ATSC-M/H (Advanced Television Systems Committee - Mobile/Handheld) is a standard in the USA for mobile digital TV, that allows TV broadcasts to be received by mobile devices. Its official appellation is A/153.
Just as the DVB-H and 1seg are mobile TV extensions to the DVB-T and ISDB-T terrestrial digital TV standards respectively, ATSC-M/H is a suggested extension to the available digital TV broadcasting standard ATSC A/53. ATSC is optimized for a fixed reception in the typical North American environment and uses 8VSB modulation. The ATSC transmission scheme is not robust enough against doppler shift and multipath radio interference in mobile environments, and is designed for highly directional fixed antennas. To overcome these issues, additional channel coding mechanisms are introduced in ATSC-M/H to protect the signal.
Evolution of mobile TV standard
Several requirements of the new standard were fixed right from the beginning:
Completely backward compatible to ATSC (A/53),
Broadcaster can use their available license without additional restrictions and
Available legacy ATSC receivers can be used to receive the ATSC (A/53) standard without any modification.
Ten systems from different companies were proposed, and two remaining systems were presented with transmitter and receiver prototypes:
MPH (an acronym for mobile/pedestrian/handheld, suggesting miles per hour), was developed by LG Electronics and Harris Broadcast. (Zenith, a subsidiary of LG, developed much of the original ATSC system.)
VSB (Advanced-VSB) was developed by Samsung and Rohde & Schwarz.
To find the best solution, the Advanced Television Systems Committee assigned the Open Mobile Video Coalition (OMVC) to test both systems. The test report was presented on May 15, 2008. As a result of this detailed work by the OMVC, a final standard draft was designed by the Advanced Television Systems Committee, specialist group S-4. ATSC-M/H will be a hybrid. Basically the following components of the proposed systems are used:
Signaling of service designed on the base of the established mobile standards
On December 1, 2008, the Advanced Television Systems Committee elevated its specification for Mobile Digital Television to Candidate Standard status. In the following six months, the industry will test the standard with their potential customers and start first product developments. Before it becomes an official standard, additional improvements will be proposed. A ratified A/153 standard will be balloted for by the ATSC members in July 2009.
Structure of mobile DTV standard
The ATSC Mobile DTV standard ATSC-M/H (A/153) is modular in concept, with the specifications for each of the modules contained separate Parts. The individual Parts of A/153 are as follows:
Part 1 “Mobile/Handheld Digital Television System” describes the overall ATSC Mobile DTV system and explains the organization of the standard. It also describes the explicit signaling requirements that are implemented by data structures throughout the other Parts.
Part 2 “RF/Transmission System Characteristics” describes how the data is processed and placed into the VSB frame. Major elements include the Reed Solomon (RS) Frame, a Transmission Parameter Channel (TPC), and a Fast Information Channel (FIC).
Part 3 “Service Multiplex and Transport Subsystem Characteristics” covers the service multiplex and transport subsystem, which comprises several layers in the stack. Major elements include Internet Protocol (IPv4), UniDirectional Protocol (UDP), Signaling Channel Service, FLUTE over Asynchronous Layered Coding (ALC) / Layered Coding Transport (LCT), Network Time Protocol (NTP) time service, and Real-time Transport Protocol (RTP) / RTP Control Protocol (RTCP).
Part 4 “Announcement”: Part 4 covers Announcement, where services can optionally be announced using a Service Guide. The guide specified in Part 4 is based on an Open Mobile Alliance (OMA) broadcast (BCAST) OMA BCAST-Electronic program guide, with constraints and extensions.
Part 5 “Application Framework” defines the Application framework, which enables the broadcaster of the audio-visual service to author and insert supplemental content to define and control various additional elements of the Rich Media Environment (RME).
Part 6 “Service Protection” covers Service Protection, which refers to the protection of content, either files or streams, during delivery to a receiver. Major elements include the Right Issue Object and Short-Term Key Message (STKM).
Part 7 “Video System Characteristics” defines the Advanced Video Coding (AVC) and Scalable Video Coding (SVC) Video System in the ATSC Mobile DTV system. Additional elements covered in this Part included closed captioning (CEA 708) and Active Format Description (AFD).
Part 8 “Audio System Characteristics” defines the High-Efficiency Advanced Audio Coding (HE-AAC v2) Audio System in the ATSC Mobile DTV system.
