HDTV Digital television also known as DTV and High Definition Television, also known as HDTV, is a technological advancement compared to the analog television most Americans have now. High definition was a marvel that was bound to come. It seems that every time a new technology emerges, it is a must have, but what is a high-definition television? That is what I’m going to find out. Television is a field of electronic technology that has a very high effect on people of the world today. It is a very specialized branch of technology. The concept of television was developed in the 1920’s and was shown in the 1930’s. Commercial broadcasting started in the 1940’s. The 30 largest broadcast markets in the U.S. began digital transmission in 1999.
Digital Television The Digital Television (DTV) has many meanings for the public. Is DTV high-definition television (HDTV)? Will this improve the quality? DTV standards are based on the standards by the Advanced Television System Committee (ATSC). This committee provides the transmission of programs into a 16X9 format and they provide a transmission of a standard definition television (STDV) format. Thus it provides a digital picture with comparable resolution to analog formats.
DTV Video The ITU-R 60 format is used to convert the analog video to a digital format. This is an international standard for digitizing component video. ITU-R 601 is the base sampling frequency and the standard is 3.375 MHz’s the sample rate is 4:2:2 of elements of a component video signal. Green, red, and blue are the colors of the video signal components. Green channel provides the luminance information. R-Y and B-Y values provide the color difference. Y, R-Y and B-Y are converted to a digital signal. Basically the luminance channel consists of R-Y and B-Y. The calculations of the sample rates are 13.5 MHz, 6.75MHz, and 6.75MHz. Bit rate are 135Mbps, 67.5Mbps, 67.5Mbps, each are *10bits/sample. Data must go under some sort of compression so that the data can fit into 6-MHz bandwidth. DTV transmissions are MPEG2, which stands for Motion Pictures Expert Group. The techniques must rely on the redundancies in the video signal. There are six redundancies in a video signal: 1. Statistical Data Redundancy - only little of a video signal is constantly changing, 2. Psych Visual Redundancy - human eye cannot see all the detail therefore things can be minimized, 3. Entropy - information must be available to provide reconstruction of the image, 4. Luminance/chrominance contrast - only can see if it has a lot of contrast, (black/white), 5. Spatial redundancies - areas that detect errors, 6. Temporal redundancies - can notice if it flickers in video signals that are very bright.
The Audio Signal The digital compression technique specified for digital television, as defined by ATSC document A/52, details the digital audio compression (AC-3) standard developed by Dolby Laboratories. This system provides five full-bandwidth audio channels 3 Hz to 20 kHz. The five channels are for the left, center, right, and left-right surround-sound channels. The standard also has one low frequency enhancement channel which reduces to 3 Hz to 120Hz. 5.1 Channel input is the new audio system, which provides for sample rates and input word lengths up to 24 bits. The six audio outputs are multiplexed together, which results in a 5.184-Mbps data stream. This data stream is then compressed to a 384-kbps data stream.
Transmission
The output of the MPEG2 video encoder is multiplexed with the AC-3 audio encoded data stream. Additional data are also multiplexed with the MPEG2 and AC-3 data streams to form a 19.39-Mbps ATSC data stream. The 19.39-Mbps multiplexed data stream is input into the exciter of an 8VSB transmitter, which fits the data stream into a 6-MHz bandwidth. 8VSB is the ATSC approved method for transmitting digital television. It is a 8-level vestigial-sideband modulator, hence the 8VSB abbreviation. The 8 VSB signal constellation somewhat resembles a 64-QAM constellation, except the 8VSB requires the decoding of only the I channel. This helps to minimize the electronics required in the receiver.
