Residential Audio & Video Systems Copyright © 2005 Heathkit Company, Inc. All Rights Reserved Presentation 18 – Digital Television (DTV) – Part 2

2 Objectives At the end of this presentation, you will be able to:

3 Explain how DTV can place three times as much information in a given bandwidth as conventional TV. Explain two ways in which DTV takes advantage of its bandwidth. Name the digital compression schemes used by DTV for video and audio. Explain the difference between spatial and temporal encoding compression techniques.

4 DTV Enabling Technologies Digital Compression Error Detection and Correction Enough processing power in the receiver to recover the original data.

5 Active Lines/ Picture Pixel Per Line Aspect Ratio Picture Rate :924p, 30p, 60i :924p, 30p, 60p :924p, 30p, 60i, 60p :324p, 30p, 60i, 60p :324p, 30p, 60i, 60p Picture Rate = Frames per second (progressive) or fields per second (interlaced) i = interlaced; p = progressive

6 HD Formats in Use 1080i – NBC, CBS, and many of the HD cable channels. 720p – Fox, ABC, and ESPN

7 1080i Versus 720p 1080i – NBC, CBS 1080 x 1920 x 30 = 62.2 megapixels per second HD 720p – Fox, ABC, ESPN 720 x 1280 x 60 = 55.3 megapixels per second

8 Pixels per second verses bit per second. 1080i requires about 62 megapixels per second while 720p requires about 55 megapixels per second. Multiple bits of data are required to fully define one pixel. Thus, 1080i requires a bit stream of about 62 million times 20 bits or about Gigabits per second

9 Raw DTV video and audio data require enormous bandwidth. HDTV – up to to gigabits per second SDTV – up to 270 megabits per second 5.1 Channel Dolby Digital Audio – up to 9 megabits per second.

10 Each TV Channel is: 6 MHz wide Capable of handling bits rates up to 19.3 megabits per second. Less than 2% of the speed required by HDTV.

11 Digital Compression Reduces bandwidth by discarding data. The greater the compression, the lower the bandwidth. The greater the compression, the more likely the loss will be noticeable. Goal: use as few bits as possible while preserving the video and audio quality.

12 Digital Compression Lossy – A discernable difference between the original uncompressed data and the recovered decompressed data. Lossless – No discernable difference between the original uncompressed data and the recovered decompressed data.

13 Lossy Compression

14 Lossy Compression

15 Lossless Compression (Spatial Encoding)

16 Spatial Encoding Discards duplicate data. Each frame broken into its frequency components. Some high frequency details discarded.

17 Lossless Compression (Temporal Encoding)

18

19 Temporal Encoding Adjacent frames are compared. Unchanged pixels and changed pixels. Unchanged pixels transmitted once. Changed pixels transmitted as needed.

20 Compression Ratio Number of bits before compression divided by number of bits after compression. Compressing 1000 bits of data into 500 bits gives a 2:1 compression ratio. Compressing 1000 bits of data into 250 bits gives a 4:1 compression ratio. Compressing 1000 bits to 80 bits gives a 12.5:1 compression ratio.

21 MPEG-2 Eliminates redundant data by examining successive frames and discarding information that does not change. (Temporal encoding) Compression ratios of 50:1 are common. Too much action produces objectionable visual problems when the compression scheme can’t keep up. The compression scheme used with Digital TV.

22 Audio Compression DTV uses Dolby AC-3 Compression Audio separated into its frequency components. Some high frequency details discarded.

23 JPEG JPEC – Uses a mathematical algorithm to eliminate data that the viewer is unlikely to miss. Works within a single frame rather than comparing successive frames. Compression ratios of 15 to 1 are common. The most common format for transmitting photographs via the Internet.

24 Using the 19.3Mbps Data Rate Single HD Channel Three SD Channels One HD Channel and one SD Channel

25 WETA TV During the day time WETA broadcasts three different standard definition TV programs simultaneously. At night WETA broadcasts one high definition program and one standard definition program simultaneously.

