Electronic Design

Stars Of The Really Small Screen

Electronic addictions abound, due mainly to the surging popularity of gadgets like MP3 players and video games. In the face of this electronics mania, television remains a leading pastime. Even average viewers spend several hours a day watching the tube. But the type of viewing is changing. Instead of sitting in front of the latest LCD or plasma screen, viewers are on the go with their portable devices. New standards have been established, services are available, and hit shows soon will be arriving in the palm of your hand.

Most mobile TV will reside in cell phones. In fact, a high percentage of the more than a billion new phones sold per year will have mobile TV. Other formats will include embedded TV receivers in laptops and netbooks or USB dongles and mini-cards. Some devices will be dedicated to mobile TV. It could even show up in personal navigation devices and in-vehicle installations like current backseat DVD players. Some of these devices are available now in various formats and standards. But beginning in 2010, a parade of new products is expected to arrive.

Mobile TV screens are mostly LCDs, but we’ve seen a gradual adoption of newer organic LED (OLED) screens. Sizes range from a little over 2 in. to about 3.5 in. in cell phones with a variety of different formats (Table 1). The width measurement indicates the number of pixels per horizontal scan line, and the height measurement is the number of horizontal scan lines. Other formats are available as well.

Another related issue is frame rate, which is the total number of completely scanned frames or pictures that occur per second (fps). Movies are commonly 24 fps, while most fixed TV is 30 fps. Slower rates produce an annoying flicker, creating a jerky and unnatural picture. Mobile TV rates tend to be slower because faster rates severely impact bandwidth needs.

The minimum acceptable rate is about 15 fps, but you may see some mobile TV at 10 or 12 fps, while 15 to 20 fps is far better. To see how this affects the streaming of video to the screen, assume a QVGA screen with 320 × 240 = 76,800 pixels. Using 24-bit color per pixel and a 15-fps rate, the bit stream rate is:

76,800 × 24 × 15 = 27,648,000 bits/s

This will require a bandwidth of about 28 MHz minimum using basic binary phase-shift-keying (BPSK) modulation, which is impractical. That’s why video compression is employed.

Standard digital TV almost universally uses MPEG2 compression. Mobile TV, on the other hand, takes advantage of MPEG4 and similar H.264 compression methods. Compression ratios greater than 100 are possible. Typical compressed video data-stream rates are usually in the 128- to 768-kbit/s range. Commonly available cellular bandwidths permit 256- to 384- kbit/s rates for reasonable resolution and frame rate.

There are five basic ways to get video into a cell phone or other mobile device.

  • Unicast: Video brought to your phone over the cellular network on a one-to-one basis
  • Multicast: Using the cellular network to deliver video to multiple users simultaneously
  • Over-the-air (OTA): Broadcast video wirelessly over a separate spectrum
  • OTA download: Video downloaded wirelessly for storage and viewing later
  • Offline download: Getting the video from the Internet via a PC and stored in the device for viewing at another time

The two most common forms are unicast and OTA broadcast.

Unicast delivers video over the existing cellular network, but at the expense of voice capacity. A common 3G basestation can typically handle an average of about 200 voice calls simultaneously. One lowresolution video stream (128 kbits/s) is the equivalent of about 10 calls, while higher resolution at 384 kbits/s would use as much bandwidth for 30 calls. So video very quickly consumes a basestation’s capacity, though it can be manageable if the resolution is low and video segments are short.

Increased-capacity basestations using 3.5G to 3.75G technology like HSPA help the situation, but they don’t completely solve the problem. Luckily, video isn’t widely used now. If every subscriber jumped on YouTube at the same time, the network would collapse and no voice calls would be possible. Clever management is needed to maintain some balance between voice and video.

Don’t forget that a significant and growing segment of that capacity goes to messaging and e-mail, not to mention Internet browsing. Juggling that combination is a major challenge. And now, with the Federal Communications Commission (FCC) on the verge of mandating net neutrality, carriers will not be able to restrict, subdivide, or otherwise manage their network capacity in a way that permits them to make even a modest profit. Politics and a lack of common sense rule these days.

The bottom line is that video uses too much of the available capacity and is far less profitable than any other service. Messaging is the most profitable use of the network, with voice second. Video is far less profitable unless the network operator charges as necessary to make it possible to provide that service. But in the end, there is still only so much capacity in a basestation.

