Hello

I had a 2-way splitter to split cable (already split once outside using a 3-way symmetrical splitter) between a cable modem and and ATI 650 PCIe to feed Vista Media Center with TV Pack.

Sometimes (especially on hot days) the cable modem was unreliable. It seemed to have a problem on the return path (power level>50 dBmV). Taking the splitter out and directly plugging the cable to the modem usually solved the problem (back to around 45 dBmV) but then of course I could not get the TV signal.

So I decided to try replacing the splitter by a low (3 dB) forward path signal amplifier with active return / 0 dB reverse loss (Electroline EDA-FT08300) to help on the return path.

It is not clear to me that the active return is helping much as it was not constant before but it did increase the forward path received signal strength of the cable modem and it is now on the high side (+8 dBmV). In any event the modem seems OK with it (SNR>38, bit error rate 0%).

On the other hand the TV signal (Media Center live TV) seemed to suffer from the amplification as I could observe sporadic macro-blocking that I did not recall observing before. I wondered if the signal were too strong so I added the splitter back between the amplifier and the ATI 650 PCIe (with an empty leg, just to reduce the power it by 3.5 dB) and that seemed to help...

This long story to explain my need: I would like to have a somewhat scientific approach to compare the quality of the image before putting the amplifier, after putting it, and to confirm that reducing the power by adding a splitter makes a difference.

To the best of your knowledge, is there a way to somehow obtain information such as signal strength or bit error rate on Clear QAM channels? Ideally I would like to display this as an overlay on top of live TV to compare the images to the signal in real time...

Thanks!

Hi

I believe I fixed the issue I had with my ATI 650 PCIe showing macro-blocking on HD Clear QAM channels and I am posting what I did to gather comments or to suggest a path to explore to others who might have the same problem.

I did not find a way to obtain signal strength information or bit error rate on Clear QAM channels directly in Media Center... but I used a great device: the Pace Digital Transport Adapter (DTA) that Comcast provides for free to its customers in (some) markets that are moving to all-digital cable.

The DTA can give you a table of the frequencies used by all the SD channels. And for each SD channel you get get the incoming power level and the Signal Noise Ratio (SNR). You just have to press the info button for about 5 seconds to get to the system menu. It's very convenient.

My understanding of the basics (at least where I live) thanks to a lot of googling is that:

Cable devices receive signals in the 54-1002 MHz range, and if they are bi-directional (cable modem, STB) communicate back in the 5-42 MHz range. Incoming channels used 256-QAM modulation and return is done using 16-QAM.

A SNR of 35 dB or above is necessary for a reliable communication using 256-QAM. Ideally a signal should be received at a power level of about 0 dBmV. Bi-directional devices trying to send a signal back using 16-QAM with a power level above 50 dBmV are a sign that the other side cannot get a reliable signal back... In other words if your cable modem SNR is under 35 dB or if the power level for the return path is above 50 dBmv you might experience what feels like a slow internet connection. That was my problem (on the return path).

For every 2-way splitter you loose about 3.5 dB. If your incoming SNR is bad and your power level is low you should try removing splitters or if not practical you should consider an amplifier. The important point to pay attention to when picking an amplifier to be used with a bi-directional device is that it should not negatively impact the return path. In other words it should not just amplify the forward path (54-1002 MHz range) but also the return path (5-42 MHz range).

I was pretty sure my problem was linked to the return path but I was unsure I wanted to install a return amplifier. I decided to try replacing a splitter by a low forward path signal amplifier with active return, namely the Electroline EDA-FT08300. My cable modem seems to like it. The return path power level has so far never reached above 46 dBmV and the internet connection always feels fast.

On the other hand I started noticing a lot of macro-blocking in Media Center when tuning to HD channels (using a ATI 650 PCIe). Since the amplifier was the only change introduced in the chain I suspected it was amplifying the signal and/or noise too much... but at the same time I was not sure I really was getting more macro-blocking than before. That's when I realized that I needed a way to measure signal strength information and SNR in Media Center.

I could not find a way to objectively measure quality in Media Center... so I got a DTA out of the box and checked what it was reporting. The DTA was always happy with a SNR above 35 dB on all SD channels regardless of how many splitters I connected it to behind the amplifier. But I could see that the power level was changing as expected (in a +6 to -1 dBmV range which is a little bit lower than what the cable modem reports when I connect it at the same spot). What was interesting to learn is that my forward cable modem signal is located between TV channels at 705 MHz. The DTA reports that I have some SD channels coming as low as 93 MHz and some as high as 753 MHz. I don't know where the HD channels are since the DTA only shows SD channels but that gives me the impression that downstream internet is handled pretty much like a TV channel.

At this point I knew that the cable modem forward path and the DTA channels were fine regardless of where I plugged them. The SD channels looked fine too in Media Center. I didn't have any info on the HD channels but I decided to assume that they should somehow be reaching me at about the same power level than the SD channels. Therefore I decided that attenuating the TV signal to reach around 0 dBmV on the SD channel was the only thing I could do. I knew the SD channels should be fine but I was quite sure it should not negatively impact the HD channels. So now I have two 2-way splitters with an empty terminated leg behind the amplifier to feed the ATI 650 PCIe. Looks ugly but I don't see any macro-blocking on the HD channels anymore. Of course the cable modem stayed just behind the amplifier.

My conclusions:

The DTA is a cool tool to check the quality of the incoming video signal.

A cable modem can probably give you a good estimate of what the DTA would tell you.

Paying attention to the return path is important.

The ATI 650 PCIe does not seem to like it when the signal is not close to 0 dBmV.

We really need a way to measure power level and SNR in Media Center!

What do you think?