8VSB Exciter The 8VSB exciter consists of six parts. The purpose of an exciter is to prepare the input information for transmission. In the case with 8VSB, the input information is the multiplexed 19.39-Mbps ATSC digital data stream. This input data must be properly conditioned to comply with the ATSC standard. Each part of the 8VSB exciter is described as Frame Synchronizer: This is used by the 8VSB exciter to synchronize the MPEG2 data packets to the 8VSB circuitry. Synchronization is accomplished by using the first byte in each MPEG2 packet. MPEG2 was eventually discarded and replaced by the ATSC segment sync. Data Randomizer: This is used to make the 8VSB data stream appear to be completely random. This is done to make the 8VSB RF spectrum to be flat across the entire channel. This is done so the 8VSB data will fit into the allocated DTV 6-MHz channel bandwidth. REED SOLOMON Encoder: A forward error correction (FEC) technique called REED SOLOMON Encoding is applied to the 187 bytes of incoming MPEG2 data packet. Twenty REED SOLOMON parity bits are added to the 8VSB.The parity bits are used to determine if a byte was received correctly. If an error is detected, the receiver can use the parity bits to correct the data. Data Interleaver: This order scrambles the order of the 8VSB data stream. In effect, this spreads the data out over the data stream, minimizing its sensitivity to burst type interference. Burst type interference is when large pieces of data are lost in transmission because of atmospheric disturbances or other types of temporary interference. Trellis Encoder: This is used by the exciter to provide another form of forward error correction. Trellis coding breaks up an 8-bit byte into four 2-bit words. The 2-bit words are then compared to past 2-bit words. A 3-bit code is generated from the 2-bit comparisons and the 3-bit words are transmitted. This is called a 2/3 encoder. At the DTV receiver, the transmitted 3-bit words are used to reconstruct the original 2-bit words. Pilot/Sync Insertion: The last piece of the 8VSB exciter is insertion of the pilot and sync signals. ATSC Pilot: The pilot provides a clock for the 8VSB receiver to lock onto. The pilot is located at the zero-frequency point of the 8VSB spectrum is 524-530 MHz, then the pilot is located at 524 MHz’s The pilot signal is similar to the color-burst signal in NTSC transmissions, which provides a stable clock reference for color signal reconstruction. Segment Sync: This repetitive 1-byte pulse is added to the front of the data segment. The frame sync is used by the 8VSB exciter to generate the receiver clock and ultimately recover the data. The segment sync is similar to the horizontal sync pulse in an NTSC signal. Frame Sync: The ATSC data frame consists of 313 data segments. The frame sync is repeated once per frame. In an NTSC world, this is comparable to the vertical sync. The frame sync has a “known” data pattern, which can be used to help the receiver remove signal irregularities, such as ghosting, that introduced by poor reception.
Reception and Coverage Area Associated with DTV reception is whether or not the licensed coverage area will be the same as it was for analog broadcast. Distortion in the received signal will impair the quality of the received signal. ATSC has recommended a signal-to-noise (SNR) ratio of 27 dB or better at the reception point. For analog transmissions, poor SNR values result in frozen pictures and repeated pixel values. This is called pixelate, where the picture freezes into a series of patterns even when there is motion in the video. Also, multipath or noisy reception can lead to no signal at the insert a blue screen to substitute for the lost data. When this happens, many receivers insert a blue screen to substitute for the lost data. This screen is humorously called the “blue screen of death” by broadcasters. New channels are adjacent channels therefore transmissions DTV and NTSC could interfere into the NTSC transmission. TV receivers reverse the procedure introduced at the DTV transmitter. The receivers must have the capability to display 1920X1080, 1280X720 or 720X480 pixel formats. They support the aspect ratio of both HDTV and SDTV.
Monitoring the DTV Signal The maximum modulating rate for the video signal is 4 MHz because it is amplitude-modulated onto a carrier, a bandwidth of 8 MHz is implied. However, the FCC allows only a 6-MHz bandwidth per TV station, and that must also include the FM audio signal. The lower visual sideband extends only 1.25 MHz below its carrier with the remainder filtered out, but the upper sideband is transmitted in full. The audio carrier is 4.5 MHz above the picture carrier with FM sidebands as created by its plus, negative 25-kHz deviation. The lower sideband is mostly removed by filters that occur near the transmitter output. While only one sideband is necessary, it would be impossible to filter out the entire lower sideband without affecting the amplitude and phase of the lower frequencies of the upper sideband and the carrier. Thus, part of the 6-MHz bandwidth is occupied by a “vestige” of the lower sideband. This is commonly referred to as vestigial - sideband operation. It offers the added advantage that carrier reinsertion at the receiver is not necessary as in SSB because the carrier is not attenuated in vestigial-sideband systems.
Once the entire TV signal is generated, it is amplified and driven into an antenna that converts the electrical energy into radio waves. These waves travel through the atmosphere to be intercepted by a TV receiving antenna and fed into the receiver once again as an electrical signal. That consists of the video, audio, and synchronizing signals. The synchronizing signals are contained in the video signal.
