In what may be the first broadcast of its kind, KXII, a digital television station in Sherman, Texas, is simultaneously transmitting two HD and one SD (standard definition) programs within a single ATSC signal stream. The station, which serves viewers in both Texas and Oklahoma, is transmitting a CBS 1080i stream, a Fox 720p signal, and Fox’s MyNetworkTV in SD at 480i.

Aldo Cugnini
Analyst

How does this work?  The multi-channel HD delivery capability is made possible through the combined use of high-quality encoding, statistical multiplexing and dynamic service management.   Statistical multiplexing, or “statmux” is a technology that dynamically allocates channel capacity to multiple simultaneous program streams within the same transport.  In other words, you “rob Peter to pay Paul” on a moment-by-moment basis, depending on which channel needs more bits. Theoretically, the priority of the different channels can also be varied according to a schedule, so for instance a high-ratings sports program could, in principle, get more bits - and therefore have higher quality.  Statmux is routinely used for satellite video transmission and is now common on digital cable systems as well.

According to KXII chief engineer Randy Wells, the station allocates 5Mbps to 15Mbps for the CBS stream, 4Mbps to 14Mbps for the Fox stream, and between 1Mbps and 6Mbps for the SD stream. (It’s not apparent how contractual obligations may impact these figures.)  A big factor in the success of this scheme is the ultimate quality of the video.  Because of the dynamically varying bitrate allocation, it’s not as simple as the constant bitrate case, wherein MPEG-2 at 10Mbps and 1920i generally looks bad.  While reports at simpler multicast sites - perhaps without a good statmux - give varying reviews for video quality, at KXII, Wells says, “It looks good.  I was watching “CSI Miami” and it just looks beautiful.”

KXII is using two MPEG-2 encoders for the high definition services and a third encoder for standard definition.  According to KXII vice president and general manager Rick Dean, “As we were planning our digital upgrade, the ability to support multiple HD channels within a single ATSC stream was an absolute necessity.  This breakthrough implementation of two HD services plus one SD channel in 19.4 Mbps dramatically enhances our business proposition and revenue opportunities as consumer demand for HD continues to grow.”

ATSC requires the use of MPEG-2 compression, a situation that will not change for many years.  In contrast, satellite service providers are transitioning to MPEG-4/AVC technology, as it affords a quality similar to MPEG-2 with about 50% of the bandwidth.  MPEG-4/AVC has also become the key solution for IPTV (TV over telco) services.  In response, ATSC is now considering supporting MPEG-4/AVC and VC-1 within the ATSC standard by means of a compatible transmission mode.  Of course, any currently deployed ATSC receiver will not be able to display an MPEG-4 encoded broadcast.

Meanwhile, KALB-TV in Alexandria, LA, has announced a similar service.  Starting in January, the DTV station will simulcast NBC-HD and CBS-HD - two network affiliations on the same station.

What does this all mean for TV viewers?   For one, expect more DTV broadcasters to experiment with statmux technology as a way of upping the number of channels they can transmit.  Unfortunately, as sophisticated as these systems are, we are pretty much at the limit of what MPEG-2 can deliver for HD pictures.  With the bottom line driving the utilization of digital capacity, expect video quality over terrestrial services to become challenged.  At the same time, the broadcasters are in no mood to lose viewers to cable and satellite, even with cable penetration currently at a 16-year low.