19 Apr 2003 12:26:47 -0400, "Chuck W"
<cwiley101@comcast.spamdeath.net> wrote:

>Use PCM if you can afford it.
>

Absolutely.

Most PCM systems can be switched to PPM if you want.

That way if you decide you don't _like_ your throttle going to idle
while you're in a hover 50 feet up, you can try PPM on your next heli.

David

Apr 20, 2003, 05:44 AM

Fred,
A PCM receiver cannot be changed to PPM, all that can be done is to switch
off the fail-safe functions in-built with PCM.
PCM modulation is radically different to PPM.
David

"Fred McClellan" <modelbof@bellsouth.net> wrote in message
news:b704av8b8q8kia8vd6fqui422a269c0ttb@4ax.com...
> On Sat, 19 Apr 2003 12:26:47 -0400, "Chuck W"
> <cwiley101@comcast.spamdeath.net> wrote:
>
> >Use PCM if you can afford it.
> >
>
> Absolutely.
>
> Most PCM systems can be switched to PPM if you want.
>
> That way if you decide you don't _like_ your throttle going to idle
> while you're in a hover 50 feet up, you can try PPM on your next heli.
>

Beav

Apr 22, 2003, 04:02 AM

"Paul Aspinall" <paul@nospamaspy.co.uk> wrote in message
news:9tcoa.694$H65.248@news-binary.blueyonder.co.uk...
> Thanks for this info.
> It is very interesting.
>
>
> Now my confusion, is.... Do I use PCM or PPM??
>
> If you are going to build an expensive heli, which would you choose??

No contest Paul. PCM every time.

--
Beav

Please note my E-mail address is "beavis dot original at ntlworld dot com"
(with the obvious changes)

Beavisland now lives at
www.beavisoriginal.co.uk

David Williams

Apr 22, 2003, 04:02 AM

Chuck W <cwiley101@comcast.spamdeath.net> wrote in message
news:Pi6dndrwFOBU5TyjXTWcqw@comcast.com...
> Use PCM if you can afford it.
>
Just bought one of the new JR R770 RXs - 7 channel, same size as the R700
but SPCM. Inwoods are selling them for GBP54.99, so not that
expensive.........

David

gavin@mindless.com

Apr 22, 2003, 04:02 AM

On Sat, 19 Apr 2003 12:26:47 -0400, "Chuck W"
<cwiley101@comcast.spamdeath.net> wrote:

>Use PCM if you can afford it.
>

It's probably worth checking the with your local club, I bought a PCM
system off the bat for my trainer and then found it caused a lot of
problems buddying up, (I've a ff7 they had ff8) all of which were down
to using PCM, and it's not as simple for the two transmitters to buddy
with. Bought a PPM and all's well, put the pcm rx on the side for
another plane once I've got my A

Having said that it's more than likely you won't be buddying for a
heli, but probably worth checked as club rules might mean you have to
for a while

>"Paul Aspinall" <paul@nospamaspy.co.uk> wrote in message
>news:9tcoa.694$H65.248@news-binary.blueyonder.co.uk...
>> Thanks for this info.
>> It is very interesting.
>>
>>
>> Now my confusion, is.... Do I use PCM or PPM??
>>
>> If you are going to build an expensive heli, which would you choose??
>>
>> Thanks
>>
>>
>>

Fred McClellan

Apr 22, 2003, 04:02 AM

On Sun, 20 Apr 2003 09:20:09 +0000 (UTC), "David"
<david@noname.bt.com> wrote:

>Fred,
>A PCM receiver cannot be changed to PPM, all that can be done is to switch
>off the fail-safe functions in-built with PCM.
>PCM modulation is radically different to PPM.

I _knew_ that . . . .

I was thinking about the expensive bit, the Tx.

Didn't consider the Rx, particularly since I have both flavors in all
my systems, that is, PPM _and_ PCM receivers.

Doesn't everyone do it that way . . . ?
<g>

duh . . .

Tramm Hudson

Apr 22, 2003, 04:02 AM

Paul Aspinall <paul@nospamaspy.co.uk> wrote:
> Can anyone offer an explanation or a summary of the differences
> between PCM and PPM?

