[NJARC] Hedy Lamarr

[al at ar88.net]

The general term for such systems is "spread spectrum."  The two main  
categories are frequency-hopping, and direct-sequence.

Lamarr's original inspiration came from music:  If we both know the  
song, we both know where the next note (frequency) is going to be.   
The military uses frequency hopping, one application is back-pack  
field radios.  Microprocessors and digitally tuned radios make this  
relatively easy.

Direct-sequence involves combining the signal with a pseudo-random  
data stream.  The result appears to be white noise.  A closely related  
technique is OFDM (Orthogonal Frequency Division Multiplexing)formerly  
known as discrete multi-tone.  Here digital data is spread across  
multiple carriers in a limited bandwidth.  Redundant data is added, so  
that if you miss a carrier of two, the computer at the far end can fix  
thing up.  This is known as FEC (Forward Error Correction.)  These are  
the sort of techniques used in cell phones.

You can hear examples of OFDM on 700 and 720KHz,  WOR's HD sidebands.

To parody the late Arthur C. Clark:  Any sufficiently advanced  
modulation scheme is indistinguishable from noise.

Enough for now,
Al

Quoting NICHOLAS SENKER <ns539 at embarqmail.com>:

> Visit our web site - See http://www.njarc.org
> _______________________________________________
> Thanks Harry, This is very interesting.  Is this technique still   
> used, say for cell phones?
> Nick
>
> ----- Original Message -----
> From: Harry Klancer <klancer2 at comcast.net>
> To: New Jersey Antique Radio Club <njarc at mailman.qth.net>
> Sent: Wed, 2 Apr 2008 21:39:27 -0400 (EDT)
> Subject: Re: [NJARC] Hedy Lamarr
>
> Visit our web site - See http://www.njarc.org
> _______________________________________________
> Nick,
>
> There was an article about Hedy Lamarr in "Invention and Technology"
> magazine about
> 7 or 8 years ago.
>
> In frequency hopping, instead of using a fixed carrier frequency, a
> transmitter uses a number
> of carrier frequencies in the same band, first it sends info using one
> carrier frequency for a
> period of time, then it changes to use another carrier frequency for a
> period of time, then another
> frequency for a period of time, etc. The transmitter and receiver have
> to have an agreed-upon
> arrangement for what frequencies will be used, and for how long. In
> addition, they must be
> synchronized so that the receiver is looking at the right frequency at
> the time when the transmitter
> is using that frequency.
>
> The agreement between sender and receiver can be in the form of a fixed
> arrangement of frequencies,
> for example, f1 then f4 then f2 then f5 then f3 then etc. Or you can use
> something called a
> pseudorandom number generator, and I'm sure there are other schemes. My
> understanding is that Hedy's
> scheme used a piano roll to tell the transmitter and receiver what
> series of frequencies to use.
>
> Some advantages of frequency hopping is that the receiver is less
> susceptible to narrowband noise.
> Suppose that there is a lot of noise around frequency f4. The receiver
> is only listening around frequency
> f4 for a short time, and then it moves on to another (presumably
> quieter) frequency. Another advantage
> is against eavesdropping - unless an intercepting eavesdropper knows the
> pattern of carrier frequencies
> that will be used, it can't follow the communication between transmitter
> and receiver since it doesn't
> know where to look.
>
> Hope this helps.
>
>                                                      Harry K
>
>
>
> NICHOLAS SENKER wrote:
>> Visit our web site - See http://www.njarc.org
>> _______________________________________________
>> An article in yesterday's Newark Star Ledger about Hedy Lamarr (not  
>>  to be confused with Hedley Lamar in 'Blazing Sadles') discribes  
>> the  sultry beauty as not only a  great actress but also a 'brainy   
>> beauty'.  This is not an April Fool's joke!
>> To quote the article (p 29):
>> 'in the early 40's Lamarr came up with the concept of frequency   
>> hopping. Her invention was 40 years ahead of its time. It was a   
>> secret communication system.  It was the first time wireless radio   
>> transmissions could be sent and received without being intercepted.  
>>   Her children stress how the wireless world uses frequency hopping  
>>  today with PDAs, wireless phones, and Wi-Fi. Composer George   
>> Antheil implemented Lamarr's concept.  He and the Vienna born   
>> actress received a patent which they gave to the US government.'
>> Amazing!
>> Can someone explain frequency hopping?
>> Nick Senker
>> _______________________________________________
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>> NJARC at mailman.qth.net
>> http://mailman.qth.net/mailman/listinfo/njarc
>>
>>
>
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