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rockotman
...Blown On The Steel Breeze
Emerging Research
join:2000-08-06
DSotM
kudos:2

1 edit

SNR on digital cable.

I am having a bit of an issue with some hi-def channels on my digital cable...

On a few of the hi-def channels, every so often the sound will cut out.

I have a DCT6416/III provided by Comcast as the STB. It only does this when I am viewing these channels via the STB. The same channels (all local broadcast channels) when viewed through the TV ASTC tuner, do not exhibit this behavior.

According to some online documentation that I found, related to the DCT6400 series STBs, one of the causes for this can be less than adequate SNR.

After performing the checks of the information in the diagnostics screens, as recommended, it appears that my problem is indeed a low SNR. Everything else checks out "good". The SNR in the "d03 OOB Status" is the only item of the several items to be checked that is not "good"... it is "fair", and typically is at between 17.5 and 18.3 dB.

I went through and redid all connections in the line - there are two splitters, both good quality high-bandwidth splitters, in the path. The first is a three way that splits off to a cable modem and to two televisions (one being the HDTV). The second is at the HDTV, splitting one signal to the STB, the other directly to the TV tuner itself.

(I realize that the second one is not necessary, but I have it there to pick up on local channel that has an HD signal on the cable that is not included in the HD channels available via our local Comcast office (for those in the Pittsburgh area - Channel 22, WMYP). Not sure why it isn't in the normal HD lineup via the STB, but it isn't, so this is the only way to get it on the TV). But I digress...

All cable is RG-6. All splitters are high quality and like new. Any other suggestions on improving the SNR?

One other thing I should mention... the cable for the cable modem from first splitter goes through a filter into another 3-way splitter, which feed the cable modem and two other rooms. One room has an old analog TV hooked up directly to the cable, the other does not (no terminator either - when we removed the TV, I couldn't find any - but I didn't really look too hard, either) I wouldn't think this unterminated cable, off on a separate run, would have an impact, but I am by no means an expert. Opinions?
--
"I was the bad guy once again. I laughed at their sorrows. Ha ha ha ha ha."



Greg_Z
Premium
join:2001-08-08
Springfield, IL

Personally, if you can get away without using multiple Splitters (figure if a 3.5, with two you are getting a 7db loss), I would go with one Splitter made for the number of connections you are expecting to use. As for the d03 screen, are you checking it on the channels that are showing suspected issues, or just a default channel?



rockotman
...Blown On The Steel Breeze
Emerging Research
join:2000-08-06
DSotM
kudos:2

said by Greg_Z:

As for the d03 screen, are you checking it on the channels that are showing suspected issues, or just a default channel?
On the channels that are having issues, as instructed by the referenced guide. I did check other channels, and they seemed about the same, though.
--
"I was the bad guy once again. I laughed at their sorrows. Ha ha ha ha ha."


owlyn
Premium,MVM
join:2004-06-05
Newtown, PA
Reviews:
·Comcast
reply to rockotman

At your 3-way,you are probably losing about 6db. At the next splitter, you are losing at least 3.5, but I suspect more. Even if you add the 9.5 to your SNR, it is still a bit low. You want to get up above 32 if possible. I would replace the first splitter with a bi-directional amp, and if you can, get rid of the other splitter.

Before doing anything, have Comcast come out and check the signal going in to your house. It could be low.



rockotman
...Blown On The Steel Breeze
Emerging Research
join:2000-08-06
DSotM
kudos:2

Thanks -

Note that since I redid the connections yesterday, it hasn't cut out once, but the SNR still seems to be the same. Go figure...

If it happens again, I think I will eliminate the second splitter for a while and see if that helps.
--
"I was the bad guy once again. I laughed at their sorrows. Ha ha ha ha ha."



gchris2203

@insightbb.com
reply to owlyn

Unfortunetly owlyn that isnt the way SNR works. It doesnt have flat loss from splitters. It is a ratio of the signal to noise. Granted if you remove the splitters from the scenario your signal lvl will rise and so will your SNR. But not exactly the loss from the splitters. Not that it matters from 2 splitters it shouldnt drop to 18.

