Today's RF Trivia

Whenever I make a call on my V-tech cordless phone I think of how I'm using a system invented by Hedy Lamarr, and I feel so much closer to Zippy.

I wouldn't say SS prefigures TDMA or CDMA, as SS is frequency-division multiplexing. Not really the same thing, or even close (in my view), as TDMA / CDMA and SS serve different purposes.

According to the article in Technology Heritage (that isn't the right name, but I can't think of it), the invention got nowhere, because the patent used a player-piano metaphor to explain how it worked, and the companies reading it just couldn't see the connection.

CDMA is Spread Spectrum.

Bob

They must have been rather stupid.

I spent last night reading about the history of mobile radiotelephone right up to the current and future generations of communication techniques. Your views are flawed, Bill. Some good reading:

Cellular/PCS Management ISBN: 0071346457 Illustrated Telcom Dictionary Bluetooth Demystified ISBN: 0071363238

techniques.

This article has a number of errors, such as stating that the 1946 Dick Tracy had a videophone on his wrist, and that the first transistor radio used silicon transistors. * The author also seems to be very young. For example, he states

"The July, 1999 Popular Electronics outlined a simple F.M. transmitter kit. It used only one transistor, eleven other parts, and took up no more than a square inch or two. F.M. is now everywhere, developed largely by one man."

I built an FM transmitter from a Popular Electronics article 35 years earlier. FM was a common form of transmission well before that time. FM is not new.

He says that "older" telephone mics use a "thin metal sheet". They don't. They use carbon-granule mics.

Worse, he says that all digital telephone systems use "vocoders". As far as I know, this is absolutely untrue. A vocoder models the original sound, then synthesizes a close approximation. That isn't the way digital transmission works.

At least he understands that digitization requires quantization. However, his gives an overly terse, shallow explanation, then he asks the reader to reread the explanation to make sure they understand it.

Then he shows he doesn't understand the difference between analog and digital by stating "A digital signal is made up of discrete units but an analog signal is a continuous unit." Absolutely and totally wrong. A "discontinuous" signal can be every bit as "analog" as a continuous signal. It is the sample quantization that makes information digital, not sampling, per se.

When I see stuff of this sort, I naturally question everything else in an article. (I've CC'd this posting to the author.) Other people have questioned it -- there's a lot of Discussion criticism. Anyhow...

The spread-spectrum system was originally intended to increase the security of the transmission. But it has another advantage -- if multiple transmitters are working in the same area, but using different spread patterns, the probability that two transmissions will interfere becomes very small -- and when they do interfere, it is only for the period of a single sample (assuming reasonable randomness among the patterns).

CDMA also offers this advantage. However, there is a code attached to each CDMA sample. This makes it possible to reconstruct the original data in a straightforward fashion, something that would be more difficult with spread-spectrum, especially when there are multiple transmissions. CDMA is not, as far as I know, a highly secure method of transmission.

I'm going to stop arguing and read the Wikipedia article when I have a chance, just to make sure I'm not (too) wrong. But I naturally cringe at such statements as "CDMA is a form of "spread-spectrum" signaling, since the modulated coded signal has a much higher bandwidth than the data being communicated." By that reasoning, commercial FM broadcasts are spread-spectrum.

• If this is true, I'll be very surprised. Silicon transistors did not become commonly available until about a decade later. I remember the first audio products that proudly proclaimed "all-silicon" -- a noteable advance, as silicon output devices were not subject to thermal runaway and other failure modes that destroyed germanium output devices.

Does he mean the first commercially successful personal transistor radio? That certainly used germanium transistors. The 'first' transistor radio ever constructed (breadboarded) certainly used point contact transistors.

Michael

In 1956 I was working for the semiconductor division of Sylvania Electric in the test equipment area. We were making germanium transistors for AM radios. At that time we had not developed a good test for mixers. The test bed we used was a radio made (IIRC) by Motorola. This was fitted with a socket. If the radio played the transistor was shipped.

Ah, the good old days.

Charlie

in

s.

I worked for Bell Labs in that era, and we actually used a high- powered audio amplifier to check diodes. A ramp of DC current was appled to the diode while it was also AC connected to the audio amplifier. If you heard snap, crackle, pop as the current increased, it was indication of an unstable diode and the diode was tossed out. If the diode was "quiet", it went on to be part of the first AT&T telephone central office in Succasunna, New Jersey.

The following e-mail has been sent both to this group, and to Thomas Farley, the author of the cell-phone technology article referred to by another poster.

I gave up -- or more accurately, outgrew -- making personal attacks several years ago. However, when Mr. Farley sent a response to my posting I discovered something very disturbing. He writes professionally about technical subjects, but is technically incompetent.

The following is my reply to him, along with a personal attack at the end.

You're probably right about the earliest telephones using metal sheets. But you said "older", not "oldest". And the carbon mic -- invented by Edison, not Watson -- came in not long after the original telephone -- less than a decade, I believe.

Look at the Wikipedia article on vocoders. Vocoders date back to the '30s. They analyze the speech sounds, then synthesize them at the receiving end. This has nothing to do with digital transmission, which samples and quantizes the waveform.

Sorry, no. The original description should be detailed and clear enough that the reader "gets it" the first time.

The way to avoid "rusing people along" is to write the material in sufficient detail.

The difference between analog and digital is that analog information varies continuously, whereas digital information is quantized. Sampling has nothing whatever to do with it, other than that sampling is a necessary prerequisite for quantization. Simply sampling a signal doesn't make it digital.

But one can have highly secure analog transmissions, of which SS is an example. CDMA is not necessarily more secure.

I prefer to be constructive and avoid "personal" remarks. But I didn't reali ze until I reached the end of your letter that you are a professional writer. I can't believe that you're actually paid to write articles about technology, because you just aren't qualified. I'm a well-qualified technical writer who's verging on bankruptcy and foreclosure because I can't find a job, and you, a technical ignoramus, earn a living doing what I do, badly. It's outrageous.

If I didn't know any better, It sounds like you have some personnel issue with the author ?

I think you'll find that the history of electronics documented over the years has a lot of errors and variants in it.

I'm not sure that I could pretend to be following who said what to whom in that exchange ... !

Arfa

Quite true. If only all such pissing contests were re-directed to personal email.

I just concede that Bill Sommerwerck knows everything about everything and that usually suffices for me :) I've been reading some of Bell Lab's Dr. William Lee's writings and they differ considerably from what Bill Sommerwerck has to say.

The issue is whether something is true or not -- not who says it.

Too many people take the word of "experts" at face value, without thinking. I would rather be wrong because I don't believe something (that happens to be true) simply because it doesn't make sense -- than to blindly believe something false.

The author of the piece referred to is lacking in technical knowledge and should not be writing articles about cell-phone technology without confirming the accuracy of what he writes.

As to CDMA versus SS -- I stand by my original comments, solely on the basis of "common sense". CDMA has certain features in common with spread spectrum, but it is not spread spectrum.