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; some needed to rescan. All, however, ultimately were able to receive and parse the required information.
The bitrate used for PSIP was set at 40 kbps in consultation with the manufacturer. The rest went to video and audio. For the tests, audio was allocated 128 kbps per stereo pair, 64 kbps for dual mono and 384 kbps for 5.1 surround. All SD channels were configured for stereo, with only the HDs assigned 5.1.
Video compression was approached from a 1080i versus 720p perspective, since TV sets support both. Retransmission through cable and satellite systems and set-tops was also considered. Given that “broadcasters have no control over the final display format in the home,” KLCS and KJLA opted for the most “efficient encoding structure for final distribution over the air.”
The stations found that 720p had a better DMOS than 1080i had half the bitrate by comparing broadcasts of the “NewsHour.” A baseline was then established using 720p at a constant bitrate of 18 Mbps for two clips. The first was 12-minute video of Las Vegas ranging in encoding complexity from “moderate to difficult.” Average DMOS was 2.16. A second clip from the Ken Burns “National Parks” documentary yielded a 2.31.
Putting the two in a single 6 MHz channel, transmitting each at 9 Mbps, yielded a 3.86 and a 3.03 respectively. Statmuxing the two yielded a 3.07 and a 3.03 respectively with both encoders set at 11-2.5 Mbps.
Throwing in a third HD stream—a clip of underwater footage—brought the scores to 3.42 (Las Vegas), 4.11 (National Parks) and 3.6 (Underwater), with the encoders still set at 11-2.5 Mbps.
At the high end, the two stations packed one HD and seven SD signals into one channel, using “Vegas” and the “NewsHour” for different versions of the test. The HD signal encoding range remained 11-2.5 Mpbs, while the SD encoders were set at 2.5-0.85. In this scenario, the highest DMOS for SD was 3.16. “Vegas” yielded a 4.55, while the “NewsHour” scored 2.81.
All numbers were within the acceptable DMOS viewing range, the report noted. It concluded by noting that channel-sharing is technically feasible, that statmuxing is preferable and advanced video compression is desirable.
“Whether two entities can successfully combine their required program streams and business models can only be answered by the stations interested in sharing,” it said.
The full report is available here.
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