I'm a Comcast customer with a Surfboard 5100. From the wall of my apartment I use a two-way splitter, one port goes to the modem and the other to an RCA VH140 4-way amp. All four amped ports are used, and any unused cable connectors in the system are capped.
Nominal picture quality is good to excellent depending on the channel. However, any activity by the modem will always inject one or more bands of horizontal noise into the video. If it's a big file download, the disruption can cause the video signal to be dropped by the receiver entirely.
Any help or suggestions would be much appreciated.
You may have a cheap splitter that isn't offering enough isolation between the legs. Also, are the amp and modem on the same power outlet, or very close together? If so, try to separate them. You may even want to use a power line filter on one or the other. You can also try capping/terminating any unused lines at the amp instead of at the end of the cable. Double-check all connections and make certain they are tight and clean and that the cables aren't kinked or damaged near or at the connectors. Use RG6 or RG6QS cable whenever possible. If your amp is a cheap, plastic Radio Shack thing, find something of higher quality.
Also, go into the diagnostics menus for your modem (most commonly http://192.168.100.1) and see what the uplink strength is. If it's in the upper 40's or 50's, that is very strong and could be causing some problems.
There is always downstream traffic coming to your cablemodem. Arp traffic, and other noise, including packets being sent to your neighbors are nearly continuous. Those signals exist even if you don't have a cablemodem. Upstream traffic only comes from your cablemodem at certain times, such as when you're actively using the connection, but the channel being used for your upstream is also used by others, so there is always something going on there, too. The only difference is the signals your modem are adding to the channel may be stronger because of the short distance to modem.
Use of the correct filters on the leg going to the TV may help. But there's other important factors that need to be considered: What is the proximity of the cablemodem to the amp? Could the ingress be transmitted between the relatively unshielded cases of the equipment in close proximity? Are they plugged into the same electric circuit? Is the circuit properly grounded?
If you disconnect the amp, does the problem go away? TV's and set-top boxes usually don't need as strong of a signal as cablemodems do. If the signal coming in is enough for reliable cablemodem service, are you sure you even need to amplify the signal to the TV's even if you split it four times? Remember, an amp is going to amplify what comes to it, including the noise. The amp could even result in a worse signal to noise ratio. Take away the extra noise, and the weaker signal may be acceptable as is without amplification.
Thanks Andrew (Ed and Warren too). I appreciate the help.
I definitely needed the amp, for while the picture wasn't bad without it, it was significantly better with. I did have everything plugged into the same (high quality) surge protector, but separating didn't make a difference. The terminating caps were all used on device connectors, never at the end of a cable. That is, the 4-way amp was used for a TV and three DVRs. The caps were for the unused Ins and Outs on those devices. I also used RG6 throughout and the 2-way splitter ahead of the cable modem was a Monster.
Before futzing with the problem setup, the stats for the Surfboard typically showed a downstream S/N of 30 dB and power level of -19 dB. The upstream power level was around 54 dBmV. I'm new to cable modems, so I'm not sure if those are good numbers or not, but the upstream power level clearly fell into the potentially troublesome range you mention.
I started by searching for the entry into the house. It was on a 4-way splitter, one branch going downstairs (the one I was using) and the other three going to bedrooms upstairs (unused, at least for now). I took the splitter out so the outside line was going directly to my downstairs jack. The S/N on the modem changed to around 36-37 dB, with a downstream power level of -8 to -11 dB (it's usually better than -10). The upstream power dropped to -43. Still had the noise.
I tried taking out the 4-port amp again. The noise went away but the picture was still noticeably degraded. I had another amp lying around, an RCA rated at +10 dB with a single output. I tried that, along with the four-way splitter I'd removed from the house-in, and that worked. The picture was great across the board and the noise was gone. Are 4-way amps just inherently more susceptible to this problem than a single-port amp or was it more likely a quality issue?
I suppose this will become a problem again when I decide I'd like to use the upstairs jacks. Is the signal coming into the house less than it should be? Should I ask Comcast to put an amplifier on it? Would that be placed inside or outside the house? Are the numbers I'm getting now on the cable modem acceptable?
Very poor. Incoming level should be between -15 and +15, with SNR over
Outgoing should stay below 50 if possible.
That incoming is still weak for being directly connected (I assume you were NOT going through any splitters?). You may want to have your cable company come out and check the lines. At my parents, I persuaded them to get a cable modem. The problem is, it wouldn't connect at all. They ended up having the line (probably 20+ years old) between the house and the pole replaced. I'm still going to replace the line from where it enters the house to the distribution point so it is all fresh and RG6 (the old stuff looks to be some form of dual RG59.)
A multi-port amp is simply an amp with a built-in splitter. Many old and/or cheap splitters have rather high losses of 4-5db per split. Newer ones are roughly 3.5db. A 4-way splitter means the signal goes through 2 splitters (1 to 2, then each further splits giving 4), giving a minimum
7db loss. On a 3-way splitter, one will be 3.5db and the other two will be 7db.
The ideal setup should be: A 2-way splitter as close as possible to where it comes into the house.
Run FRESH RG6 or RG6QS cable to the modem and connect it to one side of the splitter.
Use the other side of the splitter to feed your TV's.
Also, be aware that basic amps are not designed to allow a reverse feed. That is needed for cable modems and digital cable boxes. If you must run a modem or box through an amp, get a broadband (bidirectional) amp. Even so, you will lose a few db upstream due to insertion loss.
Also, try to keep physical connections to a minimum. Each connector is a leak point that could let noise in or cause signal reflections that make the signal tougher to decode. If you want to mount a splitter directly to the output of a single-output amp, buy a male/male coupler at Radio Shack or similar instead of using a short cable, unless you want the splitter to be far away from the amp. Also, always put the amp as close to the incoming signal as possible. It is there to boost signals to compensate for line or splitter loss, not amplify a weak signal.