Cable Signal Levels and Troubleshooting
Digital cable either works or it does not. When the signal is good, the picture is perfect. When it slips below what the tuner can handle, you get pixelation, frozen frames, stuttering audio or missing channels. Knowing what the signal readings mean helps you find the cause.
The readings that matter
Power level is how strong the signal is, measured in dBmV (decibels relative to one millivolt across 75 ohms). Too little and the tuner cannot lock on. Too much and it can overload, which causes errors too.
Signal-to-noise ratio (SNR), or the closely related MER (modulation error ratio), compares the wanted signal to the background noise, in dB. Higher is better. Dense constellations like 256-QAM need a higher SNR than 64-QAM, because their constellation points sit closer together.
Error counts show how many errors the tuner corrected and how many it could not. Uncorrectable errors are what you see on screen.
Typical ranges
Cable modems receive the same kind of QAM signal as TV tuners, and modem makers publish guidance on acceptable levels. ARRIS, for example, lists these downstream figures for its cable modems:
| Reading | Guidance |
|---|---|
| Downstream power | −15 to +15 dBmV |
| SNR, 64-QAM | 23.5 dB or better |
| SNR, 256-QAM, power −6 to +15 dBmV | 30 dB or better |
| SNR, 256-QAM, power −15 to −6 dBmV | 33 dB or better |
These are modem figures. TV tuners differ, and their on-screen meters often use their own scales, so treat them as a rough guide. A reading comfortably inside the range leaves margin for changes in weather, temperature and wiring.
Reading an HDHomeRun signal screen
SiliconDust's HDHomeRun tuners show three percentages. Its documentation describes them this way:
- Signal Strength is the raw power level. It does not say whether the signal is usable.
- Signal Quality is how clearly the data can be read, similar to SNR. This is the one to maximize.
- Symbol Quality is the share of data received correctly or corrected. 100 percent means no uncorrected errors.
Fixing common problems
- Check every connection. Loose, corroded or poorly crimped F-connectors are a common cause of noise and dropouts. Tighten them and replace damaged cables.
- Count the splits. Each splitter cuts the signal. Remove splitters you do not need and cap unused splitter ports. See Coax, Splitters and Amplifiers.
- Be careful with amplifiers. An amplifier can make up for splitter loss, but it also amplifies noise and adds some of its own, and too much gain can overload the tuner. Use one only if power is actually low.
- Test at the entry point. Connect the tuner where the cable enters your home. If the problem disappears, it is in your inside wiring. If not, it may be on the provider's side.
- Look at several channels. Problems on only the highest channels can point to cable or splitter loss, which rises with frequency.
- Call your provider if levels are poor at the entry point. Problems on the line up to your home are generally theirs to fix.
Sources
- ARRIS consumer support: Cable Signal Levels
- HDHomeRun: Signal Strength and Quality troubleshooting
- Primex coaxial splitter specifications