Rafts, Ping-Pong Balls, and Radio Waves

[First off, I’m not a signal transmission engineer. I’m a mechanical engineer who knows a lot about metals and corrosion, a little about electrical stuff, and not so much about this (IMHO, unbelievably complex and arcane) signal transmission stuff.
My thanks to all of you in advance for not asking the myriad questions about the physics of signal transmission that I couldn’t  answer to save my life.]   
Waves rolling in

Waves Rolling In


(This is a truly wild and ridiculously oversimplified analogy, but it might be of some help to someone somewhere.)

A broadcast wave (radio, TV, etc) is really two sets of waves: the carrier wave, which is the one you tune your radio to (e.g. 91.5 MHz), and the audio wave, which is at a much higher frequency. 

You might be able to imagine it as something like if you guys were all on the beach, and I were out on a raft too far from shore for you to be able to hear me, but I wanted to tell you elephant jokes. If I had a bucket of red ping-pong balls on the raft, and a bucket of white ones, I could translate my elephant jokes into binary code and place a series of ping-pong balls on the passing waves, using red for one and white for zero (somehow also making sure that they would arrive in their proper sequence). If someone there on the beach with you (let’s call him “Bill”) could then translate them back from binary to English, you’d all get to hear why elephants wear red tennies. :) 

(Because nine-ies are too small, and eleven-ies just flop around on their feet.)  

 The thing to realize here is that the ocean waves are too large to be affected by the ping-pong balls, they roll on in just the same. But the ping-pong balls are carried from the transmitter (me) to the receiver (Bill) by those waves, and as long as they roll in to where you are, I can transmit information on them. [There are lots of ways in which ocean waves are very different from the broadcast waves we’re really talking about, but this is the relevant-to-this-analogy way in which they are similar.] 


Radio Repeaters:

A radio repeater just receives the radio signal (much like your bedside clock radio does, but in a bigger and better sort of way), and then rebroadcasts (just like the original station transmitter) the signal to some area where the signal is otherwise blocked. They are often placed on top of tall mountains to re-broadcast into deep valleys.

 In the raft-&-ping-pong-ball analogy,  this would be as if some of you were not on the ocean beach, but on the far shore of a bay blocked off from the ocean waves by a narrow sand spit. The radio repeater would consist of two guys: Pete, who picks up the red and white ping pong balls and tosses them (in their correct binary sequence) across the sand spit to (who else?) re-Pete, over on the near shore of the bay. Re-Pete then re-launches them into an (extraordinarily convenient) current that carries them over to the group on the far bayshore, where Bill’s twin brother Phil translates them back into elephant jokes for the rest of the group.


A Touch of Jargon:

In signal transmission “noise” means “everything the receiver takes in other than the intended signal”. That little hiss that those of you playing vinyl records on your resurrected turntables hear is officially called “surface noise”. For radio signals, noise is static, squeals, etc., (and the “motorboating” sometimes heard on AM radio). In audio signals, noise is feedback, sixty-cycle hum, car alarms, barking dogs, and very definitely your rap-loving neighbor’s boom box drowning out the intended signal of what your  literate-but-soft-spoken friend has to say about Jane Austen. 

And, in terms of rafts and ping-pong balls, “noise” would be any and all flotsam, jetsam, seaweed, driftwood, deceased marine fauna, not-yet-deceased marine fauna, floating trash, rubber duckies, and anything else other than red or white ping-pong balls that comes floating around Bill’s feet.


To get very thoroughly carried away with this analogy:

Let’s say I’m not the only ping-pong-ball-transmitter out there. There’s a guy a hundred yards down the beach doing light bulb jokes in purple and yellow balls, and one fifty yards the other way doing knock-knock jokes in blue and orange. To keep us from interfering with each other’s transmissions, the government sets up a Floating Comics Commission (FCC) and gives it the right to regulate ball colors and auction off raft mooring sites. Pretty soon the whole thing catches on, and there’s  a nearly solid line of rafts up and down the beach, broadcasting good, bad and mediocre humor in every ping-pong ball color imaginable.

Trouble might arise if the FCC sold the raft next to mine the right to broadcast in tan and pink, and the raft three down the other way the right to use ecru and maroon. It was easy enough for Bill to sort out the purple, yellow, blue, and orange balls from the red and white ones, right along with all the other “noise” on my broadcast signal. But the tan and ecru balls could easily be confused the white ones, and the pink and maroon balls with the red ones. Just how far apart on the AM and FM radio broadcast spectrums (yes, I know it should be “spectra,” but that’s way too hoity-toity for this sort of analogy) any two stations must be is something that broadcasters and that other FCC get to spend lots of time arguing about.


