AVS Forum banner
Status
Not open for further replies.
1 - 19 of 19 Posts

·
Registered
Joined
·
37 Posts
Discussion Starter · #1 ·
I recently purchased a Monster cable coax splitter. It was VERY inexpensive. about $9. However, I believe that the quoted signal loss was a -3.5 db. Now, does that mean 3.5% of the signal will be lost as a result of the split? I don't think so because isn't there some type of doubling in effect for each db?


I needed a splitter because the Sony SAT-HD100 requires both coax inputs if you are using UHF/VHF In and UHF/VHF In (DTV) even if the signal is from the same antenna which it is in my case. So, is a -3.5db loss signifcant and if it is should I buy an active (powered) splitter? If the answer is yes, can anyone recommend a good one?


Thanks,

Tom
 

·
Registered
Joined
·
599 Posts
3db is half power

.5db is the loss of the splitter

your signal will be "split in two"

each output will have half of the original signal plus the splitter loss

which makes the new signals -3.5db down from the original level

Does it make sense?


hook it up and see if it works?


------------------

Studio Broadcast Engineer

KET
 

·
Registered
Joined
·
1,525 Posts
3.5db is the loss per leg. If you start with a 12db signal and split it, each leg will now have a 8.5db signal. I believe the 4-way splitters have a 7db loss.

The minimun requirement, as told to me by a cable installer, is -5bd min signal is acceptable, but not to me. At my house, I had 21db at the pole before splitting. Not sure what the satellite signal strength is?



------------------

-Glenn


"Obstacles are what you see when you lose sight of your goals"
 

·
Registered
Joined
·
37 Posts
Discussion Starter · #4 ·
WooWoo:


Unfortunately, it doesn't make sense. I did hook it up and

of course it works. It's just that the cables are difficult to reach and change and I really wanted to know what the loss of signal is. You aren't saying that each leg now as half the original signal strength are you? I would expect that that would result in a noticeable loss of quality in the picture and it doesn't in reality.


I just figured that an inexpensive passive splitter would not be as good as an active splitter. I guess that I probably mistakenly thought that a good powered splitter would experience very little loss and therefore would result in a noticeably better picture thereby justifying the expense.


What is a 1 db loss or gain of signal really mean from a percentage point of view? Maybe my problem is that a loss or gain at a db level can't be expressed as a percentage?


Thanks,

Tom



3db is half power

.5db is the loss of the splitter

your signal will be "split in two"

each output will have half of the original signal plus the splitter loss

which makes the new signals -3.5db down from the original level

Does it make sense?

hook it up and see if it works?

 

·
Premium Member
Joined
·
2,564 Posts
Tom:


Yes, power in each leg is half of the original signal (since this is a passive splitter). Actually, a little less, since there is a bit of loss.


The power coming out of the two legs combined is equal to the power coming in. 1/2 + 1/2 = 1. Its that simple.


Keep in mind, we're talking about signal power, NOT signal/noise ratio. The quality of reception is, within reasonable parameters, governed by the signal to noise ratio, NOT the absolute level of the signal.


------------------

Alex
 

·
Registered
Joined
·
35 Posts
dB is based on a log function

dB, % loss

1.25, 10

1.5, 18

2, 30

2.5, 40

3, 48

3.5, 54

4, 60

4.5, 65

5, 70

5.5, 74

6, 78

6.5, 81

7, 85

7.5, 88

8, 90

8.5, 93

9, 95

9.5, 98

10, 100



[This message has been edited by BrianRL (edited 08-09-2001).]
 

·
Registered
Joined
·
1,525 Posts
I think my example was the signal/noise ratio?



------------------

-Glenn


"Obstacles are what you see when you lose sight of your goals"
 

·
Premium Member
Joined
·
2,564 Posts
Hi glenn


A passive splitter will split both the "signal" and "noise". The converse is also true; you can buy a 30 db signal amplifier that will amplify the signal and noise.


You DO need to worry about the absolute signal level when running a long length of cable. This is because the cable itself picks up an additional amount of fixed noise. You can use an amplifier at the pole to counteract that. But thats not toms problem.


