On the Road to ATSC 3.0

Yes, we’re still alive!

Since CES we’ve been on the road to ATSC 3.0, heading out to Phoenix to sample the Pearl model market, Las Vegas for NAB, and made several diversions to Santa Barbara to work with NPG’s broadcast.

Depending on who you ask, ATSC 3.0 will be the greatest revolution in television since digital television (i.e. ATSC 1.0), or – being incompatible with the current ATSC 1.0 broadcast standard and embracing web and streaming technologies perhaps a little too much – its  greatest boondoggle.

ATSC 3.0 has a great many features for broadcasters, such as the opportunity for targeted advertising and content protection.  But what does it have for the average consumer?

In all probability, the greatest value to your everyday TV viewer is better picture. And we’re not talking HDR or 4K – both of which are enabled in the ATSC 3.0 specifications. Instead we’re talking about rather mundane 720p and 1080p, but received more reliably and with fewer encoding artifacts.

Better Codecs

A significant contributor to the improved picture quality is the H.265 codec. To put this in perspective, take a step back and look at ATSC 1.0. It was deployed a bit more than 20 years ago and its video encoding is based on MPEG-2. In the 20 years which have elapsed video encoding has advanced phenomenally (or, strictly speaking, the amount of memory and computational power that can be thrown at encoding and decoding has increased phenomenally). Where an ATSC 1.0 broadcast in the late ’90s and early ’00s needed a good fraction of the 19.4 Mbits available to be artifact-free, today’s ATSC 1.0 can squeeze a couple HD and few SD broadcasts into that same bandwidth – still using MPEG-2, but with enhanced encoding algorithms. ATSC 3.0 skips over the H.264 codec, which introduced in the early ’00s and used in Blu-ray and some digital cable systems. H.265 provides an enhanced encoding toolset allowing video to be compressed further than with MPEG-2 but with the same visual quality. A 1080p service can be encoded in 3-4 Mbit/s.

OFDM Modulation

A second enhancement which many OTA viewers will appreciate is the improved robustness of OFDM modulation. ATSC 1.0 uses 8-VSB, which is notoriously prone to multipath interference. Put simply, an ATSC 1.0 signal can be reflected by buildings, trees, cats wandering on rooftops, and other obstructions. What the tuner in your TV sees is the original signal and one or more time-delayed reflections. Demodulators need to try to make some sense of the mash of signals, and when they can’t you get macroblocked (or pixelated) video.  This was a step back from good old analog television. While a snowy (weak) or ghosting (multipath reflected) analog broadcast was mildly annoying, digital television had a more knife-edge quality: either you got a clean picture or it devolved into blocky mess.

ATSC 3.0 utilizes the OFDM modulation scheme, which is rather more resilient to multipath interference. There is some argument in the field that over the past 20+ years demodulator designers have done so much work in overcoming 8-VSB’s deficiencies that it is now almost on par with OFDM. It is certainly the case that modern demodulators recover signals that older demodulators couldn’t make heads or tails of. So we’ll see how much actual improvement there is. But its reassuring that ATSC 3.0 is starting off with a modulation scheme that takes into account real-world reception issues.

(And you’re probably wondering why ATSC 1.0 didn’t use OFDM to begin with. Rumor has it that it had to do with coexistence of analog and digital broadcasts during the transition period. It was imperative that digital broadcasts not interfere with the existing analog broadcasts. This could be accomplished with 8-VSB, but more challenging with OFDM which required a higher transmission power. If anyone can verify this please drop me a note. Of course today analog broadcasts are a relic of the past so ATSC 3.0 is free to use OFDM.)

Single-Frequency Networks

A side-benefit of  OFDM is the ability to deploy single-frequency networks, or SFNs. SFNs shine where broadcasters need to use repeaters or translators today. Repeaters extend the reach of a broadcast to areas that otherwise cannot receive the primary transmission. For example, here in the SF Bay Area most primary transmitters sit atop Sutro Tower in San Francisco. Those in the South Bay may be challenged to receive these signals either because of distance, or because of shadows cast by the foothills. Repeaters in the South Bay retransmit the broadcast, though on a different frequency. For example the primary ABC broadcast is on RF 7 from Sutro Tower in San Francisco. A translator in the hills above Fremont provides this broadcast on RF 35 for South Bay viewers.

The challenge with ATSC 1.0 translators is that they cannot broadcast on the same frequency as the main broadcast. If both did broadcast on the same frequency they would destructively interfere with each other. So ATSC 1.0 requires multiple channels, each with their own license, and competing for the constantly shrinking spectrum dedicated to over-the-air television. Not to mention that viewers will need to know which broadcast is best for them, something that is yet another source of frustration. (Keep in mind that viewers see the virtual channel number, which has nothing to do with the actual broadcast frequency. Is that 7-1 from RF 7 on Sutro Tower? From RF 35 above Fremont? How is the casual viewer to know?)

As its name implies, single-frequency networks all operate on a single frequency and transmitters can be configured so that they do not destructively interfere with each other.  So, to continue using our ABC example, both the Sutro Tower and Fremont transmitters could use RF 7. And to further improve coverage,  additional transmitters could be set up at select locations around the SF Bay Area. All on RF 7. The casual user still won’t know what RF channel they’re receiving 7-1 on (whether 7, 35, or 22), but since the SFN is effectively a single transmission there’s no need for the user to know.

Multiple Modulation and Coding Schemes in a Single Transmission

A third enhancement that may further help viewers receive signals is the ability to transmit services with different levels of robustness. Even with OFDM, some broadcast regions (or some areas in a broadcast region) will prove more challenging than others. ATSC 3.0 provides the option of tailoring the broadcast for those difficult areas. ATSC 1.0 in contrast is a one-size-fits-all sort of scheme. There’s one way of transmitting the bits.  If you’re in an area where the signal propagates well, great. If you’re in an area with a lot of buildings reflecting and/or attenuating the signal, well you may be out of luck. With ATSC 3.0 broadcasters can opt to use more robust modulation and coding schemes so even viewers in difficult areas can reliably receive a broadcast. The more robust modulation and coding schemes tend to come hand in hand with lower effective bitrates, which in turn means less channels per frequency or lower resolution. Viewers who are in areas with better reception won’t necessarily be thrilled by this. ATSC 3.0 has an answer for this as well in the guise of SHVC- a base layer can be broadcast with a robust modulation and coding, while an enhancement layer is broadcast with a less robust modulation and coding. The enhancement layer basically adds additional resolution, frame rate, or other attributes – for those viewers that can receive it. For example, consider a station that has a 1080p service. It may choose to broadcast a SD base layer delivered with a very robust modulation and coding, so everyone in its service area can watch something. It can broadcast the 1080p enhancement layer using a less robust (but higher effective bitrate) modulation and coding for those viewers in less challenging areas.

Time will tell whether, and to what extent, broadcasters are willing to sacrifice some amount of bitrate to improved robustness. There will always be the temptation to cram just one more diginet into the broadcast and rake in a little more advertising revenue. But at least ATSC 3.0 provides the knobs allowing broadcasters to juggle their competing priorities.

There are many more features in ATSC 3.0, for example 4K broadcasts, high-dynamic range (HDR) video, and most intriguingly the ability to deliver HTML5 apps and data to receivers. The latter has the potential to turn what we conceive of as a “channel” on its head. Today a channel is a non-interactive set of audio and video generated by a station and pushed to your TV. In the future a “channel” may be an application that serves as a portal to a the audio, video, and data that a station may make available, all selected and presented in a way you, the viewer, want to consume it. We’ll get back to these in a future post.

 

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