# LA Channel 13 OTA Signal Strength Question...



## Lee Bombard (Dec 27, 2001)

With my roof top antenna I get pretty good signal on all my locals...except KCOP channel 13. I've adjusted the antenna to try and fine-tune the signal but seems I've gotten it just about as strong as I can. But here's the odd part - When I access the HD-Tivo's signal strength meter it shows that antenna input #1 is in at around 65. Not bad. However - input #2 is much lower and rises and falls a good deal. None of the other stations do this. They all seem to be about the same on both inputs...

Any thoughts as to why?

Thanks all,
Lee


----------



## mitchrc (Jun 12, 2000)

That kind of behavior usually indicates overmodulation. Are you using an amp. Remember signal strength is irrelevant with ATSC. As long as you have lock the picture will look the same whether it's at 35 or 98.

KCOP was transmitting at lower power until recently. They still pass no HD though. I imagine that will change with the launch of Fox's new MyTV though.


----------



## good.deals (Jan 28, 2006)

Question for you mitch;
I receive abc,ktla and fox in the evening but never before noon. Any idea why? (I'm using a normal antenna on my roof)

thx


----------



## mitchrc (Jun 12, 2000)

Interesting. I have no idea. Stations don't vary their transmission power other than for maintenance so it's not the stations.


----------



## Lee Bombard (Dec 27, 2001)

mitchrc said:


> That kind of behavior usually indicates overmodulation. Are you using an amp. Remember signal strength is irrelevant with ATSC. As long as you have lock the picture will look the same whether it's at 35 or 98.
> 
> KCOP was transmitting at lower power until recently. They still pass no HD though. I imagine that will change with the launch of Fox's new MyTV though.


No amp. Just straight thru...


----------



## dogdoctor (Feb 20, 2006)

mitchrc said:


> Interesting. I have no idea. Stations don't vary their transmission power other than for maintenance so it's not the stations.


Mind you that this is a topic way outside of my knowledge base...but how sure are you that they don't change transmission power. I have an indoor antenna (straight to the TV) in northern california and during the day I get HD but if I walk (or even the cat) in front of the antenna I lose the station lock where as at night or in particular on main events like the superbowl, I can hop, skip, or jump in front of the antenna and never lose a lock. I was atributing that to increase in signal output for primetime and major events. Any thoughts?


----------



## mitchrc (Jun 12, 2000)

I worked at local TV stations for 10 years, including MCO operations, which handles things like transmitter power.

I think what you are experiencing is the presence and absence of the RF interference and atmospheric effects that comes from the sun. You're receiver is right on the edge of signal lock and the presence of the sun and its effects are affecting your ability to get the signal.


----------



## TyroneShoes (Sep 6, 2004)

mitchrc said:


> ...Remember signal strength is irrelevant with ATSC. As long as you have lock the picture will look the same...


 You are half right (the second half). The picture will look the same, as long as you have lock. But the amount of signal strength is hardly irrelevant. If it were, folks in Honolulu could be picking up the NYC stations OTA. "As long as you have lock" is really just another way of saying "as long as you have enough signal strength". If you have less than 15 dB above noise or interference, you will not have signal lock, and you will have no picture. Having that margin of signal level above either the noise floor or interfering signals at the same frequency is exactly what you need to achieve signal lock. Without it, no reception, so unless not having any reception at all is irrelevant, having the proper amount of signal strengh is very relevant. And you need to be within that operational window. Too much signal equals no reception as well.

How DT differs from analog NTSC is what seems to cause people to make this incorrect assumption. If you have signal at a strength at least 15 dB above noise or interference (for DT), the PQ will be perfect, just as it exactly is if you are 30 or 40 dB above the noise floor. At any single point in time you either have enough signal to achieve that perfect picture or you have not enough signal and no picture, or you are on the ragged edge where the picture pops in and out and pixellates, which is an indicator that you have lock one second and lose it the next as you bob above and below that 15 dB line in the sand.

NTSC on the other hand needs a signal level of +43 dB above noise to have what is commonly thought of as a "good", noise-free picture, and anything less degrades gradually and correspondingly. The picture gets even better as you approach +50 dB above noise. Once you get to about 48-50 the picture may get incrementally better the higher you go, but probably not noticeably or significantly.

And NTSC tolerates multipath and mainfests it as ghosting artifacts, while ATSC does not tolerate it at all if it is strong enough to reduce that ratio of signal to interference below 15 dB. So signal strength is very directly tied to the ratio of signal to noise or interference, and is very relevant indeed for ATSC.


