Quick Takeaways
- SNR is the ratio (in dB) between the desired RF carrier power and the noise floor in the same bandwidth
- In digital CATV (QAM256), DOCSIS requires minimum SNR of ~33–35 dB for reliable 256-QAM, ~30 dB for 64-QAM
- In analog NTSC video, every ~6 dB drop in SNR roughly doubles the visible “snow” on screen
- Real-world culprits: ingress from LTE/citizen-band radios, impulse noise from appliances, amplifier thermal noise, and micro-reflections
Think of SNR the way you think of trying to hear a conversation in a crowded bar. The “signal” is your friend’s voice; the “noise” is the music, clinking glasses, and everyone else talking. If the voice is only slightly louder than the background racket, you catch every third word and fill in the rest (or just nod and pretend). That’s exactly what happens when your cable box receives a weak 256-QAM channel with an SNR hovering around 31 dB—bit errors creep in, the picture stutters, or the modem starts correcting errors.
In a modern HFC (hybrid fiber-coax) plant, noise mostly accumulates in the return path (5–42 MHz) from your house back to the node) and from cascaded amplifiers in the forward path (54–1002+ MHz). A single loose F-connector acting as an antenna can let in 700 MHz LTE signals and tank your upstream SNR from 38 dB down to the low 20s—suddenly your Zoom call sounds like a 1990s dial-up handshake. On the downstream side, if the node’s optical link is running hot or the coax has water damage, thermal noise and distortions rise, causing frozen picture or intermittent data streams.
Bottom line: technicians live and die by SNR numbers on their meters. Keep it high with good shielding, proper levels, and clean connections.