NTSC TRANSMITTER PRINCIPLES
An NTSC transmitter is actually two separate transmitters. The aural or sound transmitter is an FM system similar to broadcast FM radio. It is still a high-fidelity system because the same 30-Hz to 15-kHz audio range is transmitted. The major difference between broadcast FM and TV audio systems is that TV uses a +-25-kHz deviation. Recall that broadcast FM uses a +-75-kHz deviation. Thus, the TV aural signal has the same fidelity but is less effective in canceling the indirect noise effects. The video, or picture, signal is amplitude-modulated onto a carrier. Thus, the composite transmitted signal is a combination of both AM and FM principles. This is done to minimize interference effects between the two at the receiver because an FM receiver is relatively insensitive to amplitude modulation and an AM receiver has rejection capabilities to frequency modulation. The TV camera converts a visual picture or scene into an electrical signal. The camera is thus a transducer between light energy and electrical energy. At the receiver, the CRT picture tube is the analogous transducer that converts the electrical energy back into light energy. The microphone and speaker are the similarly related transducers for the sound transmission. There are actually two more transducers shown, the sending and receiving antennas. They convert between electrical energy and the electromagnetic energy required for transmission through the atmosphere. The diplexer feeds the transmitter antenna both the visual and aural signals to the antenna while not allowing either to be fed back into the other transmitter. Without the diplexer, the low-output impedance of the other transmitter’s power amplifier would dissipate much of the output power of the other transmitter. The synchronizing signal block will be explained in the next section.
TV Cameras
The most widely used image pickup device is the charge couple device (CCD). CCD cameras are used in many applications such as broadcasting, imaging, scientific studies, security, and military applications. The CCD is a solid-state chip consisting of thousands or millions of photosensitive cells arranged in a two-dimensional array. When light (photons) strike the CCD surface, the light information is converted to an electronic analog of the light. The electronic information is then shifted out of the device serially in what is called a bucket brigade. The clocking of the bucket brigade is controlled by the timing of the particular system being used. An important limitation of CCD devices is the maximum speed at which the information placed on the device can be serially shifted out to storage. Undesired characteristics such as smearing can result if the information is not transferred correctly.
Scanning
In this simplified system, the camera focuses the letter “T” onto the photosensitive cells in the CCD imaging device, but instead of a million cells, this system has just 30, arranged in 6 rows with 5 cells per row. Each separate area is called a pixel, which is short for “picture element.” The greater the number of pixels, the better the quality (or resolution) of the transmitted picture. The letter “T” is focused on the light-sensitive area so that all of rows 1 and 6 are illuminated, while all of row 2 is dark and the centers of rows 3, 4, and 5 are dark. Now, if we scan each row sequentially and if the retrace time is essentially zero, the sequential breakup of information. The retrace interval is the time it takes to move from the end of one line back to the start of the next lower line. It is usually accomplished very rapidly. The variable light on the photosensitive cells results in a similar variable voltage being developed at the CCD’s output. The visual scene has been converted to a video (electrical) signal and can now be suitably amplified and used to amplitude-modulate a carrier for broadcast. The picture for broadcast National Television Systems committee (NTSC) TV has been standardized at a 4:3 ratio of the width to the height. This is termed the aspect ratio and was selected as the most pleasing picture orientation to the human eye.
