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A Compendious Description of CD PLAYER

播放机的网站,有助于我们理解光盘是如何在光盘播放机器中工作的, 这是一个关于 CD 播放机的网站,有助于我们理解光盘是如何在光盘播放机器中工作的,以及我们在 CATS 测试仪上得到一些测试参数的有关内幕。 下来并作了蹩脚的翻译,给大家分享。 测试仪上得到一些测试参数的有关内幕。因此

down 下来并作了蹩脚的翻译,给大家分享。建议同时阅读 英文原文,以防止被不确切的翻译中所误导。 英文原文,以防止被不确切的翻译中所误导。

刘世皓

Clickable Imagemap of a CD Player
可点击查看详细信息的

CD 播放机图示

Just click on the part of the CD player you are interested in, and read about it. You can also click some areas around the Cd player to learn other information. 播放机图示中点击你感兴趣的部分,即可阅读有关信息。 在下面的 CD 播放机图示中点击你感兴趣的部分,即可阅读有关信息。 working, 如果无法看到 无法看到下面的 If the image is not working, click here for a text version /如果无法看到下面的 图形,点击此处转到文字版本。 图形,点击此处转到文字版本。

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This is a text version of the imagemap
这是图示说明的文字版本

The links below are the ones that are in the imagemap. 下列每个连接都是一个图示说明 The History of CD players /CD 播放机的历史 Laser Pickup System / 激光拾取系统 The fundamental ideas of a CD player /CD 播放机的基本内幕 The disc drive system / 盘片驱动系统 Data Decoding system 数据解码系统

The pit edges of a CD / CD 的坑形边缘 The servo system / 伺服系统 3-beam tracking 3-beam pickup / 三光束寻轨 / 三光束拾取 / 三光束自动聚焦

3-beam autofocusing

Contol and display system / 控制和显示系统 Quarter-wavelength pits 四分之一波长坑深

Encoding process of digital audio / 数字音频编码过程 Pits and common object sizes / 坑形和一些物体的尺寸对比 Reading the pit / 读取坑形

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History 历史
In 1983 compact disc (CD) players entered the consumer market. By 1986, CD players were selling at the rate of over one million per year, making the CD player the fastest growing consumer electronic product ever introduced. So whose idea was it to reproduce music digitally on a CD? 1983 年 CD 播放机进入消费市场。到了 1996 年,CD 播放机的销售量以每年超过一百万台的 速度增长,使 CD 播放机成为一种快速增长的消费电子产品。那么将音乐以数字化方式记录到 CD 上是谁的主意呢? The design and development of the CD player was a collaboration of two companies: Philips and Sony. Philips was the first to come up with the idea of optical-disc audio reproduction. They had developed a laser-scanned videodisk player called LaserVision -- which lead them to the idea of developing a similar kind of system to reproduce sound. CD 播放机的设计和研发是两家公司协作的结果:Philips 和 Sony。Philips 首先提出使用光 盘进行音频再生的思想。 此前 Philips 已经研发了一种激光扫描的视频光盘播放机, 称为 LV, 从而进一步的产生了另一个主意:研发一种类似的系统用于声音再现。。 Philips decided to produce a prototype and present it to manufacturers. In the process of building their prototype, they found that error detection and correction was imperative but they did not know an efficient way of implementing it. They decided to present their prototype, anyway, to several manufactures in Japan. Of the five manufacturers present at the demonstration, Sony was the only manufacturer who decided to work with Philips on the compact disc player. Philips 决定制造一个原型机并导入生产。在制造原型机的过程中,他们发现误码的探测和 纠正问题必须得到解决,但他们没有找到有效的措施。于是 Philips 决定通过各种渠道向日 本的几个制造商展示原型机。在看过原型机的五个厂商中,只有 Sony 决定和 Philips 一起进 行光盘播放机的研发工作。 Sony was the leader in magnetic-tape recording and digital conversion techniques. Because of the complimentary knowledge between the two companies, they where able to solve the error correction and detection problem along with developing an industry standard for the format of compact discs. In 1981, thirty-five electronics manufacturers agreed on the Philips/Sony standard, and the race was on to produce the first compact disc player. Sony 在磁带记录和数字转换技术领域处于领先地位。通过两家公司之间的技术共享,在 CD 格式工业标准建立过程中的误码探测和纠正问题才得以解决。 1981 年,35 家电子制造商采 纳了 Philips/Sony 的标准,并开始了看谁先生产出第一台 CD 播放机的竞赛。

