Reception/signal question - Understanding the numbers... - AVS Forum
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post #1 of 6 Old 12-30-2013, 02:09 PM - Thread Starter
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Hi all - hopefully this is the right place to post this. A little background:

Dropped cable TV (ATT U-Verse, actually) a month or so ago. So far, so good - very happy with my decision.
I live in Orange County - Costa Mesa - and discovered that I can get very good reception overall with relatively little effort - i.e.: by literally just sticking a flat (Winegard FlatWave) antenna on the wall in my office. I have 3 devices connected to this antenna through a ChannelMaster distribution amplifier. The devices are 2 Sony TVs and a Home Theater PC (Hauppauge WinTV-2250 dual tuner card).

Here's my problem/question: the Sony TV and HTPC in my office (upstairs) both get excellent reception with very few to no glitches, dropouts, etc. The Sony TV downstairs in my primary viewing area, gets good reception, but with lots of random glitches & (mostly audio) dropouts on pretty much all channels with the exception of KTTV, channel 11.1 - it's solid/stable on both TVs.

I'm running the antenna through my home coax from upstairs, to drive the Sony TV downstairs, so my first thought was that this was the problem - bad wiring. So I took the wiring out of the equation by connecting an antenna directly to the TV downstairs and it still exhibits the same glitches/dropouts. So it's not the wiring.

I reconnected the antenna to all 3 devices, and using the signal diagnostics on both Sony TVs, I observed the following:
- Signal to noise (SNR) is almost identical on both TVs
- AGC is reading around 60% higher on the TV upstairs (closer to the antenna) than the one downstairs (ex.: channel 2.1 is 34 downstairs and 55 upstairs)
- Signal strength is also pretty much the same - slightly higher (as expected) upstairs, but very strong downstairs - ex.: channel 2.1 is 85 upstairs and 82 downstairs. All other channels are similar differences between upstairs and downstaris.

So to sum up, SNR and signal strength are basically the same - the only metric that's significantly different is AGC. And I really don't understand what impact that has on reception and whether a lower AGC number would cause the dropouts I'm experiencing. I thought SNR was the critical metric...

Sorry for the long post - would like to hear any/all thoughts on all this & if anyone has any ideas about what could be causing the glitches on my TV downstairs, and better yet, if anyone has any ideas on how to eliminate them!! smile.gif

Thanks for your help!

Barry
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post #2 of 6 Old 12-31-2013, 05:29 AM
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KTTV operates on VHF Channel 11.......most other stations are UHF.

Take a look at this thread:
http://www.avsforum.com/t/1508454/ota-hd-signal-interfering-with-cable-signal

There could be interference from a cell phone, and the amplifier could be making it worse (intermodulation that gets in to much more of the UHF band).

Ken English, Sr. Engineer, KSL-TV.
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post #3 of 6 Old 12-31-2013, 06:00 AM
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In LA, ch 7, 9, 11, & 13 are all VHF. The rest are UHF. Costa Mesa is about 45 miles south of Mt Wilson and you might not have enough signal power and quality at the indoor antenna for reception to be reliable. Try placing the antenna on a window that directly faces Mt Wilson to your north as a first step.

If you need actual assistance with reception, please include a TVFool plot.

A mod may wish to move this to the Los Angeles thread.

The AGC numbers simply indicate how much AGC is being applied to the RF amplifier in the first stage of the tuner. Since Sony doesn't tell us what they actually mean, they're probably not particularly relevant
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post #4 of 6 Old 12-31-2013, 09:50 AM
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Quote:
Originally Posted by ProjectSHO89 View Post


The AGC numbers simply indicate how much AGC is being applied to the RF amplifier in the first stage of the tuner. Since Sony doesn't tell us what they actually mean, they're probably not particularly relevant

I have a Sony TV. An AGC of 100 means no signal is present. The lower the number, the stronger the signal. I typically see AGCs around 30 at my location. The TV also displays signal-to-noise ratios. When it gets to 15-16, my picture starts to pixellate. I'm typically getting 25 or above with no pixellation. I experimented with a variable attenuator and I don't get good signal-to-noise ratios unless there is a strong signal.
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post #5 of 6 Old 01-01-2014, 11:31 AM - Thread Starter
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Thanks for the replies!

I think I found the problem: Google Chromecast...

Dropouts were getting so bad this morning that I decided to try 'casting' the live stream from my PC through the Chromecast. When I switched back to OTA, the dropouts didn't come back immediately (difference in it's behavior after actually connecting to it, I guess??), but this made me realize that the only difference between my 2 Sony TVs was the Chromecast plugged into the TV downstairs. But subsequent experiments confirmed: when I turned the Chromecast off (removed power), dropouts/glitches GONE! Plugged it back in, and they're back. Chromecast is definitely the problem.

A quick internet search confirmed that others are having the same problem - seems this little device not only interferes with OTA signals, but in many cases, WiFi as well.

Anyway, problem appears to be solved & since I have a Roku & an AppleTV, I really don't need the Chromecast. Picked it up out of curiosity & since it was cheap. The only thing I really would use it for is to mirror web sessions from my PC and if I want to do that, I'll plug it back in - otherwise, it stays off & my OTA reception is perfect...

Barry
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post #6 of 6 Old 01-01-2014, 01:10 PM
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Interesting! It must be radiating in the VHF band or its wifi signal was getting into an unshielded section of the tuner and swamping the front end.
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