Questions and Answers about HDTV

1.  What is HDTV?     HDTV is an acronym for High Definition Television. HDTV was first introduced in the early 1980's in Japan. It was known there as the MUSE system, which was an analog HDTV storage and delivery system. MUSE had great picture quality but required immense amounts of bandwidth for storage and broadcasting, so it never really caught on. HDTV is now possible because of digital encoding which allows compression of the signal to make it easier to store and transmit. Digitally encoding a television picture is known as DTV, or Digital Television. A signal that is DTV is not necessarily HDTV - think of small dish systems and the digital channels on the 4DTV system. However, all HDTV signals are DTV - or digital.
There is one addition acronym the consumer needs to know, that is EDTV or Extended Definition Television. EDTV is a digital television picture that is a little better than the current NTSC (National Television Standards Committee - the formal name of the system we all currently watch) but is not quite up to HDTV standards. This includes such transmission schemes as 480p DVD.
 
2.  What is the difference between HDTV and Standard TV (NTSC)?     To the point - it's about the picture and sound. HDTV offers about twice as many "lines" of resolution and better quality, digitally encrypted five-channel sound as well. Our current television system is more than 60 years old. It works entirely in the analog domain, much like an AM radio receiver. Although the ability to add color was included in the 1950's, the basic standard for transmitting and decoding a television picture has not changed since.
HDTV works entirely in the digital domain and so has more in common with your computer than with your current television set. HDTV signals are transmitted or stored in the form of data bits. Because these data bits are stored and transmitted with parity checking and error correcting ability, the picture is always outstanding - as long as you can receive the data stream you will get the best picture without ghosting, noise, sparkles or static.

In terms of what you can see, your current television system, NTSC, is capable of a picture approaching 500 lines of resolution. That is, the system can resolve details to 1/500 the picture width. HDTV will provide you with twice that amount of picture information both vertically and horizontally.
 
3.  What are aspect ratios?     The aspect ratio of a television display or television picture describes the ratio of the picture width to height. Current NTSC pictures are 4 units wide by 3 units tall, nearly square. This came about because that ratio, 4X3 or 1.33:1, was the ratio used for early motion pictures. also called the "Academy Ratio" for the Motion Picture Academy that established it, it was chosen so movies could be shown on television without having to change their basic composition and shape
As television gained in popularity, people found less of a reason to go to the movie theater. In response to this decline in business, moviemakers created new aspect ratio standards to create ever-larger images that use more of the viewer's peripheral vision. This immersed the viewers into a "virtual reality" experience long before that term was even coined.

HDTV, being a system designed for the home viewing of movies and other programming, will use a wide-screen aspect ratio that is similar to, but still a little narrower than, major movies. Many movies are released in aspect ratios as wide as 2.20:1. HDTV will be presented in a 16:9 format (1.77:1).
 
4.  Why are aspect ratios important?     Aspect ratios are part of the cinematography of the film. Imagine what 2001, A Space Odyssey would look like if the picture were as square as the Academy Ratio. This movie was intended to subtend a large angle of view in order to put the viewer into the action. The aspect ratio helps to create the effect the director envisioned. It is part of the art of film.
It is impossible to take all the information in a movie like 2001 and put it on a standard television screen. If the total height of the movie screen is used (so you don't cut off the heads and feet of the actors) then you must "crop" the image that falls outside of the 3X4-image area. If you use the total width of the image, then you have black bars at the top and bottom of the screen. This is known as "letterbox." The wide aspect ratio of HDTV will allow the vision of the movie artists to be delivered to your home with less compromise and greater impact.
 

5.  What is the difference between NTSC, 1080i, 720p and native?     To understand this we must have a basic understanding of how the NTSC system works. The following explanation is a generalization, for the enthusiast looking for all the details there are several good web sites which explain the complete operation of NTSC television.
The NTSC system shows us 30 picture frames-per-second (fps) using two fields. Each field consists of half the scanning lines of the frame. Thus, in the first 1/60 second the television tubes illuminate odd numbered scanning lines 1-525 (there are always 525 vertical lines, horizontal resolution is the term often used to describe the sharpness of a TV set in "lines of resolution"). In the second 1/60 second the picture tube illuminates the even lines (2-524). these two fields of scanning lines are therefore interlaced at a frequency of 15750Hz. All of this is keyed to the power line frequency (60Hz) - in Europe the televisions are keyed to 50Hz.

The "i" in 1080i stands "for interlaced" which means the picture is constructed exactly as detailed above but at a much higher rate. Instead of using 525 vertical lines of information, the 1080i system uses 1080 lines of information. 540 lines are illuminated every 1/60 second and interlaced with the remaining 540 lines at a frequency of32400Hz. This faster scanning rate, coupled with more lines of information, creates a picture that does not exhibit the graininess, flicker and visible scanning lines of NTSC. Further, the entire system carries with it a wider bandwidth (bandwidth is proportional to picture detail) allowing about twice as much information to be displayed when compared with NTSC.

The "p" in 720p stands for progressive. Progressive scan systems operate like a computer system; they show each scanning line sequentially without interlace. Thus, a 720p system would possess 720 horizontal scanning lines, which are illuminated every 1/60 second. A 720p system can be said to operate at a frequency at 43200Hz. As the scanning frequency of a set gets higher, the set gets more expensive and difficult to product. This is why you do not find too many 35" computer monitors - they can get very expensive.

