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Old 05-05-2014, 09:27 AM   #1
fyodor
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TV Distortion with New Motorola Surfboard Cable Modem/MoCA

Hi,

I am an RCN user. I have a Tivo Premiere Quad and a Tivo Preview.

I recently replaced my cable modem with a Motorola Surfboard -sbg6782 (integrated router, wireless AC, and MoCA). It works well for all these functions.

Since installing this device I've noticed some weird/distortion/pixellation effects on recordings certain channels. It's not quite pixellation-every 5-10 minutes certain blocks get kind of distorted/fuzzy.

Right now there is a powered splitter outside the house that splits the signal two ways, one going downstairs to the Preview and Cable/Modem and the other going upstairs to the recording Tivo. The Preview uses its Ethernet interface to connect to the MoCA adapter in the basement and the Tivo Premiere connects via its built-in MoCA to the network in the basement.

As far as I can tell there are three possibilities.

1. The new cable modem, in its capacity as a cable modem, is doing something to interfere with the cable TV video signal.

2. The new cable modem, in its capacity as a MoCA adapter, is doing something to interfere with the cable TV video signal.

3. Having changed/simplified my splitter arrangement in the basement to work with the cable modem instead of a separate MoCA adapter, I've changed the impedance of the system causing the signal to come in too strong/weak upstairs.

It is hard to diagnose because it's not continuous-it's every 5 or ten minutes. Any suggestions for fixes, or even how to isolate/target the problem would be greatly appreciated.
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Old 05-05-2014, 06:07 PM   #2
BigJimOutlaw
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Which channels in particular... Locals?

If you unpower the modem for a while, does the problem stop? (This might be "of course, dumbass" obvious, but I'm just making sure it's the modem.)

Could be damaged/old splitters, splitter with poor shielding, loose coax, bad/loose connectors or a high upstream power level on the modem.

Is there a way to isolate the modem from the powered splitter, or is it necessary to get reliable internet service?

Doubt it's the moca per se, but you could try moving the moca network to a higher channel/frequency to give it more separation from the TV signal.

Last edited by BigJimOutlaw : 05-05-2014 at 06:15 PM.
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Old 05-06-2014, 09:05 AM   #3
fyodor
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Quote:
Originally Posted by BigJimOutlaw View Post
Which channels in particular... Locals?

If you unpower the modem for a while, does the problem stop? (This might be "of course, dumbass" obvious, but I'm just making sure it's the modem.)

Could be damaged/old splitters, splitter with poor shielding, loose coax, bad/loose connectors or a high upstream power level on the modem.

Is there a way to isolate the modem from the powered splitter, or is it necessary to get reliable internet service?

Doubt it's the moca per se, but you could try moving the moca network to a higher channel/frequency to give it more separation from the TV signal.
The problem showed up in my HBO recordings but not my locals. I didn't record any basic cable shows that night so I don't know if those were affected. HBO is on the high end of the numerical channels, but I don't know how this correlates to frequencies.

No offense is taken for obvious suggestions- last week I wasted fifteen minutes playing with audio drivers on my computer before realizing my receiver was muted. The problem is sporadic enough (maybe three or four times per 30 minutes) that I can't easily disconnect the modem to evaluate causation. If the problem persists I'll try disconnecting overnight and scheduling a recording.

I can't isolate the cable modem (or would prefer not to) because I'm using the built-in MoCA bridge.

Sunday night, when I noticed it, I tried switching the MoCA from the lowest frequency to the highest one. I didn't see any issues on the Monday night broadcast but I only watched for about ten minutes so it's hard to say for sure.

When you say high upstream on the modem, do you mean that the modem's transmissions are bleeding into the cable TV range or that the Tivo is being affected by cable modem broadcasts in the cable modem domain (just trying to understand if a filter will help).

Thanks much
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Old 05-06-2014, 09:26 AM   #4
tatergator1
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When the issue happens, have you hit the rewind button to go back a bit and re-watch the same time-frame of the video to see if the distortion happens the same way? Likewise, if you watch the recording via the Quad and then via the Preview, do you get the distortion at the same points in the recording at both locations.

You note that's it's not so much pixelation, but fuzziness/distortion. you mention 3-4 times per 30 minutes. How long does each distortion last? Since you're dealing with Digital Cable signals, I don't think fuzziness is really possible as a result of signal interference; you either get the signal, or you see pixelation.

