QAM vs ATSC 1.0 and ATSC 3.0
In the United States, digital TV reaches homes in two main ways over radio-frequency signals: cable systems use QAM on coaxial cable, and broadcasters use the ATSC standards over the air. Many TV tuners handle both, which is why a TV menu usually asks whether you are connected to an antenna or to cable.
Side by side
| Cable QAM | ATSC 1.0 | ATSC 3.0 | |
|---|---|---|---|
| Used for | Digital cable | Over-the-air broadcast | Over-the-air broadcast |
| Modulation | 64-QAM or 256-QAM (ITU-T J.83 Annex B) | 8VSB | OFDM |
| Channel width | 6 MHz | 6 MHz | 6 MHz |
| Data per channel | about 26.97 or 38.81 Mbit/s | about 19.39 Mbit/s | Varies with how the station configures it |
| Transport | MPEG-2 transport stream | MPEG-2 transport stream | IP-based delivery |
Why cable and broadcast use different modulation
A cable network is a closed, controlled path. Signal levels are managed, there is little interference, and the operator can keep noise low, so it can use a dense constellation such as 256-QAM and fit more data into each channel. Over-the-air signals must cope with distance, terrain, buildings and reflections. ATSC 1.0's 8VSB trades capacity for robustness, carrying about half the payload of 256-QAM in the same 6 MHz.
ATSC 1.0
ATSC 1.0 is the digital broadcast standard US full-power stations switched to when analog broadcasting ended. It is described in ATSC document A/53 and uses 8VSB modulation at about 10.76 million symbols per second, giving a payload of about 19.39 Mbit/s per channel. A station can split that capacity among a main HD channel and several subchannels.
ATSC 3.0
ATSC 3.0, sometimes marketed as NextGen TV, is a newer broadcast standard. It uses OFDM modulation and delivers content over IP, which allows features such as more efficient video coding and flexible trade-offs between coverage and capacity. The FCC authorized it on a voluntary basis, and stations that adopt it must keep their primary programming available in ATSC 1.0, usually by simulcasting it on a partner station's signal. The FCC has proposed changing these transition rules, so the simulcast requirement may not last.
ATSC 3.0 is not backward compatible: an ATSC 1.0 or QAM tuner cannot receive it. Some ATSC 3.0 broadcasts are also encrypted with digital rights management (DRM). SiliconDust, which makes the HDHomeRun tuners, notes that DRM-protected ATSC 3.0 channels may not play on its devices, may limit viewing outside the home and in third-party apps, and may restrict recordings. Where a station offers an ATSC 1.0 version, that is the fallback.
What this means when buying a tuner
- For cable, the tuner must support QAM, and the channels must be unencrypted. See Clear QAM.
- For an antenna, an ATSC 1.0 tuner receives the stations most viewers watch today. ATSC 3.0 support is a bonus, subject to the DRM limits above.
- Tuner specifications often list both, for example "ATSC 1.0 & QAM64/256". See Choosing a TV Tuner.
Sources
- ATSC A/53, ATSC Digital Television Standard
- ATSC A/53 Part 2, RF/Transmission System Characteristics
- ITU-T Recommendation J.83
- FCC 17-158, Authorizing Permissive Use of the Next Generation TV Broadcast Television Standard
- eCFR: 47 CFR 73.3801, Full power television simulcasting during the ATSC 3.0 transition
- Federal Register: Authorizing Permissive Use of the Next Generation Broadcast Television Standard (proposed rules)
- SiliconDust: HDHomeRun FLEX 4K product page (ATSC 3.0 DRM notes)