[Discuss] Cable TV fundamentals (OT)

[jim at gasek.net (Jim Gasek)]

I'd like to give a quick background- makes it easier 
to understand.

Cable TV started in Denver a long time ago.

Think of the cable as you would think of "off air"
signals, except the cable signal is "contained" within the
(then completely coax ) wire.   Anywhere the wire can
go, the signal can go.  And it can be made to be 
"uniformly strong" (like, you're equally near the antenna, 
for the off-air analogy).

The cable systems bypassed many FCC regulations, as their
signals were "contained" in private enclosed spaces.

The wire has 3 components.

The core.  Solid condictor.  Signal rides here.

The insulator.   A plastic like material.

The outer layer (think tinfoil or screen), which 
"keeps the signal in, and keeps the noise OUT".

Ok, so what was the initial design?

TV.   Broadcast.   One to many.

This is referred to as "THE FORWARD".   The signal
LEAVING the headend.

It didn't take long for businessmen to understand that
this same wire that carried the FORWARD, could also carry
the RETURN (from the consumer TO the headend).

R = RETURN = REVENUE.   

But, there was a problem.  A big problem.   Since it wasn't
in the original design, the RETURN would need to be placed
in electromagnetic real estate that wasn't already claimed
by the FORWARD.   This meant below about 52MHz, right below
the carrier for analog Channel 2.

So, frequencies allocated for the return were scarce and 
weak.   Prone to interference and "ingress".   Signals 
which creep back into the system, especially where 
junctions ("tap plates") where inadvertantly corroded 
or otherwise grounded.   

Your cable modem "receives" the forward signal usually 
at abundant and fast electromagnetic real estate (see 
192.168.100.1 for your signal type), but replies at much
mode dear, crowded and noisy places.  

It matters less the modulation type (digital or analog),
and much more the electromagnetic real estate available 
for the RETURN.

The RETURN must be shared by all "two way" services,
telphony,  impulse pay per view, video on demand, monitoring,
and each channel (think "ethernet") which is on every 
"branch" (fiber node) from the headend.   Each branch
must aggregate the RETURN traffic from that neighborhood.

Fiber transmissions are pristine from the headend to the
place it is "stepped down" (converted) into RF signals
over coax.  

the best (remaining) elsectromagnetic real estate was
used for data- from about 20-40 (?) MHz.  5-10MHz was used
for monitoring.   Poor real estate.  

This is where all the discussion about LEVELS comes into 
play.   You need to have enough SIGNAL so that it rides
above the noise and does the job.   SIGNAL strength,
think "volume" in a conversation, must ride above the 
background chatter.  

Splitting cables causes signal loss.   Usually not a huge
deal.    Amplification can overcome signal loss, but 
remember, you're amplifying signal and noise.   Crappy 
real estate is still crappy real estate, amplified or not.

All this ancient history is understood well by the DATA
techs.   they understand the RETURN.  Regular cable monkeys
mostly look at the FORWARD.  

Poorly maintained systems will allow signal (HAM radio)
to creep back in, at spots where junctions are bad.  

This will (momentarily, while mike button is down) knock
out the return signal right at around 25MHz.  

The "ethernet" in your neighborhood is probably shared 
by dozens, if not hundreds of other households.
Luckily for the cable operators, not all users are
online at once.  

The switch from (off-air) analog to digital transmission
was primarily to recover the very valuable elecromagnetic
real estate for more important and lucrative services.  
"Auctioned off" to high bidders, making revenue for the 
government.  


Thanks,
Jim Gasek

--- cra at WPI.EDU wrote:

From: Chuck Anderson <cra at WPI.EDU>
To: discuss at blu.org
Subject: Re: [Discuss] Extra splitter (OT)
Date: Tue, 6 Sep 2011 08:42:11 -0400

On Sun, Sep 04, 2011 at 05:53:30PM -0400, edwardp at linuxmail.org wrote:
> The recent thread regarding RCN, reminded me of the AT&T BB installation  
> 10 years ago.  They installed an additional splitter (two-way) with one  
> cable going into the cable modem and the other cable going into another  
> splitter (three-way) going to the TV's.  At the time of the  
> installation, they also installed a filter on the other cable going from  
> the two-way to the other splitter, but eventually Comcast removed it as  
> it was no longer required.
>
> With the technological advances made since then, is this extra two-way  
> splitter still required today, or could everything now go into one 
> splitter?

You can use a three-way splitter.  Those are usually marked with a
"high" output with lower dB loss--connect that one to the cable modem.
For each two-way splitter, you lose a bit more than half the signal
(3.5 dB up to 1 GHz for good splitters, worse for cheap ones).  Any
bigger splitters are internally just networks of two-way splitters,
and the same thing holds true.  E.g. a three-way splitter looks like
this inside:

     __-3.5dB__Out
In--|             __-7dB__Out
     ~~-3.5dB~~~~|__-7dB__Out

So the first output's power is less than half the input power, and the
other two are less than 1/4 the input power.

Just make sure you get a "good" splitter, 5 MHz - 1 GHz, 3.5 dB, $1-$3.
You don't need to spend extra for the "Monster" one though.
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