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From: jeh@cmkrnl.uucp
Newsgroups: sci.electronics
Subject: Re: Cable
Message-ID: <1991Jun3.132526.67@cmkrnl.uucp>
Date: 3 Jun 91 20:25:26 GMT
References: <1991Jun3.044218.16908@milton.u.washington.edu>
Organization: Kernel Mode Consulting, San Diego CA
Lines: 25

In article <1991Jun3.044218.16908@milton.u.washington.edu>,
 amigo@milton.u.washington.edu (The Friend) writes:
>      Also in #2, can the technician actually see what channel a TV is on?
> The spec sheet on a RF detector only says it can pick up "mid band TV" - 
> meaning it (I think) finds only leakage (bad connections, extras, etc).

there are several ways this can be done in theory.  

For example, when a tv set is tuned to a particular channel, the local 
oscillator in its tuner will be set to about 4.5 MHz higher (assuming 
that tv tuners still work the way they used to, and that the IF is still 
4.5 MHz).  

Another interesting idea is to pick up the hash that is radiated by the 
set's scanning circuits.  The 60 Hz vertical scan is well-modulated into
this hash (put an ordinary AM radio near an operating tv set and you'll 
get a sample).  Now, the different stations carried on the cable are
NOT gen-locked to one another.  One could pick the 60 Hz out of the set's
leakage and compare its phase with that of each signal on the cable to tell
which station was being watched.  

	--- Jamie Hanrahan, Kernel Mode Consulting, San Diego CA
Chair, VMS Internals Working Group, U.S. DECUS VAX Systems SIG 
Internet:  jeh@dcs.simpact.com, hanrahan@eisner.decus.org, or jeh@crash.cts.com
Uucp:  ...{crash,scubed,decwrl}!simpact!cmkrnl!jeh