Anyone use ZPCM (on a JR radio)? Is it really worse than SPCM? Is it a difference that you can actually perceive?

[user]

[Peter Christy]

Yes, I use it. The average pilot will be unable to detect the difference. If
you are using a lot of mixing (ccpm?) you *might* notice that it doesn't run
quite so smoothly on the ground. Whether you would be able to tell in the air
is a moot point......!

The only big difference an average pilot will notice is that you can't set
failsafe on or off for individual channels. Its either on or off for all
channels!

--
Pete
christy@NOattglobalSPAM.net
(make the obvious amendments to reply!)

[Steve R.]

Ok, from the subject line. ZPCM? SPCM??

These are phrases I've never seen before. JR is mentioned after the ZPCM.
I know that JR encodes their PCM information differently than Futaba does.
Don't know about anyone else, Airtronics for example.

Does ZPCM, SPCM refer to those differences?

Just wondering.

Fly Safe,
Steve R.

[Beav]

"user" <user@here.com> wrote in message
news:st5vk0dl1v449t9gr2mb5sof26hasvi65s@4ax.com...

Odd question. What makes you think the SPCM is "worse" enough to make an
even "worser" comparison to the ZPCM?

Who says the Z is "worse" than the "S" and what makes them think the S has
any problem?



--
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

[Beav]

"Steve R." <srhodes13@houston.rr.nospam.com> wrote in message
news:OdX3d.11594$Qb.561@fe2.texas.rr.com...
> Ok, from the subject line. ZPCM? SPCM??
>
> These are phrases I've never seen before. JR is mentioned after the ZPCM.
> I know that JR encodes their PCM information differently than Futaba does.
> Don't know about anyone else, Airtronics for example.
>
> Does ZPCM, SPCM refer to those differences?

The difference is simple Steve. The orginal JR PCM system (the Z series)
was/is a 512 bit, whereas the "S" is a full 1024 bit. Like the later Futaba
(VHP onwards) which was 1024, their latest PCM system is 2048. Both the "S"
and the Z"Z are JR's names, and no-one else uses them.

The result is more steps, better resolution and like Pete says, bloody hard
to tell the difference unless you're using a raft of programmable mixers.


--
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

[Steve R.]

Thanks, Beav! I appreciate the clarification.

Fly Safe,
Steve R.


"Beav" <beavis.original@ntloxoworld.com> wrote in message
news:10l2vqtccmhkq72@news.supernews.com...
>
> "Steve R." <srhodes13@houston.rr.nospam.com> wrote in message
> news:OdX3d.11594$Qb.561@fe2.texas.rr.com...
>> Ok, from the subject line. ZPCM? SPCM??
>>
>> These are phrases I've never seen before. JR is mentioned after the
>> ZPCM.
>> I know that JR encodes their PCM information differently than Futaba
>> does.
>> Don't know about anyone else, Airtronics for example.
>>
>> Does ZPCM, SPCM refer to those differences?
>
> The difference is simple Steve. The orginal JR PCM system (the Z series)
> was/is a 512 bit, whereas the "S" is a full 1024 bit. Like the later
> Futaba
> (VHP onwards) which was 1024, their latest PCM system is 2048. Both the
> "S"
> and the Z"Z are JR's names, and no-one else uses them.
>
> The result is more steps, better resolution and like Pete says, bloody
> hard
> to tell the difference unless you're using a raft of programmable mixers.
>
>
> --
> 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
>
>
>

[Peter Christy]

Just one other point about PCM systems in general that isn't widely
appreciated....

The more "bits" you use, the slower the frame rate.

The channels spacing on a 35MHz system is 10 KHz, which means that the useable
signal bandwidth is about 3 KHz. The more information you try and shove down a
narrow band channel, the slower you have to do it! Think of it as water going
down a pipe! If you try and push too much water through a narrow pipe, you
have to use so much pressure that the pipe will burst!

PPM systems have the highest frame rate because they make best use of the
bandwidth. However, they are also the most susceptible to disruption from
interference. PCM gets round this by transmitting extra, redundant
information, so that the receiver can tell if the signal its got is corrupted
or not, and take the appropriate action (failsafe or hold). However, because
you are transmitting more information, you must do it more slowly.

Similarly, when you go from 512 to 1024 bits, you are trying to cram even more
information into a very narrow channel! Something has to give, and it will be
response time.

If you slam the stick hard over (on the ground!) on a PPM system, the response
is instantaneous for all practical purposes. Now try it with a PCM system. You
will notice a small but perceptible delay before the servo moves.

Modern systems have got this delay down to a very small value, but it is still
there! On early PCM sets it was quite a problem........!

--
Pete
christy@NOattglobalSPAM.net
(make the obvious amendments to reply!)