Forthcoming Long-Term Evolution (LTE) basestations will be far more able to handle video, but they too have an upper limit. Analysis of the profitability issues and capacity, as well as capital expenditures and operating expenditures, have led carriers to minimize video offerings and price them as necessary to manage their capabilities. That’s opened up an opportunity for over-the air TV.

Over-the-air TV is a broadcast method that’s completely separate from the cellular network. Cellular carriers aren’t typically involved except in how they select the cell phones they sell with OTA TV capability.

In OTA TV, broadcast stations in each city or metro area transmit the video to separate TV receivers built into cell phones or other devices. OTA TV is like standard broadcast TV but designed to be more robust under mobile conditions. The broadcast stations transmit at very high power levels, so they cover a very large geographic area.

OTA TV comes in two versions—free and fee-based. Both are implemented around the world in some form, and more models are on the way. Japan and Korea lead the world in mobile handset TV adoption, followed by Europe. The U.S. is well behind but catching up with two standards: FLO TV, which is a fee-based service, and the soon-to-arrive simulcast of currently available ATSC digital TV.

There is an unusually large number of mobile TV standards (Table 2):

  • ATSC-M/H: Advanced Television Standards Committee-Mobile/Handset
  • CMMB: China Mobile Multimedia Broadcasting
  • DVB-H: Digital Video Broadcast- Handset
  • MediaFLO: Also called FLO TV
  • ISDB-T: Integrated Service Digital Broadcast-Terrestrial
  • T-DMB: Terrestrial-Digital Multimedia Broadcast

Most of these mobile standards were derived from initially established terrestrial digital standards. The terrestrial standard is based on fixed operation and can use a higher-order modulation scheme like 64-state quadrature amplitude modulation (64QAM) to achieve high definition.

Under mobile conditions, which could mean speeds up to 60 mph, a higher carrier- to-noise (C/N) ratio is needed as well as some way to overcome Doppler fading effects. The solution is to use a simpler and more robust modulation scheme like quadrature phase-shift keying (QPSK) and to add extra error-correction coding to enhance overall performance.

All of the standards use coded orthogonal f requency-division multiplexing (COFDM) as the main modulation scheme, except for the ATSC-M/H standard in the U.S., which relies on the 8-VSB eight-level vestigial sideband AM scheme.

COFDM is still miles ahead of other modulation methods for mobile high-speed digital applications because of its greater immunity to fading and multipath effects. Yet the U.S. stuck with its less robust standard to ensure backward compatibility with the original ATSC HDTV standard. The mobile version also uses more error correction and coding to improve gain and ensure solid reception under mobile conditions.

FLO TV, the first OTA mobile TV system to be implemented in the U.S., uses the MediaFLO technology. (FLO stands for “forward link only.”) A subsidiary of Qualcomm, FLO TV is implemented in more than 100 major markets. The pay TV system uses the recently abandoned UHF TV channel 55 operating in the 716- to 722-MHz band.

MediaFLO can offer from 15 to 30 channels of AVC/H.264 compressed video. The audio compression is ACC+/HEACC. It delivers frame rates up to 30 fps with a 340x240 QVGA format, depending on the number of channels implemented.

In the MediaFLO system, a central network operations center receives the video content from multiple locations, including satellite and over the Internet (Fig. 1). It processes the content, including compression, and assembles the schedules of the various programs. Then the channels are multiplexed together for distribution to the existing broadcast stations around the country.

Distribution varies with the stations. Several methods are used, including fiber, microwave, or satellite. The broadcast stations typically have a 300-m tower to cover a wide area. The maximum power transmitted is 50 kW effective radiated power (ERP). ERP includes the antenna gain factor.

The signal is transmitted using COFDM with 4096 subcarriers. The signal uses convolutional coding plus Reed Solomon error-correction coding. It’s encrypted to prevent reception by unauthorized users. The typical bit rate of the compressed video is 256 kbits/s using QPSK.

Cell phones receive the signal, and the subscriber can select a desired channel from the program guide provided. Changing the channel takes about two seconds. Subscribers will have a 3G cell-phone connection back to the MediaFLO system for authorization, as well as to provide feedback in a variety of ways.

FLO TV is available now for cell phones through AT&T and Verizon. It will also come directly from FLO TV via its Personal Television unit (Fig. 2). Furthermore, it will be available as an in-vehicle system under the Audiovox Advent brand. Such systems can be installed in new vehicles or retrofit into older vehicles at more than 12,000 car dealers nationwide.