------------------

Alex
 

·
Registered
Joined
·
599 Posts
Think of the splitter as a "Y" valve on a water hose with a small leak.

Each side gets 1/2 of the water (-3db)

And the leak is .5db


A passive splitter will not add to the Signal/Noise ratio


A power'd splitter has an amp built in

So it would be unity gain

What goes in comes out on all legs

The noise figure of the amp is what's important

so that it does not add noise

If you use an amp... Get a good one



db's are a logrithmic funtion

+3db is 50% more signal

+10 db is 10 times the original signal

+20 db is 100 times

+30 db is 1000 times

etc..........


------------------

Studio Broadcast Engineer

KET
 

·
Registered
Joined
·
37 Posts
Discussion Starter · #10 ·
WooWoo:


Is there a reasonably easy way to effectively measure the signal/noise ratio? I don't suppose that it is displayed as part of the DirecTV setup screen that shows signal strength is it? I guess that would be too easy.


I am going to unhook the splitter, attach the coax input directly into the UHF/VHF In (DTV) and see if I can detect any noticeable difference on any of the different formats be it analog OTA, digital OTA or satellite and finally HDTV OTA and satellite.


Would you expect to see a difference in the quality of the picture with a -3.5 db loss?


Also, what would a good active amplifier cost and what would be a good specification from the signal/noise ratio?


Thanks,

Tom
 

·
Registered
Joined
·
599 Posts
I dont think you will be able to measure s/n at home

as for the loss

If the reciever maintains a lock than all is well

The only difference you should see is in the analog OTA


don't worry


Be happy


Enjoy your TV


------------------

Studio Broadcast Engineer

KET
 

·
Registered
Joined
·
735 Posts
From my experience, tom, the general public probably wouldn't notice much of a degrade in picture quality from one passive splitter. Two splitters is definitely noticeable.


I suggest as the others do. Buy an active splitter. Even a radio shack/best buy splitter is usually better than passive. The only problem with the cheap active splitters is that they usually use half wave rectifiers and can introduce noise to the signal. Most of them aren't noticeable, but I have found a few that blatantly do.
 

·
Registered
Joined
·
801 Posts
Wow, lots of information/misinformation in this thread, let me see if I can help inform/misinform.


The ultimate requirement for successful DTV reception is a signal that is greater than the noise by a certain amount (about 12 dB in theory, that is 16 times more signal than noise). Noise is caused by the random motion of electrons in any conductor that is at a temperature above absolute zero. Noise in excess of this is created in electronic devices such as amplifiers, both the amplifier in your STB, and the preamplifiers used at your antenna. Noise Figure is the amount of excess noise created by an amplifier above that of that in any conductor. Once noise is added to a signal, it is impossible to remove by further amplification, the noise is amplified as well as a signal.


An antenna is, for all practical purposes, a perfect device as far as signal to noise is concerned. Antenna gain is “freeâ€, it doesn’t incur a noise penalty, it doesn’t add additional noise in the process (a slight simplification, but for our purposes adequate).


Antenna preamplifiers do add noise when they amplify the signal. A 3 dB noise figure means the noise at the input of the amplifier is twice that that would be present of a resistor at the same point. The only purpose of a pre- amplifier is to overcome cable loss and improve on the noise figure of the STB, which is closer to 6 to 10 dB. An amplifier in isolation driving a perfect STB (0 dB noise figure), would actually degrade SNR rather than enhance it.


As the signal travels through the coax, and splitters, it is decreased in power. When the signal is decreased such that it is only 14 dB above the noise figure of the STB, it will no longer be decoded.


A 2 way Hybrid splitter (which is different from a resistive splitter) drops the power by half (3 dB) plus splitter loss, which is usually 0.5 dB.


The Decibel is used to denote a ratio between two signal levels. It is defined as 10*Log of the ratio between 2 power levels. For Instance 10 dB means 10 times the power, 20 dB means 100 times the power, etc. It has no absolute meaning, just an indication of the ratio. Dbm is an absolute level, as is dBuV. The first means the power above a milliwatt, the second means the voltage above a microvolt. A 3 dB splitter means half the power is lost, a 30 dB amplifier means the signal is 1000 times stronger.


