2009-7-27 P,1
Unit 11
Data Compression
Chapter Three
&Part4 Extensive Reading@ Part 1 Notes to Text
@Part5 Notes to Passage& Part 2 Word Study
@Part3 Practice on Text @Part6 Practice on Passage
2009-7-27 P,2
@ Part 1 Notes to Text
1) In computer science and information theory,data compression or
source coding is the process of encoding information using fewer
bits than an unencoded representation would use through use of
specific encoding schemes,在计算机科学和信息理论中，数据压缩或信源编码其实就是指采用比未经编码的信息表现方式更少的字节，通过专门的编码系统进行编码的过程。
注释,此句是主系表句型。 data compression or source coding 是主语，
the process of encoding information是表语,using fewer bits和
through use of specific encoding schemes是修饰 encoding
information的状语。 than an unencoded representation would use
是比较状语从句。
2)For example,this article could be encoded with fewer bits if one
were to accept the convention that the word "compression" be
encoded as "comp".例如，假如我们接受这样一种约定，也即是：将
"compression"编译为 "comp"，那么用较少的位数就可以给这篇文章完成编码。
注释,if one were to… 是条件状语从句,were是虚拟语气，表示假设；
that引导的从句是同位语，用以解释 convention的具体内容。
Data Compression
2009-7-27 P,3
3) One popular instance of compression with which many
computer users are familiar is the ZIP file format,which,as well
as providing compression,acts as an archiver,storing many
files in a single output file,ZIP文件格式就是众多计算机使用者都很熟悉的一种普及型压缩实例，它不但提供压缩，而且充当文件存储器，
在一个 ZIP输出文件中就可以存放很多文件。
注释,此句是主系表句型。 One popular instance of compression
是主语； the ZIP file format 是表语； with which many computer
users are familiar是介词提前的定语从句,which指代前面的
instance of compression； which,as well as providing
compression,acts as an archiver,storing many files in a single
output file.是非限制性定语从句,which指代前面的 the ZIP file
format； providing compression和 storing many files in a single
output file都是这个定语从句里的状语。
2009-7-27 P,4
4) A compression method called Huffman coding is frequently used
in data communications,and particularly in fax transmission.有种叫哈夫曼编码的压缩方法经常被用在数据通信，特别是传真传输上。
注释,Huffmancoding（哈夫曼编码）是一种根据字符出现的概率来构造平均长度最短的编码。它是一种变长的编码。在编码中，若各码字长度严格按照码字所对应符号出现概率的大小的逆序排列，则编码的平均长度是最小的。（注：码字即为符号经哈夫曼编码后得到的编码，其长度是因符号出现的概率而不同，所以说哈夫曼编码是变长的编码。）
5) Were the letter made up of randomly distributed black ink
covering 50 percent of the white paper surface,data compression
would hold no advantages,如果构成信件的那些随意分布的黑色墨迹占据了白纸表面的 50%，那么，数据压缩在此时就没什么优势可言了。
注释,Were the letter made up of…= If the letter were made up of… 。
在书面语中，条件从句可以不用连词 if，而将谓语中的过去式 were,had
或 should等移至主语前。这属于虚拟语气中的一种特殊结构形式；
covering 50 percent of the white paper surface是现在分词短语做定语，
修饰前面的 black ink。
End
2009-7-27 P,5 End
1,representation n,表现法，表现 8,alter v,改变
2,convention n,惯例，协定 9,restore v,恢复
3,archiver n,档案存储器 10,procedure n,过程
4,statistically adv,统计上地 11,decompression n,解压缩
5,character n,字符 12,consecutive string 连贯的字符串
6,binary adj,二进位的，二元的 13,randomly distributed 随意分布的
7,assign v,赋值，指定
& Part 2 Word Study
2009-7-27 P,6
Task 1,Listening training
Dictation of some key words
Directions,You will hear some key words twice,most of which are taken from
the Word Study,Then finish the Crossword Puzzle,(See you book p,)
2,Dictation of some key expressions
Directions,You will hear some key expressions twice,most of which are
taken from this text,Then you are asked to complete the missing words.
