TiVo Community
TiVo Community
TiVo Community
Go Back   TiVo Community > Main TiVo Forums > TiVo Roamio DVRs
TiVo Community
Reply
Forum Jump
 
Thread Tools Rate Thread
Old 09-16-2013, 11:58 AM   #1
kbmb
Registered User
 
Join Date: Jun 2004
Location: NH
Posts: 714
Roamio Plus - Comcast - SNR/Signal Level

Not having any issues.....but after looking this morning to see if my Tivo rebooted last night (separate issue) from the DVR Diagnostics screen....I noticed all my my tuners had 100 signal level and generally a high 41-43 SNR.

Now after doing a bunch of reading online....seems Tivo recommends the following ranges:

Minimum SNR
QAM 64 - 23 dB
QAM 256 - 29 dB

Maximum SNR
QAM 64 - 30 dB
QAM 256 - 35 dB


Now, the only real reason I care is some people say those high levels of mine, not so much the SNR but the signal, can cause the Tivo problems over the long run. I've read somewhere that Tivo has protection built in that in extreme cases will cut the signal if it's too hot?

Most people I read say that Tivo is a little picky with the SNR numbers. Some say why would a high SNR be bad......only thing is that a high SNR could mean a high signal level (which I think is what I had).

My setup
For my house I have the cable that enters the house split once (-3.5dB each) to feed the cable modem on a clean run and the other to feed a 4 port amp that boosts (7dB) for each port. One of those ports feeds the Tivo (about a 26 foot run).

So first thing I did was take the split before the amp and split it again with a -3.5dB splitter. I fed the Tivo directly from this, and the amp with the other side. That was line into house -3.5dB, -3.5dB and then a run to the Tivo. This was OK....but overall I saw some issues with some channels having a real low SNR level of 26 and a low signal level. This caused severe pixelation and tons of errors.

So next I reverted everything back to the amp. From the amp port that feeds the Tivo....I ran a cable into a 3-way splitter and used one of the -7dB ports. This to me is basically using the amp to go +7dB and the splitter to go -7dB. This really didn't make much difference. All my signal levels stayed at 100 and the SNR levels stayed basically around 41-43dB. I read this as the signal was still to hot.

So I took that -7dB run and split it again with a -3.5dB splitter. Now, SNR levels down into the upper 30's (36-39dB)....but what I think is more important are the signal levels are now all under 100....most in the 96-99 range.

What's weird is by splitting this after the amp....I'm taking -10.5dB off the signal (while adding 7dB)....that technically is about the same as splitting the line before the amp....but for some reason splitting before the amp dropped the signals too much? No idea why.

Anyway.....I think I'll order some attenuators and replace my franken-splitter setup.

I'm assuming splitters do the same thing as an attenuator would correct? Just that the attenuator is a cleaner install?

Anyone know why the pre-amp split would cause the bad signal levels?

-Kevin
kbmb is offline   Reply With Quote
Old 09-16-2013, 12:44 PM   #2
Scooby Doo
Registered User
 
Join Date: Dec 2002
Posts: 95
There is no such thing as too high SNR. I don't know where you read about this, but it was nonsense.

As you have observed, a splitter before an amplifier reduces SNR more than a splitter after an amplifier. This is because the noise in the amplifier is fixed, so if you attenuate the incoming signal you will reduce the SNR. In contrast, when you attenuate the outgoing signal you reduce both the signal and the noise and so the SNR remains more or less constant. The lesson you should draw from this is that the best place to locate your cable amplifier is at the building entry before any splitters.
Scooby Doo is offline   Reply With Quote
Old 09-16-2013, 12:51 PM   #3
kbmb
Registered User
 
Join Date: Jun 2004
Location: NH
Posts: 714
Quote:
Originally Posted by Scooby Doo View Post
There is no such thing as too high SNR. I don't know where you read about this, but it was nonsense.

As you have observed, a splitter before an amplifier reduces SNR more than a splitter after an amplifier. This is because the noise in the amplifier is fixed, so if you attenuate the incoming signal you will reduce the SNR. In contrast, when you attenuate the outgoing signal you reduce both the signal and the noise and so the SNR remains more or less constant. The lesson you should draw from this is that the best place to locate your cable amplifier is at the building entry before any splitters.
Most "normal" people agree that there is no such thing as a too high SNR. It's only Tivo support that continues to say this:
http://support.tivo.com/app/answers/detail/a_id/150

My amp is basically the first thing in the house. The only reason there is a single splitter before it is to feed the cable modem without an amped signal.