ATSC-M/H is a service for mobile TV receivers and partly uses the 19.39 Mbit/s ATSC 8VSB stream.
Block Diagram Head-end
ATSC-M/H BW consumes fixed chuncks of 917kbit/s out of the total ATSC Bandwidth. Each such chunk is called M/H Group. A data pipe called parade is a collection of one to eight M/H groups. A parade convey one or two ensembles which are logical pipes of IP datagrams. Those datagrams in turn carry TV services, System Signaling tables, OMA DRM key streams and Electronic Service Guide. ATSC-M/H improves design by detailed analyse of experience with other mobil DTV standards. The SMT table replaces PAT-PMT-INT and SDP of DVB-H which created huge complexity for the network and receivers. The time interleaving of ATSC-M/H is 1 second comparing to DVB-H 100-200msec.
ATSC-M/H Layer Model
ATSC-M/H protocol stack is mainly an umbrella protocol that uses OMA ESG, OMA DRM, MPEG-4 in addition to many IETF RFCs.
The ATSC-M/H standard defines a fixed transport stream structure, based on M/H Frames, which establishes the location of M/H content within the VSB Frames and allows for easier processing by an M/H receiver. This is contrary to the legacy ATSC transport stream, defined in A/53, in which there is no fixed structure to establish the phase of the data relative to VSB Frames.
One M/H Frame is equivalent in size to 20 VSB Frames and has an offset of 37 transport stream (TS) packets relative to the beginning of the VSB Frame. Each M/H Frame, which has a fixed duration of 968 ms, is divided into five M/H sub-frames and each sub-frame is further subdivided into sixteen M/H Slots. Each slot is the equivalent amount of time needed to transmit 156 TS packets. A slot may either carry all main ATSC data (A/53) or 118 packets of M/H data and 38 packets of main data. The collection of 118 M/H packets transmitted within a slot is called an M/H Group. Each of the 118 M/H packets within an M/H Group are encapsulated inside a special TS packet, known as an MHE packet.
An M/H Parade is a collection of M/H Groups and can carry one or two M/H Ensembles. These Ensembles are logical pipes for IP datagrams. Those datagrams in turn carry TV services and the signaling of mobile content. The M/H Groups from a single Parade are placed within M/H Slots according to an algorithm defined in A/153 Part 2. The Number of Groups per M/H Sub-Frame (NoG) for an M/H Parade ranges from 1 to 8 and therefore the number of Groups per an M/H Frame for a Parade ranges from 5 to 40 with a step of 5. The data of a Parade are channel coded and distributed by an interleaver during an M/H Frame.
Mobile Data are protected by an additional FEC, as Interleaving and Convolutional Codes. To improve the reception in the receiver, training sequences are introduced into the ATSC-M/H signal to allow channel estimation on the receiver side.
Time slicing is a technique used by ATSC-M/H to provide power savings on receivers. It is based on the time-multiplexed transmission of different services.
ATSC-M/H combines multiple error protection mechanisms for added robustness. One is an outer Reed Solomon Code which corrects defective Bytes after decoding the outer convolutional code in the receiver. The correction is improved by an additional CRC checksum since Bytes can be marked as defective before they are decoded (Erasure decoding)
The number of RS parity symbols can represent 24, 36 or 48. The symbols and the additional checksum form the outer elements of a data matrix which is allocated by the payload of the M/H Ensemble. The number of lines is fixed and the number of columns is variable according to how many slots per Subframe are occupied.
The RS Frame is then partitioned into several segments of different sizes and assigned to specified regions. The M/H data in these regions are protected by an SCCC (Series Concatenated Convolutional Code), incorporating a code rate of 1/2 or 1/4, and is specific to each region in a group. A 1/4 rate PCCC (Parallel Concatenated Code) is also employed as an inner code for the M/H signaling channel, which includes FIC (Fast Information Channel) and TPC (Transmission Parameter Channel). The TPC carries various FEC modes and M/H Frame information. Once the TPC is extracted, the receiver then knows the code rates being employed and can decoded each region at its specified rate.
A modified trellis encoder is also employed for backwards compatibility with legacy A/53 receivers.
ATSC M/H Signaling and Announcement defines three different layers of signalling. The layers are organized hierarchically and optimized to characteristics of the transmission layer.
