rrTV-PHOTO   New HD TV
HOME   rrTV-PHOTO   GALLERIES   MY GALLERY   HELP-FAQ
myHOME PM pmRR MEMBERS 386 ONLINE 45 EVENTS SEARCH REGISTER  START HERE
 
3 pages [ <<    <     1      2     ( 3 )    >    >> ]3908 viewsPOST REPLY
E-flite . Futaba-RC . Next D

.
.
Radio - Servo - Gyro - Gov - Batt > FM or PCM
 
 
charliex
Heliman
Location:

here some more info on the PCM timeout, the default is 3/10ths of a second, and is settable.
09-07-2001 Over year old.
 
 
charliex
Heliman
Location:

its too hard to judge how hard it is to jam either, RF doesn't work like that.

there are far too many factors involved, power, signal strength etc are determined by many many factors, you'll notice that most RC makers will not give you a range for their radios. There is a good reason for it.

Two strong TX's one on PCM and one on PPM will likely just swamp the receiver..

You can measure two radios, and receivers etc but it'll only be valid for the time you measured it.

The RF signal gets to the PCM/PPM the same way, so they are no different at that point, its the way the signals is handled after it gets there thats the issue.
09-07-2001 Over year old.
 
 
Dragon2115
Key Veteran
Location: New England

Re: Re: PCM vs. PPM (FM)

Quote 
Originally posted by gnichola

Well, that may be true for some radios but on my JR 8103 you select hold or failsafe on a channel by channel basis.


Let me ask you this, what is the difference between hold and failsafe when one is no control at the last commanded position and the otheris no control at a preprogrammed position? It's still no control but with the added safety feature of being able to cut the throttle.

Quote 
But it seems to me the discussion is more about what happens when one or the other gets jammed, when it should be more focused on how hard it is to jam one vs. the other.


Both can be jammed quite easily by hitting the right frequency with enough power, whether that is an external source such as the 72 MHz signal some railroads send down their rails to communicate with trains and signals, or and internal source such as a bad bearing or loose metal to metal contact.


Quote 
Originally posted by charliex

its too hard to judge how hard it is to jam either, RF doesn't work like that.

there are far too many factors involved, power, signal strength etc are determined by many many factors,


Actually it's quite predictable. RF power is decreased by a factor that is equal to the square of the distance in meters. Knowing what the sensitivity of my receiver is I can calculate what the maximum range of the system will be under nominal conditions i.e. abient background noise levels below the sensitivity threshold of the receiver.

Quote 
you'll notice that most RC makers will not give you a range for their radios. There is a good reason for it.


You're quite right but it has nothing to do with not being able to calculate the maximum range of their system. It's because they can't guarantee nominal conditions like no outside inteference such as the sources I listed above. Besides in this sue at the drop of a hat society they aren't willing to print a number and have some beanbag decide to test it with a pager tower in between the tx and the rx and then claim, "well, you said it was good up to a mile". They don't need this legal pandoras' box it opens. Better to just not give a number at all and just say it's good for a reasonable distance for the application.

Quote 
Two strong TX's one on PCM and one on PPM will likely just swamp the receiver..


Maybe, but that's where the image rejection specs start to come into play. Which rx is closer to which tx etc. PCM is better for image rejection though. But even that depends on the design of the receiver. Back in my dark years when I flew planes, my friend and I were on the same channel and both using FM. The only difference was mine was JR and his was Futaba. We turned on both receivers and the JR tx. I worked the sticks and his plane glitched and twitched. Then we switched on the Futaba tx and worked the controls. The JR just sat there. It completely rejected the Futaba signal. That was when JR had just introduced their ABC&W receivers. Now I believe they're all about the same in this respect. Whether it's because Futaba got better or JR got worse is up for speculation.

Quote 
You can measure two radios, and receivers etc but it'll only be valid for the time you measured it.


That's not true with respect to the radio system. The tx's output power doesn't vary from day to day by more than a few mWatts max, or at least shouldn't. And the receivers' sensititvity shouldn't be changing either. What is changing is the ambient background noise where, as it goes up your range goes down.
09-07-2001 Over year old.
HOMEPAGE  
 
 
charliex
Heliman
Location:

Quote 
Actually it's quite predictable. RF power is decreased by a factor that is equal to the square of the distance in meters. Knowing what the sensitivity of my receiver is I can calculate what the maximum range of the system will be under nominal conditions i.e. abient background noise levels below the sensitivity threshold of the receiver.


Maybe in classical physics, but in quantum mechanics it certainly isn't.

As you are well aware many factors affect the range of radio and strengths, whereas you can measure with a certain percentage you are not guaranteed a repeatable measurement outside of a lab (on any given day)

You say , nominal conditions, you are basically saying if you measure the range in the similar conditions, you'll get similar results, well obviously, unless something has changed in the system being measured of course.

You have to first calculate the power of the transmission, the product of the consumption and the efficiency factor, which is dependant on the quality and design of the equipment, and how many times its been damaged/crashed/mistreated.

with an omnidirectional transmitter, determine the transmitted power, then use the inverse square law to get power per unit, this is TX power 4(pi)range(2), Mark we need a scientific font , then the threshold of the power of the receiver, multiply that by the received power per unit from the TX by the RXs collector area.

Battery power, antenna placement, weather, type of landscape (ie wide open or lots of corners), pine trees etc. All affect the range of the transmitter in the real world.

with a beamed transmitter its S = 2 pi [ 1 -cos( theta / 2 ) ],

the power per unit area is TX power divided by S, divided by the square of the distance.


Quote 

That's not true with respect to the radio system. The tx's output power doesn't vary from day to day by more than a few mWatts max, or at least shouldn't. And the receivers' sensititvity shouldn't be changing either. What is changing is the ambient background noise where, as it goes up your range goes down.


Its the range we're talking about and the signals abilities to get from the TX to the RX. Its purely academic that the TX and RX keep the same characteristics, its what happens to the signal inbetween, it doesn't matter if the output power is the same, the theoretical limits stay the same, but in practice rarely do.


I'll bet you $50 that if we take measurements over the course of a year with the same radio equipment at different locations the results will vary by at least 30%.
09-07-2001 Over year old.
 
 
Dragon2115
Key Veteran
Location: New England

Well Charlie, when I worked in the EW and ECM communities we didn't have the problems making and verifiying these calculation that you seem to have encountered. We were able to calculate and prove through test data exactly what a given systems' range and susceptability to interferrence or jamming was.

Bottom line here is use whatever you want, PCM or PPM. As I stated in my original point, both have advantages and both have disadvantages. But anyone that's selling PCM as the answer to all your problems is akin to a used car saleman in my book. They're just telling you what you want to hear.
09-07-2001 Over year old.
HOMEPAGE  
 
 
3 pages [ <<    <     1      2     ( 3 )    >    >> ]3908 viewsPOST REPLY
Fast Lad Performance . Esprit Model . Thunder Power RC

.
.
Radio - Servo - Gyro - Gov - Batt > FM or PCM
  UPDATE SCREEN   PRINT TOPIC Advertisers 

Subscribe to This Topic

Wednesday, July 9 - 7:03 am - Copyright © 2000 - 2008 runryder.com | email | link to rr | runryder needs cookie