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Guest
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 Posted: Sun Sep 10, 2006 8:24 pm Post subject: Splitting a coax signal |
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I would like to connect 4 tvs and one cable modem to the cable signal
coming into my home. The cable company split the signal just outside
my house and brought one run to my computer and one to my tv. I would
like to add three additional tvs and would like to know the best way to
add the others.
Will I need to boost the signal and if so what type of devise would be
the best to use.
Thank you for any suggestions.
Andy |
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Timothy Daniels
Guest
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| Posted: Sun Sep 10, 2006 10:59 pm Post subject: Re: Splitting a coax signal |
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<AndyLash@gmail.com> wrote:
| Quote: | I would like to connect 4 tvs and one cable modem to the cable signal
coming into my home. The cable company split the signal just outside
my house and brought one run to my computer and one to my tv. I would
like to add three additional tvs and would like to know the best way to
add the others.
Will I need to boost the signal and if so what type of devise would be
the best to use.
|
Leave the leg to the modem unsplit. Split the TV leg with a 4-way
splitter for the TV sets. The cable TV company should give you enough
signal strength to manage a 4-way TV split despite a split for Internet.
If you see a deterioration in signal quality, have the cable company
send out a technician to boost (or unpad) the signal at the pole, at the
entrance point, or in your house. If the tech boosts the signal at the
pole, you may be able to do a 2-way split in the modem run between
Internet and TV.
*TimDaniels* |
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$Bill
Guest
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| Posted: Tue Sep 12, 2006 2:38 pm Post subject: Re: Splitting a coax signal |
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Ed Nielsen wrote:
| Quote: | Personally, I prefer to install a drop amp if there are more than 3
outlets.
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I've got a 1-4 splitter at the ground block with the modem on 1 and 3
TV lines on the others. One of the TV lines is split 1-8 farther down
(7 in use) with no problems.
| Quote: | Typically, signal strength at the groundblock is 10-15dBmV.
Say it is 10. Lose 3.5 of that for the cable modem and you are left
with 6.5 to feed the TV outlets. A 4-way splitter drops that another
7dB, and you are at -0.5dBmV just leaving the splitter. Lose another
couple of dB through the cable, and you are at -2.5dBmV at the outlet
(depending on distance from splitter). If the level at the groundblock
is 15dBmV, you are then at +2.5dBmV at the same outlet, but I think it
is better to be above +5dBmV at the outlet. FCC says no less than 0dBmV
at the outlet. The splitter for the cable modem should be prior to any
amplifier. Cable modems should not be amplified. |
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Ed Nielsen
Guest
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| Posted: Tue Sep 12, 2006 2:38 pm Post subject: Re: Splitting a coax signal |
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Personally, I prefer to install a drop amp if there are more than 3
outlets. Typically, signal strength at the groundblock is 10-15dBmV.
Say it is 10. Lose 3.5 of that for the cable modem and you are left
with 6.5 to feed the TV outlets. A 4-way splitter drops that another
7dB, and you are at -0.5dBmV just leaving the splitter. Lose another
couple of dB through the cable, and you are at -2.5dBmV at the outlet
(depending on distance from splitter). If the level at the groundblock
is 15dBmV, you are then at +2.5dBmV at the same outlet, but I think it
is better to be above +5dBmV at the outlet. FCC says no less than 0dBmV
at the outlet. The splitter for the cable modem should be prior to any
amplifier. Cable modems should not be amplified.
CIAO!
Ed N.
Timothy Daniels wrote:
| Quote: | AndyLash@gmail.com> wrote:
I would like to connect 4 tvs and one cable modem to the cable signal
coming into my home. The cable company split the signal just outside
my house and brought one run to my computer and one to my tv. I would
like to add three additional tvs and would like to know the best way to
add the others.
Will I need to boost the signal and if so what type of devise would be
the best to use.
Leave the leg to the modem unsplit. Split the TV leg with a 4-way
splitter for the TV sets. The cable TV company should give you enough
signal strength to manage a 4-way TV split despite a split for Internet.
