NOTE: In this article, I'm talking about signals at the TV, not at the TV antenna
or as part of an MATV system
There are two main concepts and three measurements to understand when you're looking at digital TV signals:
- Signal Power, Strength or Level
- Signal Quality: Noise & Interference
- Bit Error Ratio (BER) - Pre-BER & Post-BER
- Modulation Error Ratio (MER)
1. Signal Power/Strength/Level
Note: dBm and dBμV are interchangeable, just add 108.7 to get from dBm to dBμV (for 75Ω systems)
i.e. 0dBm = 108.7dBμV, -50dBm = 58.7dBμV, -70dBm = 38.7dBμV
This is the least important measurement for digital TV signals.
Most TVs will work from around 40dBμV, so don't worry too much about this value.
It is possible to have too much power. Most people see pixelation and think "signal is too low", even when the signal coming in is hot enough to turn their TV tuners into smoking pieces of silicon and copper! We had a customer years ago, living almost in the shadow of Mount Dandenong (the main TV transmitter for Melbourne) who we couldn't manage to convince that he had far too much signal, and that's why he was getting pixelation. He really wanted us to sell him a giant antenna, designed for people living hundreds of kilometres from the tower, and we were worried he'd set his house on fire with the power of the signal!
2. Signal Quality: Noise & Interference
In an analogue system, noise/interference generally had a progressive effect. As you added more noise, the signal on a screen became worse gradually. It also means that if you had a momentary spike of higher noise, the signal would resume straight away with only that moment of bad reception. If you had a customer in a really bad spot for reception, they might have had to settle for "good enough" signal.
Digital signals deal with noise & interference differently, of course, and it's certainly possible to have bad enough quality that the TV can't even tune it in, regardless of how powerful the signal is. DVB-T systems have error correction built-in (called Forward Error Correction or FEC) that helps overcome some noise, but only up to a point.
As the number of errors in the signal increase, the picture quality stays consistently good (unlike analogue), up until the digital threshold, also called the "Cliff Edge". Stray beyond the cliff edge, and you'll end up in the water. Not really, but your signals will pixelate or you'll get the dreaded "No signal" message.
There are two measurements that help us quantify the errors & the effect on the signal.
The first is Bit Error Ratio (BER), and it more or less gives you the number of error bits versus the number of correct bits.
The second is Modulation Error Ratio (MER), and the best way to think of this one is the margin you have before the signal drops off the cliff.
2.1. Bit Error Ratio (BER) - Pre-BER & Post-BER
Note: we're talking about ratios here, so 2x10^-5 is better than 2x10^-4, and 2x10^-2 is worse.
A TV signal from an antenna goes through a couple of stages before you see it on your screen. After the demodulation of the signal, it goes through the first stage of error correction, called Viterbi. If you measure the BER before the Viterbi correction, that's Pre-BER and a pretty good picture of the raw errors in the signal. If you measure it after the Viterbi correction, then you've got Post-BER.
These can also be called bBER & aBER, or CBER & VBER, confusingly. Depends on who made your signal meter or TV system, and in which country.
There's also a secondary stage of error correction, called Reed Solomon, but we're not interested in that one here. Why? Because it's kind of like the final stage of polishing on a table surface, and if the surface is rough or full of holes, the polishing doesn't make a lot of difference.
Oh, and those numbers? 2x10^4? That's showing two error bits out of 10,000 bits transmitted. In practice, that kind of number is like saying there's just one error visible every thirty minutes.
2.2. Modulation Error Ratio (MER)
This is possibly the most important measurement in a digital TV system because it gives you a real measure of the quality of the system. The way I think about MER vs. BER is that MER gives you a picture of the quality of the system BEFORE the errors start being counted by BER. An error may be recoverable by error correction methods, but some modulation impairments will make that impossible. MER looks at those impairments.
It's possible to have no errors in a signal and still have lousy MER. The opposite is also true - it's possible to have excellent MER, but lousy BER. MER isn't so good at measuring transients (fast-moving problems), so measuring BER still has value. Loose connections or intermittent RF noise is more likely to show up in BER, whereas mast-head amplifier issues might be seen with MER, for example.