My guess would be It is a combination of problems. 18snr is really bad.

It could be the drop to your house and usually is when you have tiling issues. Maybe squirel chews on the drop then water gets in it and. then you have Both Noise getting on the lines through the chews aswell as water. and water ruins your digital signal. This is probally your issue.

The problem could also be and dont take it personal cause idk what type of fittings and splitters you consider good quality. But it could be your handy work.

Heres what you do..go pick up your phone call comcast and schedule for a tech to come out and check the drop aswell as your handy work to see if they are actually good compression fittings aswell as 1ghz splitters.

To everyone who trys to be a DIY thats cool and all but I can honestly tell you out of the number of people that try and do things on their own very..and i mean very few actually succeed. Most of the time making more work for the tech. Just call your office and get a tech out it will prolly save you time in the end anyway. considering you prolly spent most of a day trying to fix it yourself and in the end its still doing the same thing..so save yourself the trouble next time



ticktack

@gci.net
reply to rockotman

the OOB SNR is for the out of band carrier which has absolutely nothing to do with the SNR on the frequency that carries a particular HD channel. While 17 is slightly low it's not that bad for a OOB carrier. you will probably never see it above 22 because it is modulated using qpsk.

your HD channels are totally seperate. You need to tune to an HD channel and check the in band status, that will show the SNR for the current frequency you are tuned to.

How high that should be depends on the modulation of the channel. assuiming it is QAM 256 it should be at least 32 but preferably 33 or more.



rockotman
...Blown On The Steel Breeze
Emerging Research
join:2000-08-06
DSotM
kudos:2

said by ticktack :

your HD channels are totally seperate. You need to tune to an HD channel and check the in band status, that will show the SNR for the current frequency you are tuned to.

How high that should be depends on the modulation of the channel. assuiming it is QAM 256 it should be at least 32 but preferably 33 or more.
I did check the in-band stati of both tuners, per the guide that I referenced, and both SNRs there were within the limits that the guide indicates are acceptable.
--
"I was the bad guy once again. I laughed at their sorrows. Ha ha ha ha ha."


rockotman
...Blown On The Steel Breeze
Emerging Research
join:2000-08-06
DSotM
kudos:2
reply to gchris2203

said by gchris2203 :

The problem could also be and dont take it personal cause idk what type of fittings and splitters you consider good quality. But it could be your handy work.

Heres what you do..go pick up your phone call comcast and schedule for a tech to come out and check the drop aswell as your handy work to see if they are actually good compression fittings aswell as 1ghz splitters.

To everyone who trys to be a DIY thats cool and all but I can honestly tell you out of the number of people that try and do things on their own very..and i mean very few actually succeed. Most of the time making more work for the tech. Just call your office and get a tech out it will prolly save you time in the end anyway. considering you prolly spent most of a day trying to fix it yourself and in the end its still doing the same thing..so save yourself the trouble next time
Well considering that the splitters were provided by comcast, I would hope that they are good quality. And as far as my handiwork with stripping/crimping cable ends... I have the proper tools (a coax stripper and a properly sized crimper for the cable ends I use). And while RF is not my forte, I am am EE, so I think I know a bit about the importance of proper technique and equipment when it comes to doing electrical/electronics work.
--
"I was the bad guy once again. I laughed at their sorrows. Ha ha ha ha ha."


Greg_Z
Premium
join:2001-08-08
Springfield, IL

This may help you »workmanship.nasa.gov/lib/insp/2%···set.html It was posted in one of the other forms. I remember some of the stuff in there from the Navy manuals, and my father's old Air Force books.



Quaoar

join:2004-08-11
Fort Collins, CO
reply to rockotman

Well, now... I have the best of tools, etc. I've always done my own wiring. I had the same sound issue on HD channels. I do the service call (after being beaten around the ears by my wife). The tech has a look at my work, and says it is pretty good, but he'll replace some fittings anyway. Damn, the problems go away, the HD fidelity improves, my wife is happy (and that's what counts!), and my dog is relaxed.