If you see a deterioration in signal quality, have the cable company
send out a technician to boost (or unpad) the signal at the pole, at the
entrance point, or in your house. If the tech boosts the signal at the
pole, you may be able to do a 2-way split in the modem run between
Internet and TV.
*TimDaniels* |
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$Bill
Guest
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| Posted: Tue Sep 12, 2006 10:13 pm Post subject: Re: Splitting a coax signal |
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Ed Nielsen wrote:
| Quote: | With ~23dB of loss just through the splitters alone (not to mention the
attenuation through the cables from the splitters to the outlets), it
sounds like you sweet-talked someone into having your tap run out
extremely hot just to keep your furthest outlet within FCC specs (or
even acceptable). Cable modem has a rather high input level as well, I
would imagine (unless it has been padded down).
|
Not at all - there has never been an installer in the house. The only
visit was to add my modem when they became available. At that time,
they cleaned everything up and ran new cable to the house from the
underground, added a ground block and grounded to my breaker box,
replaced the splitter with a new 1-4 splitter and gave me 100' of RG6
to run to the modem. The TVs are all still running RG59.
Here's my reasonable, but not great, numbers for today at modem :
Tx Power 48.2 dBmV
Rx Power -7.0 dBmV
Downstream SNR 35.0 dB
Downstream MER 33.2 dB |
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Ed Nielsen
Guest
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| Posted: Wed Sep 13, 2006 2:48 pm Post subject: Re: Splitting a coax signal |
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Cable systems are designed for a signal level of 15-20dBmV at the tap
and every tap, splitter, directional coupler, etc. is well documented.
They know just what is where. Changing out a tap in order to get more
signal at a specific location may very well affect the signal levels
downstream as well as upstream, as different value taps have different
insertion losses, not to mention that the system would go down while
that tap is being replaced. Back to one of the reasons why so many
people hated cable TV years ago -- frequent outages.
Systems that have been upgraded to provide advanced services typically
run at 20dBmV at the tap, which, after an 85' drop is 15dBmV at the
groundblock, which is fine for 4 outlets. Install a DC-9 prior to the
4-way splitter for the cable modem and everything is happy.
If there is 15dBmV at the tap, the same 85' drop turns that into 10 at
the groundblock. A 4-way splitter makes that 3dBmV leaving the splitter
to feed the outlets. A 50' run to the outlet then drops that down to
+0.1dBmV at the outlet. Still meets FCC specs and if there is a DCT or
a pretty new cable ready TV at the outlet and all of the connections are
good (includes tight), then all is well. However, if there is a 10-year
old cable ready TV set that has less than optimal shielding integrity at
the tuner, ingress takes control and locals really suck, as do some of
the other channels. If the homeowner or tenant moves the room around
and needs a longer cable to go from the outlet to the TV set, they run
down to Wally-World and buy one. It doesn't have the shielding
integrity needed and pictures are a mess. Ingress is the reason I
prefer to have no less than +5dBmV at the outlet, even though the FCC
says 0dBmV.
CIAO!
Ed N.
Timothy Daniels wrote:
| Quote: | My impression from talking with several techs in
the So. Calif. area as they did their installations for
customers in our condo building is that they'll give you
enough signal strength to accommodate a 4-way split
for TV. If they determine where the splitters are put in
the residence they'll do what's necessary to *still* give
you enough signal strength for a 4-way split on the TV
legs and assure proper signal level for the modem.
As long as they know what the setup will be inside the
home, they'll do that because it's in their company's
financial interest to:
1) Assure a good picture on the TV sets,
2) Good signal level for the modem, and
3) Avoid future callbacks.
My impression from conversations with the field tech's
supervisors is that this policy may stem more from the
supervisors' interpretation of the company's "culture"
than written company policy. Given that, I think that the
more you can tell the field tech what your ultimate
configuration will be, the more likely he'll be to accommo-
date your needs in signal strength.
*TimDaniels* |
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Timothy Daniels
Guest
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| Posted: Thu Sep 14, 2006 9:40 am Post subject: Re: Splitting a coax signal |
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Cut to the chase, Ed - what would be your advice to the OP?
