Entertainment Center: TVs, Stereos, VCRs and DVDs - splitters and amplifiers

We have just finished running in a house with 8 -- yes 8 -- adults -- each with his own computer/tv/vcr or dvd. Each of us got a home run of cable and CAT 5 from the cable entry point. Our priority now is getting the tvs right.

I have read a lot of the posts and it seems that we need a splitter first -- one to the modem for the computers and one to an amplifier and splitter. Should the amp and splitter be one unit? Do they make them together with 8 ports? (without breakiing the bank?)

I have been checking amps and splitters out and the "tech descriptions" all have numbers/ranges that we don't understand. From another post, I read a splitter should have a higher freq range (how high?) and lower db loss (how low?). And would a 25 db boost be enough? What are the important numbers for us?

Also another post made me wonder if we should add a single-line amp BEFORE the first 2-way split between the modem and the tv lines.

Thanks to anyone who can help!

I assume that you aren't trying to run cable modems because you were mentioning running cat5 cable. Perhaps you should consider connecting the cable TV coax to a distribution amplifier with at least 8 outputs, then just connect each cable TV run to the separate outputs of the DA.

How you handle the computer connections would depend on what kind of service everyone has. Is everyone going to use DSL or dialup? Perhaps some want a cable modem. In that case the DA would cause a problem.

Each time you split an rf signal with a proper splitter matched to the characteristic impedance of the line the signal will halve in power at each port. In dB terms this corresponds to a loss of 3dB. So 8 ports will result in a 8*3 = 24dB loss and you will need 24dB of gain (amplification) somewhere in the system to compensate.

There are many ways to do this but as jughead said the DA with 8 ports would be the way to go. You can also hook up a bunch of amps in parallel and then split the signal. Providing all the amplification upfront may not be a good idea as you could saturate the signal and cause overload. Also trying to compensate after splitting may also not be a good idea as it will degrade Signal to noise ratio.

The cable company said they would install a splitter for the TV line. To test for "noise", do all the tvs have to be on at the same time? Or is it enough to check with the one farthest away from the splitter?

No the TVs don't have to be on if you want to test SNR. When we refer to noise in any comms channel like cable or the atmosphere we are referring to anything unwanted. The sources for the noise are many - atmospheric disturbance, random movement of electrons in the semiconductors at the transmission end (so called "white" or Gaussian noise) etc.

As long as you keep the signal value high the signal to noise ratio (usually also expressed in dBs) will be good. If you let the signal value fall when you amplify it you will also amplify the noise and degrade the SNR performance. As good value for a radio signal is around 50-60dB SNR (not sure what it should be for cable). You can compare this to a Hi-Fi where a good SNR should be around 100-110dB for a CD player. Teh larger the figure the better the performance.

To get the best SNR you will need to amplify the signal before splitting and distribution and the worst SNR figure will be at the one furthest from the splitter as you mentioned.

How do you plan to test the SNR value - do you have a signal strength meter.

I had a problem a while back with my cable TV installation. The cable guy came out and put his field strength meter on the end of my cable and found a low signal level. After he found the problem on the pole he came back and rechecked the signal. There was now good level and good SNR on all channels. I don't recall that there was any more than about 15 db more signal than would be acceptable for reasonable performance especially on the higher frequency (upper) channels. This fact would make it unlikely that you could get away with splitting your existing cable signal 8 ways without some amplification. I recently installed a distribution amplifier at work. Before installation I opened the thing up and found a simple amplifier and a splitter all surrounded by a metal box. It wasn't very expensive. I suspect that the only way you will receive acceptable performance will be to run the cable signal into the DA and run all the other TVs from there. You will experience too much loss otherwise. Don't get the idea that you can just use a booster amp on your TV if signal levels get too low. It won't work. Once the signal is degraded by losses you can't boost the signal without boosting the noise as well.