Other posters have given you info on the differences in the user
perspective of PCM and PPM, but if you are interested in the technical
distinctions, I have done some work reverse engineering Futaba's PCM1024
protocol. Full details are here:

http://autopilot.sourceforge.net/pcm.html

I'm flying my helicopter via an onboard computer that is handling pitch
and roll stabilization, engine governing and will soon also take over
the navigation duties.

Trammell
--
-----|----- hudson@osresearch.net W 240-283-1700
*>=====[]L\ hudson@rotomotion.com M 505-463-1896
' -'-`- http://www.swcp.com/~hudson/ KC5RNF

Patrick Free

Apr 22, 2003, 04:02 AM

In article <b81g5n$v9e$1@iruka.swcp.com>, hudson@swcp.com says...
> http://autopilot.sourceforge.net/pcm.html
>
> I'm flying my helicopter via an onboard computer that is handling pitch
> and roll stabilization, engine governing and will soon also take over
> the navigation duties.
>
> Trammell
>

Trammel,

I would like to hear more about this. I do PIC programming, and it
seems like a fairly natural application.

Patrick

David Lodge

Apr 30, 2003, 04:01 AM

I believe PCM Rx's output a higher frae rate than PPM Rx's - attach a
servo to each Rx and try and move it to make it "buzz" and listen to
the pitch. It'll be higher with a PCM Rx - which means it'll hold
better.

I won't even think about using a PPM Rx for any of my helis, esp my
electric Logo 10.

David.

Patrick Free <pfree@nukem.com> wrote in message news:<MPG.190e0a5481c7ab37989753@news-server>...
> In article <b81g5n$v9e$1@iruka.swcp.com>, hudson@swcp.com says...
> > http://autopilot.sourceforge.net/pcm.html
> >
> > I'm flying my helicopter via an onboard computer that is handling pitch
> > and roll stabilization, engine governing and will soon also take over
> > the navigation duties.
> >
> > Trammell
> >
>
> Trammel,
>
> I would like to hear more about this. I do PIC programming, and it
> seems like a fairly natural application.
>
> Patrick

Six_O'Clock_High

Apr 30, 2003, 04:01 AM

You are right about the frame rates and servo hold signals, but just after
that is where our opinions part company. Having lost a couple of birds to
PCM lockout, I am not sure I buy into your thesis.

Since helicopters are just collections of part all trying to beat each other
to death while rejecting gravity, there are very frequently loose parts.
This means that there is a very high probablibility of vibration created RFI
which may cause your PCM Rx to go into lock up. Not a good thing from what
I know. I think the PPM is better for the chopper for that very reason.

Just my opinion based on experience in both realms. YMMV

Jim Branaum
AMA 1428
Six_O'clock_High@Target_Lock.guns

"David Lodge" <david@cherrybank.co.uk> wrote in message
news:6e6cfcb.0304290823.12b091a6@posting.google.co m...
> I believe PCM Rx's output a higher frae rate than PPM Rx's - attach a
> servo to each Rx and try and move it to make it "buzz" and listen to
> the pitch. It'll be higher with a PCM Rx - which means it'll hold
> better.
>
> I won't even think about using a PPM Rx for any of my helis, esp my
> electric Logo 10.
>
> David.
>
> Patrick Free <pfree@nukem.com> wrote in message
news:<MPG.190e0a5481c7ab37989753@news-server>...
> > In article <b81g5n$v9e$1@iruka.swcp.com>, hudson@swcp.com says...
> > > http://autopilot.sourceforge.net/pcm.html
> > >
> > > I'm flying my helicopter via an onboard computer that is handling
pitch
> > > and roll stabilization, engine governing and will soon also take over
> > > the navigation duties.
> > >
> > > Trammell
> > >
> >
> > Trammel,
> >
> > I would like to hear more about this. I do PIC programming, and it
> > seems like a fairly natural application.
> >
> > Patrick
>

Ed Cregger

Apr 30, 2003, 04:01 AM

I'm jumping in here late, Jim.

To address the original poster's post, the frame rate is the same for PCM or
PPM. What is different is that the PCM rig can send finer control inputs
than PPM, in most systems.