It seems my work was good enough for SD, but not quite good enough for HD.

Think about it.

Q



rockotman
...Blown On The Steel Breeze
Emerging Research
join:2000-08-06
DSotM
kudos:2

Ah - yer right - what do I have to lose?

Other than possibly sitting around all day waiting for someone that does not show up!



MadMANN3
Premium
join:2005-08-19
kudos:2

1 edit
reply to rockotman

said by rockotman :

I am am EE, so I think I know a bit about the importance of proper technique and equipment when it comes to doing electrical/electronics work.
No offense, but I wish I had a crisp $1 bill for every trouble call related to THAT statement.


rockotman
...Blown On The Steel Breeze
Emerging Research
join:2000-08-06
DSotM
kudos:2

Well, if you meant no offense, then why did you say it?


Not that I mind.

I could tell you stories about techs that I have had out from Comcast, and their predecessors ATTBI and @Home. Like the know-it-all that drilled through the phone lines. Or the one that tried three cable modems before he realized that the signal strength had gone to shit because of a bad crimp that he did. This was the same guy that installed a NIC in my PC and fried my dial-up modem in the process. No offense, though.
--
"I was the bad guy once again. I laughed at their sorrows. Ha ha ha ha ha."



MadMANN3
Premium
join:2005-08-19
kudos:2

I said no offense because I am not including you in the generalization, since I have never been at your house. I can say with confidence that any experienced tech has a story about some customer professing to know more than the tech because they were an electrician, an EE, or used to work in the cable industry 20+ years ago.

My fav story that comes to mind is when I rolled to a house a few years ago with tiling digitals and choppy internet where the customer was hesitant to let me in the house because he was an EE, he installed it himself, and claimed that the problem had to be outside because his neighbor had the same service problems. So I go to the ground block and check for common problems, not finding anything. I politely asked him to let me verify that everything is OK in the home. He reluctantly lets me in and let me know in no uncertain terms that I was "wasting my time and his".

So, I look at the picture and confirm that the pictures are indeed tiling badly. I take readings at the set to find low SNR and error rates out the wazoo. Look behind the set and find a silver Radio Shack "hybrid" cable splitter that he was using for PIP along with some screw on connectors with the braid hanging out of them. Replace the splitter and jumpers and *poof* tiling is gone.

So, I ask to look at his internet problem. Signal is bad at the modem. Go to the basement and find the same type of splitter fed AFTER a four way splitter that he added so he could add an extra set in the basement. Replace and reconfigure with new connectors, and again, *poof* internet is cruisin.

As I have him sign the paperwork, I ask him if he happened to "help" his neighbor self-install their service as well. With the answer of "yes" I reach into my truck, give him two good 2way splitters and say "Tell him to call us if he continues to have problems."

I have many more similar to that story, but I wouldn't bore anyone with them. I think I make my point.

That said, I have also had many run ins with techs that don't know any more than my mother-in-law or they do and are too lazy to apply it. They typically do not last very long.



rockotman
...Blown On The Steel Breeze
Emerging Research
join:2000-08-06
DSotM
kudos:2

I agree that there are many good techs, too. I had a problem with signal integrity when they expanded the digital package to include MusicChoice. I started having tiling problems at that time, too. Tech came out and determined that the problem was the run from the garage to the TV that was some old RG-59 (I think it was) that the previous owner had installed back in the early 80's. He told me that I needed to be using RG-6, but said he couldn't do the install since it was not originally run by Comcast (or the predecessor TCI). But then he went out to the truck and gave me about 100ft. of RG-6, and told me what type of stripper, crimper, and connectors to get at Home Depot. Problem solved thanks to knowlegable and heplful tech.
--
"I was the bad guy once again. I laughed at their sorrows. Ha ha ha ha ha."



gar187er
I do this for a living

join:2006-06-24
Dover, DE
kudos:4
reply to rockotman

-- had an electrician (20 years in the field) who though coax was like speaker wire....had his underground drop cut, so he stripped back to the center conductor then twisted the two end together......

one word....WOW



rockotman
...Blown On The Steel Breeze
Emerging Research
join:2000-08-06
DSotM
kudos:2
reply to rockotman

Hmmmm.... seems like we were all wrong...