So by me putting the splitter after the amp....I'm not reducing the SNR and signal as much? Since my SNR levels and signal levels bypassing the amp with 2 splitters was much worse that after the amp and 2 splitters.

The bad thing about the Tivo is the signal strength maxes at 100 so you really can't tell what your real signal level is.

So do you think I should be concerned with the straight run from the amp that pegged the signal levels at 100? Basically, what I found by adding the splitters was.....I needed to add at least -10.5dB to get the signal levels to ever drop below 100.

-Kevin
kbmb is offline   Reply With Quote
Old 09-16-2013, 01:01 PM   #4
DaveDFW
Registered User
 
Join Date: Jan 2005
Location: Richardson, TX
Posts: 498
Quote:
Originally Posted by Scooby Doo View Post
There is no such thing as too high SNR.
Yes, this should be true. However, in Tivo-land, the SNR displayed has historically been a proxy for absolute signal level. Tivo could avoid all this confusion by displaying both an SNR and absolute signal level instead of the nebulous "signal strength," which appears to be locked at 100 for most folks moving to Roamios.

But on the flip side, adding more technical information about the signal quality could mean more support calls for Tivo, which is probably something Tivo doesn't want to encourage.
DaveDFW is online now   Reply With Quote
Old 09-16-2013, 01:04 PM   #5
Scooby Doo
Registered User
 
Join Date: Dec 2002
Posts: 95
You should probably just forget about amplifiers and attenuators and feed all of your devices using the minimum number of splitters required to connect them. If this gives you unsatisfactory results due to low signal level, then use an amplifier at the building entry and feed all of your devices from its output, again using the minimum number of splitters necessary. Those are the only two solutions that make sense.
Scooby Doo is offline   Reply With Quote
Old 09-16-2013, 01:06 PM   #6
Scooby Doo
Registered User
 
Join Date: Dec 2002
Posts: 95
Quote:
Originally Posted by DaveDFW View Post
Yes, this should be true. However, in Tivo-land, the SNR displayed has historically been a proxy for absolute signal level.
Good point. This is probably the OP's problem: too high absolute signal level. Get rid of the amplifier!
Scooby Doo is offline   Reply With Quote
Old 09-16-2013, 01:24 PM   #7
kbmb
Registered User
 
Join Date: Jun 2004
Location: NH
Posts: 714
Quote:
Originally Posted by Scooby Doo View Post
You should probably just forget about amplifiers and attenuators and feed all of your devices using the minimum number of splitters required to connect them. If this gives you unsatisfactory results due to low signal level, then use an amplifier at the building entry and feed all of your devices from its output, again using the minimum number of splitters necessary. Those are the only two solutions that make sense.
I've always been told to never amp a cable modem run? That's why the one and only splitter I had was the single split before the amp.


Quote:
Originally Posted by DaveDFW View Post
Yes, this should be true. However, in Tivo-land, the SNR displayed has historically been a proxy for absolute signal level. Tivo could avoid all this confusion by displaying both an SNR and absolute signal level instead of the nebulous "signal strength," which appears to be locked at 100 for most folks moving to Roamios.

But on the flip side, adding more technical information about the signal quality could mean more support calls for Tivo, which is probably something Tivo doesn't want to encourage.
And this I'm guessing is what is happening. The SNR being high is really just a sign of high signal levels.


Quote:
Originally Posted by Scooby Doo View Post
Good point. This is probably the OP's problem: too high absolute signal level. Get rid of the amplifier!
I can't remove the amp without it effecting the rest of the house.

Now what's interesting is, I redid my split before the amp.....and so far signal levels and SNR seem ok...which makes more sense to me since splitting twice before the amp (-3.5dB and -3.5dB) was the same dB loss as twice after the amp (-7dB and -3.5dB). I'm wondering if maybe I didn't have something tight before.

I'll have to keep an eye on it.

Do either of you know.....can I get a real signal level if I hook up one of those Motorola Comcast DVRs to this and use the diagnostics menu?

