Digital video goes mobile
Richard A. Quinnell, Contributing Technical Editor -- Test & Measurement World, 3/1/2005
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| Mobile television receivers such as this unit from Nokia should soon be in development. Courtesy of Nokia Corp. |
At the end of 2004, ETSI adopted the DVB-H (digital video broadcast for handhelds) specification, setting the stage for commercial implementation of broadcast digital television services aimed at devices such as PDAs and cell phones. Commercial trials have already begun in Helsinki, Berlin, and Pittsburgh, PA. An additional trial is scheduled to begin mid-2005 in Australia.
The mobile video specification, available from ETSI as EB 302 304, is a variation on the DVB-T terrestrial digital television spec already adopted in Europe. Created by the DVB Group consortium (www.dvb.org), the DVB-T standard demonstrated an ability to accommodate receivers in motion, such as in an automobile or train, and spawned commercial services for mobile operation in Singapore. DVB-H was created to make mobile reception more appealing to consumers, and the two are so similar that the DVB-H signals can be broadcast along with two DVB-T channels in the same multiplex.
This is good news for test engineers who will be working with the new standard. Much of the equipment already available for DVB-T will be useable for DVB-H testing. Even more good news is that the specialized needs of DVB-H test are already being addressed by test equipment manufacturers. Rohde & Schwarz, for example, is offering a product portfolio that includes transmitters, data inserters, test systems, and receivers that are compatible with the specifications.
A different environmentThere are several differences in mobile video, however, that test engineers will need to look at when planning their test strategy. These differences stem from accommodating the mobile receiver environment by altering the broadcast signal. Three critical elements of the mobile receiver environment not addressed by broadcast digital video are ignition noise from automobiles, receiver battery life, and fashionable but functionally poor antenna design. To address these concerns, DVB-H includes:
- Time slicing. Rather than broadcast data continuously, DVB-H sends bursts. This allows the receiver to be inactive most of the time, reducing battery power by as much as 90%. It also gives the mobile unit a chance to search for other active channels to facilitate smooth handover when roaming, while needing only a single receiver.
- 4K-mode. DVB-T offers a 2K-mode for small-area use and high-speed data, and an 8K-mode for wide-area use. The 4K-mode provides a compromise of data rate and coverage. It also allows the use of signal interleaving in the broadcast, giving DVB-H better ignition interference immunity.
- MPE-FEC. Multiplexer-level forward error correction in the DVB-H data stream helps eliminate data loss under poor reception conditions.
The standard is expected to be widely adopted for commercial broadcast in Europe and Asia-Pacific countries. Its fate in the US and Japan is less clear. In the US, the digital terrestrial broadcast standard ATSC has proven unsuitable for mobile operation. In Japan, the ISDB-T system will work with mobile receivers, but concerns about battery life are unresolved. But whether or not it becomes a worldwide standard, DVB-H will be an important new communications system needing extensive testing.