Transmission Signaling System is the lowest layer and use the Transmission Parameter Channel (TPC). It provides information for the receiver needed to decode the signal
Transport Signaling System is the second layer it use the Fast Information Channel (FIC) in combination with the Service Signaling Channel (SSC). The main purpose of the FIC is to deliver essential information to allow rapid service acquisition by the receiver. The Service Signaling Channel (SSC), consists different signaling tables. The information carried within these tables can be compared to the PSIP information of ATSC. The SSC provides mainly the basic information, the logical structure of the transmitted services and the decoding parameters for video and audio
Announcement / Electronic Service Guide (ESG) is the highest layer of signaling. It use the Open Mobile Alliance (OMA) Broadcast Service Enabler Suite (OMA BCAST) Electronic Service Guide (ESG).An ESG is delivered as a file data session File Delivery over Unidirectional Transport (FLUTE) is used as delivery protocol. The ESG consists of several XML sections. With this structure, a program guide and enabled interactive services can be realized.
Signaling of video- and audio coding
Each video- or audio decoder needs information about the used coding parameters, for instance resolution, frame rate and IDR (Random Access Point) repetition rate. In MPEG-4/AVC, mobile TV systems the receiver uses information from the Session Description Protocol File (SDP-File). The SDP-file is a format which describes streaming media initialization parameters. In ATSC-M/H, the SDP-File is transmitted within the SMT-Table. Most of the information are coded as binary values and some are coded in the origin ASCII text format. In case of signaling with ESG, the complete SDP-File is transmitted
Note that while it is intended to be free to air, its spec defines service protection, therefore it may or may not be free to air based on the actual business model which does not exist yet.
Single-frequency network (SFN)
In an SFN, two or more transmitters with an overlapping coverage send the same program content simultaneously on the same frequency. The 8VSB modulation used by ATSC allows SFN transmissions. To allow regular channel approximation, ATSC-M/H provides additional training sequences. ATSC A/110 defines a method to synchronize the ATSC modulator as part of the transmitter. The A/110 standard sets up the Trellis coder in a pre-calculated way to all transmitters of the SFN. In such an SFN, the ATSC-M/H multiplexer and the ATSC-M/H transmitter are synchronized by a GPS reference. The ATSC-M/H multiplexer operates as a network adapter and inserts time stamps in the transport stream. The transmitter analyzes the time stamp, delays the transport stream before it is modulated and transmitted. Eventually, all SFN transmitters generate a synchronized signal.
Other mobile standards
While MediaFLO has become available in parts of the U.S., it is a premium service that requires subscription. ATSC-M/H would be free to air, as are regular broadcast signals.
As with any new technology, mobile TV, based on the soon to be adopted ATSC M/H (mobile/handheld) standard, has its own learning curve, and any station that plans to broadcast to mobile/handheld devices will need to start scaling that curve soon. ATSC M/H is a real departure from the stream of programs that are part of what is now called main channel ATSC (or legacy DTV), and there are many challenges that need to be addressed to successfully transmit ATSC M/H.
Bursts of data
One of the most fundamental differences in broadcasting mobile TV is that it uses a burst mode of data transmission. This was developed to save power in the mobile receiver. This means that the programs sent are not streamed to the viewer but sent in small packets, or bursts of data, that are stored and put together to form a continuous video or audio program. The entire M/H system is based on the IP data format to allow for future changes and expansions. The standard is also able to deliver data files to mobile devices, such as for programs to be viewed later.
Figure 1: Overview of ATSC M/H encoding
Encoding and multiplexing
Standard MPEG-2 encoders will not work for M/H broadcasting, because MPEG-4 or H.264 is used to compress the video and audio signals. The new M/H encoder produces an IP-encapsulated MPEG-4 signal that is connected to the next new piece of gear, the M/H multiplexer/preprocessor. The encoder’s output is connected to the multiplexer via a 100BASE-T network cable (Cat 5/RJ45). (See Figure 1.)
This multiplexer combines all of the M/H signals, including the electronic service guide (M/H speak for electronic programming guide). One of the main functions of the M/H multiplexer is to add forward error correction to the combined signals. Because of the nature of mobile reception, a great deal of FEC is required, and the multiplexer/preprocessor is where this is added. Although statistical multiplexing could help conserve the bandwidth required for M/H, the FEC required occupies much of the bandwidth that could be saved. Stations can control the picture format (the default is 416 x 240, cell phone size) as well as how much FEC is used, which depends on their particular situation and equates to what the topology of the targeted area is.