NTSC Transmitter/Receiver Synchronization
When the video signal is detected at the receiver, some means of synchronizing the transmitter and receiver is necessary. When the TV camera starts scanning line 1, the receiver must also start scanning line 1 on the CRT output display. You do not want the top of a scene appearing at the center of the TV screen. The speed that the transmitter scans each line must be exactly duplicated by the receiver canning process to avoid distortion in the receiver output. The horizontal retrace, or time when the electron beam is returned back to the left –hand side to start tracing a new line, must occur coincidentally at both transmitter and receiver. You do not want the horizontal lines starting at the center of the TV screen. When a complete set of horizontal lines has been scanned, moving the electron beam from the end of the bottom line to the start of the top line (vertical fly back or retrace) must occur simultaneously at both transmitter and receiver. Visual transmissions are more complex than audio because of these synchronization requirements. At this point, voice transmission seems elementary because it can be sent on a continuous basis without synchronization. Thus, the other major function of the transmitter besides developing the video and audio signals is to generate synchronizing signals that can be used by the receiver so that is stays in step with the transmitter. In the scanning process for a television, the electron beam starts at the upper left-hand corner and sweeps horizontally to the right side. It then is rapidly returned to the left side, and this interval is termed horizontal retrace. An appropriate analogy to this process is the movement of your eye as you read this line and rapidly retrace to the left and drop slightly for the next line. When all the horizontal lines have been traced, the electron beam must move from the lower right-hand corner up to the upper left-hand corner for the next “picture.” This vertical retrace interval is analogous to the time it takes the eye to move from the bottom of one page to the top of the next. Federal Communications Commission (FCC) regulations stipulate that U.S. NTSC TV broadcasts shall consist of 525 horizontal scanning lines. Of these, about 40 lines are lost as a result of the vertical retrace interval. This leaves 485 visible lines that you can actually see if a TV screen is viewed at close range. The number of visible lines does not depend on the TV screen size. Because this scanning occurs rapidly, persistence of vision and CRT phosphor persistence cause us to perceive these 485 lines as a complete image. Persistence is the length of time an image stays on the screen after the electrical signal is removed.
Inter-laced Scanning
The frame frequency is the number of times per second that a complete set of 485 lines (complete picture) is traced. That rate for broadcast TV is 30 times per second. Stated another way, a compete scene (frame) is traced every 1/30 s (second). Thirty frames per second is not enough to keep the human eye from perceiving flicker as a result of a non-continuous visual presentation. This flicker effect is observed when watching old-time movies. If the frame frequency were increased to 60 per second, the flicker would no longer be apparent, but the video signal bandwidth would have to be doubled. Instead of that solution, the process of interlaced scanning is used to “trick” the human eye into thinking it is seeing 60 pictures per second. The first set of lines (the first field) is traced in 1/60 s, and then the second set of lines (the second field) that comprises a full scene (485 lines total) is interleaved between the first lines in the next 1/60 s. Therefore, lines 2, 4, 6, etc., occur during the first field, with lines 1, 3, 5, etc., interleaved between the even-numbered lines. The field frequency is thus 60 Hz (the actual field rate is 59.94 Hz) with a frame frequency of 30 Hz. The illusion is enough to convince the eye that 60 pictures per second occur when, in fact, here are only 30 full pictures per second. The process of interlacing in TV is analogous to a trick used in motion picture projection to prevent flicker (non-continuous motion). In motion pictures, the goal is to conserve film rather than bandwidth, and this is accomplished by flashing each of the 24 frames per second onto the screen twice to create the illusion of 48 pictures per second.
Horizontal Synchronization
To accommodate the 525 lines (485 visible) every 1/30 s, the transmitter must send a synchronization (sync) pulse between every line of video signal so that perfect transmitter-receiver synchronization is maintained. Three horizontal sync pulses are shown along with the video signal for two lines. The actual horizontal sync pulse rides on top of so-called blanking pulse, as shown in the figure. The blanking pulse is a strong enough signal so that the electron beam retrace at the receiver s blacked out and thus invisible to the viewer. The interval before the horizontal sync pulse appears on the blanking pulse is termed the front porch, while the interval after the end o the sync pulse, but before the end o the blanking pulse, is call the back porch. The back porch includes an eight-cycle sine-wave burst at 3.579.545 Hz. It is appropriately called the color burst, because it is used to calibrate the receiver color subcarrier generator. Naturally enough, a black and white broadcast does not include the color burst. The two-lines of video picture signal can be described as follows; line 2 - It starts out nearly full black at the left-hand side and gradually lightens to full white at the right-hand side and line 4 - It starts our medium gray and stays there until one-third of the way over, when it gradually becomes black at the picture center. It suddenly shifts to white and gradually turns darker gray at the right-hand side. Since the horizontal sync pulses occur once for each of the 525 lines every 1/30 s, he frequency of these pulses will be 525 x 30 = 15.75 kHz. Thus, both transmitter and receiver must contain 15.75-kHz horizontal oscillators to control horizontal electron beam movement.
