# Improve SNR without increasing signal?



## kohai (Apr 25, 2009)

After 1 year and 5 months of using my Tivo HD, I have started to get gray screens, partial recordings, and stuttering/pixelation. I even had one random reboot. Tivo support thinks the signal to noise ratio (SNR) is too low (~28). 

Background:
I have run the reboot + 54 code SMART HD tests and they all passed. 

I have run the reboot + 57 code tests overnight and the Tivo was running in the morning.

I have a signal strength generally in the mid 90s!

I have an SNR generally 28-30.

Tivo support thinks the signal to noise ratio (SNR) is too low (~28). 

OTA antenna for HD and analog cable. No cable cards.

Tivo HD with external drive expansion.


The challenge:

I have read a lot of people talk about using pre-amps to increase signals, but my signal is already high. If I raise my SNR using a pre-amp, I believe I will also increase my signal. Tivo likes a signal level of 85. (I hear the Winegard pre-amps won't over amp a signal? Will they clean up the line?)

The Tivo tech said the signal may possibly be too high and suggested attenuating it. (The problem Verizon Fios users have...)

I added a 10db attenuator to the line and I still have gray screen problems. My SNR dropped a bit by adding this (25-28). My signal dropped to about 85.


Cabling setup:
A large antenna in my attic. (Home is a rambler.)

Cable run to the basement wiring panel is probably about 60 ft. Cabling installed in 2001.

There is only one splitter on the line in the basement wiring panel. It is a 2-way splitter to my two TVs. No unused ports.

The run from the basement to the Tivo is about 40 ft.

OTA antenna for HD and analog cable. No cable cards.


So, the crux of my post is... How do I reduce noise without increasing the signal, thus improving SNR?

Chris


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## pmiranda (Feb 12, 2003)

Amp at the antenna, attenuator at TiVo if you need it. Or, a more selective antenna but I suspect it's already a pretty good antenna.

Is there any pattern to what channels are recording when you get hangs or other problems? Could be one of your locals is sending something strange on their data stream. Also could be just one or two locals is weak and causing problems if they get tuned in but the ones you normally watch are fine.


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## kohai (Apr 25, 2009)

It is all HD channels. No problems with the analog cable (SD).

I noticed the sticky post about partial records and read that thread and realized that is exactly what I am seeing. Tivo support said it was my SNR, but I'm suspecting that may not be it. I divorced my expander last night to see if the problem goes away. If it does, Tivo support is going to get an ear full.

If it doesn't solve it, then I'm going to try a Winegard pre-amp.

Chris


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## KeithB (Dec 12, 2008)

TWC recently replaced/upgraded all my coax cable ends from crimp-on to compression fittings. It raised my SNR 2 to 3 points and eliminated several annoying problems with my TiVo HD.


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## pmiranda (Feb 12, 2003)

kohai said:


> It is all HD channels. No problems with the analog cable (SD).


I missed the bit about you having analog cable... is it a separate cable run into the cable connection on TiVo, or is it getting combined into the feed from the antenna?

Regardless, I do think you've got a bit of a long run to that antenna, so an amp up at the antenna and a check of all connections will probably do the trick.


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## DougJohnson (Dec 12, 2006)

kohai said:


> After 1 year and 5 months of using my Tivo HD, I have started to get gray screens, partial recordings, and stuttering/pixelation. I even had one random reboot. Tivo support thinks the signal to noise ratio (SNR) is too low (~28). .
> 
> OTA antenna for HD and analog cable. No cable cards.
> 
> ...


What kind of cable are you using from the antenna? Are there any kinks or sharp bends in it? What kind and how many connectors? RG-6 quad shield with compression connectors is the gold standard. RG-59 with crimp connectors is bad. Does the cable run near any noise sources (motors, fluorescent lights)? Any or all of these could be your problem.-- Doug


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## k2ue (May 9, 2002)

That the SNR only dropped slightly with a 10dB pad at the receiver suggests to me that the TiVo itself is at fault, especially if the SNR is similar for OTA and cable -- cable at least should have a high SNR, and the chances that both are similar from external causes is remote. Do you have some source that has a built-in modulator you can connect direct to the TiVo? If that too comes up with the same SNR, the diagnosis is TiVo tuner service. If much better, the focus is on bad cables and/or connectors, until cable is working properly. Then if OTA SNR is still low an antenna-mounted preamp is in order. I've been a working RF engineer for over 40 years, so my conclusions are reasonably informed.


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## kohai (Apr 25, 2009)

I'm going to try new connectors and see if it improves my SNR. Divorcing my expander from my Tivo did solve my partial recording problems. I think if I improve the SNR, that the occasional pixelation I see will go away -- it isn't enough to be a big problem but it would be nice to improve. Thanks for all of the suggestions! I'll give them a shot.


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## SCSIRAID (Feb 2, 2003)

Is the SNR you are asking about from Cable or OTA? The modulation schemes are different between cable (QAM) and OTA (8VSB). Each has different characteristics for acceptable SNR. Cable companies typically have SNR's in the 34-36 db range. QAM is designed around high bitrate and would rather be in a quiet environment. Over the Air situations are noisy and 8VSB is designed to do well in a noisy environment but can only get half as much bandwidth in the 6Mhz channel. OTA typically is fine with SNR's >20db. The best I have ever seen OTA here is 30db so at 28db you should be fine.


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## michael new (Jan 13, 2006)

So if Im reading this right, a signal streangth of 100 and snr 36-37 is good on a quam modulation? 

I noticed some change in quality lately and thought comcast may have boosted my signal for the digital upgrade in June. Does anyone here think a 10db attenuator may be needed?


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## bkdtv (Jan 9, 2003)

michael new said:


> So if Im reading this right, a signal streangth of 100 and snr 36-37 is good on a quam modulation?
> 
> I noticed some change in quality lately and thought comcast may have boosted my signal for the digital upgrade in June. Does anyone here think a 10db attenuator may be needed?


That's entirely possible. Others on this forum have found that they needed an attenuator after an infrastructure upgrade increased their signal so much that it overpowered their TiVo on some channels.


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## SCSIRAID (Feb 2, 2003)

michael new said:


> So if Im reading this right, a signal streangth of 100 and snr 36-37 is good on a quam modulation?
> 
> I noticed some change in quality lately and thought comcast may have boosted my signal for the digital upgrade in June. Does anyone here think a 10db attenuator may be needed?


The S3 seems especially intolerant to high signal levels from my experience. I have 10db attenuation on my S3 Cable input which gets my strongest channels (the SDV QAM's) to just bounce between 95 and 100 signal strength. This gives me best performance. It also gives me between 34 and 36db SNR. My weakest channels show 85-90 signal strength with 34db SNR.

Note that my cable signal level is pretty hot... about +5 dBmV. I have a distribution amp where the cable enters the house.


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