INDIAN WELLS, CALIF. — The first thing that mobile DTV had to do was not destroy the legacy delivery stream, said Thomas Edwards, vice president of engineering and development for Fox Network Engineering and Operations, at the Hollywood Post Alliance Technology Retreat. Fox is at the forefront of developing and delivering broadcast television signals to mobile devices through its involvement in the Mobile Content Venture and Dyle-branded content.
Edwards said 15 Fox-owned stations are transmitting mobile DTV signals on channels between 7 and 44. Mobile content data is broadcast as IP packets encapsulated in the MPEG transport stream, with an unreferenced packet ID, typically, 0x1FF6. The video is compressed using H.264; with HE AAC-encoded audio, sent in real-time protocol via a multicast user-data protocol. Additional coding—about four times more than for legacy receivers—is necessary for the smaller mobile antennas, Edwards said. This data stream is ignored by ordinary TV sets.
The program stream is transmitted during specific time slots. Receivers, in turn, save power by turning on only during the time slots required for service. Mobile reception is theoretically possible at speeds of up to 200 miles per hour, but this has yet to be thoroughly tested.
As for bandwidth usage, the mobile DTV payload is measured in NoGs, or number of groups of mobile data that fit into 12.1 millisecond time slots. Fox is getting a 240 kbps signal in around 917 kbps using HQQQ coding in one NoG. A NoG=1 scheme will accommodate a kids’ cartoon at a resolution of 416x240 encoded at H.264 baseline. A NoG=2 scheme will do live video at 416x240 using H.264 baseline, and a NoG=3 scheme will do “great live video” at 640x368 using H.264 main profile, he said.
“Most Fox O&O’s are transmitting at NoG=4—number of groups is four per M/H sub-frame—using HQQQ coding, which uses 3.6 Mbps of data from the 19.39 Mbps legacy 8VSB signal, and provides 970 kbps of usable payload for the mobile services,” Edwards said, explaining further.
Larcan is providing the H.264 encoder, the mobile exciter, mobile mux, GPS receiver, the electronic service guide an the Ethernet switch for Fox’s mobile TV endeavor. Triveni software is being used for transport-stream monitoring. The hardware end of the operation requires just a few rack units (picture right).
Encryption is possible with the transmission scheme, Edwards said. Dyle, which includes programming from NBC, Telemundo, Univision and Ion’s Qubo in addition to Fox, currently is available via a free decryption app used with peripheral hardware, or in limited markets on the Samsung Galaxy Lightray 4G smartphone. Network executives have said they would evaluate the business model as to whether or not to continue providing a free service, or charge for it.
The Dyle app requires an initial registration, which provides real viewership reports with anonymized demographics. It also decrypts individual stations that are not part of Dyle, but that are associated with MCV, and plays other nonencrypted stations, such as PBS member stations. The station list is pre-populated depending on the market.
Fox’s Dyle lab checks receivers for sensitivity, multipath and echo rejections, co-channel and adjacent-channel interference. Over-the-air signal monitoring is done with gear from Decontis and Acrodyne.
Over-the-air monitoring is done with gear from Decontis and Acrodyne and checks the RF and MPEG transport stream, the A/53 and A/153 program and system information; mobile IP stream validity and decoded closed captions.
Off-air recording is not yet available for mobile DTV, Edwards said. Mobile data packets have non-systematic RS encoding in the exciter that a A/53 receiver can’t decode, so a professional-grade A/153 receiver is needed. Right now, there is no such thing.
“If you record off-the-air with an A/53 receiver, you can put it into your transport stream player and play it through an exciter,” Edwards said of the legacy digital television transmission standard. “You can’t do this with an A/153 [mobile] signal. Recordings must be made right out of the mux, not over-the-air.”
When no one at a station can watch the signal on a receiver, they can’t know when snafus arise, such as when the signal goes off the air, or when lip sync is off or loudness is wrong—even though there is no dialnorm in AAC.
There is also a glitch with the asynchronous serial interface—the streaming data format that often carries the MPEG transport stream. It doesn’t run precisely at the ATSC data rate, Edwards said. There’s roughly a 54 bits-per-second disparity, causing null packets to be either added or dropped to the transport stream, which in turn “blows up” mobile DTV.
As for loudness, Fox ran some tests to find a metric preferred by listeners. Using five clips of network material at -20 and -14 LKFS, Fox tested listeners in an office environment, using a variety of speakers, earbuds and headphones, with iPhones an iPads. They found that listeners preferred an average of 9.5/16 volume for -20 LKFS, and 8.2/16 for 1-4 LKFS. Edwards said the ATSC is moving toward a -14 LKFS recommendation.