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Do you know who won? With Philips struggling on the implementation of the digital electronics, Sony's expertise in that area allowed them to produce the first CD player one month earlier than Philips. 你知道谁最终获得了胜利吗?Philips 需要在数字电子技术领域奋斗,而 Sony 依靠在该领域 的专有技术使之比 Philips 提前一个月制造出第一台 CD 播放机。

Laser Pickup System 激光拾取系统
The digital data on a CD is represented by bumps, where the edge of each bump represents a one. The bumps are read by a laser that is part of the laser pickup system. (See the section on the Optical Train) The laser pickup system includes thelaser diode, mirrors and lenses, and the photodetectors. CD 上的数字数据通过“凸起”重现,“凸起的边沿”表示“1”。“凸起”被作为“激光拾 取系统”一部分的激光读取(参见光学培训有关章节)。激光拾取系统包括:激光器,反射 镜和透镜以及光电探测器。 The laser beam (which is produced by the laser diode) is directed on to the CD via the mirrors and lenses. When the beam strikes the CD, the beam is reflected and directed to the photodetectors. Photodetectors are transducers that convert light into an electric signal. So the information reflected off of the CD is converted to an electrical signal and sent to the servo and data decoding systems via the photodetectors. 激光束(由激光器发出)经过反射镜和透镜直射到 CD 上。光束到达 CD 后被反射回来并直射 到光电探测器上。光电探测器是将光转化成电信号的变换器。这样 CD 反射回来的信息就通过 光电探测器变成电信号,然后被送入伺服系统和数据解码系统。

The fundamental ideas underlying the compact disc player 播放机的基本原理 CD 播放机的基本原理
The basic ideas behind a compact disc player are quite fundamental, but the true marvel is in the engineering and manufacturing of this consumer electronic product. CD 播放机背后的原理是非常基础的。但真正的奇迹是这一消费电子产品的的工程和制造。

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Music stored on a CD is in digital form. When music is stored digitally, it requires a tremendous amount of storage space. For example, one second of sound takes up over a million bits of digital information. If you were to try and store this information on a floppy disk it would hold less than three seconds of music! A very dense digital storage medium is needed to store digital music. 音乐以数字形式存储于 CD 上,需要大量的存储空间。例如,一秒钟的声音需要一百万位的数 字信息。如果试图将声音存储到一张软盘上,那么只能记录少于 3 秒中的音乐。存储数字音 乐需要非常密集的的存储媒介。 The problem of dense storage media was solved by using a laser to read off data bits on an optical disc. Data can be crammed much tighter on a CD than on a magnetic floppy or hard drive because a laser beam can be focused to a much smaller point than magnetic heads. One second of music can now be stored on a CD in an area the size of a pin head! 媒体的密集存储问题,是通过使用一束激光从光盘上读取数据位来解决的。数据在光盘上的 排列比在软盘或硬盘上更加紧密,原因是激光束可以比磁头聚焦到更小的点上。一秒钟的音 乐存储在光盘上只需要针孔那么大的面积。 Actually, a total of 15 billion bits of information can be stored on a music CD which equates to about 74 minutes of continuous stereo music. It would take over 1,480 floppy disks to store that much information and you certainly wouldn't get continuous stereo music! 实际上,在一张 CD 上可以存储 150 亿位的数据,相当于 74 分钟长的立体声音乐。这些数据 需要 1480 张软盘来储存,当然无法在软盘上实现不间断的音乐播放。

