wingracer

iTrader: (24)

Just did a little math. At 70mph (the typical top speed of 1/8th onroad cars) a car will travel 6.16 feet in 20ms (the latency of some cheap crappy 2.4 radios). That's about 4 inches per millisecond. So if you can't tell the difference in a few milliseconds, you can't tell the difference in 4 inches and you can't drive an RC car very well.

30Tooth

iTrader: (1)

A quick google search -that returned lots of unrelated stuff- and a reaction time test seem to give an interval of 150-250ms. That's 15m to 25m reaction distance, so a radio with 20ms of lag is around 10% total reaction time. This is a particular situation, like avoiding a wreck in front of our car while on the straight at 70mph.
Now the tricky part, the human brain compensate very well for consistent lag so a system with more lag would feel like it doesn't turn in very well and over steers coming out of corners for a driver accustomed to a faster system. That's what I feel the most when I change from a GT3B to a Hitec CRX or M8

wingracer

iTrader: (24)

True but the problem isn't reaction time. High latency radios tend to be inconsistent in their latency. So while the average latency might be 14ms, sometimes it's only 8ms and sometimes it's 20ms. Your brain can adjust to a consistent latency and turn in at the right time to hit the apex but if every 5th lap it reacts 8 inches sooner, you hit the wall and every 10th lap it gets bogged down and takes 20ms, you go wide by a foot and get passed.

30Tooth

iTrader: (1)

Absolutely true, that's why I think average latency is not as important as the difference between minimum and maximum latency.

Dave H

Might be able to compensate for latency on a smooth lap. Not so much on a unexpected slide, or reacting to traffic, etc.

theclutch

iTrader: (2)

Ok, so back to the original question, are old AM radios generally faster than today's 2.4 GHz radios? My LHS tried to convince me today that it's a night and day difference in speed and tried to push me to the Spectrum DX4R for $330 with two receivers. I'm thinking Airtronics MT-4S. But I'm not inclined to get either if they will be slower than my AM JR radio.

30Tooth

iTrader: (1)

My understanding is that old AM radios are "locked" into a 20ms frame. It changed only when digital servos and HRS receivers came out. Like the Futaba 3PK and the M8 I think.

The MT-4S is less than half of that AFAIK.

asc6000

iTrader: (15)

The old stuff is like watching black and white tv. Avoid spektrum as well. The radio is the single most important investment we make in this hobby, don't cheap out. Get an MT4s. I will say this. Having taken several spektrums out of rtrs to be replaced I have twice put in an old fm radio and it was better than the cheapo 2.4 radio but still noticeably less responsive than a high end 2.4 with a good servo. All to be expected I would think. MT4

wingracer

iTrader: (24)

An MT4 should outperform anything except the top of the line stuff like an M12 and the best of the FM, like an FM M11.

Dave H

While I agree with that aspect, I suspect there is more than frame rate to consider. The processing and transmitting of the signal to the receiver and the processing in the receiver has to be part of it I think.

I don't know specifics of the radios, but in general any digital processing does take some time, possibly even several clock cycles. Analog systems process fairly quickly, usually only limited by transistor switching speed and rise time of capacitors and inductors in the circuit, and of course speed of electricity. I see it with industrial servo and test equipment, the older analog controllers can respond quicker than the modern digital stuff in some if not many cases.

The modern radios possibly have an advantage of operating at higher broadcast frequencies, the shorter wavelength could be an advantage.

Interesting question, would be great to see thorough testing.

howardcano

iTrader: (37)

Old PPM radios, AM or FM, used many different frame rates depending on the manufacturer and the number of channels to be transmitted (the fewer the channels, the faster the frame rate could be). Some used a fixed frame rate, regardless of the control pulse widths sent; the synchronization pulse was variable to take up the remaining time. Some used a fixed synchronization pulse length, so the frame rate depended on the control pulse widths.

The fixed frame rate had several advantages on older analog servos and ESCs: The servo loop gain didn't change, and the ESC throttle position wasn't affected by steering position (or control surface position on aircraft). A variable frame rate had the least latency, but the servo loop gain changed (usually not enough to be a problem) and the throttle position changed with steering (definitely problem, at least for cars).

P.S.: The latency of a tiger beetle is about 28 milliseconds:
http://www.sciencedaily.com/releases...0415133815.htm

30Tooth

iTrader: (1)

You're right on the analog processing, in the end the design is very complex and somewhat inefficient. OTH digital is simpler and can be almost as quick, the little I know about the GT3B's main chip is that it can't handle more than one channel at a time on the input. It reads the potentiometers alternately then applies the expo or whatever function and then dispatch the stream to the 2.4GHz module. This happens in very few clock cycles AFAIK. The receiver just decodes and send the stream to the respective channel, latency at this stage would be tiny but I have no hard numbers.
On the wavelength, I seem to recall something about 8+ channel transmitters needing more than what the 27MHz could stream but I can't find that info right now. Thanks for yours and Howard's technical input!

theclutch

iTrader: (2)

So in layman's terms... AM faster or slower?

30Tooth

iTrader: (1)

In your particular case AM is slower, the MT-4 will be light years ahead in performance and features