The multiplexer/preprocessor combines the main service (legacy ATSC TS) with the new M/H service. It does this by interweaving M/H packets and main service packets in the final SMPTE 310 transport stream.
To combine the two signals, room must be made in the main ATSC TS for the ATSC M/H data. This is accomplished with the use of null packets in the main ATSC TS. But it is important to leave enough room for the M/H data or a data collision will occur, the signal will be corrupted and all programs will be lost.
The electronic service guide is the program and system information service for ATSC M/H. Any M/H services (e.g. video, audio, data) that are available will be announced through the announcement subsystem using a service guide, which is a special M/H service that is located in the service signaling subsystem. The mobile unit determines available service guides by accessing the guide access table for M/H. This table lists the service guides present in the M/H broadcast and gives information about the service provider for each guide, as well as access information for each guide.
The service guide is delivered using one or more IP streams. The main stream delivers the announcement channel, and other streams are used to deliver the guide data. A data service may be used to fill in the program information such is done now with main ATSC EPGs. Of course, many new opportunities may be available, such as having viewers respond to program content using their cell phones, and that would be controlled and monitored using the ESG encoder at the station.
In the output transport stream, the IP-encapsulated M/H data are converted to fit within the normal ATSC TS for compatibility with legacy 8-VSB receivers. The M/H service data is encapsulated in special MPEG-2 transport stream packets called M/H encapsulation packets. The M/H transmission system, which feeds the M/H exciter, can carry many types of encapsulated data including MPEG-2 video/audio, MPEG-4 video/audio and other types of data. Legacy DTV receivers will see this new M/H data as null packets and ignore them.
Time-division multiplexing of main and M/H data introduces changes to the timing of the main service stream packets. Changes are necessary to compensate for time displacements where the two data sets are combined so the transmitted signal complies with the MPEG and ATSC standards so as to protect legacy receivers. These functions are performed by the packet timing and PCR adjustment block in the M/H multiplexer. The data manipulation of the M/H data is divided into two stages: a preprocessor, contained in the multiplexer, and a post-processor, within the DTV exciter.
Figure 2: Saving power
Click to enlarge
Data for a particular M/H service is carried in what is called a parade, and these, in turn, are broken into groups of packets within the transmitted signal. These data groups are arranged into a predictable pattern that allows mobile receivers to only power on their receiver circuitry when the data groups they want are present. Of course, this only happens after it has acquired an ATSC-M/H signal. In this way, about 20 percent of battery power is conserved. (See Figure 2.)
A new digital ATSC M/H exciter is required to handle the new ATSC M/H TS. This new exciter outputs the same RF signal as the legacy DTV exciter, but the way it handles the new M/H data is different. The main ATSC TS and the M/H data are routed within the exciter to different preprocessors and then combined again before trellis coding and the modulator. Special flags within the ATSC M/H TS inform the exciter that M/H data are present.
Figure 3: Simplified antenna polarization
Transmitter and antenna
The actual transmitter required for ATSC M/H stays the same — the bandwidth requirements and power levels do not have to change. But because the target audience is mobile (sometimes very mobile at 60m/h), the antenna may need to be changed. It has been found that an elliptical polarized transmit antenna pattern is best for mobile TV reception. Some broadcast facilities that are currently upgrading or switching channels are switching to an elliptically polarized antenna, while others may have to pull down their current DTV antenna to achieve the best possible mobile TV reception.
Typically, an elliptically polarized antenna for mobile TV diverts from 10 to 30 percent of the power to the vertical and the remainder to horizontal. This, in turn, requires more transmitter power equal to the amount diverted to the vertical polarity. (See Figure 3.)
While ATSC M/H holds a great deal of promise for broadcasters, it also presents a great deal of challenges as well. ATSC M/H is still new, and both station engineers and management have much to learn about what is and is not possible. ATSC M/H may require even more changes to the broadcast facility than mentioned here to reach as many viewers as possible.
Jay Adrick of Harris, Richard Schwartz of Axcera and Richard Fiore of Thomson contributed to this tutorial.