-Kevin
kbmb is offline   Reply With Quote
Old 09-16-2013, 01:30 PM   #8
DaveDFW
Registered User
 
Join Date: Jan 2005
Location: Richardson, TX
Posts: 498
Quote:
Originally Posted by kbmb View Post
Do either of you know.....can I get a real signal level if I hook up one of those Motorola Comcast DVRs to this and use the diagnostics menu?
My Motorola cable modem and Tivo are on equal-strength segments, and I trust my cable modem to display accurate signal information. Even if it's only looking at 50mhz and down, the displayed numbers react appropriately to whatever I do to the signal. Split, amplify, attenuate, all changes reflect on the cable modem in a predictable fashion.

The information Tivo displays about signal quality and level seems to not respond predictably to these kinds of changes.
DaveDFW is online now   Reply With Quote
Old 09-16-2013, 01:33 PM   #9
kbmb
Registered User
 
Join Date: Jun 2004
Location: NH
Posts: 714
Quote:
Originally Posted by DaveDFW View Post
My cable modem and Tivo are on equal-strength segments, so I trust my Motorola cable modem to display accurate signal information. Even if it's only looking at 50mhz and down, the displayed numbers react appropriately to whatever I do to the signal. Split, amplify, attenuate, all changes reflect on the cable modem in a predictable fashion.

The information Tivo displays about signal quality and level seems to not respond predictably to these kinds of changes.
Thanks. Problem is I have an SMC gateway and it's in bridge mode, which means I can't access the web interface on it anymore. That's why I was hoping the Moto DVR would show it. I'll have to look and see.

-Kevin
kbmb is offline   Reply With Quote
Old 09-16-2013, 01:49 PM   #10
moyekj
Registered User
 
Join Date: Jan 2006
Location: Mission Viejo, CA
Posts: 8,506
Quote:
Originally Posted by kbmb View Post
I've always been told to never amp a cable modem run? That's why the one and only splitter I had was the single split before the amp.
With cable modem it needs a return path, so as long as the amp you use is a good one that provides a good return path without significant attenuation that need not be the case. The "rule of thumb" is to assume a terrible amplifier without a good return path which is why that is the general recommendation.
__________________
Roamio Pro (GigE)
Elite (MoCA)
Premiere (MoCA adapter)
Cox - Motorola CableCards & TAs
Slingbox 350 via TiVo Mini & TiVo Stream for remote viewing

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
moyekj is offline   Reply With Quote
Old 09-16-2013, 02:40 PM   #11
kbmb
Registered User
 
Join Date: Jun 2004
Location: NH
Posts: 714
First, thanks to everyone that has responded. Most of this is me being a tinkerer and being curious about the numbers.

Using a made up number for signal strength of 0.....let me ask people what they think is best. Again, 0 is a made up number just for illustration purposes.

Setup #1:
Original setup that started this was:

Line in from house = signal 0
Split -3.5 = signal -3.5
Into amp +7 = signal 3.5
out to Tivo as signal level 3.5

This is the setup that had the high SNR and always had 100 on the signal level. Wondering if the signal was too hot or not.

------------------------------------------------------------------------------------------------

Setup #2:

Line in from house = signal 0
Split -3.5 = signal -3.5
Split again -3.5 = signal -7
out to Tivo as signal level -7

This has the SNR numbers more in the 34-40dB range. Signal levels anywhere from 93-100
I'm trying this right now.

------------------------------------------------------------------------------------------------

Setup #3:

Line in from house = signal 0
Split -3.5 = signal -3.5
Into amp +7 = signal 3.5
Split out of amp -3.5db = signal 0
out to Tivo as signal level 0

Haven't tried this yet.

------------------------------------------------------------------------------------------------

Setup #4:

Line in from house = signal 0
Split -3.5 = signal -3.5
Into amp +7 = signal 3.5
Split out of amp -7db = signal -3.5
out to Tivo as signal level -3.5




What Tivo needs is to just show an actual signal level number....or at least go beyond 100. I have no idea if my original amp only setup is even too hot now. According to Tivo it probably is because the SNR is 41-43.

Also....any difference between splitting the signal with splitters vs. using an attenuator?

Any thoughts?


-Kevin



EDIT:

Also, does anyone know what the Tivo does to protect itself?