26 HD Video Surround Sound MPEG-2 Encoder AC-3 Encoder Multi- plexer 1G14M 9M 380k Uncompressed (bps) Compressed (bps)

27 HD Video Surround Sound SD Video Stereo Sound MPEG-2 Encoder AC-3 Encoder MPEG-2 Encoder AC-3 Encoder Multi- plexer 1G14M 9M 380k 250M 2M 4.5M 90k Uncompressed (bps) Compressed (bps)

28 HD Video Surround Sound SD Video Stereo Sound Data Services MPEG-2 Encoder AC-3 Encoder MPEG-2 Encoder AC-3 Encoder Multi- plexer 1G14M 9M 380k 250M 2M 4.5M 90k <1M Uncompressed (bps) Compressed (bps) 19.3 Mbps

29 Frame Synchronizer Data Randomizer Reed- Solomon Encoder Data Interleaver Trellis Encoder Sync Insertion Pilot Insertion 8VSB Modulator Amplifier/ Transmitter 19.3 Mbps RF Output

30 Frame Synchronizer Determines the starting and ending points of each incoming packet. Synchronizes packets with circuitry.

31 Data Randomizer Randomizes each incoming byte using a pseudo-random number generator that has a known pattern. Spreads the data evenly across the entire 6 MHz bandwidth of the channel. The process is later reversed in the receiver.

32 Reed-Solomon Encoding An error correction scheme in which additional bits are added to each packet. The additional bits are mathematically related to the contents of the packet. By comparing the added bits to the contents of the package the receiver can detect and correct errors that may have occurred during transmission.

33 Data Interleaver Scrambles the sequential order of the data stream. Recreates new packets out of fragments of the old, with each MPEG-2 packet being spread across several new packets. Minimizes burst interference which might otherwise destroy a complete MPEG-2 packet. When the MPEG-2 packets are reconstructed at the receiver from the scattered fragments, the noise burst is spread across many MPEG-2 packets keeping the error rate below the Reed-Solomon error correction rate.

34 Trellis Encoder Another form of error correction. Data is split into 2-bit segments. Each 2-bit segment is converted to a 3-bit segment that describes the transition from one segment to another. The code with the addition bit allows the receiver to follow the trail of the transitions automatically correcting errors along the way.

35 Sync and Pilot Insertion Helper signals are inserted that aid the receiver in recovering the original data. Sync signals are added that help the receiver recover the original system clock and thereby keep everything in sync. The data is also shifted slightly to insure that some of the carrier frequency (called a pilot) occurs in the final RF output. The pilot is needed by the receiver to lock on properly to the RF Carrier.

36 8-Level Vestigial Sideband (8VSB) Modulator Bit stream has grown to over 32Mbps. 8 amplitude levels are used to represent 3- bits each. Each change in amplitude is called a symbol and represents 3-bits. About 10 million symbols per second are amplitude modulated onto an RF carrier.

37 Frame Synchronizer Data Randomizer Reed- Solomon Encoder Data Interleaver Trellis Encoder Sync Insertion Pilot Insertion 8VSB Modulator Amplifier/ Transmitter 19.3 Mbps RF Output

38 The DTV Receiver Tune the channel Reject adjacent channels and noise sources Demodulate the 8-level VSB signal recovering the 32 Mbps bit stream. Convert 3-bit trellis code segments back to their original 2-bit format correcting errors as necessary. Unscramble the work of the data interleaver recreating the Reed-Solomon coded MPEG-2 packets. Convert the Reed-Solomon coded packets back to their MPEG-2 format correcting errors as necessary.

39 The DTV Receiver (Continued) Reverse the work of the data randomizer recreating the original multiplexed 19.3 Mbps stream. Synchronize packets Separate the sub-channels if sub-channels were used. Separate video, audio, and data services signals. Decompress the MPEG-2 and AC-3 packets into their original uncompressed form. Route video, audio, and data services signals Present the programming content.

40 The DTV Receiver Requires significant computing power. Much more complex than the NTSC receiver. More complex display. More complex audio.

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