HDMI Noise and Signal Interfernce for HTPC

Looks like a grounding problem.

Why are you using composite video for your cable box? Does it not have an HDMI output?

I realize that is what you are using. I am asking if it doesn't also have an HDMI output that you can use. That signal is digital and will be immune to the distortion you are seeing.

Even though it's a digital connection, it can still be a factor and confounder for ground loop problems as it's an electrical connection.

I'll wager the culprit is your cable co coax feed. Specifically, the feedthru coupler at the building entry isn't properly earthed.

I didn't say otherwise. I am advising him to move away from composite video for his cable box that is getting impacted by the ground loop. If we convert that connection to digital, we have a shot at solving his problem and improving his video quality immensely.

And I was trying to address a possible root-cause for his ground loop problem. Not work around it. As a trouble-shooting aid, you can (temporarily) earth the coax shield entering the STB or HTPC. If this knocks it down or eliminates it, you know what it is and how to fix it properly.

In 90% of the ground-loop problems I've seen in a system that involves a cable company connection, the problem is what I noted. I've seen >60V on cable co coax shields. In addition to a horrendous ground loop problem, it's also a big safety problem. It's why by code, the building feedthu coupler must be properly earthed. Often they are loose, missing or corroded and gunked up with decades of paint.

He said he's using '3 channel' analog which would presumably be component, not composite, and HDMI may not be so 'immensely' better PQ than component. Both done properly should yield about the same results. If the root-cause of the ground loop or ground potential is eliminated, then all the connections should be more robust.

Your HTPC is likely to be a confounder as it has a safety ground connection in the power cord. Three wire power cord vs the two wire cord the STB and TV likely has. Yes?

Do you have a volt meter and know how to use it safely? Can you test things visually with the cable co coax feed disconnected and removed as a variable? (Use STB menus/OSDs)

It is common for these problems to pop up when you add one more gear. If you swap the cable boxes and use the one with HDMI next to the TV with HTPC, does the problem go away?

++ disconnecting the Coax input and using the OSD from the cable box. Does the noise go way with the coax disconnected but the OSD displayed? Also if you have a multimeter with an AC Voltage Autorange you can put one lead to ground on the outlet and the other lead on the hex connector on the coaxial and see if you have voltage going across. This is assuming that your outlets are wired correctly.

BTW Dlarsen, the OP did say he is using component. Yellow for video and the R/W for the stereo channels. He didn't list S/PDIF or TOSLink as a connection so:

1. I doubt he is getting anything above SD out of the cable box
2. It doesn't sound like component

OP, Do you have ANY other connections from the cable box to the TV other than the three lead cable?

He said '3 channel' more than once and never mentioned audio or stereo in this connection. My HD STB has component video and I believe that most HD STBs have component vid out. Yes, the sun has set there though.

I know, just thinking that when OP posted: But, when i watch cable-TV which works via 3channel (red, blue, Yellow cables) there is heavy noise and some signal distortion

May be that he meant red/black/yellow. I dunno. But without any other mention of any other kind of cable I have to think it is composite. Yellow being video, and the other two for stereo.

Any ways clarification has been asked for. Just need to wait.