@Part3 Practice on Text
1) data________________
2) ___________information
3) a single_______________
4) an ___________________message
5) in fax___________________
6) ______________________black ink
compression
encoding
output file
uncompressed
transmission
randomly distributed
2009-7-27 P,7
Read and Reply
Directions:Students are required to read aloud the whole text after
the teacher,Then give short answers to the following questions:
a) What is data compression?
b) What would be discovered if a typical message were
statistically analyzed?
Task 2,Read,Write and Translate
Data compression is the process of encoding information
using fewer bits than an unencoded representation would use
through use of specific encoding schemes,
If a typical message were statistically analyzed,it would be
found that certain characters are used much more frequently
than others,
No,
c) Can all kinds of data be compressed to the same degree?
2009-7-27 P,8
( )1) Data compression or source coding is the process of
encoding information using more bits than an unencoded
representation would use through use of specific encoding
schemes,
( )2) The ZIP file format only provides compression,
( )3) By analyzing a message before it is transmitted,longer
binary codes may be assigned to frequently used characters
and shorter codes to rarely used characters,
( )4) Data representing a computer program may not be
reduced significantly,
( )5) Huffman coding can help reduce the total message
length for any faxed business letter,
Read and Judge
End
F
F
F
T
F
2009-7-27 P,9
Directions:Read the following short passage,Complete it by filling in the missing words.
A,require B,enough C,useful D,detrimental E,involve
F,compress G,such H,including I,decompressed J,instance
Compression is 1_____because it helps reduce the consumption
of expensive resources,2） _____as hard disk space or transmission
bandwidth,On the downside,compressed data must be 3） ___ to be
viewed,and this extra processing may be 4） ___ to some
applications,For 5） ______,a compression scheme for video may 6）
_____expensive hardware for the video to be decompressed fast 7）
_____to be viewed as it's being decompressed,The design of data
compression schemes therefore 8） _____trade-offs among various
factors,9） ______the degree of compression,the amount of
distortion introduced,and the computational resources required to
10） _____and uncompress the data.
Read and Complete
End
C
G
I
D
J
A
B
E
H
F
2009-7-27 P,10
1)他们抛弃了过去的习惯。（ convention）
2)我们使他恢复原职。（ restore）
3)魔术师叫我随便挑一张牌。 (at random)
4)因为下大雨，我们不得不改变计划。 (alter)
5)那件工作分配给我了。 (assign)
Read and Translate
Directions:Put the following sentences into English,using the words or phrases given.
End
They abandoned the conventions of the past.
We restored him to his old post,
The magician asked me to pick a card at random,
We had to alter the plan because of the heavy rain.
The task was assigned to me,
2009-7-27 P,11
Lossy Audio Compression
Lossy(有损耗的 ) audio compression is used in an extremely wide
range of applications,In addition to the direct applications (mp3
players or computers),digitally compressed audio streams are
used in most video DVDs; digital television; streaming media(流媒体 )on the internet; satellite and cable radio; and increasingly in
terrestrial(地域性的 )radio broadcasts,Lossy compression typically
achieves far greater compression than lossless(无损的 ) compression
(data of 5 percent to 20 percent of the original stream,rather than
50 percent to 60 percent),by discarding(丢弃 ) less-critical data.
The innovation of lossy audio compression was to use
psychoacoustics(心理声学 ) to recognize that not all data in an audio
stream can be perceived by the human auditory(听觉的 )system,Most
lossy compression reduces perceptual(知觉的 )redundancy(冗余 )by first
identifying sounds which are considered perceptually irrelevant(不相关的 ),that is,sounds that are very hard to hear,Typical examples
include high frequencies,or sounds that occur at the same time as
other louder sounds,Those sounds are coded with decreased
accuracy or not coded at all.
&Part4 Extensive Reading
2009-7-27 P,12
While removing or reducing these 'unhearable' sounds may
account for a small percentage of bits saved in lossy
compression,the real savings comes from a complementary
phenomenon - noise shaping,Reducing the number of bits
used to code a signal increases the amount of noise in that
signal,In psychoacoustics based lossy compression,the real
key is to 'hide' the noise generated by the bit savings in areas
of the audio stream that cannot be perceived,This is done by,
for instance,using very small numbers of bits to code the high
frequencies of most signals - not because the signal has little
high frequency information (though this is also often true as
well),but rather because the human ear can only perceive very
loud signals in this region,so that softer (noise) sounds
'hidden' there simply aren't heard.
