World TV standards
Oct 1, 2007 12:00 PM, BY ALDO CUGNINI
ATSC, ISDB-T, DVB-T and DMB-T/H stake their claims.
In the last few decades, there has been a revolution in the technologies used to deliver video and audio to users. Transitioning completely to digital delivery means the hope for a single worldwide standard has given way to several systems due to timing, receiver technology maturity, regional priorities and politics. While the United States and Europe have spearheaded the terrestrial delivery of digital programming, other nations are now following suit.
Design decisions
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MPEG2 has strongly influenced the rollout of video service. (See Table 1 on next page.) Starting with ATSC in the United States and a handful of other countries, the video and systems-layers are nearly exclusively delivered using the standard. However, the use of transmission and audio technologies has not converged in a similar fashion. While many of the umbrella standards allow the use of different components (see Table 2 on next page), exclusivity has become a deciding factor. In a nutshell, ATSC does not support COFDM or MPEG2 audio, and DVB does not support VSB (or AC-3, in most regions).
At this point in time, many of the standards have been officially decided, but deployment is still to come. Note also that while the governments of Argentina and Brazil have decided independently which digital TV standard each nation would deploy, they have also agreed to work together to implement a single standard for the Mercosur customs union.
ATSC
The overriding design goals of the ATSC system were to replicate the coverage of analog NTSC terrestrial television using rooftop antennas and to provide the capacity for digital HDTV broadcasting. As is well known to most of our readers, ATSC uses 8VSB modulation with a fixed level of error correction. While incorporating the MPEG2 systems and video standards, ATSC has several constraints to those standards, including limiting transmission to packetized elementary streams (PES), and specifying the various video profiles and levels that are supported.
ATSC has recently begun work on ATSC-M/H, expected to provide mobile and handheld transmission modes compatible with ATSC A/53. To date, two ATSC-compatible systems have been demonstrated for mobile and handheld use: A-VSB from Samsung and Rohde & Schwarz, and MPH by LG and Harris. These and several other systems have now been formally proposed to ATSC.
The ATSC Synchronization Standard for Distributed Transmission, A/110a, defines a standard method for synchronization of multiple transmitters using 8VSB modulation. The standard describes the mechanisms necessary to transmit synchronization signals to several transmitters, including the formatting of associated packets. It also provides for adjustment of transmitter timing and other characteristics through additional information carried in the specified packet structure, as well as techniques for cascading transmitters in networks of synchronous translators. Two types of single frequency networks (SFNs) are defined: digital on-channel repeaters (a daisy-chain network) and distributed transmission (a star network).
DVB-T
DVBT is the Digital Video Broadcasting standard developed for terrestrial television. Mandated for use in the European Union, the standard has also been widely adopted elsewhere. Based on MPEG video and systems, and coded orthogonal frequency-division multiplexing (COFDM) transmission, the DVB standards also define cable and satellite transmission. (The latter two media in the United States also use similar, though proprietary, standards.)
COFDM is a way of coding the data stream onto a large number of individual carriers. With DVBT, various transmission parameters can be selected, allowing the individual broadcaster to trade off parameters such as data rate, signal robustness and multipath.
DVB supports two modes for OFDM transmission: 2K (1705 carrier) and 8K (6817 carrier), with QAM carriers. While the 8K mode can allow more multipath protection, the 2K mode offers advantages for moving receivers. Five code rates are supported, from 1/2 to 7/8. The code rate 1/2 has the highest redundancy, but the highest transmission safety. A code rate of 7/8, on the other hand, has a low redundancy but a very weak error protection. Three types of nondifferential modulation schemes can be selected: QPSK, 16-QAM and 64-QAM, offering 2, 4 or 6 bits per modulation symbol, respectively.
A guard interval that protects against multipath is inserted between consecutive COFDM symbols. This immunity must be traded off with data rate, as the length of the interval affects the net data rate. Various guard intervals are available — from 1/4 to 1/32 — depending on the amount of multipath immunity desired. SFNs — where multiple transmitters operate on the same carrier frequency — can also be set up, by proper choice of the guard interval.
DVBT also includes hierarchical modulation, where the data to be transmitted can be split into two parts: one at a lower data rate with high error protection, and a second with higher data rates and weaker error protection. In this manner, SDTV and HDTV could be sent simultaneously with different coverage characteristics.
In order to support the multiple transmission characteristics that could change dynamically in some deployments, DVB also defines a Transmission Parameter Signaling for automatic receiver compatibility. Manual selection of modulation parameters at the receiver is also possible.
In Australia, several features of DVB are used in a unique combination. While television services transmitted in SDTV require an MPEG1 Layer II audio stream, the service may also contain a Dolby Digital (AC-3) audio stream. For an HDTV transmission, however, either Dolby Digital or MPEG-1 audio is allowed. In addition, the requirement of analog and digital simulcasting, together with a minimum annual HDTV programming requirement, has led to a so-called triplecast scenario.
ISDB-T
ISDB-T, standardized in Japan by ARIB, was developed after ATSC and DVB, and hence has several newer features. Band-segmented transmission-OFDM (BST-OFDM) can provide simultaneous services for HDTV and mobile and portable reception by dividing a 6MHz channel into 13 segments for signal transmission.
For example, a mobile reception service that employs one segment can be partially received, together with HDTV broadcasting on the other 12 segments. Three SDTV services could likewise occupy 12 segments. Similarly, a separate digital terrestrial sound broadcasting service can use a one- or three-segment scheme. In this manner, a common one-segment receiver can receive both TV and sound broadcasting services.
Also, an HDTV mobile reception mode was developed using reception direction-control techniques and Doppler-shift compensation techniques. Using this mode, stable HDTV reception has been demonstrated at speeds of 100km/h (62mph).
While DVB has a separate standard for handheld and mobile applications (DVB-H), ISDBT has a handheld mode as a subset of the standard (as does ATSC). By using one segment in the center of the 13 segments, the handheld service has a capacity of 312Kb/s, using QPSK modulation. MPEG4/AVC is the supported video codec for this mode.
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