Toner Cable Equipment, Inc.

• Digital Television (DTV)
• MPEG-2 Video Coding
• Formats and Bandwidth
• Digital Visual Interface (DVI)
• SDI
• ASI
• 8VSB
• QAM
• QPSK
• Compression
• Channels
• Acronym Definitions

Digital Television (DTV)

Digital television is the new method of transmitting television signals to the consumers TV set. Digital TV is actually the next evolution from Analog television.

Analog television was developed in 1939 -1941, when standards for television were adopted by the National Television Standards Committee (NTSC). This was done to ensure all televisions were compatible with one broadcasting method. This was for black and white TV. In 1953, the NTSC again adopted a standard for color TV but the standard required that Televisions also be able to receive black and white transmissions.

In 1982, a new group called the Advanced Television Systems Committee (ATSC) was formed. This is an international group created to develop standards for digital television. ATSC Digital TV Standards include digital high definition television (HDTV), standard definition television (SDTV), data broadcasting, multichannel surround-sound audio, and interactive television. In 1996, the Federal Communications Commission (FCC) adopted the major elements of the ATSC Digital Television (DTV) Standard. DTV uses digital modulation data, which is digitally compressed and requires decoding by a Digital TV, a digital set-top box, or a PC fitted with a digital television card.

The ATSC Digital Television Standard is based on the MPEG-2 Video Standard.

MPEG-2 is widely used as the format of digital television signals that are transmitted by broadcast (over-the-air), cable TV, and direct broadcast satellite systems. It also specifies the format of movies and other programs that are distributed on DVD and similar disks. TV stations, TV receivers, DVD players, and other equipment are often designed to this standard. MPEG-2 was the second of several standards developed by the Moving Pictures Expert Group (MPEG) and is an international standard.

MPEG-2 Video Coding

An HDTV camera generates a raw video stream of more than 75,000,000 bytes per second. This stream must be compressed if digital TV is to fit in the bandwidth of available TV channels and if movies are to fit on DVDs. Fortunately, video compression is practical because the data in pictures is often redundant in space and time. For example, the sky can be blue across the top of a picture and that blue sky can persist for frame after frame. Also, because of the way the eye works, it is possible to delete some data from video pictures with almost no noticeable degradation in image quality.

TV cameras used in broadcasting usually generate 59.94 pictures a second (in North America). Digital television requires that these pictures be digitized so that they can be processed by computer hardware. Each picture element (a pixel) is then represented by one luma number and two chrominance numbers. These describe the brightness and the color of the pixel. Thus, each digitized picture is initially represented by three rectangular arrays of numbers.

A common (and old) trick to reduce the amount of data is to separate the picture into two fields: the "top field," which is the odd numbered rows, and the "bottom field," which is the even numbered rows. The two fields are displayed alternately. This is called interlaced video (I). Two successive fields are called a frame. The typical frame rate is then 29.97 frames per second. If the video is not interlaced, then it is called progressive video (P) and each picture is a frame. MPEG-2 supports both options.

Another trick to reduce the data rate is to thin out the two chrominance matrices. In effect, the remaining chrominance values represent the nearby values that are deleted. Thinning works because the eye is more responsive to brightness than to color. The 4:2:2 chrominance format indicates that half the chrominance values have been deleted. The 4:2:0 chrominance format indicates that three quarters of the chrominance values have been deleted. If no chrominance values have been deleted, the chrominance format is 4:4:4. MPEG-2 allows all three options.

MPEG-2 specifies that the raw frames be compressed into three kinds of frames: intra-coded frames (I-frames), predictive-coded frames (P-frames), and bi-directionally-predictive-coded frames (B-frames).