Ed Cregger

"Six_O'Clock_High" <Target_Lock@guns.com> wrote in message
news:7ezra.47261$4P1.4414485@newsread2.prod.itd.ea rthlink.net...
> You are right about the frame rates and servo hold signals, but just after
> that is where our opinions part company. Having lost a couple of birds to
> PCM lockout, I am not sure I buy into your thesis.
>
> Since helicopters are just collections of part all trying to beat each
other
> to death while rejecting gravity, there are very frequently loose parts.
> This means that there is a very high probablibility of vibration created
RFI
> which may cause your PCM Rx to go into lock up. Not a good thing from
what
> I know. I think the PPM is better for the chopper for that very reason.
>
> Just my opinion based on experience in both realms. YMMV
>
> Jim Branaum
> AMA 1428
> Six_O'clock_High@Target_Lock.guns
>
>
> "David Lodge" <david@cherrybank.co.uk> wrote in message
> news:6e6cfcb.0304290823.12b091a6@posting.google.co m...
> > I believe PCM Rx's output a higher frae rate than PPM Rx's - attach a
> > servo to each Rx and try and move it to make it "buzz" and listen to
> > the pitch. It'll be higher with a PCM Rx - which means it'll hold
> > better.
> >
> > I won't even think about using a PPM Rx for any of my helis, esp my
> > electric Logo 10.
> >
> > David.
> >
> > Patrick Free <pfree@nukem.com> wrote in message
> news:<MPG.190e0a5481c7ab37989753@news-server>...
> > > In article <b81g5n$v9e$1@iruka.swcp.com>, hudson@swcp.com says...
> > > > http://autopilot.sourceforge.net/pcm.html
> > > >
> > > > I'm flying my helicopter via an onboard computer that is handling
> pitch
> > > > and roll stabilization, engine governing and will soon also take
over
> > > > the navigation duties.
> > > >
> > > > Trammell
> > > >
> > >
> > > Trammel,
> > >
> > > I would like to hear more about this. I do PIC programming, and it
> > > seems like a fairly natural application.
> > >
> > > Patrick
> >
>
>
>

Tramm Hudson

Apr 30, 2003, 04:01 AM

Ed Cregger <ecregger@hotmail.com> wrote:
> To address the original poster's post, the frame rate is the same for PCM or
> PPM. What is different is that the PCM rig can send finer control inputs
> than PPM, in most systems.

Actually, David Lodge was right when he wrote:

> I believe PCM Rx's output a higher frae rate than PPM Rx's - attach a
> servo to each Rx and try and move it to make it "buzz" and listen to
> the pitch. It'll be higher with a PCM Rx - which means it'll hold
> better.

The frame rate on the radio is not necessarily the same from the frame
rate generated by the receiver. In most PPM systems, the receiver has
no clock generation circuitry, so the frame rate depends on the rate at
which the signals are transmitted. In most 8 channel units, this is a
50 Hz signal, with each channel transmitted one after another. The
servos are driven directly by the received pulse train, so they are
driven in a serial fashion.

All PCM receivers hold onto the commanded positions and can regenerate
the servo commands at a rate independent of the radio transmission.
The PCM format also sends each channel twice per frame, so the RX can
update the servos twice as fast. Most of the ones with which I have
worked update two servos at a time rather than the serial one-at-a-time
of the PPM receiver.

If you're interested in details on the actual protocols, I've written
a description of my reverse engineering efforts here:

http://autopilot.sourceforge.net/pcm.html

As for the finer controls, it depends on a number of factors. PPM
is an analog signal, so the number of steps depends on the quality
of the signal, the quality of the receiver and the amount of ambient
RF noise. Futaba PCM is a digital signal with exactly 1024 positions
(10 bits of data).

My custom control board generates a PPM signal with 4192 positions (13
bits of data), which is even better than the PCM signal. However, I
send this pulse train directly to the servo driver, so there is no RF
loss involved.

Trammell
--
-----|----- hudson@osresearch.net W 240-283-1700
*>=====[]L\ hudson@rotomotion.com M 505-463-1896
' -'-`- http://www.swcp.com/~hudson/ KC5RNF

Bob Adkins

May 03, 2003, 04:01 AM

On 29 Apr 2003 09:23:07 -0700, david@cherrybank.co.uk (David Lodge) wrote:

>I won't even think about using a PPM Rx for any of my helis, esp my
>electric Logo 10.

Either are way faster than your hand movements.

Bob

Steve Simpson

May 03, 2003, 04:01 AM

> This means that there is a very high probablibility of vibration created
RFI
> which may cause your PCM Rx to go into lock up. Not a good thing from
what
> I know.

