|
Posted by TC on September 17, 2006, 11:10 pm
If you were Registered and logged in, you could reply and use other advanced thread options > The vast
> majority of cable outlets in the United States are analog only, with the
> majority of those just being a jumper from the outlet to the cable-ready
> TV or VCR. In a house with 5 outlets, 3 (maybe 2) of those are likely
> to be cable-ready TVs.
this is interesting, i run into about one non-cable ready tv every
three weeks, if that.
John Q. Public decided to rearrange his room
> where he watches a cable-ready TV. The jumper isn't long enough to
> reach where he moved his TV set to, so he goes down to WallyWorld and
> buys one that has less than adequate shielding. Source of ingress.
> Rather than have an ugly cable wrapped halfway around his room, he
> decided to relocate the outlet, so he goes to the nearest home
> improvement store and buys some cable and fittings, but he doesn't want
> to spend too much so he buys some screw-on connectors and a 99-cent
> splitter and cuts the cable that goes to the existing outlet in the room
> and installs his new splitter there. 4 sources of ingress on that one.
Granted, the ingress will be a problem, but when that customer has gone
too far, and created a disaster that he cannot repair, and god willing,
he gets the proper cable guy in his house, he will HOPEFULLY ( hah)
learn his lesson. ;)
> Most drop amps have about a 2.4-3dB noise figure. That is not Signal to
> Noise Ratio (SNR), that is the amount of noise the device itself
> generates. When you add 3dB of noise but increase the signal level by
> 15dB, you may actually improve the SNR. Simple math: Say you have a
> signal level of +10dBmV and a noise figure of 2dB. Your SNR is 10:2 or
> 5:1. Insert a 15dB gain drop amp that has a noise figure of 3dB. Your
> numbers are 25:5, or the same 5:1 that you had in the first place. The
> amp had no effect on the SNR. As long as the input level is above the
> noise figure of the amplifier, you won't experience any noise problems.
> Gotta stay be below the maximum input level specified for the amp, though.
youre right, dead on, but:
you really can't, _technically_ improve a signal to noise ratio
anywhere but at the source, can you?
you'd need a lot more that a drop amp to do that, even within amp
specs...
i mean , when you amp a signal, you are amping the noise floor too..
please correct me if i am wrong...
i have to admit, i really attempt to shy away from drop amps every
chance i get, there are too many situations i have seen where
less-than-creative techs use them to boost one lousy outlet of an in
house system, rather than do ther job properly, so as a result, i just
try to do it as a last resort. in most cases , youre right, the tap is
hot enough.
> > as for 10-15 at the groundblock, on channel 4,
> > maybe.. real world, rarely, unless you are in a lab...
>
> Cable systems are designed to run out at 15-20dBmV at the tap at their
> system's highest frequency. Not a lab thing, real world
spoken in haste, true true ... after i wrote that i instantaneously
regretted having done it, _usually_ there is more than enough to play
with at the GB.
> Cable modems operate with an input signal level of -15 to +15dBmV. That
> is not just a "happen to" thing -- that is a specification. That also
> is the level of the QAM carrier, which is either 6 or 10dBbelow the
> adjacent analog carrier (depends on whether it is a 64 or 256QAM system).
+17 on 256, +15 on 64, for most, but this is one place i have to say
that, as i see most everyday, this is not usually the case.. mostly, go
above +5 on a dht or cable modem, and usually the dht is a LOT more
forgiving...and youre gonna have some pronblems... i see it almost
every day :(
| 
Yahoo! 
Google 
Windows Live 
del.icio.us 
digg 
Netscape 
XML site map