Disc Drive System 磁盘驱动系统
<>The purpose of this system is to spin the CD at the correct speed. Unlike records, (which spin at a constant rate) CD's spin at different rates depending on where the laser beam is located. The speed of the CD varies from 500 revolutions per sec (rev/sec), on the inside, to 200 (rev/sec) on the outside. This allows the data to be picked up by the laser system at a constant rate. To illustrate why the CD must spin faster on the inner track: suppose an inner track radius is ri and an outer track radius is ro. Now, let the speed of the inner track (ri) be Si and the speed of the outer track (ro) be So. To have constant data rate pickup, the speed (rev/sec) times the distance (circumference/rev) of the different tracks must equal. To compare the inner track with the outer track: Si·2π ri=So·2π ro,check units: ,where l = length

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该系统的功能在于保证盘片的旋转速度正确,不同于记录(运行在恒定转速上),其旋转速 度因激光束处于不同的位置而不同。光盘的转速在由在内圈 500 转/秒到外圈 200 转/秒的 的范围内变化。这可以使激光系统以恒定的速率拾取数据。为什么 CD 在读取内圈时转速更快 呢?假定内圈半径为 ri,外圈半径为 ro。这样,内圈速度为 Si,外圈速度为 So。要获得恒 定的数据拾取速度,在不同的轨道上,速度(每秒多少圈)与距离(每圈的周长)的乘积必 须相等。 Si·2π ri=So·2π ro 检查单位:〔rev/sec〕〔1/rev〕=1/sec 其中 l 为长度。 units check out because speed is distance per time. 说明速度就是距离除以时间,即 Si·ri=So·ro Since ro > ri, then Si > So, hence the speed of the CD starts at 500 (rev/sec) on the inner-most track and slows to 200 (rev/sec) as it move to the outer-most track. The control system monitors the position of the laser beam and sends the speed commands to the disc drive system. 因为 ro>ri,所以 Si>So。因此 CD 的转速由内圈的 500(转/秒)开始逐渐减小到外圈的 200(转/秒)。控制系统监视激光束所在的位置并向盘片驱动系统发出速度控制指令。 Si·ri=So·ro

Data Decoding System 数据解码系统
When the photodetectors translate the reflected light to an electrical signal, the signal represents a string of ones and zeros that is encoded and modulated. The job of the decoding system is to demodulate and decode the data string and convert it to music. The data decoding system entails several subsystems: demodulator, error detection and correction (EDC), demultiplexor, and digital-to-analog converters. 光电探测器将反射回来的光转换为电信号,电信号为一连串的“1”和“0”构成,是编码和 的调制结果。而解码系统的工作就是将数据串进行解调制和解码,使之还原为音乐。数据解 码系统需要几个子系统:解调器、误码探测和纠正、信号分离器、和数字模拟转换器。 The demodulator circuit performs the opposite function as the EFM circuit. It demodulates the data string before the EDC circuitry begins decoding. After the data is demodulated, subcode is available to the control system. Subcode contains information such as track number, time left on the track, and time left on the CD. 解调制电路做与 EFM 电路相反的工作,它在 EDC 电路开始解码之前进行数据串的解调制。解 调制完成以后,控制系统就可以获得子码信息。子码中包含轨道信息、轨道剩余时间以及光 盘剩余时间等信息。 The EDC circuits decode the CIRC code that is embedded in the digital music. The decoding process detects and corrects errors that are found on the CD. Types of errors 7/20

