How Cable TV Tuners Work

A digital cable tuner turns the jumble of signals on a coaxial cable into one TV program. Whether it is built into a TV, sits on your network, or plugs into a computer, it goes through the same basic steps.

1. Selecting a channel

The cable into your home carries many channels at once, each in its own 6 MHz slot of frequencies. The industry channel plan (CTA-542) assigns those slots numbers, running up to channel 158 near 1 GHz on modern systems. The tuner's front end locks onto one 6 MHz slot and filters out the rest.

2. Demodulating the QAM signal

The demodulator recovers the stream of QAM symbols, deciding which constellation point each received symbol is closest to and turning it back into bits. A clean signal makes this easy. Noise, reflections and overloading make symbols harder to read. See What Is QAM?.

3. Correcting errors

Cable QAM includes forward error correction, so the tuner can repair many damaged bits on its own. When errors exceed what the coding can fix, you see pixelation, frozen frames or dropouts rather than the gradual snow of analog TV. That is why digital reception tends to be either fine or broken, with little in between. See Cable Signal Levels and Troubleshooting.

4. Picking out the program

The corrected data is an MPEG-2 transport stream that usually holds several programs. Tables in the stream list the programs by number. The tuner picks the program you asked for and passes its video and audio to the decoder. This is why a clear QAM channel may be labeled with a physical channel and a program number, such as 79-3.

5. Decryption, if needed

If the program is encrypted, the tuner needs the operator's conditional access system to unlock it: a set-top box, a digital transport adapter, or a CableCARD in a compatible device. A plain QAM tuner can only show unencrypted programs. See Cable Encryption and Clear QAM and CableCARD.

6. Decoding and display

Finally the video and audio are decoded and shown on screen. A network tuner may instead send the stream over your home network to an app on a TV, phone or computer, which does the decoding there.

Where channel names come from

Set-top boxes get the full channel lineup and program guide from the operator, often through out-of-band data that a retail clear QAM tuner does not use. A plain QAM tuner relies on whatever information is sent with each channel, and many systems send little. So after a scan you may see numbers without names, or channels in an unfamiliar order. See Channel Scanning and Virtual Channels.

Sources

• ITU-T Recommendation J.83
• ANSI/SCTE 07, Digital Transmission Standard for Cable Television
• CTA-542-D, Cable Television Channel Identification Plan
• ANSI/SCTE 65, Service Information Delivered Out-of-Band for Digital Cable Television