An I-frame is a compressed version of a single uncompressed (raw) frame. It takes advantage of spatial redundancy and of the inability of the eye to detect certain changes in the image. Unlike P-frames and B-frames, I-frames do not depend on data in the preceding or the following frames. Briefly, the raw frame is divided into 8 pixel by 8 pixel blocks. The data in each block is transformed by a discrete cosine transform. The result is an 8 by 8 matrix of coefficients. The transform converts spatial variations into frequency variations, but it does not change the information in the block. The original block can be recreated exactly by applying the inverse cosine transform. The advantage of doing this is that the image can now be simplified by quantizing the coefficients. Many of the coefficients, usually the higher frequency components, will then be zero. The penalty of this step is the loss of some subtle distinctions in brightness and color. If one applies the inverse transform to the matrix after it is quantized, one gets an image that looks very similar to the original image but that is not quite as nuanced. Next, the quantized coefficient matrix is itself compressed. Typically, one corner of the quantized matrix is filled with zeros. By starting in the opposite corner of the matrix, then zigzagging through the matrix to combine the coefficients into a string, then substituting run-length codes for consecutive zeros in that string, and then applying Huffman coding to that result, one reduces the matrix to a smaller array of numbers. It is this array that is broadcast or that is put on DVDs. In the receiver or the player, the whole process is reversed, enabling the receiver to reconstruct, to a close approximation, the original frame.

Typically, every 15th frame or so is made into an I-frame. P-frames and B-frames might follow an I-frame like this, IBBPBBPBBPBB(I), to form a Group Of Pictures (GOP); however, the standard is flexible about this.

P-frames provide more compression than I-frames because they take advantage of the data in the previous I-frame or P-frame. I-frames and P-frames are called reference frames. To generate a P-frame, the previous reference frame is reconstructed, just as it would be in a TV receiver or DVD player. The frame being compressed is divided into 16 pixel by 16 pixel macroblocks. Then, for each of those macroblocks, the reconstructed reference frame is searched to find that 16 by 16 macroblock that best matches the macroblock being compressed. The offset is encoded as a "motion vector." Frequently, the offset is zero, but if something in the picture is moving, the offset might be something like 23 pixels to the right and 4 pixels up. The match between the two macroblocks will often not be perfect. To correct for this, the encoder computes the strings of coefficient values as described above for both macroblocks and then subtracts one from the other. This "residual" is appended to the motion vector and the result sent to the receiver or stored on the DVD for each macroblock being compressed. Sometimes no suitable match is found. Then, the macroblock is treated like an I-frame macroblock.

The processing of B-frames is similar to that of P-frames except that B-frames use the picture in the following reference frame as well as the picture in the preceding reference frame. As a result, B-frames usually provide more compression than P-frames. B-frames are never reference frames.

While the above generally describes MPEG-2 video compression, there are many details that are not discussed, including details involving fields, chrominance formats, responses to scene changes, special codes that label the parts of the bitstream, and other pieces of information.

Formats and Bandwidth

All digital television signals start out as a digital Video / Audio signal usually from a digital camera. A digital video camera typically outputs an uncompressed signal of about 400 Mbit/s in one of many common standards such as DVI and SDI.

Digital Visual Interface (DVI) is a video interface standard designed to maximize the visual quality of digital display devices such as flat panel LCD computer displays and digital projectors. It was developed by an industry consortium, the Digital Display Working Group (DDWG). It is designed for carrying uncompressed digital video data to a display. It is partially compatible with the High-Definition Multimedia Interface (HDMI) standard in digital mode (DVI-D).

SDI The Serial Digital Interface  standardized in ITU-R BT.656 and SMPTE 259M, is a digital video interface used for broadcast-grade video.

A related standard, known as HD-SDI, High Definition Serial Digital Interface, is standardized in SMPTE 292M; this provides a nominal data rate of 1.485 Gbit/s.

An emerging interface, commonly known in the industry as dual link HD-SDI and consisting essentially of a pair of SMPTE 292M links, is standardized in SMPTE 372M; this provides a nominal 2.970 Gbit/s interface used in applications (such as digital cinema) that require greater fidelity and resolution than standard HDTV can provide. A more recent interface, consisting of a single 2.97 Gbit/s serial link, is standardized in SMPTE 424M.