Via:
http://support.tivo.com/app/answers/detail/a_id/150

Quote:
NOTE: If your signal strength is at 100 and the video looks fine - this is not an error. The signal strength meter sets 100 at a safe threshold. If the signal is at this threshold or a bit beyond, your picture quality should be excellent. If the signal strength is well beyond the threshold, it will still register as "100", but you may experience poor picture quality as some box components cycle in and out of "protect" mode to prevent damage.
Curious if anyone knows what the Tivo does? I'm pretty sure my signal is beyond the 100.....but not sure by how much. I don't have any picture issues.

-Kevin

Last edited by kbmb : 09-16-2013 at 06:02 PM.
kbmb is offline   Reply With Quote
Old 09-16-2013, 06:39 PM   #12
lessd
Registered User
 
Join Date: Jan 2005
Location: CT
Posts: 5,417
I had a problem with OD on my Roamio Plus, the cable tech came out and with his equipment said that I was about 3 db under ideal signal level, the Roamio read 100%, the problem was a bad cable card but the signal measurement was of interest, as I do use a two way amp for my home inc the cable modem and the tech could measure the back power to the head end and said that it was normal.
__________________
Les Daniels

_____________________________________________
3 Roamio Plus upgraded to 2Tb & 3Tb, and 1 Mini, Also 1 not used Humax Series 2 with a DVD burner
lessd is offline   Reply With Quote
Old 10-13-2013, 08:51 AM   #13
kbmb
Registered User
 
Join Date: Jun 2004
Location: NH
Posts: 714
So just an update. After all the research on attenuators, it ended up that I'm not using any. Yes, I still have signal levels of 100 and the SNR readings 40-43 dB, but picture quality is great and I'm not having any issues with missing channels.

What's interesting is, I'm not sold on Tivo's numbers. Yesterday I had to troubleshoot something so I brought my Comcast Moto box downstairs and plugged in into the line used by Tivo. The Moto box showed 37.5 dB SNR.

Not sure why Tivo's SNR numbers would be different unless they were boosting it internally.

-Kevin
kbmb is offline   Reply With Quote
Old 10-13-2013, 09:29 AM   #14
aristoBrat
Registered User
 
Join Date: Dec 2002
Location: Va Beach, VA
Posts: 957
Quote:
Originally Posted by kbmb View Post
What's interesting is, I'm not sold on Tivo's numbers. Yesterday I had to troubleshoot something so I brought my Comcast Moto box downstairs and plugged in into the line used by Tivo. The Moto box showed 37.5 dB SNR.
My Roamio replaced a Premiere XL4 that replaced a Premiere. All three were in the same location, using the same cable card and wiring.

Cox could not pair the CC with the Roamio. Part of the troubleshooting was to put the P4XL back in, and every time the CC would pair instantly. Put the Roamio back and the CC would not pair. Spent ~6 hours over a few nights working with Cox on this.

TiVo support noticed my Roamios SNR were mid-40s and were floored with how high that was. Out of curiosity, I checked the P4XL's SNR, and they were low 30s. Again, same location and same wires as the Roamio.

I put a splitter between the Roamio and the wall, which dropped the Roamios SNR down to mid-30s. Called Cox back and the CC paired instantly.

So in my case, lowering the SNR was required for the CC to pair.

This makes me think that Roamios might amplify the signal.
__________________
Roamio Pro + Mini (x3)
Premier XL 4 + Mini (x2) + Premiere
Premiere (x2) + Series 3 OLED (x2)
Series 3 OLED (x2)
DirecTiVo (x3)
Series 1
aristoBrat is offline   Reply With Quote
Old 10-16-2013, 10:40 AM   #15
kbmb
Registered User
 
Join Date: Jun 2004
Location: NH
Posts: 714
Hoping some of the signal experts in here can help with this question:

Why on certain channels only, does the signal and SNR fluctuate?

As I mentioned before, I have some channels like Food Network and A&E that for whatever reason are more sensitive to signal levels. When I was trying the attenuators, it would be these channels only that dropped to lower levels.

Since scrapping the attenuators, ALL my channels have basically been 100 signal level and 41-43 SNR. Perfect picture quality (well as perfect as Comcast can get) and no signal drops.

However, today I noticed in the diagnostics screen that A&E signal level dropped to 91% and the SNR to 36. Food network dropped to 98% and SNR 38. What causes signal levels to drop from day to day? For the most part ALL other channels remain at 100% and 41-43 SNR.

I remember when I had a Comcast tech out to check on their box, he briefly mentioned something like Comcast re-signaling some channels or something like that?

