With the ATSC standard for mobile DTV released last year, and the potential for a half-billion worldwide mobile TV subscribers by 2013 (according to ABI Research), this is the year that mobile video services and devices will make it into the U.S. marketplace. The ATSC Mobile/Handheld standard A/153 (also called ATSC-M/H or simply ATSC Mobile) is a backward-compatible enhancement to the existing ATSC A/53 standard for digital television transmission. (See Figure 1.) New devices can decode either the mobile or the main stream, and existing devices can continue to operate by decoding the main stream alone. This month, we'll describe the high-level features of the standard.
ATSC-M/H is organized into eight parts: system description, RF and transmission system, service multiplex and transport subsystem, announcement method, application framework, service protection, video coding, and audio coding.
RF and transmission
To improve reception reliability for moving receivers over that of ATSC A/53, ATSC-M/H uses additional training sequences and FEC. Because power consumption is a critical factor for handheld receivers, the standard also provides time slicing, which allows the receiver demodulator (a high consumer of power) to cycle on and off, resulting in power savings. This is possible because the M/H data is organized into groups called parades, wherein each parade can carry different kinds of services. It is rarely necessary that a receiver simultaneously decodes all of the services present in a stream.
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. This main data rate loss (MDRL) can be set between 0.9Mb/s and 7.3Mb/s. The efficiency of this loss will vary, however, as the M/H stream requires overhead for additional error protection. This overhead can be set, upon transmission, and presents a trade-off between the payload data rate (PDR) and error resiliency.
The heart of the M/H error protection scheme uses what is called a serial concatenated convolutional code (SCCC). The standard defines two SCCC rates, namely 1/4 and 1/2, and these can be applied separately to different arrangements of RS frames, which are selectable groupings of services providing the same QoS. Fundamentally, the SCCC rates imply data efficiency (data-to-overhead) ratios of 25 percent and 50 percent, respectively. However, the actual throughput efficiency is a more complicated function of these rates plus other overhead data. In addition, different services can be grouped together into the different RS frames. To give an idea of the boundary conditions, the total PDR can vary from 152kb/s (at the highest QoS) to 2.5Mb/s (at the lowest QoS), while the MDRL can vary between 0.9Mb/s and 7.3Mb/s. Numerous combinations of PDR and MDRL are possible; a few are shown in Table 1.
Transport and services
The ATSC mobile service multiplex and transport subsystem supports various mechanisms for delivering multimedia to portable devices. Whereas the main channel service was designed to carry A/V transport streams, ATSC-M/H was designed to carry IP datagrams. (See Figure 2.) These are 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. In other words, 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 Transmission Control Protocol/Internet Protocol (TCP/IP). TCP/IP, however, requires a two-way communications path to allow the retransmission of lost packets — something that broadcasting does not provide. For this reason, User Datagram Protocol (UDP) is used on top of IP, which is a connectionless protocol that does not set up a dedicated end-to-end connection. UDP requires only a small transport layer and provides packets with definite boundaries, which aids in synchronization. UDP does not have a method for time stamping, however, so Real-time Transport Protocol/RTP Control Protocol (RTP/RTCP) over UDP (over IP) is used.
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. One of these methods is time stamping, which specifies the sampling instant upon transmission of the first byte of the RTP packet. RTP does not guarantee arrival of packets, but the use of sequence numbers can provide an indication of packet loss. The payload type in RTP defines various codecs and allows for future codecs, but this is currently constrained for ATSC-M/H.
ATSC-M/H supports the delivery of many types of content. In IP parlance, TV broadcast would now be called streaming delivery of audio and video using a unidirectional link route (UDLR). File delivery over UDLR is also possible, providing a versatile mechanism for the transfer of data (e.g., HTML, XML and similar files) and applications, as well as nonreal-time caching of content for later consumption.
ATSC-M/H also defines an interaction channel, an out-of-band bidirectional link route (BDLR), which can be used for carrying out interactive transactions. The form of BDLR channel is not specified but would be implemented by means of a two-way communications channel, such as that provided by a cellular phone. The standard supports the delivery of a service guide over the in-band UDLR channel, or by means of the BDLR channel, as defined in the ATSC A/96 standard for ATSC Interaction Channel Protocols.
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.
Announcement and service protection
A special announcement protocol is used in ATSC-M/H to announce the content and services being delivered or scheduled for delivery. This protocol includes, but is not limited to, a service guide (or EPG) that is sent as its own service. Because this guide could be extensive, a much smaller service signaling channel (SSC) is also sent, which indicates what services are currently available. This allows receivers to quickly display information to the user regarding the immediately available content. Service protection and content protection are also available. The first is used to protect content during its delivery, and the second is used subsequent to delivery.
Video and audio
ATSC-M/H specifies MPEG-4 Part 10 AVC and SVC video coding, and MPEG-4 Part 3 HE AAC v2 audio coding. While the standard does not specify the bit rates for video and audio, we can consider an example case: Using 500kb/s for video would provide five program channels using the most efficient mode of transmission. With so many options available, there will no doubt be much experimentation and customization of broadcast configurations. Watch for more details about video and audio coding and interactivity in later installments of this column.
Aldo Cugnini is a consultant in the digital television industry.
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