*TimDaniels*
"Ed Nielsen" wrote:
| Quote: | Cable systems are designed for a signal level of 15-20dBmV at the tap
and every tap, splitter, directional coupler, etc. is well documented.
They know just what is where. Changing out a tap in order to get more
signal at a specific location may very well affect the signal levels
downstream as well as upstream, as different value taps have different
insertion losses, not to mention that the system would go down while
that tap is being replaced. Back to one of the reasons why so many
people hated cable TV years ago -- frequent outages.
Systems that have been upgraded to provide advanced services typically
run at 20dBmV at the tap, which, after an 85' drop is 15dBmV at the
groundblock, which is fine for 4 outlets. Install a DC-9 prior to the
4-way splitter for the cable modem and everything is happy.
If there is 15dBmV at the tap, the same 85' drop turns that into 10 at
the groundblock. A 4-way splitter makes that 3dBmV leaving the splitter
to feed the outlets. A 50' run to the outlet then drops that down to
+0.1dBmV at the outlet. Still meets FCC specs and if there is a DCT or
a pretty new cable ready TV at the outlet and all of the connections are
good (includes tight), then all is well. However, if there is a 10-year
old cable ready TV set that has less than optimal shielding integrity at
the tuner, ingress takes control and locals really suck, as do some of
the other channels. If the homeowner or tenant moves the room around
and needs a longer cable to go from the outlet to the TV set, they run
down to Wally-World and buy one. It doesn't have the shielding
integrity needed and pictures are a mess. Ingress is the reason I
prefer to have no less than +5dBmV at the outlet, even though the FCC
says 0dBmV.
CIAO!
Ed N.
Timothy Daniels wrote:
My impression from talking with several techs in
the So. Calif. area as they did their installations for
customers in our condo building is that they'll give you
enough signal strength to accommodate a 4-way split
for TV. If they determine where the splitters are put in
the residence they'll do what's necessary to *still* give
you enough signal strength for a 4-way split on the TV
legs and assure proper signal level for the modem.
As long as they know what the setup will be inside the
home, they'll do that because it's in their company's
financial interest to:
1) Assure a good picture on the TV sets,
2) Good signal level for the modem, and
3) Avoid future callbacks.
My impression from conversations with the field tech's
supervisors is that this policy may stem more from the
supervisors' interpretation of the company's "culture"
than written company policy. Given that, I think that the
more you can tell the field tech what your ultimate
configuration will be, the more likely he'll be to accommo-
date your needs in signal strength.
*TimDaniels* |
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Ed Nielsen
Guest
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| Posted: Thu Sep 14, 2006 2:51 pm Post subject: Re: Splitting a coax signal |
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Basically the same as yours -- a 2-way splitter (or directional coupler)
with 1 leg going to the cable modem and the other leg going to the TV
distribution system. If that TV distribution system is merely a 4-way
splitter, great. If some of the channels are on the snowy or fuzzy side
I would install a drop amp prior to the 4-way splitter.
CIAO!
Ed N.
Timothy Daniels wrote:
| Quote: | Cut to the chase, Ed - what would be your advice to the OP?
*TimDaniels*
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Timothy Daniels
Guest
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| Posted: Fri Sep 15, 2006 9:16 am Post subject: Re: Splitting a coax signal |
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Sounds good to me! :-)
*TimDaniels*
"Ed Nielsen" wrote:
| Quote: | Basically the same as yours -- a 2-way splitter (or directional coupler)
with 1 leg going to the cable modem and the other leg going to the TV
distribution system. If that TV distribution system is merely a 4-way
splitter, great. If some of the channels are on the snowy or fuzzy side
I would install a drop amp prior to the 4-way splitter.
CIAO!
Ed N.
Timothy Daniels wrote:
Cut to the chase, Ed - what would be your advice to the OP?
*TimDaniels*
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TC
Guest
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| Posted: Sat Sep 16, 2006 2:43 am Post subject: Re: Splitting a coax signal |
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Ed Nielsen wrote:
| Quote: | Personally, I prefer to install a drop amp if there are more than 3
outlets. Typically, signal strength at the groundblock is 10-15dBmV.
