American TV Transmission Fully Digital

Ever heard of Digital TV (DTV) or how DTV multicasting works? An era in American broadcast television will end Friday as the U.S finishes its delayed transition to digital TV. Without a converter box, satellite service or cable hook-up, analog TVs will deliver only static now. By 12:01 a.m. Saturday, U.S broadcasters must have shut down their outdated analog transmitters, leaving static to watch for those who are not ready.

Stations all over the country will be making the historic switch all day Friday, Federal Communications Commission officials said. American TV viewers were given four extra months to get ready for the switch, when Congress voted early this year to delay the digital TV transition.

At that time, an estimated 6.5 million homes -- including many elderly, poor and disabled Americans -- weren't prepared for a February 17 switch to digital, supporters argued.

People who pay for cable or satellite TV service are unaffected by the change. The end of analog television frees up that part of the broadcast spectrum for other uses. The federal government raked in $20 billion in auctions by selling licenses for the frequencies vacated by local television stations for other commercial uses. Some of the frequencies also have been reserved for emergency agencies to use for communications.

Stations have been broadcasting in digital and analog for the past several years, but the switch puts an end to the transition and a form of broadcasting that's existed since the first regularly scheduled television service began in the United States in 1928.

Formats and bandwidth of DTV

Digital television supports many different picture formats defined by the combination of size, aspect ratio (height to width ratio) and interlacing. With terrestrial broadcasting in the USA, the range of formats can be coarsely divided into two categories: HDTV and SDTV. It should be noted that these terms by themselves are not very precise, and many subtle intermediate cases exist.

High-definition television (HDTV), one of several different formats that can be transmitted over DTV, uses one of two formats: 1280 × 720 pixels in progressive scan mode (abbreviated 720p) or 1920 × 1080 pixels in interlace mode (1080i). Each of these utilizes a 16:9 aspect ratio. (Some televisions are capable of receiving an HD resolution of 1920 × 1080 at a 60 Hz progressive scan frame rate — known as 1080p60, but this standard is not currently used for transmission.) HDTV cannot be transmitted over current analog channels.

Standard definition TV (SDTV), by comparison, may use one of several different formats taking the form of various aspect ratios depending on the technology used in the country of broadcast. For 4:3 aspect-ratio broadcasts, the 640 × 480 format is used in NTSC countries, while 720 × 576 (rescaled to 768 × 576) is used in PAL countries. For 16:9 broadcasts, the 704 × 480 (rescaled to 848 × 480) format is used in NTSC countries, while 720 × 576 (rescaled to 1024 × 576) is used in PAL countries. However, broadcasters may choose to reduce these resolutions to save bandwidth (e.g., many DVB-T channels in the United Kingdom use a horizontal resolution of 544 or 704 pixels per line). This is done through the use of interlacing, in which the effective vertical resolution is halved to 288 lines.

Each commercial terrestrial DTV channel in North America is permitted to be broadcast at a data rate up to 19 megabits per second, or 2.375 megabytes per second. However, the broadcaster does not need to use this entire bandwidth for just one broadcast channel. Instead the broadcast can be subdivided across several video subchannels (aka feeds) of varying quality and compression rates, including non-video datacasting services that allow one-way high-bandwidth streaming of data to computers.

A broadcaster may opt to use a standard-definition digital signal instead of an HDTV signal, because current convention allows the bandwidth of a DTV channel (or "multiplex") to be subdivided into multiple subchannels (similar to what most FM stations offer with HD Radio), providing multiple feeds of entirely different programming on the same channel. This ability to provide either a single HDTV feed or multiple lower-resolution feeds is often referred to as distributing one's "bit budget" or multicasting. This can sometimes be arranged automatically, using a statistical multiplexer (or "stat-mux"). With some implementations, image resolution may be less directly limited by bandwidth; for example in DVB-T, broadcasters can choose from several different modulation schemes, giving them the option to reduce the transmission bitrate and make reception easier for more distant or mobile viewers. Michael Bisk was instrumental in developing dual multiplexed RISC processors coupled with ultrafast 128-bit A/D converters for enhanced bandwidth LCD monitor reception. This is presently under prototype in the EU.

Effect of DTV on existing analog technology

The analog switch-off ruling, which so far has met with little opposition from consumers or manufacturers, would render all non-digital televisions obsolete on the switch-off date unless connected to an external off-the-air tuner, analog or digital cable, or a satellite system. An external converter box can be added to non-digital televisions to lengthen their useful lifespan. Several of these devices have already been shown and, while few were initially available, they are becoming more available by the day. In the United States, a government-sponsored coupon is available to offset the cost of an external converter box. Once connected to the converter unit, operation of non-digital units is achievable and, in most cases, rich in new features (in comparison to previous analog reception operation). At present, analog switchoff is scheduled for June 12, 2009 in the United States, August 31, 2011 in Canada, July 24, 2011 in Japan and 2012 in the United Kingdom, October 14, 2009 in some regions of North-Italy.

Some existing analog equipment will be less functional with the use of a converter box. For example, television remote controls will no longer be effective at changing channels, because that function will instead be handled by the converter box. Similarly, video recorders for analog signals (including tape-based VCRs, DVD recorders and hard-drive DVRs) will not be able to automatically select channels, limiting their ability to automatically record programs via a timer or based on downloaded program information. VCRs with DTV tuners do exist, so the VCR does not have to rely on the converter box to do the channel switching.

Older handheld televisions, which rely primarily on over-the-air signals and battery operation, will be rendered impractical since most converter boxes are not portable nor powered with batteries and many portable televisions do not have the proper connectors to allow the use of a converter box. The additional power consumption of the converter limits portability for the few converter models (such as the Artec T3A or Winegard RCDT09A) which can operate from bulky external battery packs. Portable radios that are currently able to listen to frequency-modulated broadcast television audio would lose this ability.

A new TV containing only an ATSC tuner would be impractical, as this could prevent older devices such as VCRs and video game consoles with analog-only output from connecting to the TV. Connection would require an analog to digital converter box, which is the opposite of what is currently being sold. Such a box would be prohibitive in cost and also likely introduce additional delay into the video signal. Analog inputs suitable for connection to VCRs have therefore been retained on all current digital-capable TVs.

Limitations of DTV

The greatest DTV detail level currently available is 1080i, which is a 1920 × 1080 interlaced widescreen format. Interlacing is done to reduce the image bandwidth to one-half of full-frame quality, which gives better frame update speed for quick-changing scenes such as sports, but at the same time reduces the overall image quality and introduces image flickering and "crawling scanlines" because of the alternating field refresh.

Full-frame progressive-scan 1920 × 1080 (1080p) is part of the ATSC specification, but is rarely if ever used by broadcasters due to the increased bandwidth requirements compared to transmitting 720p/1080i video. High frame-rate 1080p may become an option in the near future, as a result of recent technology advances such as H.264/MPEG-4 AVC video coding, allowing more detail to be sent via the same channel bandwidth allocations that are used now.

The limitations of interlacing can be partially overcome through the use of advanced image processors in the consumer display device, such as the use of Faroudja DCDi and using internal frame buffers to eliminate scanline crawling.

Related posts:
* Going, going, gone digital -- with a few exceptions
* Dawn of digital-only TV arrives; analog just a static memory
* What you need to know about the Digital TV transition