CBS, it's time for a harsh reality involving HDTV quality - AVS Forum | Home Theater Discussions And Reviews
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Old 10-24-2008, 07:00 AM - Thread Starter
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Over the past ten years, you've taken a "quality over quantity" approach that has meant no subchannels have been deployed to your high-definition O&O TV stations. While I agree with the policy, this commitment may be tested by your commitment towards mobile TV. Judging by the CBS Mobile channel on the Verizon and AT&T wireless systems, you're quite interested in mobile TV.

Sometime next year, a standard currently in draft form called ATSC-M/H will be finalized. There has been a market of people who have purchased and used small, battery-operated, portable TVs designed to pick up over-the-air analog channels; these TVs could be used in things like a power outage. ATSC-M/H will be the new standard in the post-analog-shutoff world for easy mobile TV reception for a new set of portable TVs, especially ones placed into cell phones.

M/H devices would pick up a 4x3 SD simulcast of HD programming (tech specs: a quarter-rate CIF (352x288 pixel) H.264 Level 1.3 video stream encoded at around 300 kbps). In the future, an additional 600 kbps stream could be used for non-live streamed content, including pay TV. It wouldn't be enough bandwidth for live streaming of 480p material to backseat TVs, as has been suggested, unless very heavy artifacting during fast motion is acceptable to viewers, but it could technically be done.

But the CIF M/H simulcast in of itself would be too lucrative for stations across America to pass up: they could earn a combined $2 billion annually in new advertising revenue. Once the standard is finalized, any commercial TV station that doesn't quickly deploy ATSC-M/H will be at a competitive disadvantage versus the other stations that do deploy it.

ATSC-M/H is going to use up a lot of ATSC bitrate. I've noticed that at current peak HD bitrate, there's still about 1.5 Mbps of null packets sent along; some of this can be used for the M/H stream, but the rest is going to have to come from HD video bitrate. How much? That depends on how much bitrate goes into the M/H stream. In short, the CIF-resolution simulcast alone would use about 2.3-2.4 Mbps of the ATSC stream*. How I came to that conclusion is potentially complicated to understand, so I've cordoned it off inbetween the two green lines below.


Here's a page which discusses a recent prototype of the system. In that case, a 4.5 Mbps M/H stream was divided into two streams: a 300 kbps "quarter-rate" stream, and a 600 kbps "half-rate" stream. Each stream totalled 1.2 Mbps prior to ~1.1 Mbps (per stream) of "turbo coding", a method of error correction that allows for moving vehicles to pick up the signal. So by my estimation, that means each of these M/H streams used up a little less than 2.3 Mbps, but for the following demonstration-based purpose, just to be safe, I'll round it up to 2.4 Mbps in case guide data, audio encoding, and M/H null packets use up more than that.

19.39 Mbps total ATSC throughput
- 2.4 Mbps ATSC-M/H stream
- .576 Mbps HDTV audio (384 kbps 5.1 audio + 192 kbps stereo SAP)
- .414 Mbps for PSIP, digital TV Guide data delivery (if applicable), and null packets
= 16 Mbps maximum for HDTV video


By my calculations, the maximum HDTV video bitrate with ATSC M/H is limited to 16, possibly 16.1, Mbps. By now, Harris NetVX encoders have probably been ordered and installed at all CBS O&O stations, but I'm not too confident that the encoders can deliver proper picture quality at that bitrate. I could be wrong, and if I am, then great. How about this: Have a trial period simulating ATSC M/H conditions (with 16.1 Mbps HD peak video bitrate) for a Sunday football game (since that would test the encoder the most of all CBS shows) at WWJ Detroit, where Ken H, DrDon, and homcom can monitor the over-the-air signal and comment on any changes in picture quality. (Hopefully, they'll be told when the test will take place, so they can schedule viewing if need be) If the NetVX can't deliver the goods, you'll have a dilemma: Accept mediocre HDTV picture quality, or purchase new encoders at every owned-and-operated station?

As for a new encoder: If Harris releases a new NetVX HD encoder next year, that includes stuff like motion-compensated temporal filtering, and multi-pass technology, then great. But it's time to start preparing now in case that doesn't happen. I suggest performing a "bake-off" with other encoder manufacturers, including Harmonic, Tandberg, and JVC, to see if any of their encoders do well at 16.1 Mbps. If one of them can live up to the task better than Harris will (and from the reports of this forum, Harmonic is probably up to it), then if you mean what you say with quality over quantity, you'll fork over the cash.

As for other CBS affiliates, I highly suggest discussing with them M/H, how much HD bandwidth it will use up, and how to budget space appropriately, including dealing with subchannels.

* If the actual M/H bitrate is a bit less, feel free to place the bitrate into HDTV video bitrate up to 16.1 Mbps, then any extra bits into ATSC null packets.
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