2009-7-27 P,13
If reducing perceptual redundancy does not achieve sufficient
compression for a particular application,it may require further
lossy compression,Depending on the audio source,this still
may not produce perceptible differences,Speech for example
can be compressed far more than music,Most lossy
compression schemes allow compression parameters to be
adjusted to achieve a target rate of data,usually expressed as
a bit rate,Again,the data reduction will be guided by some
model of how important the sound is as perceived by the
human ear,with the goal of efficiency and optimized(使最优化 )
quality for the target data rate,(There are many different
models used for this perceptual analysis,some better suited to
different types of audio than others.) Hence,depending on the
bandwidth and storage requirements,the use of lossy
compression may result in a perceived reduction of the audio
quality that ranges from none to severe,but generally an
obviously audible reduction in quality is unacceptable to
listeners.
2009-7-27 P,14
Because data are removed during lossy compression and
cannot be recovered by decompression,some people may
not prefer lossy compression for archival storage,Hence,as
noted,even those who use lossy compression (for portable
audio applications,for example) may wish to keep a
losslessly compressed archive for other applications,In
addition,the technology of compression continues to
advance,and achieving a state-of-the-art(最先进的 )lossy
compression would require one to begin again with the
lossless,original audio data and compress with the new
lossy codec(编解码器 ),The nature of lossy compression for
both audio and images results in increasing degradation of
quality if data are decompressed,then recompressed using
lossy compression.
End
2009-7-27 P,15
@Part5 Notes to Passage
1)The innovation of lossy audio compression was to use
psychoacoustics to recognize that not all data in an audio
stream can be perceived by the human auditory system.有损压缩音频的创新在于运用心理声学来认识这一事实，即：并非所有音频流中的数据都能被人类的听觉系统所察觉。
注释,此句是主系表句型。 to use psychoacoustics是表语； to
recognize是表语里的目的状语。 that not all data in an audio
stream can be perceived by the human auditory system是宾语从句。
2)Most lossy compression reduces perceptual redundancy by
first identifying sounds which are considered perceptually
irrelevant,that is,sounds that are very hard to hear,大部分的有损压缩首先是通过识别出那些我们认为无关的声音，也就是那些很难听的声音，来减少知觉的冗余。
注释,介词短语 by first identifying sounds用作方式状语； which are
considered perceptually irrelevant和 that are very hard to hear
都是定语从句，分别修饰前后两个 sounds。
2009-7-27 P,16 End
3)While removing or reducing these 'unhearable' sounds may
account for a small percentage of bits saved in lossy
compression,the real savings comes from a complementary
phenomenon - noise shaping.去掉或减少这些“没法听”的声音固然能够在有损压缩过程中节省一小部分位数，但真正有助于有损压缩节省大量位数的是一个互补现象 — 噪音抑制。
注释,注意 While引导的是状语从句，意为“虽然、固然、尽管 …,表示让步；所谓 noice shaping,噪音抑制”或“噪音修整”，是一种处理音频噪音的专门技术
4)Most lossy compression schemes allow compression
parameters to be adjusted to achieve a target rate of data,
usually expressed as a bit rate,大多数的有损压缩方案都允许通过调整压缩参数来达到某个目标数据比率，通常用比特率来表示。
注释,usually expressed as a bit rate是过去分词做定语，相当于
which is usually expressed as a bit rate。
2009-7-27 P,17
1.Listening Challenge
Spot Dictation
Directions,The following passage will be heard 3 times,During the second time,you
are required to put the missing words or phrases in the numbered blanks according to
what you hear,The third time is for you to check your answers,Now the passage
begins.
In 1)__________ audio compression,methods of psychoacoustics
are used to 2)__________ non-audible (or less audible) components
of the signal,Compression of human 3)________ is often performed
with even more specialized 4)_____________,so that "speech
compression" or "voice 5)________" is sometimes distinguished as
a separate discipline 6)______ "audio compression",Different audio
and speech compression standards are 7)__________ under audio
codecs,8)_________ compression is used in 9)_______ telephony
for example,while audio compression is used for CD ripping and is
decoded by audio10)___________.