Thanks for the reply. I have looked at various DAs but each has different numbers in their tech specifications. I know that dB is critical and I should look for a higher number there -- would 25 be enough for me? But what about impedence, frequency range, etc??? What are the important ones and which can I overlook? And what should the numbers be?

You need to investigate the DAs a bit futher to answer these questions. If you buy an 8 way DA that consists of 4 DAs coupled to 4, 2 way splitters then you need to look at the gain of each DA. Another way to do it is to buy 4 seperate DAs with splitters to give you the 8 signals. In this case each DA should be rated at a gain of 6dB. To try and amplify the signal by 24dB upfront is not a good idea as you can overload the signal leading to distortion.

Splitting it 8 ways and then trying to give it 24dB gain is also not a good idea. My feeling would be to go for the 4 DA, 2 way splitter per DA approach. The 6dB will be there to compensate for the splitter but is small enough to not cause too much degradation in the SNR. If you feel the signal is a bit low you can go for something with a little more gain.

Impedance should typically be 50 Ohms - this is generally the characteristic impedance of rf coax. Video coax tends to be usually 75 Ohms.

Frequency - I think the upper limit of cable is around the 800MHz region so if you get a DA rated upto around 1GHz you should be fine.

Thanks for the info. Is the minimum freq range important? Also, the cable company said they supply the splitters. So what about using a DA before the split with a smaller boost -- maybe 6-10-15dB? Then after the split, if needed, add individual DAs on the lines that need it? I was thinking that this way, there would be more signal strength to split, but not the 25dB boost that might cause problems. And then those TVs with longer cable lines or weaker signals could be boosted after the split. Would this work? And if so, any recommendation for the dB before the split? Again, thanks for all your help.

Most broadband amplifiers go down to DC so it should not be an issue. You should look for one that says something like 0Hz - 1GHz or something like that. I'm not sure what the lowest frequency being transmitted by the cable signal is. You may be able to do a web search to find out. If you can get a DA that goes down to DC (0Hz) you will be fine.

Yes, the approach you mention would be the way to go. Every time you split a signal use 6dBs of gain before the split. You can split them in a "chain" kind of arrangement or you can do them in parallel.

If you can get DAs with a gain adjustment then you could provide a few dBs of extra gain on the longer runs of cable.

Thanks for your help --I am starting to get straight on this -- but what did you mean by looking for a DA with a "gain adjustment"? What is that?

Oh just another word for being able to adjust the amount of dBs of amplification. Think of it like a volume control. Gain is just another term used to refer to amplification.

So what I was saying was if you could get a DA with the dB adjustment for the longer runs you could set the value to the needed 6dB + 2 or 3 dB more to compensate for the longer run.

I installed a DA a while back that had about 25 db adjustable gain and 16 outputs. Seems to me the device only cost about 150 bucks. It was designed for 75 ohms in/out with F connectors. That would be just what you need.

Great information! Do I have this right -- that adjustable means you can adjust each of the out-ports individually?

You can certainly get ones with individual gain controls for use in professional installations. I've seen them used in Television and Broadcast studios. I'm not sure if they are widely available for use in homes. I could be mistaken here as I've never had any reason to use one in a home.

If you see a DA with 75 ohm impedance as mentioned double check the frequency ratings since 75 ohms is commonly also used for video feeds. The frequency range of a composite video signal is only a few MHz and will not work at UHF.

I used to work as an engineer in a TV studio, and yes, there are such things as video DAs. I'm refering to RF ones. They are nothing more than a amplifier and splitters all mounted inside a box. The cheaper units do not have separate gain controls for the individual outputs nor would they even be necessary for a small home installation. The only reason to use an amplifier at all is to overcome the loss of splitting the signal 8 ways. You don't need anything fancy or expensive. Just a "plain Jane" model that has 15 to 25 db gain and 8 to 16 outputs will do the job. Just be sure that you have an outlet nearby so you can plug the unit in.

I have a much better understanding of this -- thanks to your postings -- THANKS!