[Beav]

"Peter Christy" <christy@NOattglobalSPAM.net> wrote in message
news:PPC4d.282$g11.276@newsfe4-gui.ntli.net...
> Just one other point about PCM systems in general that isn't widely
> appreciated....
>
> The more "bits" you use, the slower the frame rate.
>
> The channels spacing on a 35MHz system is 10 KHz, which means that the
useable
> signal bandwidth is about 3 KHz. The more information you try and shove
down a
> narrow band channel, the slower you have to do it! Think of it as water
going
> down a pipe! If you try and push too much water through a narrow pipe, you
> have to use so much pressure that the pipe will burst!
>
> PPM systems have the highest frame rate because they make best use of the
> bandwidth. However, they are also the most susceptible to disruption from
> interference. PCM gets round this by transmitting extra, redundant
> information, so that the receiver can tell if the signal its got is
corrupted
> or not, and take the appropriate action (failsafe or hold). However,
because
> you are transmitting more information, you must do it more slowly.
>
> Similarly, when you go from 512 to 1024 bits, you are trying to cram even
more
> information into a very narrow channel! Something has to give, and it will
be
> response time.
>
> If you slam the stick hard over (on the ground!) on a PPM system, the
response
> is instantaneous for all practical purposes. Now try it with a PCM system.
You
> will notice a small but perceptible delay before the servo moves.
>
> Modern systems have got this delay down to a very small value, but it is
still
> there! On early PCM sets it was quite a problem........!

I didn't realise that was why Pete, but your description pretty much
explains it. Thanks for that.

I know I don't use any programmable mixes for the reason that it slows
things down too much, so I use as little internal mixing/programming as I
can get away with coz I like PCM more than PPM.


--
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

[Peter Christy]

Beav wrote:

> I didn't realise that was why Pete, but your description pretty much
> explains it. Thanks for that.
>
> I know I don't use any programmable mixes for the reason that it slows
> things down too much, so I use as little internal mixing/programming as I
> can get away with coz I like PCM more than PPM.

Actually, the delay happens whether you're using any mixing or not - its a
feature of the transmission medium rather than anything else...

The reason it gets notchy when you start putting a lot of mixing in is
two-fold:

Firstly, the processors in your average Tx is pretty puny - it has to be,
otherwise the batteries wouldn't last five minutes! Also, it has to share its
time out between reading the stick and switch positions, converting that into
digits, then coding it all up into a PCM stream and spitting it out to the
modulator! Quite a task for a relatively low-powered processor! Adding in
mixing adds an extra task for it to do, which doesn't help, but

Secondly, if you're using 512 bit PCM, that means that the system has a
resolution of approximately half a per cent (1/512). But that's half a percent
of the full travel available (which on most systems is 150%). By the time
you've knocked it back to the standard 100% travel, your resolution is already
down to 0.75 per cent ( (150/100)*0.5 ). Now suppose that your mix is 70/30.
The resolution on the 30% side is now down to 2.5% ( (100/30)*0.75 ). At this
level, the servos may well start to behave a bit notchy, as their resolution
is much higher than the 30% input signal.

As with any digital system, the 512 bits (or discrete servo steps for the
mechanically minded) is a fixed amount. As you reduce the amount of travel you
are actually using, those steps start to become a significant proportion of
your demanded position!

Sorry to be long-winded, but its one of those things that's difficult to
explain in just plain words, and it seems to be something that's not generally
appreciated ;-)

Cheers,

--
Pete
christy@NOattglobalSPAM.net
(make the obvious amendments to reply!)

[Beav]

"Peter Christy" <christy@NOattglobalSPAM.net> wrote in message
news:2Yi5d.142$6g4.71@newsfe3-win.ntli.net...
> Beav wrote:
>
> > I didn't realise that was why Pete, but your description pretty much
> > explains it. Thanks for that.
> >
> > I know I don't use any programmable mixes for the reason that it slows
> > things down too much, so I use as little internal mixing/programming as
I
> > can get away with coz I like PCM more than PPM.
>
> Actually, the delay happens whether you're using any mixing or not - its a
> feature of the transmission medium rather than anything else...
>
> The reason it gets notchy when you start putting a lot of mixing in is
> two-fold:
>
> Firstly, the processors in your average Tx is pretty puny - it has to be,
> otherwise the batteries wouldn't last five minutes! Also, it has to share
its
> time out between reading the stick and switch positions, converting that
into
> digits, then coding it all up into a PCM stream and spitting it out to the
> modulator! Quite a task for a relatively low-powered processor! Adding in
> mixing adds an extra task for it to do, which doesn't help, but
>
> Secondly, if you're using 512 bit PCM, that means that the system has a
> resolution of approximately half a per cent (1/512). But that's half a
percent
> of the full travel available (which on most systems is 150%). By the time
> you've knocked it back to the standard 100% travel, your resolution is
already
> down to 0.75 per cent ( (150/100)*0.5 ). Now suppose that your mix is
70/30.
> The resolution on the 30% side is now down to 2.5% ( (100/30)*0.75 ). At
this
> level, the servos may well start to behave a bit notchy, as their
resolution
> is much higher than the 30% input signal.


