crawler tech

I am reseaching the purchase of a new Tx and have narrowed it down to the Hitec Eclipse 7 Channel FM Advanced Digital Radio Control System for various reasons.
My question is what will a PCM radio do for me that an FM radio won't ?

olivier

Hi,

This will give you an idea, also on how to search :

http://www.rcuniverse.com/forum/sear...mitbutton=+OK+

or :
http://www.rcuniverse.com/forum/sear...mitbutton=+OK+

Olivier

exeter_acres

My Feedback: (2)

PCM IS FM

mattnin

PCM can be either AM or FM. I know this because for my senior project (I have a bachelors in electronic engineering), I designed a 500khz AM transmitter that used a PCM circuit to transmit data. If I am not mistaken, every R/C transmitter uses PCM or PPM for each channel. That is simply the way all transmitters work.

If you want a better transmitter, you need to choose one that is FM and has a higher frequency, better range, or one that uses frequency technologies like spread spectrum. Transmitters that use higher frequency can replicate their PPM or PCM data very well. Spread spectrum can be used to filter out stray radio noise. Even better than that, you need to choose a transmitter that is used with a very good receiver. The receiver can be of more importance than the transmitter.

dirtybird

My Feedback: (5)

AS far as I know there is no RC transmitter that uses AM to transmit PCM

Your suggestions for a better transmitter would be good except that such transmitters are not available for our use. Only for park fliers

mattnin

That is not true. I just said earlier that my senior project in college was to make a 500khz AM transmitter that used a PCM signal generator. As matter of fact, I controlled a traxxas servo with it.

The signal generators in nearly every R/C radio generate PCM or PPM. It is then that the signal gets modulated. Please understand that the dials and knobs and switches in all R/C radios either generate PCM or PPM. It is this type of square waveform that gets modulated, be either AM or FM.

andythirty7

Hi this is my first post. had a time trying to find out how to post. hope im in the right place. Ive got a Century Elite 4 transmitter. 72.780. Im wanting to set up a park flyer for the grandson, I dont have a receiver for this transmitter, but do have several receivers from past years. how can I find out what will work. I do know that Century has a universal receiver (mini) but by itself its 49.95 no servoes. what can anyone tell me. Thanks john[:@]

mattnin

Andy, you can try using identical crystals in the transmitter and one of your receivers. The receivers must be built for that same band as the transmitter though. Do you already have servos?

exeter_acres

My Feedback: (2)

but we are limited to 72mhz and 50 mhz for aircraft use.....

and 2.4ghz for spektrum......



and most people don't build their own transmitters......

andythirty7

Century claims that there universal receiver will work for all FM transmitters except PCM and all FM brands. Also just found out that JR has there JRS600 universal. I guess im trying to find a tech guy that can tell me if there are any Futaba receivers etc that will work.

BasinBum

My Feedback: (2)

Crawler,
The main differerance between PCM and PPM (often mistakenly refered to as FM when comparing to PCM) is how they respond to noise or interferance. PCM will tend to mask the noise by going into lockout for short periods of time untill the noise completely overtakes the signal from your radio and then it will go to a preset "failsafe" mode, i.e. low throttle, all sufaces neutral. PPM will tend to twitch or show a "hit" as people like to say when it encounters noise.

Many will argue that PCM is less prone to radio interferance than PPM but that is a very debatable point. PCM receivers are more expensive than PPM on average and Hitec only recently introduced a PCM receiver to thier line. Futaba transmitters for example tend to allow you to transmit with PPM or PCM, at least the higher priced ones.

Andy,
What you need to be concerned with is "shift" which can be positive or negative. The easy solution is a dual shift receiver that will work with both yet tends to be the same cost as others in it's class. I suggest a Hitec and if you need servos the flight packs are a good value.

Rodney

Mattin is correct in that there have been PCM units built using AM modulation, in fact that is what was used in the first generation of PCM back in the 1940's or 50's if my recall is correct. I do not believe any company now produces such a device on AM as it was not very successful (exhorbintantly expensive).

exeter_acres

My Feedback: (2)

Noone said he was wrong....



but since this is the case.... what does it matter in this thread

P-40 DRIVER

Just personal bias but the only radio I ever owned that gave me problems was PCM. I fly FM and have zero problems. In some instances you would want a failsafe such as when flying a jet to shut off the engine in case of a problem. I would not get to hyped up on buying to much radio with the 2.4 ghz stuff right around the corner, okay in the next coulple of years, can't wait!

pilotpete2

Rodney,

I'm a bit mystified, I know of no implementation of PCM in the 40's or 50's, certainly not for R/C.
I understand that Nyquist published the theory of PCM in 1923 ( I believe at Bell Labs ) but technolgy took a long time to catch up with theory.
Seriously, I like to know if I'm wrong. [X(]

Regards,

Pete

BasinBum

My Feedback: (2)