Vertical Synchronization The vertical retrace and thus vertical sync pulses must occur after each 1/60 s since the two interlaced fields that make up one frame (picture) occur 60 times per second. The video signal just before during and after vertical retrace. Look at the two horizontal sync pulses and the last two lines of video information of a field are initially. These are followed in succession by equalizing pulses at a frequency double the horizontal sweep rate, or 15.75 kHz x 2 = 31.5 kHz. They each have duration of about 2.7 us with a period of 1/31.5 kHz, or 31.75 us. They are used to keep the receiver horizontal oscillator in sync during the relatively long (830 to 1330 us) vertical blanking period. One vertical sync pulse with a 190-us pulse width and five serrations having duration of 4.4 us at 27.3 us intervals. These serrations are used to keep the horizontal oscillator synchronized during the vertical sync pulse interval, more equalizing pulses. Horizontal sync pulses until the entire vertical blanking period has elapsed. Notice that the vertical blanking period is variable because the number of visible lines transmitted can vary between 482 and 495 at the discretion of the station. All other aspects of the pulses such as number, width, and rise and fall times are tightly specified b the FCC so that all receiver manufacturers know precisely what type of signal their sets have to process. The vertical sync pulses occur at a frequency f 60 Hz (the exact rate is 59.94), which is the same frequency as the ac line voltage in North America. This allows for good stability of the vertical oscillator in the receiver. In Europe, where 50-Hz line voltage exists, a 50-Hz vertical oscillator system is used.
Resolution
To provide adequate resolution, the video signal must include modulating frequency components from dc up to 4 MHz This requires a truly wide band amplifier, and amplifiers that have band pass characteristics from dc up into the MHz region have come to be known as video amplifiers. Resolution is the ability to resolve detailed picture elements. We already have an idea about resolution in the vertical direction. Since about 485 separate horizontal lines are trace per picture, it might seem that the vertical resolution would be 485 lines. Vertical resolution may be defined as the number of horizontal lines that can be resolved. However, the actual resolution turns out to be about 0.7 of the number of horizontal lines, or 0.7 x 485 = 339. Thus, the vertical resolution of broadcast TV is about 339 lines. Horizontal resolution is defined as the number of vertical lines that can be resolved. A little mathematical analysis will show this capability. The maximum modulating frequency has already been stated as 4 MHz The more vertical lines to resolve, the higher the frequency of the resulting video signal. The horizontal trace occurs at 15.75-kHz frequency, and thus each line is 63.5 us (1/15.75 kHz) in durations. The horizontal blanking time is about 10 us, leaving 53.5 us. Since two consecutive lines can be converted into the highest rate video signal the number of vertical lines resolvable is 4 MHz x 53.5 us x 2 = 428. Thus, the horizontal resolution is about 428 lines. Note that the 428 vertical lines conform nicely to the 339 horizontal lines when one remembers that a TV screen has a 4:3 width to height (aspect) ratio (428/339 = 4/3). Thus, equal resolution exists in both directions, as is desirable. Increased modulating signal rates above 4 MHz allow for increased vertical or horizontal resolutions or some increase for both.
Principles of NTSC Color Television
We have thus far been concerned mainly with black-and-white or monochrome television. While color TV presents a much greater degree of sophistication, the student who has mastered monochrome principles reasonably well can advance to the color set by adding a few more basic ideas. Our system for color TV was instituted in 1953 and is termed compatible. That is, a color transmission can be reproduced in black and white shades by a monochrome receiver, and a monochrome transmission is reproduced in black and white by a color receiver. To remain compatible, the same total 6-MHz bandwidth must be used, but more information (color) must be transmitted. This problem is overcome by a form of multiplexing, as when FM stereo was added to FM broadcasting. It turns out that the video signal information is clustered at 15.75-kHz (the horizontal oscillator frequency) intervals throughout its 4-MHz bandwidth. Midway between these 15.75-kHz clusters (harmonics) of information are unused frequencies, as indicated in Figure 17-34. By generating the color information around just the right color subcarrier frequency (3.579545 MHz), it becomes centered in clusters exactly between the black-and-white signals. This is known as interleaving. At the color TV transmitter, the scene to be televised is actually scanned by three separate pickup sensors in the camera, each camera sensitive to just one of the three primary colors: red, blue, and green. Because various combinations of these three colors can be mixed to form any color to which the human eye is sensitive, an electrical representation of a complete color scene is possible. The tree color cameras scan the scene in unison, with the red, green and blue color content separated into three different signals. This process is accomplished