@Part6 Practice on Passage
End
lossy
remove
speech
techniques
coding
than
listed
Voice Internet
players
2009-7-27 P,18
End of this Unit
Unit 11
Data Compression
Chapter Three
&Part4 Extensive Reading@ Part 1 Notes to Text
@Part5 Notes to Passage& Part 2 Word Study
@Part3 Practice on Text @Part6 Practice on Passage
2009-7-27 P,2
@ Part 1 Notes to Text
1) In computer science and information theory,data compression or
source coding is the process of encoding information using fewer
bits than an unencoded representation would use through use of
specific encoding schemes,在计算机科学和信息理论中，数据压缩或信源编码其实就是指采用比未经编码的信息表现方式更少的字节，通过专门的编码系统进行编码的过程。
注释,此句是主系表句型。 data compression or source coding 是主语，
the process of encoding information是表语,using fewer bits和
through use of specific encoding schemes是修饰 encoding
information的状语。 than an unencoded representation would use
是比较状语从句。
2)For example,this article could be encoded with fewer bits if one
were to accept the convention that the word "compression" be
encoded as "comp".例如，假如我们接受这样一种约定，也即是：将
"compression"编译为 "comp"，那么用较少的位数就可以给这篇文章完成编码。
注释,if one were to… 是条件状语从句,were是虚拟语气，表示假设；
that引导的从句是同位语，用以解释 convention的具体内容。
Data Compression
2009-7-27 P,3
3) One popular instance of compression with which many
computer users are familiar is the ZIP file format,which,as well
as providing compression,acts as an archiver,storing many
files in a single output file,ZIP文件格式就是众多计算机使用者都很熟悉的一种普及型压缩实例，它不但提供压缩，而且充当文件存储器，
在一个 ZIP输出文件中就可以存放很多文件。
注释,此句是主系表句型。 One popular instance of compression
是主语； the ZIP file format 是表语； with which many computer
users are familiar是介词提前的定语从句,which指代前面的
instance of compression； which,as well as providing
compression,acts as an archiver,storing many files in a single
output file.是非限制性定语从句,which指代前面的 the ZIP file
format； providing compression和 storing many files in a single
output file都是这个定语从句里的状语。
2009-7-27 P,4
4) A compression method called Huffman coding is frequently used
in data communications,and particularly in fax transmission.有种叫哈夫曼编码的压缩方法经常被用在数据通信，特别是传真传输上。
注释,Huffmancoding（哈夫曼编码）是一种根据字符出现的概率来构造平均长度最短的编码。它是一种变长的编码。在编码中，若各码字长度严格按照码字所对应符号出现概率的大小的逆序排列，则编码的平均长度是最小的。（注：码字即为符号经哈夫曼编码后得到的编码，其长度是因符号出现的概率而不同，所以说哈夫曼编码是变长的编码。）
5) Were the letter made up of randomly distributed black ink
covering 50 percent of the white paper surface,data compression
would hold no advantages,如果构成信件的那些随意分布的黑色墨迹占据了白纸表面的 50%，那么，数据压缩在此时就没什么优势可言了。
注释,Were the letter made up of…= If the letter were made up of… 。
在书面语中，条件从句可以不用连词 if，而将谓语中的过去式 were,had
或 should等移至主语前。这属于虚拟语气中的一种特殊结构形式；
covering 50 percent of the white paper surface是现在分词短语做定语，
修饰前面的 black ink。
End
2009-7-27 P,5 End
1,representation n,表现法，表现 8,alter v,改变
2,convention n,惯例，协定 9,restore v,恢复
3,archiver n,档案存储器 10,procedure n,过程
4,statistically adv,统计上地 11,decompression n,解压缩
5,character n,字符 12,consecutive string 连贯的字符串
6,binary adj,二进位的，二元的 13,randomly distributed 随意分布的
7,assign v,赋值，指定
& Part 2 Word Study
2009-7-27 P,6
Task 1,Listening training
Dictation of some key words
Directions,You will hear some key words twice,most of which are taken from
the Word Study,Then finish the Crossword Puzzle,(See you book p,)
2,Dictation of some key expressions
Directions,You will hear some key expressions twice,most of which are
taken from this text,Then you are asked to complete the missing words.