Despite reworking my cables a few weeks ago, the problem recurred.

Anyway, a Comcast tech was at the house today to hook me up for digital voice.

I asked him about my problem with the HD channels having sound cut in and out occasionally.

I also explained what I had observed in the STB diagnostics page ("Good" SNRs for the digital channels (in band), low AGC value for the OOB combined with a "Fair" SNR for the OOB). He said that the signal may actually be "too strong" and swamping the input amp on the STB. He said to try adding another splitter to the signal to see if it fixed the problem. So I rearranged my splitters to move the HD box downstream of another splitter.

Guess what - the problem seems to be corrected.
--
"I was the bad guy once again. I laughed at their sorrows. Ha ha ha ha ha."


NoPegs
Premium
join:2006-06-14
Myerstown, PA
reply to rockotman

said by rockotman:

said by gchris2203 :

The problem could also be and dont take it personal cause idk what type of fittings and splitters you consider good quality. But it could be your handy work.

Heres what you do..go pick up your phone call comcast and schedule for a tech to come out and check the drop aswell as your handy work to see if they are actually good compression fittings aswell as 1ghz splitters.

To everyone who trys to be a DIY thats cool and all but I can honestly tell you out of the number of people that try and do things on their own very..and i mean very few actually succeed. Most of the time making more work for the tech. Just call your office and get a tech out it will prolly save you time in the end anyway. considering you prolly spent most of a day trying to fix it yourself and in the end its still doing the same thing..so save yourself the trouble next time
Well considering that the splitters were provided by comcast, I would hope that they are good quality. And as far as my handiwork with stripping/crimping cable ends... I have the proper tools (a coax stripper and a properly sized crimper for the cable ends I use). And while RF is not my forte, I am am EE, so I think I know a bit about the importance of proper technique and equipment when it comes to doing electrical/electronics work.
We don't ever crimp any more. Compression fittings only... Hex crimps are eeeeeevil!!!

You can actually pick up a decent compression tool for between $60-$100 on e-bay these days...


rockotman
...Blown On The Steel Breeze
Emerging Research
join:2000-08-06
DSotM
kudos:2

LOL - yeah they are... but for now, that is all I have. Besides - problem resolved for the time being.


rody_44
Premium
join:2004-02-20
Quakertown, PA
reply to NoPegs

i was thinking the same thing. all compression fittings suck.


toolman1955

join:2008-03-02
Pendleton, IN
reply to rockotman

Sounds to me like it's over driven. Need to reduce the signal coming to the DVR. Having to much signal is just as bad as not having enough. This will cause the SNR to be to close and cause issues that you are speaking of.


toolman1955

join:2008-03-02
Pendleton, IN
reply to owlyn

Bi-directional amp (2-way amp) not a good idea if you are having INGRESS or SNR problems. This type of amp will only increase the problem and it may cause problems for your neighbors. It can also effect the NODE and then when a truck rolls and they find out what is adding to the problems (like modems dropping out etc.) could be quite a service call bill due to customer (subcriber) induced problems. OVER AMPLIFICATION IS JUST AS BAD AS WEAK SIGNAL....


toolman1955

join:2008-03-02
Pendleton, IN
reply to rockotman

We get bad splitters NEW right out of the box every day. If the splitters you had came from Comcast were they brand new or had they been laying around in the basement for awhile unwraped from it original wrapping it could have drawn moisture, have some internal corrosion etc.
And by the way even the best installers can accidently leave one little piece of braid wraped around the stinger and cause all kinds of problems.....