within the color TV camera as shown in Figure 17-35. The lens focuses the scene onto a beam splitter that feeds three separate light filters. The red filter passes only the red portion of the scene, resulting in the R (red) electrical signal. A blue and a green filter accomplish the same process to generate the B and G signals. At the receiver, these three separate signals are made to illuminate properly groups of red, green, and blue phosphor dots (called triads), and the original scene is reproduced in color. After generation these three separate color signals are fed into the transmitter signal processing circuits (matrix) and create the Y, or luminance, signal the Chroma, or color, signals I and Q. The Y signal contains just the right proportion of red, blue and green so that it creates a normal black-and-white picture. This proportion is Y = 0.3R + 0.59G + 0.11B. It modulates the video carrier just as does the signal from a single black-and-white camera with a $-MHz bandwidth. The Chroma signals, I and Q, are used to phase modulate the 3.58-MHz color subcarrier, which the interleaves their color information the gaps left by luminance Y signal’s sidebands. The proportions for I and Q are: I = 0.6R + 0.28G + 0.32B and Q = 0.21R – 0.52G + 0.31B. This modulation by the I and Q signals is accomplished in a balanced modulator, thus suppressing the 3.58-MHz subcarrier because it would cause interference at the receiver. At the receiver, a monochrome set simply detects the Y signal and thus presents a normal black and white rendition of a color picture. The Chroma signals (I and Q) cannot be detected in a monochrome set because their 3.58-MHz subcarrier was suppressed and is not present in the received signal. Thus, a color set must have a means to generate and reject the 3.58-MHz subcarrier to enable detection of the I and Q signals. Notice in Figure 17-36 hat the Q signal is a full DSB signal with sidebands extending +500 kHz around the color subcarrier, which is 3.58 MHz above the overall carrier frequency. The I signal has a lower sideband 1.5 MHz below the color subcarrier. It is a vestigial sideband signal, however, because the upper sideband is attenuated after 500 kHz. The NTSC (National Television Systems Committee) signal and was approved by the FCC in 1953. The I and Q signals are summed with the Y signal to modulate the TV carrier frequency. The chrominance signals (I and Q) modulate the 3.58 MHz subcarrier in separate balanced modulators. The subcarriers are 90 out of phase (in quadrature). The two double-sideband signals created ( I and Q) can be separately recovered at the receiver because of this quadrature modulation process.
The Color CRT and Convergence
Color receiver CRTs are a marvel of engineering precision. As previously mentioned, they are made up of triad of red, blue and green phosphor dots. The trick is to get the proper electron beam to strike it respective colored phosphor dot. This is accomplished by passing the three beams through a single hole in the shadow mask. The shadow mask prevents the “red” beam from spilling over onto an adjacent blue or green phosphor dot, which would certainly destroy the color rendition. A typical color CRT has over 200,000 holes in the shadow mask and triads of phosphor dots. To make the three beams converge correctly on their color dot of phosphor throughout the face of the tube requires special modification to the horizontal and vertical deflection systems. Static convergence refers to proper beam convergence at the center of the CRT’s face. This adjustment is made by dc level changes in the horizontal and vertical amplifiers. Convergence away from the center becomes more of a problem and is referred to as dynamic convergence. It is necessary because the tube face away from the center is not a perfectly spherical shape (it is more nearly flat), and thus the beams tend to converge in front of the shadow mask away from the tube center. Special dynamic convergence voltages are derived from the horizontal and vertical amplifier signals and are applied to a special color convergence yoke placed around the tube yoke. The dynamic convergence of a set involves the shown magnet adjustment and several adjustments (usually 12) on the convergence board that have interaction effects. The process is quite involved and time consuming.
Conclusion
Overall High-Definition Television does have many advantages; however making the switch from analog to digital is not an easy task. Although high-digital television produces a better picture, better sound and more options many people are just not willing to make the switch if it requires purchasing a new television. Personally I think that digital television is a great concept and would benefit people in many ways once the switch it complete. The switch is the hardest part, if it could magically happen overnight that would be great; however it is not that easy. High-Digital television is just too expensive for many people right now. Some people are still not willing to go out and spend well over $1,000 on a television set if they have a perfectly good set at home. When people are ready to purchase new sets then they may be more willing to spend that amount of money. I see High-digital television being a great thing in the future, but it will defiantly take some time to get to that point.

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