Available content includes programs from CNBC, Comedy Central, MSNBC, MTV, NBC, NBC2GO, NBC News, NBC Sports, Nickelodeon, and Fox News. Subscription rates run about $30 month for cell phones and as low as $10 month for in-vehicle or handheld devices, depending on the service available locally.

The ATSC-M/H standard is an extension of the existing digital TV standard in the U.S. Of course, analog TV has been completely phased out in favor of digital.

The complex system is designed for fixed service and can’t handle mobile operation, so the standard’s committee started work on a mobile version several years ago. That work was completed as of late 2009. Only a few test systems are in operation. However, many stations will come online next year and beyond as the infrastructure is put into place and as receiver chips are created and built into phones.

The current HDTV system transmits a 19.39-Mbit/s stream of packets of coded MPEG2 compressed video and audio using the 8-VSB modulation. The M/H subsystem, which comprises a processor and multiplexer, takes the content and compresses it using the MPEG4/H.264 process.

Additional error correction is added to boost mobile immunity to multipath fading and Doppler effects. The system then creates Internet Protocol (IP) format packets that include the simulcast content and a program guide. These packets are interleaved with the regular HDTV packets being transmitted, and the multiplexed signal is sent to the transmitter.

An interesting feature of the M/H standard is that instead of streaming packets, they’re sent in a burst mode. These packet bursts are stored at the receiver and assembled into a continuous stream, which is sent for viewing. This helps save power in the receiver, because the receiver powers on only when it receives the M/H data packets. The station can control the picture format. The default in this system, though, is 416 by 240.

The transmitter and antenna remain the same. However, mobile reception can be improved if the station switches to an elliptically polarized antenna that changes the vertical and horizontal transmission pattern. This typically increases the vertical pattern, so it requires a boost in transmitter power to compensate for the loss of some horizontal range.

Putting mobile TV into a cell phone is not so easy. Designers are just beginning to solve some of these new challenges:

  • Power consumption: Adding the receiver chip is a key part of the problem. The radio is sophisticated, but the demodulator and baseband processors are complex and usually power-hungry. The new receiver chips will partially mitigate this problem, but not eliminate it. Each new feature on a smart phone increases the power demands on the battery. The overall effect is simply reduced battery life before recharge is necessary. Battery technology isn’t keeping up.
  • Processing power: Generally, a more powerful processor is necessary to deal with the additional chores related to the screen and the receiver.
  • The screen: A larger screen is needed for enjoyable viewing, with its attendant power increases.
  • More memory: Larger memory is essential to handling TV. Flash covers it, but is still expensive.
  • Software: The operating system must be able to handle multimedia, and the user interface must accommodate the video in a consumer-friendly way.
  • The antenna: This may constitute the biggest headache for designers. At TV frequencies from 47 to 862 MHz, the size of a quarter wavelength ranges from about 5 ft at the low end to 4 in. at the high end. For effective reception, the antenna is going to be a compromise length to get a range of frequencies, especially on the lower bands used with the ATSCM/ H system. An extendible whip is the preferred antenna, but no one wants that on a cell phone. Packaged antennas are generally too short to be effective, since they are at the higher frequencies. One potential solution is to use the headphone leads as an antenna, like some MP3 players and cell phones with FM radio. An active antenna design may work. So far, the problem persists.
  • Content: Content designed for the small screen is essential. Granted, some standard TV will work. But for best viewing, the format must be user-friendly. Shorter content will be needed, too. Music videos should be a big hit.

Some studies show that there’s little interest from consumers in watching TV on a small screen. Yet actual tests and usage say otherwise. With programs reformatted for the small screen, viewing a show is not bad at all. And most shows will be shorter to accommodate the battery life limitations. This will attract more subscribers than expected.

Furthermore, free OTA TV will be a big hit once the cell phones and other mobile Internet devices (MIDs) become reasonably priced. Most analysts predict mobile TV will be a big winner. Market research firm TeleAnlytics expects the broadcast mobile-TV market to reach $2.8 billion and serve 50 million users in North America by 2013.

Mobile TV will push us to watch even more TV. As it is, many people think we watch TV too much now. It makes you and your kids less social. Kids are already overly preoccupied with computers and video games. Mobile TV is just one more electronic addiction that will shorten our attention span. Of course, free OTA TV will also bring advertisements. And if you think texting while driving is bad, wait until mobile TV is in full swing.

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