Mobile DTV is continuing to roll out in the United States. The Open Mobile Video Coalition (OMVC) recently projected that the new service will reach more than 77 million households in the next 12 months. Also, a new RF signal capture project by the group is designed to further enhance reception, by collecting and recording relevant signals for manufacturers to use when designing new reception devices. Meanwhile, ATSC M/H is already evolving to provide greater functionality. First, below is a summary of ATSC M/H.
ATSC M/H specifies MPEG-4 Part 10 AVC (Advanced Video Coding) and SVC (Scalable Video Coding). SVC offers a layered approach to encoding. The program is coded into different layers, and all layers are transmitted in a bit stream. The simplest decoder will only decode one service (the base layer) and form pictures at that associated quality level, while ignoring the higher layers. A more sophisticated decoder will decode multiple services (base and enhancement layers) and use the additional information to produce a higher-quality program.
ATSC mobile has been designed to carry IP datagrams, network layer packets that contain data together with source and destination information. Intended for transmission over a network where a reception acknowledgment is not relayed back to the source, IP datagrams map the concept of an IP network onto a lossy medium such as broadcast transmission. IP-based delivery allows for the implementation of an interactive application software stack that is compatible with existing mobile delivery standards. IP-based networking forms the backbone over which the Internet operates, by means of TCP/IP.
A/153 includes a streaming delivery mechanism, including UDP encapsulation of real-time streaming data, RTP encapsulation of real-time streaming data, and a buffer model for real-time streaming data. Designed for real-time, end-to-end transfer of multimedia data, RTP/RTCP provides methods to achieve jitter compensation and to detect and compensate for out-of-sequence packet arrival — conditions that are common in an IP network. The payload type in RTP defines various codecs and allows for future codecs, but this is currently constrained for ATSC M/H.
Because the primary objective of the ATSC Mobile standard is to define the delivery of video and audio services, a specific method or middleware for handling “apps” is not currently described. The A/153 standard does, however, provide a framework for the delivery of auxiliary (graphical) components, based on the Open Mobile Alliance Rich Media Environment (OMA-RME) specification, written specifically for mobile devices. ATSC M/H also supports A/65 captions and parental controls, and provides for graphical overlay support through a standardized Application (also called Presentation) Framework.
New addition to ATSC M/H in progress
ATSC has been developing a new Scalable Full Channel Mobile Mode (SFCMM), which provides an extension to the ATSC mobile DTV system that enables use of increased channel capacity for mobile services. Adding a new section to the existing standard, the new mode is described in ATSC A/153 Part 9. To conform with the specification, a legacy A/153 transmission (Core Mobile Mode, or CMM) requires a minimum of 4.7Mb/s to be allocated to main (i.e., non-mobile) services. SFCMM, on the other hand, can scale capacity up to the total available from the channel; it has capabilities compatible with existing A/53 receivers, and an incompatible extension capable of using the full 6MHz channel.
Because the total bit rate for an ATSC transmission is fixed at 19.4Mb/s, the M/H data stream must borrow bandwidth from the main channel, resulting in a main data rate loss (MDRL) that depends on how much payload data rate (PDR) is desired. The actual throughput efficiency depends on a number of variables.
In CMM, mobile DTV services are transmitted while reserving at least 38 of the 156 packets in an M/H slot for main A/53-compatible services. In SFCMM, mobile DTV services are transmitted while reserving fewer than 38 of the 156 packets in an M/H slot for main A/53-compatible services. SFCMM may apply to some or all of the slots in a mobile DTV transmission. As with CMM, SFCMM supports time slicing, providing a reduction in power consumption by “turning off” power-hungry portions of the receiver during irrelevant portions of the transmission.
Both CMM and SFCMM define two serial concatenated convolutional code (SCCC) rates (¼ and ½) that can be applied separately to different arrangements of “RS frames,” which are selectable groupings of services providing the same quality of service (QoS), i.e., transmission robustness. The actual throughput efficiency is a function of these rates and the bandwidth needed for overhead data, plus the fact that different services can be grouped together into the different RS frames.
To give an idea of the boundary conditions, the total PDR using CMM can vary from 152kb/s (at the highest QoS) to 2.5Mb/s (at the lowest QoS), while the MDRL using CMM can vary between 0.9Mb/s and 7.3Mb/s. Numerous combinations of PDR and MDRL are possible. (See Table 1.)
Payload data rate, Mb/s Main data rate loss, Mb/s Efficiency QoS CMM 0.15 0.92 16.9% highest 1.26 7.33 17.2% 0.31 0.92 34.0% 2.53 7.33 34.5% lowest SFCMM 0.20 1.21 16.2% highest 3.19 9.70 32.9% lowest
Table 1. Exemplary ATSC M/H data rates and efficiencies
Note that each of the capacities listed in Table 1 pertains to a single M/H Parade, i.e., a collection of M/H Groups that have the same FEC parameters. An M/H Parade can carry one or two M/H Ensembles, which are logical pipes for IP datagrams, and an M/H Parade can be either a CMM Parade or an SFCMM Parade.
The highest PDR possible is thus 5.0Mb/s with CMM and 6.2Mb/s with SFCMM, if two parades are used in each case. Note that the MDRL in these situations is thus 14.6Mb/s and 19.4Mb/s, respectively. The latter case represents complete use of a 6MHz channel for mobile broadcasting, something currently outside of the FCC broadcast rules, providing an operating basis for a nonbroadcast licensee of spectrum.
There is one more special, dedicated, full-channel mode of operation with SFCMM, specified as Scalable Mode “111” within the standard. In this mode, the packet header and RS parity byte locations within the transport stream are repurposed to contain mobile data, resulting in the availability of all packets for mobile. In such a case, the transport stream no longer conforms with an ATSC A/53 service, as the comprising packets consist of only payloads.
The ATSC membership recently voted to elevate A/153 Part 9 (SFCMM) to Proposed Standard status. An Interim (prepublication) Version was released June 1.
Aldo Cugnini is a consultant in the digital television industry.
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