Say it is 10. Lose 3.5 of that for the cable modem and you are left
with 6.5 to feed the TV outlets. A 4-way splitter drops that another
7dB, and you are at -0.5dBmV just leaving the splitter. Lose another
couple of dB through the cable, and you are at -2.5dBmV at the outlet
(depending on distance from splitter). If the level at the groundblock
is 15dBmV, you are then at +2.5dBmV at the same outlet, but I think it
is better to be above +5dBmV at the outlet. FCC says no less than 0dBmV
at the outlet. The splitter for the cable modem should be prior to any
amplifier. Cable modems should not be amplified.
CIAO!
Ed N.
|
-2.5 isnt "realworld" bad, actually, regardless of FCC requirements...
you could gain that back on an extra chilly day.. lol, i think the FCC
would forgive 2.5 db +/- I would rather err on the neg side than the
positive...IMO, drop amps Are (mostly) A bad idea. 8 times out of 10,
you are amplifying garbage anyway...you typically lose 3DB snr through
them, and that , as i am sure you would agree , is a more dangerous
loss than the Db..... as for 10-15 at the groundblock, on channel 4,
maybe.. real world, rarely, unless you are in a lab... now before i am
beaten and flogged with multiple posts, let me add that there are
plenty of times when an amp is quite necessary.. HOWEVER... this sounds
like a situation where i don't agree.. that 10 to 15 db is NOT going to
be uniform across the spectrum, and if it is, well then, man, youve got
a pretty clean system..and you got lucky. As for plus 5 OR MORE at the
outlet? most DTV boxes (and modems) are rated for -11 to +5 db.. above
that, the first thing that usually happens is that audio gets goofy, or
serious blocking issues.. mostly from signals being too hot, something
that gets overlooked way too much...
HOWEVER, at 10db, I would personally put a 6 or 9db coupler at the
GB, send the tap to the modem, which puts you at a nice, +3 or +1 at
the modem, and somewhere around Zero at the sets. |
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Ed Nielsen
Guest
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| Posted: Sun Sep 17, 2006 8:19 pm Post subject: Re: Splitting a coax signal |
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No flogging here, just a few responses...
TC wrote:
| Quote: | Ed Nielsen wrote:
-2.5 isnt "realworld" bad, actually, regardless of FCC requirements...
|
That is with the assumption that every outlet in the house has a DCT and
that ALL of the cabling (including jumpers) is high quality and ALL of
the fittings are high quality and are made up properly and tightened
appropriately. Unfortunately, that is seldom the case. 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. 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.
With a DCT and assuming that any channels any watched at that outlet,
there probably wouldn't be many picture problems with a signal level of
-2.5dBmV. With a cable ready TV set though, picture problems would
abound. VHF locals would have multiple images (ghosting) and local UHF
channels would cause interference in cable channels from the mid-60s on
up. Channels in the upper teens through 21 or 22 would exhibit
herringbone and other such lines. Channels 95-97 would have several
local FM radio stations making pictures less than watchable. Ingress
problems are not limited to poor cable and/or connectors and/or
splitters, either. There are alot of cable-ready TVs out there that
have less than adequate shielding in their tuners (unfortunately, I have
2 of them). The distribution system itself in the house may be the
tightest on the planet, but with a poorly shielded tuner ingress still
can wreak havoc.
| Quote: | you could gain that back on an extra chilly day.. lol, i think the FCC
would forgive 2.5 db +/- I would rather err on the neg side than the
positive...IMO, drop amps Are (mostly) A bad idea. 8 times out of 10,
you are amplifying garbage anyway...you typically lose 3DB snr through
them, and that , as i am sure you would agree , is a more dangerous
loss than the Db.....
|
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.
| Quote: | 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.
| Quote: | now before i am beaten and flogged with multiple posts, let me add that
there are plenty of times when an amp is quite necessary.. HOWEVER...
this sounds
like a situation where i don't agree.. that 10 to 15 db is NOT going to
be uniform across the spectrum, and if it is, well then, man, youve got
a pretty clean system..and you got lucky. As for plus 5 OR MORE at the
outlet? most DTV boxes (and modems) are rated for -11 to +5 db..
|
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).
| Quote: | above that, the first thing that usually happens is that audio gets goofy, or
serious blocking issues.. mostly from signals being too hot, something
that gets overlooked way too much...
