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Posted by TC on September 15, 2006, 10:16 pm
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Ed Nielsen wrote:
> 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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Posted by Ed Nielsen on September 17, 2006, 11:19 am
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No flogging here, just a few responses...
TC wrote:
> 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.
> 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.
> 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.
> 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).
> 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.
> 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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Posted by Timothy Daniels on September 17, 2006, 12:57 pm
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> 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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Posted by Ed Nielsen on September 17, 2006, 3:35 pm
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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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Posted by Timothy Daniels on September 17, 2006, 11:46 pm
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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:
> 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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