@Part3 Practice on Text
1) data________________
2) ___________information
3) a single_______________
4) an ___________________message
5) in fax___________________
6) ______________________black ink
compression
encoding
output file
uncompressed
transmission
randomly distributed
2009-7-27 P,7
Read and Reply
Directions:Students are required to read aloud the whole text after
the teacher,Then give short answers to the following questions:
a) What is data compression?
b) What would be discovered if a typical message were
statistically analyzed?
Task 2,Read,Write and Translate
Data compression is the process of encoding information
using fewer bits than an unencoded representation would use
through use of specific encoding schemes,
If a typical message were statistically analyzed,it would be
found that certain characters are used much more frequently
than others,
No,
c) Can all kinds of data be compressed to the same degree?
2009-7-27 P,8
( )1) Data compression or source coding is the process of
encoding information using more bits than an unencoded
representation would use through use of specific encoding
schemes,
( )2) The ZIP file format only provides compression,
( )3) By analyzing a message before it is transmitted,longer
binary codes may be assigned to frequently used characters
and shorter codes to rarely used characters,
( )4) Data representing a computer program may not be
reduced significantly,
( )5) Huffman coding can help reduce the total message
length for any faxed business letter,
Read and Judge
End
F
F
F
T
F
2009-7-27 P,9
Directions:Read the following short passage,Complete it by filling in the missing words.
A,require B,enough C,useful D,detrimental E,involve
F,compress G,such H,including I,decompressed J,instance
Compression is 1_____because it helps reduce the consumption
of expensive resources,2） _____as hard disk space or transmission
bandwidth,On the downside,compressed data must be 3） ___ to be
viewed,and this extra processing may be 4） ___ to some
applications,For 5） ______,a compression scheme for video may 6）
_____expensive hardware for the video to be decompressed fast 7）
_____to be viewed as it's being decompressed,The design of data
compression schemes therefore 8） _____trade-offs among various
factors,9） ______the degree of compression,the amount of
distortion introduced,and the computational resources required to
10） _____and uncompress the data.
Read and Complete
End
C
G
I
D
J
A
B
E
H
F
2009-7-27 P,10
1)他们抛弃了过去的习惯。（ convention）
2)我们使他恢复原职。（ restore）
3)魔术师叫我随便挑一张牌。 (at random)
4)因为下大雨，我们不得不改变计划。 (alter)
5)那件工作分配给我了。 (assign)
Read and Translate
Directions:Put the following sentences into English,using the words or phrases given.
End
They abandoned the conventions of the past.
We restored him to his old post,
The magician asked me to pick a card at random,
We had to alter the plan because of the heavy rain.
The task was assigned to me,
2009-7-27 P,11
Lossy Audio Compression
Lossy(有损耗的 ) audio compression is used in an extremely wide
range of applications,In addition to the direct applications (mp3
players or computers),digitally compressed audio streams are
used in most video DVDs; digital television; streaming media(流媒体 )on the internet; satellite and cable radio; and increasingly in
terrestrial(地域性的 )radio broadcasts,Lossy compression typically
achieves far greater compression than lossless(无损的 ) compression
(data of 5 percent to 20 percent of the original stream,rather than
50 percent to 60 percent),by discarding(丢弃 ) less-critical data.
The innovation of lossy audio compression was to use
psychoacoustics(心理声学 ) to recognize that not all data in an audio
stream can be perceived by the human auditory(听觉的 )system,Most
lossy compression reduces perceptual(知觉的 )redundancy(冗余 )by first
identifying sounds which are considered perceptually irrelevant(不相关的 ),that is,sounds that are very hard to hear,Typical examples
include high frequencies,or sounds that occur at the same time as
other louder sounds,Those sounds are coded with decreased
accuracy or not coded at all.
&Part4 Extensive Reading
2009-7-27 P,12
While removing or reducing these 'unhearable' sounds may
account for a small percentage of bits saved in lossy
compression,the real savings comes from a complementary
phenomenon - noise shaping,Reducing the number of bits
used to code a signal increases the amount of noise in that
signal,In psychoacoustics based lossy compression,the real
key is to 'hide' the noise generated by the bit savings in areas
of the audio stream that cannot be perceived,This is done by,
for instance,using very small numbers of bits to code the high
frequencies of most signals - not because the signal has little
high frequency information (though this is also often true as
well),but rather because the human ear can only perceive very
loud signals in this region,so that softer (noise) sounds
'hidden' there simply aren't heard.