----------



## TyroneShoes (Sep 6, 2004)

mitchrc said:


> ...I think what you are experiencing is the presence and absence of the RF interference and atmospheric effects that comes from the sun. You're receiver is right on the edge of signal lock and the presence of the sun and its effects are affecting your ability to get the signal.


Here's a great example of what we're talking about, Mitch. Whether it's the sun or not, reception needs to have that S/N ratio of 15 dB. That is the minimum standard as determined by the ATSC, and the recommended guideline they distribute to tuner manufacturers. IOW, the tuner is designed to lock at that exact S/N ratio threshold.

In reception, the transmitted power may remain exactly the same, while at the receive antenna two things can happen: The received power can change slightly up or down due to atmospheric issues, and the noise floor can also move slightly up and down, either from weather-related issues, secondary interference issues, or changing multipath interference issues. So both the amount of received signal and the amount of noise in the environment can change dynamically from moment to moment, even though the station's EIRP does not normally change. If the ratio is typically only about 18 dB or so, and the received signal level briefly drops 2 dB at the same time that the noise floor briefly rises 2 dB, guess what happens. The S/N ratio drops to +14 and digital lock, along with the picture, vanishes.

So the bottom line is that you will have reception if you are above the 15 dB threshold of S/N, and you will not have it if you are below it. The trick is to increase that ratio drastically so that atmospheric and dynamically interfering issues can't close the gap, making the worst-case scenario still well above the 15 dB threshold. IOW, a fade margin that insures reliable reception over time, by insuring that that ratio remains high even under the worst of conditions. All of the typical ATSC reception tricks are geared exactly toward that goal.


----------



## n8. (Feb 26, 2006)

I'm not too technical so I dont understand half of what you guys are talking about but I do know I get all of my digital channels OTA after 9pm. Before that it's in and out.. before 1pm its not even there. Doesnt help when I have rabbit ears behind my tv; inside an oak-walled room that was built 30 years ago. I'll be hooking up an ota antenna and it should be pretty awesome as I am only 24 miles away from my towers (mt. wilson) with a pretty clear line of sight.


----------



## n8. (Feb 26, 2006)

dogdoctor said:


> Mind you that this is a topic way outside of my knowledge base...but how sure are you that they don't change transmission power. I have an indoor antenna (straight to the TV) in northern california and during the day I get HD but if I walk (or even the cat) in front of the antenna I lose the station lock where as at night or in particular on main events like the superbowl, I can hop, skip, or jump in front of the antenna and never lose a lock. I was atributing that to increase in signal output for primetime and major events. Any thoughts?


I just experienced what you stated here, I heard a car drive by and my signal cut out... came back when it drove by the point where my antenna was pointed. Maybe I should tilt it back a little and get an upward LOS to avoid interference from cars etc.


----------



## FlugPoP (Jan 7, 2004)

I'm 35 miles from Mnt Wilson, and I have always had issue with KCOP DT. Signal has always been poor for me. All my other LA stations come in great. I am using a radio shack uhf antenna. I know its not my tivo, my Sony HD100 had issues with it also.


----------



## n8. (Feb 26, 2006)

FlugPoP said:


> I'm 35 miles from Mnt Wilson, and I have always had issue with KCOP DT. Signal has always been poor for me. All my other LA stations come in great. I am using a radio shack uhf antenna. I know its not my tivo, my Sony HD100 had issues with it also.


Signal strength? Indoor or outdoor antenna?


----------



## FlugPoP (Jan 7, 2004)

n8. said:


> Signal strength? Indoor or outdoor antenna?


Tuner 1 all over the place 20's to 30's
Tuner 2 better 40's to 50's

Outdoor antenna


----------



## kepper (Nov 28, 2003)

FlugPoP said:


> I'm 35 miles from Mnt Wilson, and I have always had issue with KCOP DT. Signal has always been poor for me. All my other LA stations come in great. I am using a radio shack uhf antenna. I know its not my tivo, my Sony HD100 had issues with it also.


I'm about 43 miles from Mt. Wilson (in OC) and I consistently get signal strengths of 80-85 on KCOP-DT. I just checked it a few minutes ago to be sure nothing had changed.

Kevin


----------



## marcello696 (Jun 18, 2004)

KCOP has always been marginal for me as well (35% to 45%) but quite honestly UPN has never had anything I've cared to watch so really I've never cared 

With the CW network coming everything will be on KTLA anyways.


----------



## Lee Bombard (Dec 27, 2001)

FlugPoP said:


> Tuner 1 all over the place 20's to 30's
> Tuner 2 better 40's to 50's
> 
> Outdoor antenna


This is exactly what is happening on mine... I don't understand why if we are simply splitting the signal into the back if the HDTivo that it should not read the same signal strength on both tuners?


----------