There is no such thing as 'lock up'. The receiver is not 'locked', it is
still receiving a signal just as a PPM receiver.

The difference is that the PCM receiver recognizes interference and does no
act on it, giving the impression that it is 'locked' or not functioning. In
fact it is functioning, looking at the incoming signal and trying to get a
good command at which time it immediately resumes moving the servos.

Most interference is momentary so while the PCM is patiently waiting for a
clear signal, the PPM receiver is blindly moving the servos according to
whatever the interference dictates . . . which can be full travel on any
servo.

> I think the PPM is better for the chopper for that very reason.
>

During the period of interference, you have no control of the heli with
either system. It may SEEM like you have some control with PPM because the
heli is still reacting, but it is not reacting to you. The interference is
flying your bird.

Static bad enough to cause a PCM Rx to ignore scrambled commands will
certainly be making a PPM go nuts also. The difference is that there is no
way to predict what the PPM is going to do. If the interference makes the
signal look like 'full left aileron', that is what the PPM will do.

Steve Simpson

May 03, 2003, 04:01 AM

> The expression "locked out" is not technical in its origin.

Well, that is certainly true. Neither is 'lock-up'.

It would be interesting to know how these phrases actually got started,
since neither is accurate.

>
> If I move the sticks and the servos don't move, for all practical
purposes,
> I am locked out.

I'd agree with that assement, Ed.

However, what is at issue is the differences in PCM vs PPM and by your
definition, you would be equally 'locked out' of PPM because you would not
actually be controlling those servos either.

> I am interested in controlling my model, not whether the computer in my
> receiver is occupied with problem solving.

It wouldn't matter if the receiver was occupied baking brownies. The point
is that in terms of pilot control, there is no 'lock out' mode that is
specific to PCM that precludes control that would otherwise be available
under PPM.

So for 'all practical purposes' radio interference 'disconnects' the sticks
from the servos in either system. The difference is in the consequences of
that disconnect.

It is a difficult subject. A google search on this group would net every
conceivable definition of any term used to describe the function of PCM. The
best thing to do is read the technical papers published by the
manufacturers.

For example, at one time it seemed widely believed that the 'time delay'
inherent in PCM was the amount of time that the receiver would remain
'locked' during ANY interference event . . . or that this was the amount of
time needed before the receiver would resume functioning after the
interference cleared. Both are admittedly scary thoughts, but neither is
actually the case.

As to PCM supposed 'rejection' of interference, my understanding (which may
not be technically accurate) is that because PCM uses the carrier only to
transmit digital information, it is less sensitive to interference than
would be the analogue of PPM. While it 'sees' exactly the same interference,
PCM is only effected by distortions severe enough to make a 0 into a 1 or
vice versa whereas PPM is degraded by all interference. Someone well versed
in electronics or radio (i.e. not me) perhaps could explain it better.

Ed Cregger

May 03, 2003, 04:01 AM

At one time, there was a significant time delay, Steve. I forget the
particulars these days, but some of the early PCM rigs were beasts and did
things in a most awkward manner.

I have been in situations where I felt that I had more control with PPM
while experiencing interference than with PCM. This is wildly subjective and
is not to be taken as a firm belief on my part.

Flying low to the ground with very maneuverable and light aircraft with high
speed servos can give you the impression that PCM has a small, but noticable
built-in delay, even without interference. I have flow both Futaba and JR
1024 PCM and have had the feeling I am describing.

I fly both types of receivers, when I can fly at all.