These standards are used for transmission of uncompressed, unencrypted digital video signals (optionally including embedded audio) within television facilities; they can also be used for packetized data. They are designed for operation over short distances; due to their high bitrates they are inappropriate for long-distance transmission. SDI and HD-SDI are currently only available in professional video equipment; various licensing agreements, restricting the use of unencrypted digital interfaces to professional equipment, prohibit their use in consumer equipment.

ASI Asynchronous Serial Interface, is streaming format much like an MPEG Transport Stream MPEG-TS. It can contain a single program or be a multiplex of multiple programs, each consisting of video and audio. It is electrically identical to an SDI signal. There are two elementary streams that are addressed by the ASI interface, the 188 byte format and the 204 byte format. The 188 byte format is the more common ASI transport steam. When optional data is included, the packet can stretch an extra 16 bytes to 204 bytes total.

In current practice, high-definition television (HDTV), which is usually used over DTV, uses one of two formats: 1280 x 720 pixels in progressive scan mode (abbreviated 720p) or 1920 x 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 x 1080 at a 60 Hz progressive scan frame rate - known as 1080p60 .)

The ATSC standard, used in the United States, uses MPEG-2 video at the Main Profile @ High Level, with additional restrictions: The maximum bitrate of the MPEG-2 video stream is exactly 19.4 Mbps for broadcast television, and exactly 38.8 Mbps for the "high-data-rate" mode (e.g., cable television). (The practical limit is somewhat lower, since the MPEG-2 video stream must fit inside a transport stream, with overhead, sent out at 19.3927... Mbps for broadcast.)

8VSB is the 8-level vestigial sideband modulation method adopted for terrestrial broadcast of the ATSC digital television standard in the United States, Canada, and other countries.

In the 6 MHz channel used for broadcast ATSC, 8VSB carries 19.39 Mbit/s of usable data, although the actual transmitted bit rate is significantly higher due to the addition of forward error correction codes. The eight signal levels are selected with the use of a trellis encoder. There are also the similar modulations 2VSB, 4VSB, and 16VSB. 16VSB was notably intended to be used for ATSC digital cable, but quadrature amplitude modulation (QAM) has become the de facto industry standard instead

QAM is the modulation method adopted for Cable television transmission of the ATSC digital television standard in the United States, Canada, and other countries.

Quadrature amplitude modulation is a modulation scheme which conveys data by changing (modulating) the amplitude of two carrier waves. These two waves, usually sinusoids, are out of phase with each other by 90 degrees and are thus called quadrature carriers-hence the name of the scheme.

Like all modulation schemes, QAM conveys data by changing some aspect of a carrier signal, or the carrier wave, (usually a sinusoid) in response to a data signal. In the case of QAM, the amplitude of two waves, 90 degrees out-of-phase with each other (in quadrature) are changed (modulated or keyed) to represent the data signal.

Phase modulation (analog PM) and phase-shift keying (digital PSK) can be regarded as a special case of QAM, where the amplitude of the modulating signal is constant, with only the phase varying. This can also be extended to frequency modulation (FM) and frequency-shift keying (FSK), for these can be regarded a special case of phase modulation.

QPSK is the modulation method adopted for Satellite television transmission of the ATSC digital television standard in the United States, Canada, and other countries.

Quadrature Phase-Shift Keying is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave).

Any digital modulation scheme uses a finite number of distinct signals to represent digital data. PSK uses a finite number of phases, each assigned a unique pattern of binary bits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data. This requires the receiver to be able to compare the phase of the received signal to a reference signal - such a system is termed coherent (thereby term CPSK for it).

QPSK uses four points on the constellation diagram, equispaced around a circle. With four phases, QPSK can encode two bits per symbol. Although QPSK can be viewed as a quaternary modulation, it is easier to see it as two independently modulated quadrature carriers. With this interpretation, the even (or odd) bits are used to modulate the in-phase component of the carrier, while the odd (or even) bits are used to modulate the quadrature-phase component of the carrier.

Compression

Off air Broadcast (8VSB) and Cable (QAM) both utilize digital video compression to transmit more than one program in a normal 6 MHz television channel.

