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#159646 - 01/11/05 03:34 AM TV Distribution Systems?(dB)
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Does this area encompass Television Distribution systems?.
In particular, how many of you guys actually understand the in's and out's of Decibels (dB) (as in Loss and Gain)?.
I learnt enough about it a few years back to sit my Radio Ham's ticket, but I've wondered about it since.
Can anyone give me a decent idea of how this system works?.
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#159647 - 01/11/05 08:01 AM Re: TV Distribution Systems?(dB)
bandb12 Offline
Member

Registered: 01/09/05
Posts: 23
Loc: Crown Point, NY, USA
Yes it does....all CATV dist. is measured in db. db is actually a measurement of the signal strength. An example of gain would be the increased signal level on the output of am amp. The losses are thru the cables and other passive devices such as splitters. This is why the CATV amps out on the poles are actually spaced by db, not per se, in feet. 100ft of RG-6 coax cable has a loss of 1db / 100ft. So, say you have +3db of signal strength on your incoming CATV drop from the pole, and you have a 200 ft drop, you'd only have +1db at the ground block before it even went inside. Believe it or not, 0db (or higher) is an ok signal strength right at the back of your tv...

hope this helps!

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#159648 - 01/11/05 10:03 AM Re: TV Distribution Systems?(dB)
dereckbc Offline
Member

Registered: 10/08/03
Posts: 158
Loc: Tulsa, OK
I will try. Decibels or db, when used in signal circuits are used to express a power level to a known reference level of 0db at known circuit impedance. 0db = 1 milliwatt @ some impedance. The impedance depends on the type of circuit. For CATV the impedance is 75 ohms, RF @ 50 ohms, audio @ 600 ohm’s, you get the idea.

Decibels are a log rhythmic expression of large changes in power levels. Forget the math but this ought to help:

0db = 1 mw
+3db = 2 mw
+6db = 4 mw
+9db = 8 mw
+10db = 10 mw
+20db = 100 mw
+30db = 1 W
-3db = .5 mw
-6db = .25 mw
-9db = .125 mw
-10db = .1 mw
-20db = .01 mw
-30db = .001 mw

Hope that helps… Dereck

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#159649 - 01/11/05 11:31 AM Re: TV Distribution Systems?(dB)
hbiss Offline
Member

Registered: 12/16/03
Posts: 893
Loc: Hawthorne, NY USA
The nice thing about using decibels, and the reason it's used, is that you don't need to work with voltages and impedances which would require a lot of complicated computations. With decibels you only need simple addition and subtraction.

A little background:

An important thing to remember is that a decibel has no absolute value much like the terms "twice" or two times. Only when you set a value for "once" will twice have a value. 6db for example expresses a voltage ratio of 2:1 or twice. If you set 0db equal to 1 volt, 6db would then represent 2 volts. If you set 0db equal to 3 volts, 6db would represent 6 volts. The formula for converting a voltage ratio to DB is 20 log E1/E2. That's 20 times the log of one voltage divided by the other.

Since this is a logarithmic function you will see that, if we take our 6db= 2 volts example above, 3db will not be half of 2 volts.

You don't need to remember any of this though to be able to calculate levels for CATV distribution. Simple addition and subtraction is all it takes.

The thing to remember is that ideally you want to provide a level of 0db to about +10db at each set or jack. Less than 0db and you risk picture degradation. Much more than +10db can cause overload on some sets or cable boxes which can show up as interference.

Your starting point for calculations is the level at the drop coming to the building. This needs to be measured with a signal level meter. This connects to the drop cable and is tunable through all the channels (each video and audio carrier) on the system. It displays the level of the carrier it is tuned to in DB. Generally you will measure the video carrier of the lowest channel carried and the highest. Good idea to then sweep though the whole spectrum to see if there is anything particularly hot. Because cable attenuation is not a constant for all frequencies (attenuation at low frequencies is less than at high frequencies, called cable tilt or slope) you may find that the high end is less than the low end, hopefully by not much. Generally the CATV system design goal is to provide +10db at the drop but this varies widely in the real world.

Once you know what you have to work with you can decide how much you can split the signal. We want to provide a minimum of 0db at the set. A 2-way splitter introduces a loss of 3.5db. A 4-way, which is actually a 2-way feeding two more 2-ways all in one case would then cost you 7db.