Ed Cregger

"Steve Simpson" <simpsons34@NIXcox.net> wrote in message
news:G7Bsa.19$fm4.1@fed1read06...
>
> > The expression "locked out" is not technical in its origin.
>
> Well, that is certainly true. Neither is 'lock-up'.
>
> It would be interesting to know how these phrases actually got started,
> since neither is accurate.
>
> >
> > If I move the sticks and the servos don't move, for all practical
> purposes,
> > I am locked out.
>
> I'd agree with that assement, Ed.
>
> However, what is at issue is the differences in PCM vs PPM and by your
> definition, you would be equally 'locked out' of PPM because you would not
> actually be controlling those servos either.
>
> > I am interested in controlling my model, not whether the computer in my
> > receiver is occupied with problem solving.
>
> It wouldn't matter if the receiver was occupied baking brownies. The point
> is that in terms of pilot control, there is no 'lock out' mode that is
> specific to PCM that precludes control that would otherwise be available
> under PPM.
>
> So for 'all practical purposes' radio interference 'disconnects' the
sticks
> from the servos in either system. The difference is in the consequences of
> that disconnect.
>
> It is a difficult subject. A google search on this group would net every
> conceivable definition of any term used to describe the function of PCM.
The
> best thing to do is read the technical papers published by the
> manufacturers.
>
> For example, at one time it seemed widely believed that the 'time delay'
> inherent in PCM was the amount of time that the receiver would remain
> 'locked' during ANY interference event . . . or that this was the amount
of
> time needed before the receiver would resume functioning after the
> interference cleared. Both are admittedly scary thoughts, but neither is
> actually the case.
>
> As to PCM supposed 'rejection' of interference, my understanding (which
may
> not be technically accurate) is that because PCM uses the carrier only to
> transmit digital information, it is less sensitive to interference than
> would be the analogue of PPM. While it 'sees' exactly the same
interference,
> PCM is only effected by distortions severe enough to make a 0 into a 1 or
> vice versa whereas PPM is degraded by all interference. Someone well
versed
> in electronics or radio (i.e. not me) perhaps could explain it better.
>
>
>
>
>
>
>

Tramm Hudson

May 03, 2003, 04:01 AM

Steve Simpson <simpsons34@NIXcox.net> wrote (in part):
> As to PCM supposed 'rejection' of interference, my understanding (which may
> not be technically accurate) is that because PCM uses the carrier only to
> transmit digital information, it is less sensitive to interference than
> would be the analogue of PPM. While it 'sees' exactly the same interference,
> PCM is only effected by distortions severe enough to make a 0 into a 1 or
> vice versa whereas PPM is degraded by all interference.

PCM is even more robust than that. It uses a 6B10 encoding that has a
small amount of redundancy as well as a CRC checksum. The frame is
then transmitted again inverted. If any of these fail to check out,
the previous frame data is reused.

After some number of invalid frames, the receiver switches to its
fail safe settings. This is a special frame that is transmitted
roughly once every ten seconds that tells the receiver what to do
if the transmission is lost. The servos do not jitter or lose their
holding power during this process.

PPM, however, will lose sync with a single pulse error. Since it
is clocked with the falling edges (with Futaba -- JR is on the rising
edge), any interference strong enough to cause the comparator to
see a falling (or rising) edge will send bogus data to the servos
for that frame. There is no error detection or correction at all.

Hopefully the next sync pulse (> 10 ms) will allow the receiver
to sync back up with the transmitter and all that will happen is
just a momentary jitter in the servos. If the transmitter is lost,
the servos are likely to start jumping around if there alot of
ambient RF noise or just lose their holding power if there is not
much noise.

Trammell
--
-----|----- hudson@osresearch.net W 240-283-1700
*>=====[]L\ hudson@rotomotion.com M 505-463-1896
' -'-`- http://www.swcp.com/~hudson/ KC5RNF

Steve Simpson

May 03, 2003, 04:01 AM

> After some number of invalid frames, the receiver switches to its
> fail safe settings.

Is this not actually the programmable 'delay' which is so often confused
with 'lock out duration', 'resumption delay' or some other imaginary
sinister feature?

I am not real sharp with electronics, but my understanding from reading both
the Futaba and Jr descriptions of PCM functionality is that the only 'delay'
in the system is the amount of time between the start of continuous
interference until the Rx goes to the pre-defined fail safe positions.

Servo hold (not failsafe) is instantaneous with loss of useable signal and
servo resumption is instantaneous with receipt of a good frame.

So there is literally no 'delay' built into the system from a response
standpoint (save normal codec time). It has been a couple of years since I
studied this, but I am fairly sure that I have it right.

> If the [PPM] transmitter is lost, the servos are likely to start jumping
around if there alot of
> ambient RF noise or just lose their holding power if there is not
> much noise.
>

I can attest to that. I started with a Cobra (way noisy) on PPM. I figure it
was undoubtedly female . . . . . it would do exactly as I asked . . . .
provided it didn't have a better idea of it's own.

Steve Simpson

May 03, 2003, 04:01 AM

> Flying low to the ground with very maneuverable and light aircraft with
high
> speed servos can give you the impression that PCM has a small, but
noticable
> built-in delay, even without interference.