What all this means is that you have to look at antennas/amplifiers/splitters as a system, none of them in isolation have any meaning. A Preamplifier needs to provide enough gain to overcome the cable/splitter loss/ and noise figure of the STB. If the cable loss is 5 dB, the splitter is 3 dB, and the noise figure of the STB is 10 dB, you need a total of 18 dB of gain to overcome the losses between the antenna and the STB. If the combination of cable/splitter/STB is less than 10 dB, an amplifier with a 10 dB noise figure and any amount of gain will buy you nothing.


As I said, Signal to Noise ratio is everything, and the “Signal Strength†indicator in your STB is an indication of SNR. It actually measures Bit Error, and extrapolates that to a signal strength. It doesn’t care how much gain or loss is present between the antenna and the STB, it only cares about the resultant SNR. That is why, in a good signal environment, the indication of the meter will be the same with or without a preamplifier. It isn’t the level, it’s the quality of the signal that matters.


If after adding the splitter to the STB, the signal indicator shows the same number, there will be no problem putting the splitter in line.


I realize this is a bit wordy, but I hope it clears up some of the questions.


Bob Smith
 

·
Registered
Joined
·
165 Posts
OK, now, a related question:


I have a preamp on my antenna on the roof, running in to my DISH 6000 box. Without the preamp I had low signal strength readings on the New York stations, and with them I get every station at good strength readings.


Anyway, I want to split the signal so I can feed it to an AccessDTV card in my HTPC as well.


My installer tells me that to do that will defeat the preamp completely. Reading Bob Smith's reply above seems to indicate otherwise.


Can I do this with a reasonable likelihood of keeping my signal strength reading and station reception, and if so, which splitter model should I buy and where should I buy it?


Thanks for any help.
 

·
Registered
Joined
·
2,463 Posts
Woo! Your way off.


Andrew. Lets say your Ant. has a output of -2db for those digital signal. Adding a preamp will boost the signal from 1db to (what every the amp is rated). Lets say a 10db gain, so you now have 11db. Taking that signal into a 2 way splitter will net you about 7.5db output on each leg. Not a huge loss in analog signal terms but digital may/maynot be so forgiving. A 7.5db signal is a strong signal. Not perfect but strong. That 7.5 maybe for the channel that is average. You may have another channel that is stronger and/or one weaker this is a one channel example. Just something to keep in mind.


Typical losses with splitters.


2 way - 3.4 to 4 db loss

4 way - 7db loss

8 way - 11 to 12db loss


Dave



------------------

Watch HDTV!!! Nothing else comes close!


[This message has been edited by David Richardson (edited 08-11-2001).]
 

·
Registered
Joined
·
1,525 Posts
I've had good results with the Winegard SP-2052, 40-2050MHz splitter.



------------------

-Glenn
 

·
Registered
Joined
·
801 Posts
You should be able to stand the 3.5 dB splitter loss. Look at the signal strength before and after adding the splitter, if it is the same, the SNR hasn't been affected, and everything should be just fine. Chances are slim you are that close to the margin. A standard amplifier gives about 20 dB of gain, if you have 10 dB of cable loss (unlikely), you will probably have enough left with no problem. The splitte won't "undo" what the amplifier has done. The amplifier has about 20 dB of gain, the splitter has about 3.5 dB of loss, this still leaves a net 16.5 gain with the amplifier/splitter combination. What sort of amplifier are you using??


Bob Smith
 

·
Premium Member
Joined
·
2,564 Posts
If you want a nice big signal boost, pick your self up a Channel Master Super Titan http://www.starkelectronic.com/titan.htm


It still won't help toms problem for the reasons Bob Smith has stated. You ususally use a preamp to compensate for signal loss from the antenna to the stb, NOT to boost the absolute signal level of the antenna itself. This can be an issue with OTA, but it is a greater issue with DBS satellite.


------------------

Alex
 
1 - 19 of 19 Posts
Status
Not open for further replies.
Top