found on CD's are random types and burst types. Random errors entail a couple of damaged bumps at a time and usually occur during the manufacturing process of the CD. Burst errors are many consecutive damaged bumps that can span up to a couple thousand bumps. An example of these are scratches, animal hair, or finger prints. The error detection and correction circuits can correct, conceal , or in extreme circumstances mute errors on the CD. EDC 电路将包含于数字音频中的 CIRC 码解码。解码过程探测并修正在 CD 中找到的误码,包 括随机误码和突发误码。随机误码由一对损坏的“凸起”导致,一般在 CD 制造过程中形成。 突发误码由连续的损坏“凸起”导致,其跨度可能包含几前个“凸起”。例如划伤、宠物毛 发或者指纹都可导致严重的突发误码。误码探测和纠正电路可以纠正、隐藏,甚至在极端情 况下将这些误码变成哑音。 After the EDC circuits, the data is almost in its original music form. At this point, both the right and left channels are in the same continuous stream of data. This stream is sent to a demultiplexor where the two channels are separated and each one is sent to a digital-to-analog converter (DAC). Some system have only one DAC so the demultiplexor is after the DAC. The DAC is a circuit that converts a digital signal to an analog signal. This is the final circuit in the CD player, and it outputs the electrical signal to your stereo, headphone, or speakers. 经过 EDC 电路之后,得到几乎与原始音乐一样的数据。此时,左右声道为相同的连续数据流。 该数据流被送到信号分离器,分成两个声道,然后两个声道的数据流都被送到一个数字模拟 转换器(DAC)。另有一些系统只有一个 DAC, 因而信号分离器被放到 DAC 之后。DAC 电路的 作用是将数字信号转换为模拟信号。这是 CD 播放机中的最后一部分电路,它把电信号输出到 你的立体声系统、耳机或者扬声器中。

Pit Edges
A CD disk contains a long string of pits written helically on the disk. The edges of the pits correspond to binary "1"s. CD 上有一条很长的坑形串列,它以螺旋线方式排列于盘面。坑形的边沿对应于二进制“1”。

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Servo System 伺服系统
The servo system is responsible for thefocusing and tracking of the laser beam on the CD. This is not an easy task since the distance between the pits is extremely small (1.6 micrometers) and the beam must be focused to a tiny point of 0.7 millimeters. Also, the CD is not completely flat -- so when it is spinning the CD wobbles. So how does the laser pickup system stay on track and in focus? 伺服系统负责激光束在光盘上的聚焦和轨道跟踪。这可不是一个简单的工作,因为坑形之间 的距离特别的小(1.6 微米),并且激光束必须聚焦成小到 0.7 毫米的光斑。同时,光盘不 是完全的平整--旋转起来以后,CD 会摇晃。那么,激光拾取系统如何聚焦并实现轨道跟踪 呢? As mentioned before, the signal from the photodetectors goes to both the data decoder system and the servo system. The photodetectors provide feedback to the laser pickup system via the servo system. The servo system uses servomechanisms (servos) to make minute changes in tracking or focusing. The servos are typically moving-coil actuators. These actuators can be found in the laser pickup system. The actuators move the objective lens either toward or away from the CD for focusing, and sideways for tracking. As you listen to the music from the CD player, the servo system is continuously making minute adjustments to the tracking and focusing so that you can hear error free music. 前面提及,光电探测器中接受到的信号不但送到数据解码系统,也送到伺服系统。光电探测 其器根据伺服系统信号向激光拾取系统反馈信息。伺服系统使用伺服机构来实现跟踪和聚焦 的微小变化。伺服机构一般为动圈制动器,可以在激光拾取系统中找到它。制动器移动物镜 使之靠近或远离光盘以实现聚焦,同时通过侧向移动保持跟踪。当用 CD 播放机播放音乐的时 候,伺服系统不停的调整跟踪和聚焦以保证在播放中不会产生错误。

3-beam tracking 三光束轨道跟踪
When the laser beam goes through the diffraction grating, it is split up into a central bright beam plus a number of side beams. The central beam and one beam on each side are used by the CD for the tracking system. 当激光束射入衍射光栅后,被分成一个中间的明亮光束和一些处于旁边的光束。主光束和两 边各一个副光束被用于轨道跟踪系统。

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Consider a segment of the CD player containing several tracks. 下图是光盘的一部分,包括几条轨道。

If the optical head is on track, then the primary beam will be centered on a track (with pits and bumps) and the two secondary beams will be centered on land. The three spots are deliberately offset approximately 20 microns with respect to each other. 如果光学头在轨道上,主激光束将处于轨道中心,两个副光束处于轨道之间。三个光斑被故 意错开 20 微米以防相互干扰。

Two additional detectors are placed alongside the main quadrant detector in order to pick up these subsidiary beams. If the three beams are on track, then the two subsidiary photodetectors have equal amounts of light and will be quite bright because