Anyway....so far no issues with those dropping too much, but I'm more curious as to why it happens.

-Kevin
kbmb is offline   Reply With Quote
Old 10-16-2013, 06:08 PM   #16
kbmb
Registered User
 
Join Date: Jun 2004
Location: NH
Posts: 714
Ok, so I just checked those 2 channels again, and the signal was a little lower than this afternoon. On A&E it was down to 88%.

Out of curiosity, I went downstairs to the amplifier that feeds all the outlets in the house. I simply removed the connection to the Tivo, and plugged it back in. It was nice a secure before as well so it wasn't loose.

But what's weird is the signal on both those shot back up to 100%. Why is that? Could it be the amplifier going bad? Could it be something in the Tivo causing it to amp the signal after it loses it?

I noticed in all my testing with bypassing the amp and even the attenuators, that the signal would start off strong....but then over a few days (week) it would dip.

Any ideas?

EDIT: I can't imagine it would be the amp going bad since this is only effecting certain channels not everything. Still can't explain why simply unplugging and plugging back in would cause the signal to go back up.

-Kevin

Last edited by kbmb : 10-16-2013 at 06:59 PM.
kbmb is offline   Reply With Quote
Old 10-17-2013, 04:17 PM   #17
mchief
Morey
 
Join Date: Sep 2005
Location: Alexandria, VA
Posts: 291
After 3 weeks of 6 tuners on the new Plus, tuner 5 dropped out. The message was this channel not authorized. Got the too hot signal (all tuners at 100) from Tivo and a replace the cable card from Comcast. By this time 3 tuners were gone. I put a 3 way splitter that had out at -3db, -3db and -7db. I hooked the Tivo to the -7 out and within half an hour all the tuners were working again. I then hooked the Comcast DTR to a -3 and dropped the signal to 95 to 98 and on one tuner to 87. All seems well now, will leave everything in place for a few weeks to see if this is a problem and solved.
__________________
Tivo Roamio Plus 2TB WD20EARS (SA M-Card) Ethernet Tivo HD 1TB WD10EVCS (SA M-Card) Ethernet Tivo HD 1TB WD10EADS (SA M-Card) Wireless. SAMSUNG HDTV LN55B650 DLNA. Hitachi HDTV 42HDS69
mchief is offline   Reply With Quote
Old 10-18-2013, 12:58 PM   #18
kbmb
Registered User
 
Join Date: Jun 2004
Location: NH
Posts: 714
Quote:
Originally Posted by mchief View Post
After 3 weeks of 6 tuners on the new Plus, tuner 5 dropped out. The message was this channel not authorized. Got the too hot signal (all tuners at 100) from Tivo and a replace the cable card from Comcast. By this time 3 tuners were gone. I put a 3 way splitter that had out at -3db, -3db and -7db. I hooked the Tivo to the -7 out and within half an hour all the tuners were working again. I then hooked the Comcast DTR to a -3 and dropped the signal to 95 to 98 and on one tuner to 87. All seems well now, will leave everything in place for a few weeks to see if this is a problem and solved.
I responded to your other post in the update thread as well.

It will be interesting to see your signal levels over time. I have no idea why the Tivo seems to report wonky numbers at times and why things seem to work fine and then go bad.


I have always had 100% signal levels and have not ever had an issue with missing tuners (knock on wood).

Also, Tivo's SNR readings are different from the Comcast DVR, even on the same outlet. The Comcast DVR will report SNR in the 37.5dB range, while the Tivo will report 41-43dB.

Also have no idea why I had those 2 channels go below 100% the other day, and then by replugging in the cable, they went back to 100%. The cable wasn't loose, and none of the other channels were low.

-Kevin
kbmb is offline   Reply With Quote
Reply
Forum Jump




Thread Tools
Rate This Thread
Rate This Thread:


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Advertisements

TiVo Community
Powered by vBulletin® Version 3.6.8
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
vBulletin Skins by: Relivo Media

(C) 2013 Magenium Solutions - All Rights Reserved. No information may be posted elsewhere without written permission.
TiVoŽ is a registered trademark of TiVo Inc. This site is not owned or operated by TiVo Inc.
All times are GMT -5. The time now is 06:29 PM.
OUR NETWORK: MyOpenRouter | TechLore | SansaCommunity | RoboCommunity | MediaSmart Home | Explore3DTV | Dijit Community | DVR Playground |