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Originally featured on BroadcastEngineering.com
Apr 11

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4/11/2010 7:35 PM  RssIcon

Tore Nordahl sent me a copy of his latest report, "Fighting for M/H & 3D dollars: Why 720p60 may rise again." The entire paper is available at his website, www.coax.tv.

I can summarize his paper’s premise: If your station is transmitting in 1080i60, you may want to consider moving to 720p60 to maximize available bandwidth for new services such as M/H.

Nordahl states that comparing format-to-format, 720p60 gives viewers much, if not most of what they need in image quality. That's especially the case for 37in sets or smaller. Some would agree that 720p60 does better for high-motion images and 1080i60 does well on large screens for less active images. However, let's look at the sizes of HDTV sets American consumers are buying.

Last year, 30 million HDTV sets sold, and two-thirds, 20 million, had 37in or smaller displays. Of that 20 million, half had displays smaller than 32in.

Three years ago, the average HDTV set had a screen size of around 47in. By the end of 2010, the most common TV set screen may be only 37in, according to Nordahl.

So, the argument that the public is buying huge TV sets seems incorrect. This sets the stage for rethinking what type of OTA signal needed to drive those TV sets.

It’s a progressive world
Clearly progressive displays rule the computer and non-TV display world. And—as shown in any Sunday sales ad—progressive, especially the newer 1080p60 displays, claims to be the "format of choice." But the issue, says Nordahl, is that broadcasters are facing new challenges and need additional bandwidth to meet them.

This is where the progressive format of 720p60 comes to favor. According to Nordahl, a 1080i60 interlaced signal requires about 16Mb/s of the available 19.4Mb/s, whereas a 720p signal can be encoded using about 12Mb/s.

The advantage of using 720p60 transmission is that the station gains about 4Mb/s for other services. Using his calculations, a 1080i60 station would have about 2Mb/s for M/H and other channels after allowing for PSIP and system data. A 720p60 station would have more than 6Mb/s for M/H and SD services.

Is it time to choose?
The Nordahl report suggest that after all the FEC, PISP, and overhead is included in the ATSC signal, a 1080i60 station has sufficient bandwidth to provide one HD signal at 16Mb/s and one SD signal leaving little room for M/H. Of course, the station could drop the SD channel and transmit M/H, but that means losing the second video broadcast channel.

On the other hand, a 720p60 station can transmit an HD signal at about 12Mb/s and an SD signal at 2.5Mb/s. This configuration leaves abut 3.5Mb/s for M/H transmission.

All these arguments may unsettle some golden eyes out there. Many engineers and even vendors at this year's NAB think 1080p60 is the future. Certainly there are plenty of vendors claiming 1080p60 upgrades and capability available on this show's exhibit floor.

If so, then sticking with a 1080i60 infrastructure makes some sense because an upgrade to 1080p60 might be easier than changing from 720p60 to 1080p60 infrastructure.

Nordahl argues that moving to a 720p60 OTA format makes sense for several reasons. Encoding is easy and efficient. Additional bandwidth for M/H services result. Even ENG may become easier because at least one company, JVC, offers a 19Mb/s encoder that makes backhaul easy and perhaps less expensive.

Does this mean broadcasters should forgo any drive to an ultimate 1080p60 transmission goal? Nordahl thinks so. Stations will need to decide, but he makes a good case to use 720p60 if you are interested in building out new M/H services.

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