Don't hold your breath. Do you think Horizon started out to design a park flyer system? Of course not but when they found that the range was not reliable enough for larger stuff they came up with marketing genius with the park flyer system. There are still some big hurdles to get the ultra high frequency stuff to work with aircraft at a distance and it may not be as soon as many expect.

pilotpete2

Hi Basinbum,
I hope your wrong regarding 2.4GHz, anecdotal[X(] evidence seems that they are just playing it safe. Seems like a brilliant marketing ploy to me, but that's just my $.02.
I'm planning to drive down to the WRAM show this Saturday, it will be interesting to hear what the radio manufactures reps are saying or not saying. I'm sure there are as many theories as to who Spektrum actually is, as there are of who shot JFK! Or JR for that matter Did I say JR[X(] Enough of the conspiracy theories for now, but this Spektrum deal is going to be fun to watch unfold, I just hope I don't have to hold my breath tooo long[&:]

Pete

mattnin

Are you even sure that your FM radio doesn't use PCM? I bet it does. PCM is actually a very common circuit, there is nothing really special about it, and I don't know why people are saying that PCM radios are higher tech, or cost more. It is simply a solid state circuit with a form or RC circuit to control the pulse code. PCM is no more difficult to construct than a PPM, or PWM generator.

Pete, Nyquist did not publish a theory of PCM. PCM was around before Nyquists theory. Nyquist only said that the minimum sampling rate must be twice the frequency being sampled (nyquist theory). It was the application of Nyquists theory that enabled us to convert audio (or whatever analog waveform that needed to be sampled) into a digital form such as PCM.

I have used Nyquists theorem in the past working at a company developing a data logger to map corrosion potentials on natural gas transmission pipelines by sampling transformer/rectifiers. It has absolutely nothing to do at all with PCM.

Jim_McIntyre

The straight answers;

+ improved range due to improved signal to noise ratio
+ be less susceptible to being shot down (for same reason as above)
+ failsafe (ability to pre-program channels to move to pre-set { e.g. cut throttle } ) in the case of control loss due to interference or Transmitter fialure - no it won't save the aircraft but, it might save a person....
+ low battery warning (buy you time to land before control is lost)

exeter_acres

My Feedback: (2)

the only correct one is failsafe.....

HarryC

My Feedback: (1)

You're all wrong!! PCM does not go into hold and failsafe. The failsafe feature that is bolted onto the receiver does all that, PCM does not do it. PCM code can be sent in smoke signals, by clapping my hands, flashing a light and so on. None of those PCM systems have failsafe, failsafe is not an inherent part of PCM. Failsafe is a separate system that is added on. It is perfectly possible to have a PCM system without failsafe which would go just as out of control during interference as a plain PPM without failsafe. It is also perfectly possible to add failsafe onto PPM, and several manufacturers do. My models, including my jets, all fly PPM with failsafe that does hold and preprogrammed failsafe on all channels, just like the PCM with failsafe systems do.

So it is not true that the difference between PCM and PPM is that PCM does failsafe and PPM glitches when interference hits them. The difference is whether a failsafe system is fitted or not. Failsafe is fitted as standard in all PCM systems, failsafe is fitted as standard in some but not all PPM systems.

Harry

Rodney

Right on HarryC

exeter_acres

My Feedback: (2)

true... so I was incorrect in that... as it is standard on my PCM setups.....

mattnin

Harry is absolutely correct. The best radios usually have the most features built in. Just like Harry said, you can mix and match to your hearts content, a PCM can work with FM or AM, same as PWM or PPM, spread spectrum can use PCM, PWM or PPM, but it may not have a failsafe, etc.....

However, from a radio frequency standpoint, the higher the frequency, the better a signal (pcm, ppm, pwm) can be transmitted and reproduced at the receiver, but there is a tradeoff. The higher the frequency, the less range or more power is required to achieve the same range. A benefit to having a higher frequency is smaller antennas, etc.... Also, the receiver is just as important at the transmitter. If the receiver is of a bad construction, what point is it to have a good transmitter if the receiver is introducing glitches into the setup?

All in all, it is best to pick a transmitter and receiver that has a solid record, and important features that you may need.

Spread spectrums inherent feature is that you do not have to worry about stray radio interference because spread spectrum uses a very wide band of frequencies to transmit signals. Even the US military used to use a form of spread spectrum to transmit data as it was so secure and could not be interfered with.

PCM, PPM, or PWM all achieve the same effect, to generate a signal that can be used by a servo. There is no real difference between them, the difference comes down to the quality of the radio and receiver at this point.

The lower the frequency, the greater the range, but the poorer quality of signal.

The higher the frequency, less range, but a higher quality signal.

FM is better than AM for signal quality, you can notice the difference in audio quality for sure! Thats why we listen to radio stations on FM due to the increased clarity.

However, AM has much better range than FM. AM is the only type of radio that can be used as communcation across the Atlantic Ocean.

HarryC

My Feedback: (1)

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