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they are only tracking on land. The central beam will be reduced in brightness because it is tracking on both land and pits. 两个附加的探测器被置于四分成像探测器两边,用于拾取副光束。如果三束激光处于轨道的 正确位置,因为两个副光束只射到轨道中间的“台”上,因而两个附加探测器上将得到等量 的光。主光束亮度将降低,因为它在台和坑上进行跟踪。

However, if the optical head is off track, then the center spot gets more light (because there are fewer pits off track) and the side detectors will be misbalanced. 但是,如果激光头脱离轨道,中心光斑将得到更多的光(因为光斑照射到少量的坑形部分) 并且附加探测器将失去平衡。

3-beam pickup 三光束拾取
The most common optical train in modern CD players is the three beam pick-up, depicted below. 11/20

现在大多数 CD 播放机的光路都使用三光束拾取系统实现,如下图所示:

The light is emitted by the laser diode and enters a diffraction grating. The grating converts the light into a central peak plus side peaks. The main central peak and two side peaks are important in the tracking mechanism. 激光束从激光二极管中发出,射入衍射光栅。光栅将入射光束转化成一个中心峰值和一系列 副峰值。其中心主峰值和两个副峰值对于轨道跟踪机构非常重要。 The three beams go through a polarizing beam splitter. This only transmits polarizations parallel to the page. The emerging light (now polarized parallel to the page) is then collimated. 三个光束射入偏振光束分光器,只有平行偏振光穿过,然后被校准。 The collimated light goes through a 1/4 wave plate. This converts it into circularly polarized light. 经过准直镜校准后的光线射入四分之一波板。平行偏振光又被转化为圆偏振光。 The circularly polarized light is then focused down onto the disk. If the light strikes "land" it is reflected back into the objective lens. (If the light strikes the pit, now a bump, it is not reflected.) 圆偏振光经物镜聚焦后射到光盘上。如果射到“台”(land)上,光线会被反射回来,进入 物镜。(如果光线射到坑上,实际为凸起,就不会有反射光回来。) The light then passes through the 1/4 wave plate again. Since it is going the reverse direction, it will be polarized perpendicular to the original beam (in other words, the light polarization is now vertical with respect to the paper). 反射光穿过物镜,再次进入四分之一波板。因为从反向射入,将产生垂直于原光束偏振方向 的偏振。 12/20

When the vertically polarized light hits the polarizing beam splitter this time, it will be reflected (not transmitted as before). Thus, it will reflect though the focusing lens and then the cylindrical lens and be imaged on the photodetector array. The cylindrical lens is important in the auto-focusing mechanism. 当垂直偏振光射到偏振光束分光器上,它将被反射(而不是像前一次那样穿过)到聚焦透镜 和柱面镜上,然后在光电探测器阵列上成像。柱面镜在自动聚焦机构中非常重要。

3-beam auto focusing 三光束自动聚焦
If the objective lens is closer to the compact disk than the focal length of the object lens, then the cylindrical lens creates an elliptical image on the photodetector array. 如果物镜与光盘之间的距离短于物镜的焦距,柱面透镜就会使在光电探测器阵列上的成像变 成椭圆影像。

If the objective lens is further away from the compact disk than the focal length of the object lens, then the cylindrical lens again creates an elliptical image on the photodetector array. However, this elliptical image is perpendicular to first image. 如果物镜与光盘之间的距离长于物镜的焦距,柱面透镜也会使在光电探测器阵列上的成像变 成椭圆影像。但这次的成像与前一次的垂直。

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Of course, if the disk is right at the focal length of the objective lens, then the cylindrical lens does not affect the image and it is perfectly circular. 当然,如果正好处于物镜焦距位置,柱面镜就不会影响到光电探测器上的成像,因而会获得 一个完美的圆性影像。

So, if the disk is too far away -- then quadrants D and B will get more light than quadrants A and C. Similarly, if the disk is too close -- then quadrants A and C will get more light than D and B. A simple circuit generates an autofocus signal based upon the output of the photodetector. 所以,当光盘太远,像限 D 和 B 中将比 A 和 C 得到更多的光,同样的,如果光盘太近,像限 A 和 C 中将比 D 和 B 得到更多的光。这样就可以使用一个简单的电路按光电探测器上的输出 进行自动聚焦。

