Understanding Digital vs. Analog Television Broadcasts

The first Television broadcast was tested in the U.S. in 1928, with CBS beginning regular broadcasts in 1931, with Kate Smith and George Gershwin. When did the first High Definition broadcast take place? In 1936, California station W6XAO demonstrated what they called “high definition television,” improving the original 60-line image to 343 lines, and later to 441 lines. Eventually, the National Technical Standards Committee (NTSC) called for 525 lines of horizontal resolution, displayed at 30 frames per second.NBC Peacock Logo

Now the definition of High Definition means a lot more than 343 or 525 lines! High definition television (HDTV) includes a number of variations, including 720p, 1080i, and 1080p. Waiting for future technology to support it, HDTV includes a 2160p format, but it will be likely many years before hardware supports this resolution, other than perhaps cinema (movie theater) applications.

The “p” in these numbers refers to progressive scan, which means that the entire picture is sent from the source (DVD player, digital cable, DTV/ATSC tuner) 30 times per second. The “i” stands for interlaced, which is the way existing analog television is broadcast. Because NTSC standards were developed before many of our modern technological advancements, the television signal could not be transmitted fast enough to send the information rapidly enough. So a compromise was made, sending only half the information in each frame transmission. But rather than sending only the top half of the picture and then the bottom half for the next frame, the concept of interlacing was developed to send first the odd-numbered lines, 1, 3, 5, 7, 9, 11, up to 525, during the first frame, and then sending the even-numbered lines, 2, 4, 6, 8, 10, up to 525, during the second frame. This alternating continues continuously, odd/even, odd/even, displayed rapidly enough that the viewer perceives the illusion of motion.

As manufacturers develop larger and larger displays for televisions, monitors, and projectors, the flickering of an interlaced signal became more objectionable. Technology improved, making higher/faster data rates possible, and computer monitors developed without the need for interlacing. These were called NI, non-interlacing monitors, or de-interlaced monitors. Because this seems like a double negative, the term progressive scan took its place.

Wikipedia interlacing graphic

Interlacing graphic from Wikipedia 

Digital Television (DTV) refers to the new method of broadcasting television signals, sending digital data using radio frequencies, rather than analog signals. DTV will require a new television tuner, which is included in nearly all TV’s sold today. Older TV’s can be utilized with DTV signals, with the use of an external digital ATSC tuner box. Older TV’s will not be able to display HDTV unless they were HDTV-compatible, and not all digital signals are wide-screen or HDTV. But the signal quality is improved (both video and audio).

DTV is currently in operation, as most broadcast television stations have added the new equipment required to send out digital signals. In fact, the FCC has mandated that the existing VHF and UHF television frequencies must cease operation on February 17, 2009. The new digital television frequencies are UHF frequencies that were unused by the old UHF channels. But the old channels (which have specific Megahertz, such as 187 MHz for channel 9, or 549.25 MHz for channel 27) must be vacated in 2009, and will be re-sold and reassigned by the FCC for other uses.

Cable television reception, satellite television receivers, and DVD players are not effected by the DTV change, in that they will still function. But the older television tuners (and the older tuners in VCR’s) will not tune in the new frequencies and digital signals without the ATSC converter boxes. From revenue generated by the sale of the use of the new digital frequencies, the U.S. government is offering one or two $40 coupons to use towards purchase of an off-air DTV tuner, allowing an older TV to display broadcast television signals after February, 2009. See the website, http://www.ntia.doc.gov/otiahome/dtv/, for more information and to request a coupon to be mailed to you.

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