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Sample rate conversion

Sample rate conversion. At times, it will be necessary to convert the sampling rate in a source signal to some other sampling rate Consider converting from a CCIR 601 signal (a digital video standard) to an MPEG SIF signal MPEG is a compression standard for video

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Sample rate conversion

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  1. Sample rate conversion • At times, it will be necessary to convert the sampling rate in a source signal to some other sampling rate • Consider converting from a CCIR 601 signal (a digital video standard) to an MPEG SIF signal • MPEG is a compression standard for video • SIF is the source input format for the compression) • CCIR 601 is an interlaced signal with a 50 Hz field rate • The signal consists of three components: Y, U and V

  2. Sample rate conversion • The Y component (luminance) is sampled at a resolution of 720x480 • The U and V components (chrominance) are sampled at a resolution of 360x480 • The sampling pattern is shown on the next slide

  3. CCIR 601 Sampling Pattern

  4. Sample rate conversion • MPEG SIF samples luminance at a resolution of 360x240 and chrominance at a resolution of 180x120 • The sampling pattern is different as well, as shown on the next slide

  5. MPEG SIF Sampling Pattern

  6. Sample rate conversion • The conversion to the lower sampling resolution begins with the discarding of one of the interlaced fields • This reduces the picture rate to 25 Hz (non-interlaced) and reduces the vertical resolution by one half

  7. Sample rate conversion • Now, the luminance is decimated by one half in the horizontal direction • One possibility is to subsample the values, however better results are obtained by applying an FIR filter before subsampling • One filter which has been found to give good results in decimating the luminance is shown on the following slide

  8. Luminance subsampling filter weights

  9. Sample rate conversion • The results of multiplying the filter weights by the original values is divided by 256 • Use of a power of two allows a simple hardware implmentation • An example of the use of this filter followed by subsampling is shown on the next slide • At the ends of lines, some special technique such as renormalizing the filter or replicating the last pixel must be used • In the example below, the data in the line is reflected at each end

  10. Example of filtering and subsampling of a line of pixels 10 12 20 30 35 15 19 11 11 19 26 45 80 90 92 90 32 32 23 9 12 49 95 95

  11. Sample rate conversion • The chrominance samples have to be placed at a horizontal position in the middle of the luminance samples • A linear filter with a phase shift of half a sample is used for this task e.g.

  12. Chrominance subsampling filter weights

  13. Sample rate conversion • The result is divided by eight • The chrominance values are vertically decimated after they have been horizontally decimated • The conversion process is pictured on the next slide

  14. CCIR to SIF Conversion

  15. CCIR to SIF Conversion • After this conversion, further compression takes place and the MPEG video is transmitted • At the receiver, the signal must be uncompressed and then reconverted • After zeroes have been inserted between samples, a linear phase FIR filter is applied for upsampling • For reconversion, both luminance and chrominance components are upsampled both horizontally and vertically

  16. Upsampling filter for luminance

  17. Upsampling filter for chrominance

  18. Temporal resampling • We may also find it necessary to convert from one picture rate to another • Consider film digitized at 24 frames per second which is to be shown on television at 60 fields per second • The required conversion can be accomplished by the technique of 3:2 pulldown as shown

  19. 3:2 Pulldown

  20. Temporal resampling • Video coded at 25 pictures per second can be converted to 50 fields per second by displaying the original decoded lines in the odd fields and the interpolated lines in the even fields • Video coded at 24 pictures per second may be converted to 50 fields per second by speeding it up by about 4% and decoding it as if it had been encoded at 25 pictures per second

  21. Temporal resampling • The decoded pictures can be displayed in the odd fields, and interpolated pictures in the even field • The audio must be maintained in synchronization, either by increasing the pitch, or by speeding it up without a pitch change

  22. High Definition Television • High-Definition Television (HDTV) is high-resolution digital television (DTV) combined with Dolby Digital surround sound (AC-3) • HDTV is the highest DTV resolution in the new set of standards

  23. High Definition Television • History of HDTV in the United States • The Federal Communications Commission (FCC) formed the Advanced Television Systems Committee (ATSC) to examine proposals for a standard for HDTV in the United States • In March 1990, the FCC announced that it would consider only simulcast systems for a standard - systems in which one broadcast channel carries an NTSC signal and a second channel carries an HDTV signal

  24. High Definition Television • There were five proposals • Four are from American groups • All of these employ digital broadcasting • The fifth proposal, from the Japan Broadcasting Corp. (NHK) is an analog system called MUSE (Multiple Sub-Nyquist Encoding) • MUSE was used on a test basis in Japan • The simulcast signals must make use of the currently unoccupied and unavailable taboo channels • These channels are those which would cause interference if they were used for broadcasting NTSC signals

  25. High Definition Television • With UHF channels, for example, the minimum distance between co-channel transmitters (those with the same channel allocation) varies between 250 and 355 kilometers (155-250 miles) • For adjacent channels, the minimum specified distance is 90 km (55 miles)

  26. High Definition Television • The digital proposals all advocate using the taboo channels due to the fact that digital signals can be transmitted with far less power than can analog signals • The required average transmitter power for the digital portion of an HDTV signal can be less than 10 percent of that required by an NTSC transmitter with the same service area

  27. High Definition Television • All of the proposed digital systems used some form of video bandwidth compression after analog-to-digital conversion • All systems also used some form of error correction coding so that the digital signal input to the source decoder portion of HDTV decoders will be accurate

  28. High Definition Television • HDTV signals require compression because they are much larger than NTSC signals • The signals have twice as much luminance definition, both vertically and horizontally (four times as many luminance pixels) • Further additional pixels are needed for the wider screen (16:9 aspect ratio instead of 4:3) • The increase in luminance detail requires about five times the video bandwidth of conventional television systems

  29. High Definition Television • When the additional chrominance detail is factored in, the total bandwidth required is 6 to 8 times that of conventional systems • Eventually, the 4 competitors joined into a “Grand Alliance” which combined the best of the proposals into a single proposal • This proposal was accepted in 1996

  30. High Definition Television • Of the 18 DTV formats, six are HDTV formats, five of which use progressive scanning and one interlaced scanning • Of the remaining formats, eight are SDTV (four wide-screen formats with 16:9 aspect ratios, and four conventional formats with 4:3 aspect ratios), and the remaining four are video graphics array ( VGA ) formats • Stations are free to choose which formats to broadcast

  31. High Definition Television • The formats used in HDTV are: • 720p - 1280x720 pixels progressive • 1080i - 1920x1080 pixels interlaced • 1080p - 1920x1080 pixels progressive

  32. Digital Television • DTV uses MPEG-2 compression to fit in the 6 MhZ bandwidth used by analog television • MPEG-2 is also used in • DVD videos • Some satellite TV broadcast systems • The FCC has mandated that all stations be capable of broadcasting HDTV by 2006

  33. DTV Transition Timeline • November 1999 - Affiliates from the top 30 markets (which reach 50 percent of U.S. households) must have constructed digital facilities. • December 1999 - Stations in some of the major markets, including New York, Los Angeles, Atlanta and Chicago, were broadcasting digital signals • May 2002 - The remaining markets after the top 30 (there are 211 total) will have constructed digital facilities. • April 2003 - All stations must simulcast at least 50 percent of their NTSC programs on their digital TV channel.

  34. DTV Transition Timeline • April 2004 - Stations must simulcast 75 percent of their NTSC programs on their digital TV channel. • April 2005 - Stations must simulcast 100 percent of their NTSC programs on their digital TV channel. • 2006 - The Federal Communications Commission has targeted this year for the complete conversion from analog to digital broadcasting.

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