Based on what you describe, it seems like this is either something with the Tivo or TV and how the image is being processed and displayed that results in the fuzziness, or there is an inherent issue with how RCN encoded the Mpeg2 video, perhaps too low of a bit-rate for the level of action/detail in the video.
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Old 05-06-2014, 09:50 AM   #5
fyodor
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Quote:
Originally Posted by tatergator1 View Post
When the issue happens, have you hit the rewind button to go back a bit and re-watch the same time-frame of the video to see if the distortion happens the same way? Likewise, if you watch the recording via the Quad and then via the Preview, do you get the distortion at the same points in the recording at both locations.

You note that's it's not so much pixelation, but fuzziness/distortion. you mention 3-4 times per 30 minutes. How long does each distortion last? Since you're dealing with Digital Cable signals, I don't think fuzziness is really possible as a result of signal interference; you either get the signal, or you see pixelation.

Based on what you describe, it seems like this is either something with the Tivo or TV and how the image is being processed and displayed that results in the fuzziness, or there is an inherent issue with how RCN encoded the Mpeg2 video, perhaps too low of a bit-rate for the level of action/detail in the video.
IIt's definitely in the signal and shows up when rewound.

It's hard to articulate-it's not quite like pixellation from bad decodes, but more like a single chunk of the video becomes an unformed mass. I think that "macroblocking" is the right term. If the problem persists I'll do a screenshot.

I've certainly heard of cable modem/moca interference causing macroblocking when the Tivo receives an incomplete signal and is unable to properly decode sections of the video. See here.

http://www.avsforum.com/t/1528061/mo...gnals-at-times
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Old 05-06-2014, 06:30 PM   #6
BigJimOutlaw
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Quote:
Originally Posted by fyodor View Post
I can't isolate the cable modem (or would prefer not to) because I'm using the built-in MoCA bridge.

...

When you say high upstream on the modem, do you mean that the modem's transmissions are bleeding into the cable TV range or that the Tivo is being affected by cable modem broadcasts in the cable modem domain (just trying to understand if a filter will help).
By isolating the modem I meant adding a plain splitter, with one leg going to the modem, and the other leg going to the powered splitter. Moca should still work if the splitter's frequency is high enough but I get that may defeat the point of the powered splitter. btw, is the splitter only powering one leg?

By high upstream power level I mean the internet upstream could bleed into TV. Theoretically.

I'm not sure which options the RCN Tivo removes, but if you go into Settings> Account & System Info > DVR Diagnostics, what kind of Signal Strength and SNR levels are you seeing on your tuners?
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Old 05-07-2014, 09:20 AM   #7
fyodor
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Quote:
Originally Posted by BigJimOutlaw View Post
By isolating the modem I meant adding a plain splitter, with one leg going to the modem, and the other leg going to the powered splitter. Moca should still work if the splitter's frequency is high enough but I get that may defeat the point of the powered splitter. btw, is the splitter only powering one leg?

By high upstream power level I mean the internet upstream could bleed into TV. Theoretically.

I'm not sure which options the RCN Tivo removes, but if you go into Settings> Account & System Info > DVR Diagnostics, what kind of Signal Strength and SNR levels are you seeing on your tuners?
I was seeing signal strength in the 90-95 range. Not sure about SNR.

The splitter is powering both legs, but is really only needed for the bottom leg. Correct me if I'm wrong, but I don't think that MoCA can pass from the output to the input of a powered splitter, which it'd need to do to connect in the configuration you've described (but can communicate among common outputs of a powered splitter).

Switching the MoCA frequency from the default of the lowest (1150) to the highest (1500) seems to have fixed the problem for now. Either that or it really was just a coincidence, with weird recording behavior coming in just as I replaced the modem/bridge.

Or the modem/bridge is just taunting me by pretending to be fixed and will continue to cause problems in an erratic way that makes it impossible for me to diagnose reliably, and will behave itself whenever I try to show it to tech support, and I will go mad from trying to fix it and convince others of the problem, like a character in a Hitchcock movie.
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Old 05-07-2014, 09:28 AM   #8
CuriousMark
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Originally Posted by fyodor View Post
Or the modem/bridge is just taunting me by pretending to be fixed and will continue to cause problems in an erratic way that makes it impossible for me to diagnose reliably, and will behave itself whenever I try to show it to tech support, and I will go mad from trying to fix it and convince others of the problem, like a character in a Hitchcock movie.
Just move the TV away from the Rear Window.

:-)
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Old 06-09-2014, 07:23 PM   #9
alleybj
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Was your problem solved? I'm experiencing something similar. Thanks
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