HOWEVER, at 10db, I would personally put a 6 or 9db coupler at the
GB,
|
Yes, a DC-9 or 6 would be preferred.
| Quote: | send the tap to the modem, which puts you at a nice, +3 or +1 at
the modem, and somewhere around Zero at the sets.
|
CIAO!
Ed N. |
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Ed Nielsen
Guest
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| Posted: Sun Sep 17, 2006 9:34 pm Post subject: Re: Splitting a coax signal |
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Logarithmic
In your example, Tim, you interchange signal level and signal power.
They are 2 completely different things -- signal power is an absolute
value while signal level is a logarithmic value. Hence, an doubling of
power from 1W to 2W (or 1mW to 2mW or whatever) equates to a signal
level increase of 3.01dB (due to imperfections in the manufacturing
process, it becomes 3.5dB.
I've just added some rather interesting links that go into power, noise,
etc to my website. Take a look at them
<http://www.cencom94.com/links.html>.
CIAO!
Ed N.
Timothy Daniels wrote:
| Quote: |
This implies that "Signal to Noise Ratio" is the ratio of the
dB levels, i.e. the ratio of the logarithms, not the ratio of the
absolute amplitudes. Is that true?
For example,
if signal A has a signal strength of 0dBmV, and that
of signal B had twice the signal strenght, there would
be a difference of 3.5dB (since the log of 2.0 = 0.35,
which is 0.35 Bells, or 3.5deciBells). The ratio of the
absolute amplitudes would be 2.0, but would your SNR
be infinite (i.e. 3.5dB divided by 0)? As you can see,
it would change with the value of the reference absolute
amplitude (the "0dB" level). Is that what engineers want?
If I were an engineer, I *think* I'd be interested in the ratio
of absolute signal levels, and when a signal is amplified
(i.e. multiplied) I'd just add the noise levels that are
expressed in dB of the signal and the amplifier, just as
one adds the exponents when multiplying some power
of ten. This would imply that the noise in dB is exressed
as the dB DIFFERENCE between the signal and the noise.
So a signal with 2dB of noise has an SNR of 2dB, and
amplifying that signal with an amp having a 3dB noise
factor gives the resulting signal an SNR of (2 + 3)dB.
IOW, the noise level expressed in dB is additive, which
I suspect makes engineering sense.
*TimDaniels* |
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Timothy Daniels
Guest
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| Posted: Sun Sep 17, 2006 9:34 pm Post subject: Re: Splitting a coax signal |
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"Ed Nielsen" wrote:
| Quote: | 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....
|
This implies that "Signal to Noise Ratio" is the ratio of the
dB levels, i.e. the ratio of the logarithms, not the ratio of the
absolute amplitudes. Is that true?
For example,
if signal A has a signal strength of 0dBmV, and that
of signal B had twice the signal strenght, there would
be a difference of 3.5dB (since the log of 2.0 = 0.35,
which is 0.35 Bells, or 3.5deciBells). The ratio of the
absolute amplitudes would be 2.0, but would your SNR
be infinite (i.e. 3.5dB divided by 0)? As you can see,
it would change with the value of the reference absolute
amplitude (the "0dB" level). Is that what engineers want?
If I were an engineer, I *think* I'd be interested in the ratio
of absolute signal levels, and when a signal is amplified
(i.e. multiplied) I'd just add the noise levels that are
expressed in dB of the signal and the amplifier, just as
one adds the exponents when multiplying some power
of ten. This would imply that the noise in dB is exressed
as the dB DIFFERENCE between the signal and the noise.
So a signal with 2dB of noise has an SNR of 2dB, and
amplifying that signal with an amp having a 3dB noise
factor gives the resulting signal an SNR of (2 + 3)dB.
IOW, the noise level expressed in dB is additive, which
I suspect makes engineering sense.