Understood Pete, and the delay gets silly when you DO use mixing. I tried to
use my ancient JR (and 512) PCM 10 in my Huey with eCCPM mixing and it was
absolutely awful. I ended up using my equally ancient (but 1024) Futaba FC
28 which showed no sign of distress at all.
>
> As with any digital system, the 512 bits (or discrete servo steps for the
> mechanically minded) is a fixed amount. As you reduce the amount of travel
you
> are actually using, those steps start to become a significant proportion
of
> your demanded position!
>
> Sorry to be long-winded, but its one of those things that's difficult to
> explain in just plain words, and it seems to be something that's not
generally
> appreciated ;-)

It's appreciated Pete, believe me mate.



--
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

[Steve R.]

"Peter Christy" <christy@NOattglobalSPAM.net> wrote in message
news:2Yi5d.142$6g4.71@newsfe3-win.ntli.net...
> Sorry to be long-winded, but its one of those things that's difficult to
> explain in just plain words, and it seems to be something that's not
> generally
> appreciated ;-)
>
> Cheers,
>
> --
> Pete

Actually, like Beav, I think I learned some stuff here too. Thanks Pete,
for the explanation. I think the main reason it's not generally appreciated
is that the average modeler (whom I sometimes like to think I'm a little
better than, but probably not! ;-) ), will never feel this anyway. I know
I've not felt any outward delay that I'm aware of but I've not flown a 512
system in "years" and the 1024's have got to be better about this, at least
I'd hope so.

The first time I saw a set of Futaba digital servos plugged into one of
their PCM systems, the guy pulled the aileron stick hard over and let it go.
When the stick rebounded and bounced back and forth for a second or so on
the centering springs, the servo pretty much kept up. It was impressive to
watch and certainly, "way" faster than the best analog servos out there at
the time.

If you can feel this phenomenon, I'm impressed. For me, it's not an issue.

Fly Safe,
Steve R.

[Beav]

"Steve R." <srhodes13@houston.rr.nospam.com> wrote in message
news:_hk5d.2498$dh7.1726@fe2.texas.rr.com...
>
> "Peter Christy" <christy@NOattglobalSPAM.net> wrote in message
> news:2Yi5d.142$6g4.71@newsfe3-win.ntli.net...
> > Sorry to be long-winded, but its one of those things that's difficult to
> > explain in just plain words, and it seems to be something that's not
> > generally
> > appreciated ;-)
> >
> > Cheers,
> >
> > --
> > Pete
>
> Actually, like Beav, I think I learned some stuff here too. Thanks Pete,
> for the explanation. I think the main reason it's not generally
appreciated
> is that the average modeler (whom I sometimes like to think I'm a little
> better than, but probably not! ;-) ), will never feel this anyway. I
know
> I've not felt any outward delay that I'm aware of but I've not flown a 512
> system in "years" and the 1024's have got to be better about this, at
least
> I'd hope so.
>
> The first time I saw a set of Futaba digital servos plugged into one of
> their PCM systems, the guy pulled the aileron stick hard over and let it
go.
> When the stick rebounded and bounced back and forth for a second or so on
> the centering springs, the servo pretty much kept up. It was impressive
to
> watch and certainly, "way" faster than the best analog servos out there at
> the time.

I've got to say Steve that I've never seen ANY PCM system work the servos as
fast as PPM system.
>
> If you can feel this phenomenon, I'm impressed.

It's not easy to feel when you're flying, but when you can SEE the
"stepping" during the set up, it's a bit disconcerting and this results in
you THINKING you can feel it.

For me, it's not an issue.

I doubt it is for any mortal (that discounts those who shall remain
nameless:-))


--
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
>
> Fly Safe,
> Steve R.
>
>

[elektromodely]

Peter Christy - 1/512 is not a half of per cent, it is a 2,5-times smaller (less than 0,2%) ;-)

[Alan]

I used ZPCM when I had my original setup in the Xcell 46 Graphite earlier
this year. I had to replace all the electronics in that heli over a period
of a couple of months so bought an SPCM Rx then

Can't say I noticed any major differences...

--
Alan
alandotrait@btopenworlddot.com
Remove the dots to reply
http://heliweb.users.btopenworld.com/


"elektromodely" <elektromodely.1hiaoc@rcgroups.com> wrote in message
news:elektromodely.1hiaoc@rcgroups.com...
>
> Peter Christy - 1/512 is not a half of per cent, it is a 2,5-times
> smaller (less than 0,2%) ;-)
>
>
> --
> elektromodely
> ------------------------------------------------------------------------
> elektromodely's Profile:
> http://www.rcgroups.com/forums/member.php?action=getinfo&userid=32983
> View this thread:
> http://www.rcgroups.com/forums/showthread.php?threadid=277701
>

[Barry]

works ok for me ! no problem for the past 5 years
"elektromodely" <elektromodely.1hiaoc@rcgroups.com> wrote in message
news:elektromodely.1hiaoc@rcgroups.com...
>
> Peter Christy - 1/512 is not a half of per cent, it is a 2,5-times
> smaller (less than 0,2%) ;-)
>
>
> --
> elektromodely
> ------------------------------------------------------------------------
> elektromodely's Profile:
> http://www.rcgroups.com/forums/member.php?action=getinfo&userid=32983
> View this thread:
> http://www.rcgroups.com/forums/showthread.php?threadid=277701
>