Assume you have a level of +10db (channel 2)and +9db (high carrier) at the drop. Best you can do is feed four sets with a 4-way splitter- +10-7.5=+2.5 and +9-7.5=+1.5, all levels are above 0db so you are OK, at least at the splitter.

You do need to consider the length of the cable to the set from the splitter also. Rule of thumb for 100 feet of RG-6 is -1.5db @ channel 2 (55Mhz) and -5.6db @ 750Mhz which is the high end for most systems.

What do you do when you need to split a gazilion times and you only have +10db to work with? Amplifiers are available to provide gain. Some are set at an amount like 15db, others, particularly the larger ones are adjustable. These you will need your signal level meter to adjust. They often will have individual gain and slope controls. The slope control modifies the amplifier gain to give more at the high end, less at the low so you can compensate for a non-flat signal from the drop and/or adjust for your long cables. Amplifiers always are located on the drop feeding your splitters.

-Hal
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#159650 - 01/11/05 11:41 AM Re: TV Distribution Systems?(dB)
bandb12 Offline
Member

Registered: 01/09/05
Posts: 23
Loc: Crown Point, NY, USA
Dereck is exactly right...I was just trying to keep it simple. Also, keep in mind that power levels (signal strength) can be expressed in milliwatts, but not to be confused as electrical power. This is RF (radio frequency) power / strength...not electrical.

Bill

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#159651 - 01/11/05 12:01 PM Re: TV Distribution Systems?(dB)
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Everybody has covered the logarithmic nature of decibels well, but it bears emphasizing again that the dB is a relative measurement, so you must know the reference point. Stating a level as, say, 6dB without having a reference point is as meaningless as saying something is four times as big. (As what, a mouse or an elephant? )

You'll sometimes see a suffix added to explicitly state that reference point, such as dBm (1 milliwatt) or dBV (relative to 1 volt).

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#159652 - 01/11/05 06:15 PM Re: TV Distribution Systems?(dB)
e57 Offline
Member

Registered: 05/27/03
Posts: 2837
Loc: S.F.,CA USA
Hal that was very impressive info! Kudos to you! I have known a bit of how that was done (with slightly differnt values), but never would I be able to explain it with that clarity.

Now I have a question....

Say you had a large, lets say 1-8 splitter, or amp and you're not using all of them, yet still want to leave all of the connections live. (to avoid call back to connect) How would Db level be measured/adjusted with the un-used ends? Open, or with a terminator?

People always want all of the hooked up.

What Db loss value would an average set represent? would the amount of total sets represent a loss on the whole install.
Often I am long gone before the occupant moves back in with thier sets, and have to go back and add an amp later. Something I am trying to avoid. As for some reason that set that is furthest away, is the all important M.Bed and I get called back for weak signal.
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#159653 - 01/11/05 07:30 PM Re: TV Distribution Systems?(dB)
bandb12 Offline
Member

Registered: 01/09/05
Posts: 23
Loc: Crown Point, NY, USA
Nice job Hal...so much for keeping it simple!

One thing to consider is that when adding an amp at the "end of line" or users interface is that the amp not only boosts the signal strength, it also amplifies "everything" in the bandwidth including the noise level which is highly undesirable. They do make amps with special filters which suppress the noise, (keeping the signal to noise ratio in check} while still amplifing only the desired frequencies, but these ain't cheap, and are usually only used commercially. The ones you pick up at RadioShack for $30 isn't what I mean ... some of these do provide a gain adjust, but again you'll be amplifing "everything". There are also attenuators that can be untilized to lower your dbs but these are usually tuned for specific frequencies, so you would need to know its specs.

Simply put, if there isn't enough signal strength on your drop to accomodate the entire house, then the cable guy needs to come out and do some adjustments out on the line such as lowering the tap value for starters....

Bill

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#159654 - 01/11/05 11:09 PM Re: TV Distribution Systems?(dB)
hbiss Offline
Member

Registered: 12/16/03
Posts: 893
Loc: Hawthorne, NY USA
How would Db level be measured/adjusted with the un-used ends? Open, or with a terminator?

Common question, people ask also if it makes any difference if the sets are on or off.

You might be surprised to hear that it makes essentially no difference to the levels on the other splitter ports/jacks if one or more jacks are connected to a set, left open or terminated (which is a 75 ohm resistor). Get a signal level meter and see for yourself.