In the case of Broadcast, the broadcaster will transmit from 2 to 8 different programs in the space of one TV channel. What determines how many channels are transmitted normally depends on whether the channels are standard definition TV or high definition TV. Also, the nature of the programming will determine the number of channels. A channel with "talking head" such as a new reader, can be compressed a lot more than a sporting event because each frame is almost exactly the same as the preveious frame and the next frame. In this case, the amount of data that needs to be transmitted for each frame is low compared to a channel with a lot of movement. In a sporting event such as a football game with a lot of motion, the compression that can be accomplished is much less because so much from frame to frame changes. When there is a large difference between frames, there is a large amount of data that needs to be transmitted for each frame. This limits the amount of compression. Cable systems which use QAM Modulation can compress as many as 14 standard definition channels into one 6 MHz channel, although 8 to 10 is more typical.

Channels

The compression of multiple programs into one 6 MHz channel complicates the notion of a "channel" in digital TV. The traditional 6 MHz channels, such as channel 7 or channel 29, now actually carry multiple programs. This has necessitated that we re-think what we refer to as channels and has resulted in a new scheme.  We now have the Major channel number and Minor channel number.

The Major number is also known as a virtual channel number. This is a number that the broadcaster chooses that can display on your television. For instance, a broadcaster broadcasting on channel 29 (6 MHz channel) would use 29 as the Major channel number. Then, if the broadcaster is transmitting 6 different programs on the 6 MHz channel, each program would get a Minor number such as 1 or 3 or 7 or whatever number is chosen. The programs are then identified 29-1, 29-3 and so on.

Technically, there can be up to 1,024 Minor channels in a Major channel, though in practice only a few are used (since the bandwidth must be divided among the Minor channels).

For digital cable, the cable companies assign a channel such as 29-3, a physical channel number on their set top box or converter such as channel 732. This is referred to as channel mapping and allows the cable company flexibility in setting up their system and to maximize available bandwidth.

This also allows the cable company to change the frequency of a channel without changing what the customer sees as a channel number. In such arrangements, the Major/Minor channel number are called the "QAM channel", and the alternative channel designation is called the "mapped channel", "virtual channel", or simply "channel".

Acronym Definitions

8VSB 8-level Vestigial Sideband Modulation (Off air broadcast)

QPSK Quadrature Phase-Shift Keying Modulation (Satellite transmission )

QAM Quadrature Amplitude Modulation (Cable TV systems)

ATSC Advanced Television Systems Committee

MPEG Motion Picture Experts Group

MPEG-2 The second standard created by MPEG for Digital images and sound, a compression standard

MPEG-4 The fourth standard created by MPEG, an advanced version of MPEG-2

PIXEL The basic and smallest picture element, A 1080P TV displays 2,073,600 pixels

HDTV High definition TV: 1280 x720 pixels (720P) or 1920 x1080 pixels 1080P

SDTV Standard Definition TV: Resolutions below that of HD such as 480I or 576P

SDI Serial Digital Interface: A digital video interface of broadcast grade, uncompressed, Unencrypted digital video signals

HD-SDI High Definition Serial Digital Interface: A digital video interface of broadcast grade, uncompressed, Unencrypted, High definition digital video signals

ASI Asynchronous Serial Interface: a streaming format of a compressed digital video signal. It may contain single or multiple programs with both video and audio and can be standard definition and or high definition.

CODEC A device that converts analog A/V signals to digital signals or digital A/V signals to analog, "an encoder"

RGB A video connection or Component video where the Red, Green and Blue signals are on a separate connector or cable. The sync signal must also be sent typically on a separate cable but occasionally on the Green cable. RGB is often confused with YPbPr component video because these connectors are also Red, Green and Blue. Y (luminance) is on the Green connector, Pb (blue component) is on the Blue connector and Pr (red component) is on the red connector.

SMPTE Society of Motion Picture and Television Engineers
SMPTE 259M is named and known as SDI (standard definition)
SMPTE 292M is named and known as HD-SDI
SMPTE 424M is named and known as 3G-SDI
SMPTE 372M is named and known as Dual Link HD-SDI