While a detailed discussion of the merits of progressive vs. interlaced design is beyond the scope of this document, it can be said that 1080i is currently the dominant form for HDTV transmission and display. For the sake of flexibility, the Motorola HDD200, for example, offers the user the ability to choose, via a back panel switch, whether the system should output information in the form of 1080 interlaced only, 720 progressive only, or "native" (meaning what comes in is what goes out).
 
6.
 What is the importance of having AC-3?     AC-3 is one form of Dolby Digital surround-sound which allows the user to experience a virtual reality of sound created with a matrix of 5 speakers and a subwoofer (known as a Low Frequency Effects channel). Currently the 4DTV IRD will allow the user to listen to Dolby Digital sound in monaural, stereo and pro-logic surround sound. .On the surface of it Pro-logic and AC-3 seem very similar: they both use five speakers and a subwoofer, speaker positioning is identical, both systems offer total immersion into the movie sound-field, wide dynamic range, and good detail.

Look first at Dolby Pro-logic (which is identical to Dolby surround sound and THX in a conceptual context). Dolby Pro-logic is a system that provides left, center, right and surround channels from a 4-2-4-matrix. It works like this...

The sum and difference between the two channels are used to derive the center and surround sound channels algebraically. If we label the two storage channels as A and B:

A + B = center channel sound
A - B = rear channel sound
(A + B) + (A - B) = Channel R (right channel)
(A + B) - (A - B) = Channel L (left channel)

Dolby AC-3, on the other hand, is a digital encoding system that offers 5.1 discrete channels of sound. The five channels are right front, center, left front, right rear, left rear and a low frequency effect (subwoofer) channel. The addition of the 1.1 channels, and the fact the system offers discrete channels with greater separation than the matrix system described above, allows the sound to mimic directional cues and spatial effects to a very high degree.
 
7.
 What is SPDIF?     SPDIF stands for Sony Philips Digital Interface. This is an interface standard that is specified in the compact disc "red book". The "red book" describes in detail the workings of digital audio transmission, storage and replay within a compact disc digital audio environment. SPDIF protocol uses a coaxial copper cable to connect a digital audio source to a digital-to-analog (D/A) converter or digital processor. Toslink (Toshiba) is another interface protocol that specifies the use of a fiber optic cable between digital components.
Most consumer audio products that offer AC-3 sound decoding also offer an SPDIF interface. In addition, many audiophiles and industry professionals feel that the SPDIF protocol allows for better sound quality than TosLink. ..
 
8.
 What is S-Video?     S-Video is an interface protocol that allows a video component to pass a video signal along to a monitor or recording device in a segregated chrominance (color) and luminance (black and white detail) format. This interface is very effective at eliminating "dot crawl" and other picture anomalies on signals that are received from a component quality source. If the original source is composite (black-and-white and color information have been combined) then the S-video connection offers essentially no advantages. Let's examine this.
On a digital picture, such as the digital channels on your 4DTV IRD, the signal can be sent directly from the programmer to your system with the color and black-and-white information separated. This allows your monitor to display the picture with all the fidelity possible.

If the signal is an analog source, such as an analog VCRS or clear feed from your 4DTV, then the color and black-and-white information have been combined at the programmer level and must be separated again before your TV can display the image. This separation is done by a "comb filter" located in the television. While it is possible to add a comb filter to an analog satellite receiver (or laser disc player, for that matter) and feed an S-video signal to your monitor, the question becomes which component has the best comb filter - the monitor or satellite receiver? Since a comb filter is present in every television, and since comb filter design and performance are directly dependent on the requirements of the monitor, the best comb filter is usually the one found in your television monitor. This is especially true on modern HDTV monitors.

For the best performance the signal from an analog source should be sent to the monitor via the RCA composite output, the signal from digital channels sent via S-Video connection and the signal from HDTV programs via the YPrPb connections located on the back of the HDD200 decoder.
 
9.
 What is the difference between RGB and YPrPb?     RGB stands for red, green and blue: the base colors from which all the colors your television set monitor can display are derived. The HDD200 decoder offers RGB + sync (horizontal and vertical sync) connections so you can use front projection and professional quality CRT monitors to display the high definition image.
A new interface is also supported. This is called YPrPb (also known as 770.3) and is the HDTV equivalent of an S-Video connector. YPrPb transmits the picture information in a luminance and phase-opposite chrominance pair over three coax cables. Most new HDTV monitors offer YPrPb connections.
 
10.   Who invented HDTV?     High Definition Television, as we know it today, was created by a Grand Alliance of companies working together to create the next generation of television. Members of the Grand alliance included General Instrument, Zenith, Sarnoff Labs (RCA), the Massachusetts Institute of Technology (MIT) and AT&T Labs and Philips. The Grand Alliance was formed in May of 1993 under the auspices of the FCC and was tasked with producing a single set of standards that would allow HDTV transmission to be phased into use in the United States. 

  11.  When will all my channels be HDTV?     It is very unlikely that there will be a day when every channel is HDTV. HDTV is just one option among many for the transmittal of television picture information. Even though the entire television universe will likely become a digital universe in the years to come, HDTV will be reserved for programming where extreme picture quality is required. Many less critical programs will remain in SDTV (standard definition television) format. 

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