Channel Sharing Found Feasible, Limited

LOS ANGELES Channel-sharing between two TV stations is feasible but limiting, according to a report from KLCS and KJLA. The two stations recently completed a channel-sharing pilot project, which they presented as a potential baseline assessment of sharing.

Due to limitations of both time and equipment, it was not possible to review every scenario sharing could entail, the report stated.

The stations tried out combinations of high- and standard-definition channels, but not mobile broadcasting with ATSC M/H. Mobile was mentioned only as a potential transmission format in the report's background summary on video encoding.

Every broadcast entity chooses its format: HD, SD, Mobile Handheld (M/H) and data, it said.

Channel-sharing is similar to multicasting in that more than one TV signal is transmitted in a 6 MHz channel. Several stations currently multicast as many as six or seven standard-definition signals in a single 6 MHz allocation, though the video quality tends to be diminished with each additional stream.

Channel-sharing throws in the complication of multicasting from dual facilities and coordinating the bandwidth payload of the shared content.

The channel-sharing setup used by KLCS and KJLA combined a statmux pool with eight, late-model encoders, a standalone PSIP generator, a broadcast-quality decoder, broadcast monitors, a quality-of-experience analyzer, stream analyzers and recorders, and several TV sets and set-top boxes. The encoders were fed by video servers, satellite receivers and fiber feeds from the two stations. A variety of distribution amplifiers and routers were used as well.

The report concluded that channel-sharing was possible on both the physical level, referring to transmission, and the virtual level, referring to whether or not receivers could decode the signals properly. Findings were based on empirical data.

On the virtual level, we found that all the TVs and tuners tested were able to receive and correctly parse all the required information, it said. This included virtual channel, both major and minor, ratings, audio configuration, codecs, program titles and descriptions.

On the physical or transmission level, the stations could transmit two 720p signals in a single channel, depending on the relative digital complexity' of the video content, and how the bandwidth is managed. Two stations can either use dynamic or fixed bitstream allocation. With dynamic allocation or statistical multiplexing, aka statmuxing the most action-heavy video would dominate its less visually frenetic neighbor. E.g., fast-action sports such as NASCAR racing would be more bit-hungry than a singer standing still on a stage in The Voice.

KLCS and KJLA managed to squeeze two SD streams in with two 720p HD streams, without major impact to the quality of experience of the overall material, and said that more were possible. Three HD signals were transmitted, as well as one HD and seven SD streams. Testing indicated these combinations were technically feasible, depending on what level of picture quality the sharing stations would accept.

The report did not specify what encoders were used, only that a more efficient use of MPEG-2 was found with the latest generation of encoders. And since these newer models also employ MPEG-4 H.264, it was also tested. MPEG-2 is the federally mandated broadcast TV transmission format that TV set manufacturers must accommodate. MPEG-4 is used more commonly for DVDs and Internet video.

We found that some consumer televisions decoded H.264 off-air, the report said. While this data is anecdotal, if the majority of future television sets can utilize H.264, greater bandwidth efficiency and/or quality of experience may be possible. H.264 offered a 10 to15 percent greater efficiency in bitrate utilization.

As for picture quality, the stations used the Differential Mean Opinion Score, or DMOS, based on an algorithm from the University of Texas used to approximate and predict the scores people would report if they saw two pictures side by side. The scores weighted as follows:

Zero is no defects;

0.01 to 1.0 is considered production quality;

1.0 to 7.5, broadcast quality;

7.5 to 8.5, annoying;

8.5 to 10, unwatchable;

10 and above, probably not aligned.

The stations measured DMOS with test equipment from VideoClarity while engineers watched the transmissions on professional HD monitors.

We found that our engineers, knowing what to look for and prompted by the spikes found by the VideoClarity, were able to see some faults when the DMOS score exceeded 6.0, the report said. However, these faults were only discovered with the aid of a seamless split or side-by-side comparison. Without the comparison, the picture appeared to be nearly perfect at 6.0 because the picture area where the faults were observed were generally soft or blurred in the original.

The testing also revealed the efficacy of newer encoders. Neither station knew its DMOS going in. KLCS scored between 7.0 and 8.5 with a peak of 9.0 with 10-yaer old encoders. KJLA scored between 6.0 and 7.0 with six-year-old encoders. The newer gear brought KLCS to a 1.5 and KJLA down to 0.69 with a high of 1.56.

Another crucial element of the testing involved PSIP Program and System Information Protocol. PSIP is the data used by receivers to find channels. The stations used a new version of the PSIP generator provided by an unidentified vendor.

KLCS transmits on Ch. 41 and appears on virtual Ch. 58 on receivers. KJLA transmits on Ch. 49 and shows up on Ch. 57.

The testing of the PSIP generator was designed to determine how televisions would react to physical 41 signaling both virtual 58 and 57, the report said.

Some sets found all information on Ch. 58.1 with no prompting; some had to be tuned to Ch. 41.1 to get the information;

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