By the way, it's always a good idea to terminate unused splitter ports (and jacks but that's never going to last) to eliminate leakage, nothing else.

They do make amps with special filters which suppress the noise, (keeping the signal to noise ratio in check} while still amplifing only the desired frequencies, but these ain't cheap, and are usually only used commercially. The ones you pick up at RadioShack for $30 isn't what I mean ... some of these do provide a gain adjust, but again you'll be amplifing "everything".

Bill, never heard of a special filter that will preserve the S/N ratio of an analog signal. The inventor would be richer than Bill Gates! You can't get something for nothing and this would be like perpetual motion. Sad fact is that when you amplify a signal the amplifier will add some noise. This is usually white noise which is made up of all frequencies and becomes a part of the signal you are amplifying. It's impossible to remove the noise without removing the signal also.

There are all kinds of amplifiers, the good ones will contribute the lowest possible amount of noise. No magic filters, they just don't generate much noise to begin with. They will have a very low noise figure on their spec sheet and it's undoubtedly these that you are talking about.

Amplifier noise can become a problem in a number of cases. Cascading amplifiers will eventually cause noise to build up beyond an acceptable level even with the best amps so there is a limit to how far you can go. For what we are talking about here there is usually no reason to cascade amplifiers.

An instance that you are more likely to encounter is feeding an amplifier with too low a signal to begin with- trying to make something from nothing. The ratio of signal to the noise is unacceptable resulting in a snowey picture. A classic example of this is an antenna that produces a weak signal and adding an amplifier. Depending on the actual signal level from the antenna and the amplifier noise figure, using an amplifier may actually increase the noise and snow.

There are also attenuators that can be untilized to lower your dbs but these are usually tuned for specific frequencies, so you would need to know its specs.

Anything called an attenuator would attenuate all frequencies equally. While there are adjustable ones the majority are fixed in value. Something that can be tuned as you describe would be a filter. These would not normally be used to reduce levels but reduce or eliminate individual frequencies or a band of frequencies, however broad and deep the filter was designed. Other than perhaps reducing interference you would normally have no use for these.

Simply put, if there isn't enough signal strength on your drop to accomodate the entire house, then the cable guy needs to come out and do some adjustments out on the line such as lowering the tap value for starters....

Well, maybe, but depending on where the tap is on the line that may not be possible. I've done that when I worked for the cable company a few times to get a few more splits but that's just me. Most likely, unless the level is below what it should be they won't want to do it. When you do this it lowers the levels on the taps down the rest of the line. If you do it for everybody that asks you will wind up building a bigger system, the whole thing snowballs.

For large buildings like apartment houses, condo's etc. I would run a new hard cable feeder directly from a trunk distributon amp into the building. If you had a large enough house or it was far from the street this would work also.

-Hal






[This message has been edited by hbiss (edited 01-12-2005).]
_________________________
www.myphonetechs.com

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#159655 - 01/12/05 03:17 PM Re: TV Distribution Systems?(dB)
bandb12 Offline
Member

Registered: 01/09/05
Posts: 23
Loc: Crown Point, NY, USA
Sorry Hal, but I tend to disagree with some of what you have said. But it is also obvious that you worked in CATV and have a good knowledge of it. Prior to all my years in the electrical industry, I worked as an asst. engineer doing SATCOM. From there I went into CATV. Now I'm not going to lie...it's been a LONG time, but I do remember a lot of it too...so i will have to do some homework.

One item I feel confortable in addressing is:

When you do this it lowers the levels on the taps down the rest of the line.

Changing a tap value should not affect the levels past that tap. The feeder signal goes "through" the tap virtually unaffected by the faceplate. The value of the tap faceplate is what determines the actual output of that tap to the drops only.

If the trunk / line extenders are balaced correctly, sometimes the only solution was to lower the tap value to increase the signal to the customer. Did it more than once myself.

My mistake on the attenuators...I was thinking of filters! Honest! And yes they are tunable. A perfect example would be those old style HBO traps that were installed at the tap. Remember those? They'd filter out only the frequency that HBO was set at.

It's great talking about all this stuff again! I'm glad they added this forum! And Hal, I am going to look into some info on those amps for ya!

Bill

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