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Old 6-Sep-2020, 2:14 PM   #30
rabbit73
Retired A/V Tech
 
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,747
Quote:
Originally Posted by verder
Not all tuners are equal
(If you are becoming tired of copying these PMs to the main thread, let me know and I can try bringing this conversation over to the AVS forums if you know of a good spot for them.)
It doesn't take a lot of time to post your messages, but it does take a lot of my time to think of useful answers. However, I find your reception problem interesting and I always learn something new when helping a poster.

I am willing to stick with it a little longer; it's kind of peaceful here. When I run out of ideas, it's time for you to go to AVS for some new ideas.
Quote:
Today's experiments consisted of some simple bench top testing and some garage rafter testing. The bench top testing was literally bench top. I've got a workbench in my garage and just placed the Gray-Hoverman build facing due south. I did that to perform some sanity checking with the Airspy R2 on my build. Turns out that I've apparently got some issues with the R2 where I see spikes every 8MHz no matter what center frequency I'm at. These appear at a mid-gain setting. This happens with no antenna, with antenna, USB cable swapped, computer swapped, within SDR# and within SpectrumSpy. I've got a support request in to Airspy US to see what may be the problem... could be between the chair and the keyboard (ha!), but I don't think I'm doing anything different from before and the behavior is certainly different.
It sounds like RF Interference from a switchmode power adapter (SMPS) or a battery charger. The USB cable that I ordered with my R2 has two ferrite beads on it. Without a signal, I get some noise spikes when using my RTL-SDR dongle, but I don't let it bother me. The noise spikes are greatly reduced when a signal is present.



Quote:
Unfortunately, the Gray-Hoverman frame turned out to be too big for my rafters, so I switched over to the Mclapp. I used a 50' dual-shield RG6 coax line to run from my rafters to a splitter on my workbench. The splitter feeds 2x identical Dynex TVs. One was set to the internal digital tuner and the other was connected to a cheap MediaSonic digital tuner via HDMI input. The Dynex TVs have a "Channel Strength" reading and the MediaSonic has a "Quality" reading that I referenced.

It turns out that Channel Strength and Quality don't amount to much. In the rafters with the antenna pointed towards the northwest towers, I'd have "Good" (highest Dynex reading) and would see pixelation / drop-out. With the MediaSonic quality almost always about 50%, I'd see it drop-out down to 0% (signal loss).

Testing involved tuning to each 'main' channel for 2 minutes (that is, I didn't bother checking any of the multiplexed channels from a particular tower). One minute was spent watching the channel, looking for signal loss, pixelation, or audio issues, and another minute was spent bringing up the Strength/Quality reading and looking for the worst case value over that period of time.

I had both TVs going simultaneously. One displayed the MediaSonic decoding and the other the internal TV tuner. (I realize the splitter produces -3.5dB loss, but I assume a 50ft run and nothing else wouldn't impact things too much and an amplifier was not necessary. Tell me if I'm wrong.)
Using a splitter is a good way to make a real-time comparison, but you have a signal loss of about 5 dB for the splitter and coax combined. That's too much loss; your signals are already too weak. Try a moderate amount of amplification before splitting.
Quote:
Here's where things get... interesting. After about 15 minutes of watching KVRR/CH19 (I was sitting there having supper in my garage and decided to do an extended watch), I saw the MediaSonic drop out a few times. During this time the internal Dynex tuner did not miss a beat. The MediaSonic seemed to have trouble whereas the Dynex didn't. Hm. I watched a bit longer. I saw a couple of occasions where they both dropped out. Okay. That sucks but made sense. It seemed like the MediaSonic was a piece of junk where the Dynex dominated. I then changed the channel to KRDK/CH24. The reverse happened. The MediaSonic dominated and the Dynex internal tuner crapped out -- even so bad that I had to power cycle the television. On top of that, I saw both having issues -- but they were intermittent. It took ~15 minutes of watching for the thing to show up. I never caught these problems in my quick onesie/twosie test.

This means that testing could be a massive time-sink. The dropouts on KVRR/CH19 didn't happen right away. Things were great for about 15 minutes like I said and then they flopped out. To me this means that I could perform the antenna aiming and think that everything works -- only to come back later and see that things die for an inexplicable reason. Needless to say, this is a bit frustrating and deflating.
Most of your signals are marginal, which means they are very close to the digital cliff. It is true that all tuners are not equal, but it only takes a small difference in a signal to make a big difference at the cliff. Don't sweat these small differences; OTA signals are constantly changing in strength. You need more signal margin before dropout. Focus on making all your signals stronger as they come out of the antenna before any amplification.
Quote:
I do plan to test out the Gray-Hoverman still (I shortened the frame after I finished my testing for the night), but I'm kinda thinking that what I'm getting out of the 4Max is probably as good as it's going to get. Yeah, it drops out every once in a while, but this is probably just a nature of the OTA reception? Without adequate tools to monitor the various signal components (strength / quality / symbol quality / bit error rates / etc.), getting a good signal from my location is a bit of crap-shoot.
The tools that you already have are adequate. You can measure relative signal strength and SNR. The HDHR GUI will tell you how close you are to the critical 50% signal quality point, and tell you when you have 100% symbol quality which means no uncorrected errors.

Use those tools to improve signal strength and signal quality.
__________________
If you can not measure it, you can not improve it.
Lord Kelvin, 1883
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Last edited by rabbit73; 7-Sep-2020 at 5:26 PM.
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