I've recently moved to a bigger apartment and I'm trying to get my cable internet and digital tv working properly. First let me explain the problem.

I've moved to a building which has a shop on the ground floor, and two apartments on the first floor. They all share a single (CATV) cable entering the building in the shop on the ground floor. There the cable enters a splitter/amplifier. This amplifier has 4 outputs (with return path) with two outgoing cables connected. One goes to the first apartment (my neighbor), the second to another amplifier which doesn't have a return path. From that amplifier it goes to a splitter. From the splitter, one cable goes to the front of my apartment, the other to the back:



Amp 1 is a Tratec FDU-44 and has an output gain of 4dB on all output ports.
Amp2 is a Hirschmann GHV 20E and has an output gain of 20db but has no return path.

I don't know the length of the cables going from the splitter up to my apartment.

When I hooked up my Set Top Box (STB) in my apartment, I got digital TV just fine but none of the interactive services (like On Demand) worked - which was to be expected since the Hirschmann amplifier has no return path. When I hooked up my cable modem, it found a downstream and upstream frequency without a problem, but it didn't get online. Again as expected.

The first thing I tried was connecting the two cables to my apartment directly to amp 1:



My cable modem came online, and the interactives services of my STB worked intermittently. When I called up my cable modem's signal status, this is what I got:



For those who know what they are looking at, you can see the modem has it's hearing aid on full and is shouting to get itself heard. So it's barely able to receive/transmit data.

I happened to have another Tratec FDU-44 amplifier so I hooked that one up too to see if it made a difference:



I immediately noticed that interactive services on my STB now worked (seemingly without problems), but some channels were pretty bad or could not be displayed at all. When I checked the signal status of my cable modem, I got this:



The difference was minimal. I decided I needed a better amplifier so I bought a König AMP-PRO20M. This amplifier has a 20dB gain on all 4 output ports and all 4 output port have a return path. So now my situation was this:



I first tested my STB and all channels that had problems before were now perfect. But when I switched on my cable modem, it had no problems finding a downstream and upstream frequency but it didn't come online. This was not what I expected. Not what I expected at all. I couldn't get the signal status of the modem (sometimes it just won't respond, no idea why) so I couldn't check the power levels.

And before I was able to do some more testing, the shop downstairs closed so I didn't have access to where the cable enters the building. I was able to quickly replace the new amp for the old again so I at least have internet again. But it has to wait until tomorrow before I can do more testing.

The new amp has a gain control and I was wondering if it would help it I would turn the gain down a notch. Perhaps the modem uses a lookup table for adjusting the outgoing power levels, meaning if the incoming signal is good, it will turn down the outgoing power level. Or is that not how cable modems work?

And does anybody have an idea what else I can try to get everything working properly?

just a guess but why not just use the konig and drop the other unit out
I suspect the cable Co doesnot like two amps on one line

an other thought does internet connection gain from the amp
could you split before the amp and just boost the tv/cable part

or maybe get a pure internet cable amp and use that separately if the net cable needs a boost to

[This message has been edited by ray b (edited 05-27-2010).]

2 apartments, does each have internet, or just yours?

I would see about splitting the cable, and running one line straight to your apartment for internet, and then running off the amp for everything else. As long as you had cable working through the amp that is.

Brad

I'm not a CATV expert but it seems that you have an adequate digital TV signal. A 30 dB S/N ratio sounds great for digital but maybe European standards are higher than here.

Here's some info from the Sencore site:

quote Typically the more signal level, the more dependable is the digital TV channel reception. Proper receiver decoding is possible with signal levels below 0 dBmV. Receivers may be able to receive and decode digital signals down to -25 dBmV. However, for a good window of decoding to prevent a loss of reception, keep the signal level greater than -20 dBmV. A digital signal level of -15dB serves as a good minimum level and is comparable to a 0 dBmV analog level.

And:

quote The carrier-to-noise ratio test indicates the difference in power in dB between the TV digital channel power and the noise in the frequency spectrum near the channel. A larger value indicates a stronger signal or less noise. C/N ratios over 20 dB are good for digital channel reception. C/N ratios decreasing and approaching 15 dB can intermittently begin to cause digital decoding difficulty and occasional signal dropout. C/N ratios of 15dB or less cannot be properly decoded by the receiver resulting in loss of reception.

I don't understand the +58.2 dbmV (+9.4 dBm) modem transmit level. That does seem very strong. Does it sync up with the other end?

My wireless modem receives a signal at around -78 dBm (-31 dBmV) with a 15 dB CNIR. Download speeds are around 3 Mbps. Others with better CNIR's report 10 - 12 Mbps downloads using the same modem.

Edit: Page 2 of the Sencore article has some good info on amps and splitters.

[This message has been edited by spark1 (edited 05-28-2010).]

quote Originally posted by Cliff Pennock: ... Perhaps the modem uses a lookup table for adjusting the outgoing power levels, meaning if the incoming signal is good, it will turn down the outgoing power level. Or is that not how cable modems work?

It's possible. That's how a lot of cellular devices work.
If the incoming signal is strong, it will assume that the tower is nearby, and will dial back the transmitter power since it doesn't think it has to transmit as far.

Also, it might be possible that the high signal levels are overloading the receivers or crosstalking the signal going in the opposite direction.
In radio terminology we called it "desense" (as in de-sensitize.) The transmit side of a full duplex signal would leak over and essentially raise the noise floor at the receiver of the same device, which required a more powerful signal from the far end to provide the same signal to noise ratio.

I'm NOT saying that that's how this stuff works. I really don't know. Just a thought.

Edit - Is there no technical data for those amplifiers available on the web?

[This message has been edited by Raydar (edited 05-28-2010).]

quote Originally posted by ray b: just a guess but why not just use the konig and drop the other unit out I suspect the cable Co doesnot like two amps on one line

Always put the stronger amp first......but I'm assuming that you don't want to do anything to affect the other tenant's signal.

quote an other thought does internet connection gain from the amp could you split before the amp and just boost the tv/cable part

This is what I would try. Cable modems don't like being double amped, if at all, because each amp amplifies the noise in the line as well. They can usually cope with low signal levels if the noise level is low though (just like it's easier to hear a whisper in a quiet room than a noisy one).

Also if the cable lines are RG59, replace them if possible with RG6 cabling which is shielded better and has less loss per foot.

Yeah. Definitely replace the RG59 with RG6.