*TimDaniels* |
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Timothy Daniels
Guest
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| Posted: Mon Sep 18, 2006 5:45 am Post subject: Re: Splitting a coax signal |
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Sorry to resort to Wikipedia, but it came up as the 1st hit:
http://en.wikipedia.org/wiki/Signal-to-noise_ratio .
It says:
"Signal-to-noise ratio is an engineering term for the power ratio
between a signal (meaningful information) and the background
noise: SNR = Psignal/Pnoise = (Asignal/Anoise)**2
where P is average Power and A is RMS Amplitude. Both signal
and noise power are measured within the system bandwidth.
Because many signals have a very wide dynamic range, SNRs
are usually expressed in terms of the logarithmic decibel scale.
In decibels, the SNR is 20 times the base-10 logarithm of the
amplitude ratio, or 10 times the logarithm of the power ratio:"
Thus the power ratio and the signal amplitude ratio are related in
their logarithms merely by a factor of 2 (20 in the case of dBs).
This may, or it may not, conflict with your view, but to say that SNR
is the ratio of logarithms seems a bit too complex and seems to
have no direct engineering meaning that I can see.
Here's another webpage:
http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci213018,00.html.
It says:
"...signal-to-noise ratio, often written S/N or SNR, is a measure of
signal strength relative to background noise. The ratio is usually
measured in decibels (dB).
"If the incoming signal strength in microvolts is Vs, and the noise level,
also in microvolts, is Vn, then the signal-to-noise ratio, S/N, in decibels
is given by the formula S/N = 20 log10(Vs/Vn) .
"...As an example, suppose that Vs = 10.0 microvolts and Vn = 1.00
microvolt. Then S/N = 20 log10(10.0) = 20.0 dB ."
Notice that the SNR is calculated from microvolts (absolute signal
amplitude) not as dB's above a reference level.
BTW, your website seems to be down, now, so I can't check out
your links.
*TimDaniels*
"Ed Nielsen" wrote:
| Quote: | Logarithmic
In your example, Tim, you interchange signal level and signal power.
They are 2 completely different things -- signal power is an absolute
value while signal level is a logarithmic value. Hence, an doubling of
power from 1W to 2W (or 1mW to 2mW or whatever) equates to a signal
level increase of 3.01dB (due to imperfections in the manufacturing
process, it becomes 3.5dB.
I've just added some rather interesting links that go into power, noise,
etc to my website. Take a look at them
http://www.cencom94.com/links.html>.
CIAO!
Ed N.
Timothy Daniels wrote:
This implies that "Signal to Noise Ratio" is the ratio of the
dB levels, i.e. the ratio of the logarithms, not the ratio of the
absolute amplitudes. Is that true?
For example,
if signal A has a signal strength of 0dBmV, and that
of signal B had twice the signal strenght, there would
be a difference of 3.5dB (since the log of 2.0 = 0.35,
which is 0.35 Bells, or 3.5deciBells). The ratio of the
absolute amplitudes would be 2.0, but would your SNR
be infinite (i.e. 3.5dB divided by 0)? As you can see,
it would change with the value of the reference absolute
amplitude (the "0dB" level). Is that what engineers want?
If I were an engineer, I *think* I'd be interested in the ratio
of absolute signal levels, and when a signal is amplified
(i.e. multiplied) I'd just add the noise levels that are
expressed in dB of the signal and the amplifier, just as
one adds the exponents when multiplying some power
of ten. This would imply that the noise in dB is exressed
as the dB DIFFERENCE between the signal and the noise.
So a signal with 2dB of noise has an SNR of 2dB, and
amplifying that signal with an amp having a 3dB noise
factor gives the resulting signal an SNR of (2 + 3)dB.
IOW, the noise level expressed in dB is additive, which
I suspect makes engineering sense.
*TimDaniels* |
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TC
Guest
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| Posted: Mon Sep 18, 2006 5:45 am Post subject: Re: Splitting a coax signal |
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| Quote: | 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.
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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
| Quote: | 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.
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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. ;)
| Quote: | 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.
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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.
| Quote: | 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
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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.
| Quote: | 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).
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+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  |
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