I suspect that delay may be the coding/decoding (codec) time.

As with all things computer, processors get faster and smaller and cheaper,
so I suppose it is reasonable to assume that there have been improvements in
PCM processing speed just like your friendly desktop PC.

Carl Farrington

May 11, 2003, 04:01 AM

Tramm Hudson wrote:
> Steve Simpson <simpsons34@NIXcox.net> wrote (in part):
>> As to PCM supposed 'rejection' of interference, my understanding
>> (which may not be technically accurate) is that because PCM uses the
>> carrier only to transmit digital information, it is less sensitive
>> to interference than would be the analogue of PPM. While it 'sees'
>> exactly the same interference, PCM is only effected by distortions
>> severe enough to make a 0 into a 1 or vice versa whereas PPM is
>> degraded by all interference.
>
> PCM is even more robust than that. It uses a 6B10 encoding that has a
> small amount of redundancy as well as a CRC checksum. The frame is
> then transmitted again inverted. If any of these fail to check out,
> the previous frame data is reused.
>
> After some number of invalid frames, the receiver switches to its
> fail safe settings. This is a special frame that is transmitted
> roughly once every ten seconds that tells the receiver what to do
> if the transmission is lost. The servos do not jitter or lose their
> holding power during this process.
>
> PPM, however, will lose sync with a single pulse error. Since it
> is clocked with the falling edges (with Futaba -- JR is on the rising
> edge), any interference strong enough to cause the comparator to
> see a falling (or rising) edge will send bogus data to the servos
> for that frame. There is no error detection or correction at all.
>
> Hopefully the next sync pulse (> 10 ms) will allow the receiver
> to sync back up with the transmitter and all that will happen is
> just a momentary jitter in the servos. If the transmitter is lost,
> the servos are likely to start jumping around if there alot of
> ambient RF noise or just lose their holding power if there is not
> much noise.
>
> Trammell

wowzers. Your PCM knowledge is a great treasure :-) Thanks for explaining
that to us Trammell.

Lyman Slack

May 11, 2003, 04:01 AM

Tony Stillman has an article published in the current R/C Report magazine
covering PPM v. PCM.

--
Cheers -- \__________Lyman Slack_________/
\______AMA6430 IMAA1564___/
\____Flying Gators R/C______/
\__Gainesville FL _________/
Visit my Web Site at: http://www.LymanSlack.com

"Carl Farrington" <carl@000compsup000.net> wrote in message
news:b9if8g$57h$1$8302bc10@news.demon.co.uk...
> Tramm Hudson wrote:
> > Steve Simpson <simpsons34@NIXcox.net> wrote (in part):
> >> As to PCM supposed 'rejection' of interference, my understanding
> >> (which may not be technically accurate) is that because PCM uses the
> >> carrier only to transmit digital information, it is less sensitive
> >> to interference than would be the analogue of PPM. While it 'sees'
> >> exactly the same interference, PCM is only effected by distortions
> >> severe enough to make a 0 into a 1 or vice versa whereas PPM is
> >> degraded by all interference.
> >
> > PCM is even more robust than that. It uses a 6B10 encoding that has a
> > small amount of redundancy as well as a CRC checksum. The frame is
> > then transmitted again inverted. If any of these fail to check out,
> > the previous frame data is reused.
> >
> > After some number of invalid frames, the receiver switches to its
> > fail safe settings. This is a special frame that is transmitted
> > roughly once every ten seconds that tells the receiver what to do
> > if the transmission is lost. The servos do not jitter or lose their
> > holding power during this process.
> >
> > PPM, however, will lose sync with a single pulse error. Since it
> > is clocked with the falling edges (with Futaba -- JR is on the rising
> > edge), any interference strong enough to cause the comparator to
> > see a falling (or rising) edge will send bogus data to the servos
> > for that frame. There is no error detection or correction at all.
> >
> > Hopefully the next sync pulse (> 10 ms) will allow the receiver
> > to sync back up with the transmitter and all that will happen is
> > just a momentary jitter in the servos. If the transmitter is lost,
> > the servos are likely to start jumping around if there alot of
> > ambient RF noise or just lose their holding power if there is not
> > much noise.
> >
> > Trammell
>
> wowzers. Your PCM knowledge is a great treasure :-) Thanks for explaining
> that to us Trammell.
>
>