flatlander_14

i posted this in the MT forum too, but i forgot there was a radios one too :stupid: anyways, here it is:

ok, i have a couple of questions about those topics...

concerning digital servos:

1a: if i buy a digital servo, can i run it on 4.8v safely? i prefer more runtime over more than enough power.

2a: i heard something that digital servos stop functioning under 4.8v, is this true?

3a: would a digital servo work with any reciever, if the connector fits of course...?

concerning PCM:

1b: what exactly is PCM? i know it is like FM and AM, but better from what i hear...

2b: ive never heard of a PCM crystals, are all PCM radio systems synthesized? and how many channels? would you recommend a PCM radio over, say a JR XS3 or something?

3b: what does the PCM stand for? something something modulation....




thanks in advance

BUGGIES_R_US

PCM: Pulse Code Modulation.

This is the same technology used to record .WAV files.. pretty much.

Its a digital way of modulating FM signals.. PCM is a modulation type on an FM signal. FM merely indicates that the Frequency is being modulated.

So PCM IS a special FM signal.

This information as far as I know is true. PLEASE CORRECT ME IF THIS IS INCORRECT!



Ram

JohnW

My Feedback: (6)

1. Every servo is different. Some digitals work fine at 6V, some can't be run over 4.8 volts. Always check the specs for each servo.

2. Most servos will have response issues under a certain threshold voltage, analog or digital. How the servo responds to low voltage has more to do with the signal detection design and not analog vs digital. However, you shouldn't be running much under 4.8 volts anyway because your RX will drop out in the mid 3 volt range.

3. Signal to the digital servo is the same as what analog servos use. Any RX will run them.

1b. PCM is a digital encoding method. PPM is an analog encoding method. Both use FM. With PPM, a series of pulses are sent to the RX. The length of the pulse determines the servo position. With PCM, servo positions are turned into byte words and a digital stream is sent to the RX with a checksum. It's basically analog vs digital. Both work fine; however, PCM has a better noise rejection and failsafe features compared to PPM.

2b. No such thing as a PCM crystal. Synth has nothing to do with PPM vs PCM. PCM radios available from 6 to 10+ channels. For PCM to work, both the TX and the RX must be PCM. PCM is more reliable than PPM.

3b. Pulse Code Modulation.

Rodney

For an explanation of the different types of control (PCM, PPM, PWM etc.) check out www.torreypinesgulls.org/Radios.htm

3D Joy

At 4.8 volts, you will have more run time than at 6 volts. At lower voltages, there is less current drain so the pack lasts longer. At higher voltages, there is more current. For the same resistance (servo circuit) you get higher current with higher voltage. A 5 cell pack of the same capacity of a 4 cell pack should need to be recharged sooner.

flatlander_14

thanks for clearing all of that up if i get a digital servo for my steering, i guess it wont matter as much if it cuts out.. its the throttle im afraid of :stupid:

Lynx

PCM is not necessarily more reliable than PPM encoding. PCM has built in error detection, so it will be less likely to glitch than a PPM receiver. At the same time you may not realize the PCM receiver is having receiving issues until you try to turn the plane and nothing happens. It's very much a personal issue, but just because a PCM receiver stop's the appearance of glitches doesn't mean it gets rid of them. They're still there just masked from the user. Do a search for PCM vs FM and you'll find more opinions than you can shake a stick at.