sample rate conversion at times, it will be necessary to convert the sampling rate in a source...

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


• 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

CCIR 601 Sampling Pattern


• 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

MPEG SIF Sampling Pattern


• 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


• 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

Luminance subsampling filter weights

-29 0 88 138 88 0 -29


• 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

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


• 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.

Chrominance subsampling filter weights

1 3 3 1


• 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

CCIR to SIF Conversion

720

480CCIR 601 Y Odd Field Only

240

HorizontalFilter andSubsample

360

240SIF

a. Luma

CCIR 601U, V

360

480

Select1 Field

Select1 Field

OddField

360

240

HorizontalField andSubsample

180

240

VerticalFilter andSubsample

180

120SIF

b. Chroma

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

Upsampling filter for luminance

-12 0 140 256 140 0 -12 / / 256

Upsampling filter for chrominance

1 3 3 1 / / 4

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

3:2 Pulldown

. . .

1 2 3 4

24 frames/second

. . .

1 2 2 3 4

30 frames/second

1/6 second

1/6 second

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

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

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


• 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


• 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


• 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)


• 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


• 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


• 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


• 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


• 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


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

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

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.

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.

sample rate conversion at times, it will be necessary to convert the sampling rate in a source...

Documents

following slide slide

luminance slide

chrominance slide

sif conversion slide

half slide

v slide

end slide

sampling rate