2009-7-27 P,13
If reducing perceptual redundancy does not achieve sufficient
compression for a particular application,it may require further
lossy compression,Depending on the audio source,this still
may not produce perceptible differences,Speech for example
can be compressed far more than music,Most lossy
compression schemes allow compression parameters to be
adjusted to achieve a target rate of data,usually expressed as
a bit rate,Again,the data reduction will be guided by some
model of how important the sound is as perceived by the
human ear,with the goal of efficiency and optimized(使最优化 )
quality for the target data rate,(There are many different
models used for this perceptual analysis,some better suited to
different types of audio than others.) Hence,depending on the
bandwidth and storage requirements,the use of lossy
compression may result in a perceived reduction of the audio
quality that ranges from none to severe,but generally an
obviously audible reduction in quality is unacceptable to
listeners.
2009-7-27 P,14
Because data are removed during lossy compression and
cannot be recovered by decompression,some people may
not prefer lossy compression for archival storage,Hence,as
noted,even those who use lossy compression (for portable
audio applications,for example) may wish to keep a
losslessly compressed archive for other applications,In
addition,the technology of compression continues to
advance,and achieving a state-of-the-art(最先进的 )lossy
compression would require one to begin again with the
lossless,original audio data and compress with the new
lossy codec(编解码器 ),The nature of lossy compression for
both audio and images results in increasing degradation of
quality if data are decompressed,then recompressed using
lossy compression.
End
2009-7-27 P,15
@Part5 Notes to Passage
1)The innovation of lossy audio compression was to use
psychoacoustics to recognize that not all data in an audio
stream can be perceived by the human auditory system.有损压缩音频的创新在于运用心理声学来认识这一事实，即：并非所有音频流中的数据都能被人类的听觉系统所察觉。
注释,此句是主系表句型。 to use psychoacoustics是表语； to
recognize是表语里的目的状语。 that not all data in an audio
stream can be perceived by the human auditory system是宾语从句。
2)Most lossy compression reduces perceptual redundancy by
first identifying sounds which are considered perceptually
irrelevant,that is,sounds that are very hard to hear,大部分的有损压缩首先是通过识别出那些我们认为无关的声音，也就是那些很难听的声音，来减少知觉的冗余。
注释,介词短语 by first identifying sounds用作方式状语； which are
considered perceptually irrelevant和 that are very hard to hear
都是定语从句，分别修饰前后两个 sounds。
2009-7-27 P,16 End
3)While removing or reducing these 'unhearable' sounds may
account for a small percentage of bits saved in lossy
compression,the real savings comes from a complementary
phenomenon - noise shaping.去掉或减少这些“没法听”的声音固然能够在有损压缩过程中节省一小部分位数，但真正有助于有损压缩节省大量位数的是一个互补现象 — 噪音抑制。
注释,注意 While引导的是状语从句，意为“虽然、固然、尽管 …,表示让步；所谓 noice shaping,噪音抑制”或“噪音修整”，是一种处理音频噪音的专门技术
4)Most lossy compression schemes allow compression
parameters to be adjusted to achieve a target rate of data,
usually expressed as a bit rate,大多数的有损压缩方案都允许通过调整压缩参数来达到某个目标数据比率，通常用比特率来表示。
注释,usually expressed as a bit rate是过去分词做定语，相当于
which is usually expressed as a bit rate。
2009-7-27 P,17
1.Listening Challenge
Spot Dictation
Directions,The following passage will be heard 3 times,During the second time,you
are required to put the missing words or phrases in the numbered blanks according to
what you hear,The third time is for you to check your answers,Now the passage
begins.
In 1)__________ audio compression,methods of psychoacoustics
are used to 2)__________ non-audible (or less audible) components
of the signal,Compression of human 3)________ is often performed
with even more specialized 4)_____________,so that "speech
compression" or "voice 5)________" is sometimes distinguished as
a separate discipline 6)______ "audio compression",Different audio
and speech compression standards are 7)__________ under audio
codecs,8)_________ compression is used in 9)_______ telephony
for example,while audio compression is used for CD ripping and is
decoded by audio10)___________.
@Part6 Practice on Passage
End
lossy
remove
speech
techniques
coding
than
listed
Voice Internet
players
2009-7-27 P,18
End of this Unit
