DigitalCompressionCh1

DigitalCompressionCh1 - D IG I T AL C OMPRESSION for M...

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DIG I TAL COMPRESSION for MULTIMEDIA Principles and Standards Jerry D. Gibson Toby Berger Tom Lookabaugh Dave Lindbergh Richard L. Baker Morgan Kaufmann Publishers, Inc. San Francisco, California Southern Methodist University Cornell University DiviCom PictureTel Corporation PictureTel Corporation iI!
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1 Introduction to Data CHAPTER Compression 1.1 Why Compress? Entertainment, telecommunications, the Internet-all are part of our daily lives. We enjoy them, use them in our businesses, surf them. We read about them in magazines and newspapers. We hear about them on television. We invest in them. But we've had radio, TV; music, and telephones for decades. What's new now? Is it all hype? Not really-at least, not all of it! The new word is "digital." Today we are talking about digital communications systems and networks and digital representations of movies, TV; music, images, and voice. Why digital? Digital signals are easy to store and easy to transmit over long distances without accumulating distortion, and stored digital representations (for example, of music) are highly resistant to minor degradations. But there is a downside. Digital versions of important signals, like voice, music, TV; and movies, require more bits per second of signal to store or transmit, which translates into higher costs. For example, Table 1.1 presents the raw (uncompressed) data rates of several important source signals (Jayant, Johnston, and Safranek 1993). Certainly, many of the numbers seem large, but these numbers are only significant in comparison to the storage capacity available or the rate that can be sent over a chosen communications link. To get some idea of the significance of the rates in Table 1.1, note that the current common telephone modem rate is 28.8 kbps and the bit rate allocated to voice in North American digital cellular is 8 kbps, so the uncompressed 96-kbps requirement for telephone bandwidth voice is too high by about 12:1. Further, CD-ROM capacity is roughly 650 megabytes, and the capacity of one version of the evolving digital versatile (or video) disc (DVD-S) is roughly 40 gigabits.
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2 Introduction to Data Compression ____________________________lIIiIlj . m!l~amW1iml [email protected]· ·_ TAB L E Approximate Bit Rates for Uncompressed Sources 1.1 Telephony 8000 samples/second x 12 bits/sample = 96 kbps (200-3400 Hz): Wideband speech 16,000 samples/second x 14 bits/sample 224 kbps (50-7000 HZ): Wideband audio 44,100 samples/second x 2 channels x 16 bits/sample (20-20,000 Hz): == 1.412 Mbps Images: 512 x 512 pixel color image x 24 bits/pixel = 6.3 Mbits/image Video: 640 x 480 pixel color image x 24 bits/pixel x 30 images/second = 221 Mbps HDTV: 1280 x 720 pixel color image x 60 images/second x 24 bits/pixel - 1.3 Gbps Thus, for uncompressed video, the CD-ROM could store 23.5 seconds, and the DVD-5 could store about 3 minutes. If
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This note was uploaded on 12/29/2011 for the course ECE 241 taught by Professor Gibson,j during the Fall '08 term at UCSB.

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DigitalCompressionCh1 - D IG I T AL C OMPRESSION for M...

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