Debugging Odd ATSC Locking

[keeneye]

How much do antennas gain correlate with S/N figures for digital OTA channels?

Let's assume that with my current setup, I am able to pick up a digital station on channel 13 at 10dB S/N.

Can I really expect that with an antenna that has a 5dB gain over my current setup for channel 13, I'll then get the signal at 5dB stronger, so 15dB S/N?

Does the math always hold? Will increasing the antenna gain by X dB increase the received S/N ratio at the OTA receiver by X dB as well?

[stampeder]

Gain and S/N in a laboratory are completely inter-related and predictable. In the "real world" things get very complicated and the mathematical relationship cannot hold. Interference in its many evil forms is the chief cause, as well as signal scatter. The focus has shifted from improving antennas and amps to improving the tuners to suppress or reject such noise.

The ATSC 8VSB standard spells out some minimum performance guidelines for field reception, but now after six generations of ATSC chipsets the ratio of signal to noise required for a digital lock has drastically dropped. In the real world, DHCers who have upgraded their tuners have reported big improvements in reception.

S/N is only one of the constraints on ATSC: the major one is the ratio of good data packets to bad, which is the real measuring tool of ATSC "signal strength" meters.

I wish I knew of a chart that would display the delta in signal lock capabilities between the six different ATSC chipset versions. If anyone finds something like that, please post a link.

[blakew]

I can vouch for that. I don't know what gen. my built-in ATSC tuner is, but it needs the signal strength to be around 60%+ for a lock.

My dad's brandnew Samsung 260F can get a lock with only 30-40% signal strength (approximation because it uses bars instead of a percentile).

[Kevin S]

My one year old Hitachi only needs 20% to lock.

[stampeder]

Also there's a post in the OTA Knowledge Base & FAQ that deals with how ATSC measures "signal strength".

[El Gran Chico]

I hope the question isn't too dumb but I'm having some difficulty connecting the dots on why below is happening...

On some stations I might get 50-something percent signal and then picture is mostly stable. On other stations, I get say 60%, but the picture is either pixilated, or drops out entirely for moments at time.

Is this a question of not the "right" n%, or different stations requiring different levels to maintain the lock, or perhaps the signal strengh indicator is not so accurate?

[Frost55]

My expressvu 9200 needs around 60% to watch. I've watched my 42" Samsung plasma on 1/10 bars for hours with not a problem.Probably 5th gen in it.

[recneps77]

I think its a lack of guidelines/consistency for signal meters.
Some could be basing it on overall strength (i.e. 0 = NO signal, 100 = the most it can handle) others could be basing it on how strong the signal is on lock (0 = just enough to view, 100 = max).
This is pure speculation, but they all seem to be completely different, and I doubt a tv could actually display the picture receiving only 10% of the data (or 1 good packet to 9 bad ones) - that's a very basic representation of it, the actual 'signal strength' is a complicated mess of things.

[stampeder]

Originally Posted by recneps77

I think its a lack of guidelines/consistency for signal meters.

Yep, that is exactly the situation.

See the ATSC Tuner Signal Meters - What Do They Measure? post in the OTA Knowledge Base & FAQ:

[El Gran Chico]

Thanks everyone, but I'm talking about on a SINGLE piece of equipment, namely the ATSC tuner on my Sharp Aquos. Last weekend while experimenting with my antenna, I rotated in more southerly to see if I could pick up CHCH. I was able to get it with signal strength oscillating between 53 and 54. It was solid at that level - no dropouts, no pixilation. I thought it was finally in luck to get both Toronto and Hamilton stations from my location.

But then checking some Toronto Stations, Namely Global and SunTV - they had signal levels oscillating around 59/60, and it was very choppy!

Just trying to make sense of that?!?

[thenewdc]

In my case, CHCH-DT is a problem because of either co-channel or multipath. I often have to attenuate down to bare minimum just to get it to lock. Its the only channel I have this type of issue with. Even the weaker WNYO-DT is stable for me almost always.