And now for Pirate Radio, and our Exciting Action Sequence:

A black flag rises swiftly over the horizon, and a dark ship swoops towards the line of moored rafts. Hearts sink into beach sandals as the ping-pong transmitters recognize the race-built pinnace captained by the dread pirate Black Anne Cash. Soon she is hove to just seaward of the rafts, and suddenly her crew are at the rail, heaving barrels of Day-Glo Orange and Emergency Yellow-Green onto the ocean swells.

 The flood of ping-pong balls from the pirate transmitter overwhelms my much smaller broadcast stream, so that poor Bill on receiving end can’t make out my elephant jokes through Black Anne Cash’s binary blast demanding: “WHY DID THE CHICKEN CROSS THE ROAD?”

This is as illegal as all get-out, of course; but before the Coast Guard can arrive, she has dumped out barrels of ping-pong balls forming the signal “BECAUSE IT WAS A FOWL PROCEEDING,” hoisted sail, and slipped away into the gathering sunset mist.


 (“Waves Rolling In” image modified from photo by MUmland at morguefile.com)


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11 Responses to “Rafts, Ping-Pong Balls, and Radio Waves”

  1. Katia Says:

    Hi Evelyn! Very cool blog! I can’t believe I missed it the first time around.

  2. Evelyn Says:

    Thanks, Katia! :)
    You have the (dubiously indubitable) honor of being my very first commenter.

  3. Yella Ojrak Says:

    Does everything you explained apply to mobile phones as well?

    Anyway, this kind of stuff is interesting.

  4. Evelyn Says:

    Well, sort of – for mobile phones you have to imagine a *lot* more rafts and *one great big heck of a lot* more ping-pong balls. :)

    I’m very glad you found it interesting.

  5. Yella Ojrak Says:

    Wow! Must be a very busy ping-pong world up there…

    Another question, how do you translate something into binary codes, or binary codes into something like, English?

  6. Evelyn Says:

    Well, for cel phone transmissions you might imagine a bunch of 8-year-old boys in treehouses throwing ping-pong balls at the radio recievers in each of our phones, and a bunch of storks (like the kind that are said to be thr bringers of babies) flying back and forth between the treehouses with their pink or blue bundles full of carefully-kept-separate ping-pong-ball packets of voice transmission signals –

    – but analogies can only be taken so far, and this one really is starting to creak under the strain. :)

    As for binary coding of English, the theory’s fairly simple (it gets extremely complex in practice), but not quite simple enough to do as a comment.
    I’ll put something together on that (and then try to figure out whether it should be as a post or a page :) ).

    – Evelyn

  7. Yella Ojrak Says:

    Oh, great! That would be sooooo great!

    You know, right after I sent you my last comment, I googled ‘binary codes’ and found a Wikipedia page on it. But you know Wikipedia… It gives you links to almost every word, its info’s are not always valid, and most importantly, it’s boring. It would be great to learn it from you!

    Gosh, I love this blog a lot already!

  8. Evelyn Says:

    Aw, thanks, Ojrak :)

    I was just going to refer you to the Wikipedia page; but fortunately I Googled it up too and took a look first. Oops – those are the guys who understand all those “complexities in practice” explaining things to each other: which particular kind of code was used when, and why, and who used a different type of code at what time…
    Not a word anywhere about just plain “How does it work?”

    (And it was boring, and hard to follow even for someone who already knows the basics behind it.)

    I may not be able to get my own take on binary posted right away, so I hope you’re not holding your breath. :)

    – Evelyn

  9. Yella Ojrak Says:

    Oh, please write whenever you want to. Next month, next year, would be fine with me. Just make sure you keep writing your blog, and I’ll keep reading you.

  10. Stonehead Says:

    Your analogy makes me feel old. I first learned about radio waves by building a crystal radio and later a spark gap transmitter. Now, kids just go and buy a mobile phone.

  11. Evelyn Says:

    No need to feel old, Stoney – I’m sure that rafts predated you, and ping-pong balls probably do also. :)

    But telling that story (in my comment on your blog) about calling all over town trying to track down a bit of information, and realizing that I needed to explain why I didn’t just Google it – that one made me feel pretty damned *old.*

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