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The output of this correction signal can be used to drive a simple auto-focus servo. A typical example of such a servo is illustrated below. 修正信号的输出可以用于驱动一个简单的自动聚焦伺服器。下图为一个自动聚焦伺服器的示 意图。

Control and Display System 控制和显示系统
The control system processes the subcode that is encoded on the CD. The subcode tells information like: how many tracks are on the CD, what track it is presently on, time left on the song, or time left on the CD. With this information, control can send speed up or slow down commands to the disc drive motor. 控制系统处理编码于 CD 中的子码部分。子码提供这样一些信息:CD 上有多少个轨道,当前 是那一个轨道,当前曲目的剩余时间,整张光盘的剩余时间。根据这些信息,控制系统可以 向盘片驱动系统发送提速或降速的指令。 Because of the subcode information, the CD player has many features that simply cannot be accomplished on record players or tape decks. Some examples of various features are the following: random memory programming, manual searches (skipping forward or backward with the touch of a button), random playback, and pausing. The control system can display quite a bit of information also: what the present track is, the time left on the track, the time left on the CD, and the time left in the memory program (if you did memory programming). 因为有子码信息,CD 播放器可以实现许多在录放机或磁带卡座上无法实现的特征,例如:随 机存储器编程,手动搜索(通过按钮向前跳转或向后跳转),随机播放,以及暂停等。控制 系统也可以显示一些信息如:当前轨道,当前轨道的剩余时间,整张光盘的剩余时间以及内 存程序的剩余时间(如果你作了内存编程的话)。

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Finally, the control system provides an interface with the control buttons and knobs on the CD player. When a user presses the 'skip' button, the control system senses the command and sends control signals to the various subsystems to perform the 'skip' command. It also displays the requested task at hand. 最后,控制系统提供了 CD 播放机器上的一些控制按钮等控制界面。当使用者按下“skip”按 钮时,控制系统得到命令然后发送控制信号给各子系统来完成“skip”命令。同时显示所请 求命令的信息。

QuarterQuarter-wavelength Pits 四分之一波长坑深
<>The CD disk is actually read from the bottom. Thus, from the viewpoint of the laser beam reading the disk, the "pit" in the CD is actually a "bump". 实际上, CD 光盘是从下面读取的,因此,激光读取光盘的时候,在光盘上我们所说的的“坑 形”实际上是“凸起”。

The pit/bump is carefully fabricated so that it is a quarter of a wavelength (notice a wavelength INSIDE the polycarbonate) high. The idea here is that light striking the land travels 1/4 + 1/4 = 1/2 of a wavelength further than light striking the top of the pit. The light reflected from the land is then delayed by 1/2 a wavelength -- and so is exactly out of phase with the light reflected from the pit. These two waves will interfere destructively -- so effectively no light has been reflected. 坑形或凸起要非常仔细的制作,其高度为四分之一的读取激光波长(注意在聚碳酸脂中的激 光波长)。光束射到台(land)上走过了 1/4 + 1/4 = 1/2 波长。这样在台上反射光比从延 迟了 1/2 波长,这样恰好与从坑(pit)上反射回来的光的相位相反。这样,两束光干涉抵消 --实际上没有光反射回来

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Encoding process of digital audio 数字音频的编码过程
As mentioned before, the music on a CD is stored digitally (in other words, in 1's and 0's). The recorded music, however, is encoded in a format that allows it to be stored on the CD. So, if you were to read off the bits of information on the CD player without decoding it you would not get music. Encoding is necessary because of algorithms that are incorporated in the music at recording time (see the section on Error Detection and Correction). 之前我们提及:音乐在 CD 上以数字方式存储(换句话说就是用“1”和“0”表示)。录制好 的音乐只有经过编码后才能按照特定的格式存储在 CD 上。所以如果从 CD 播放机直接读出信 息位而不进行解码,就没法得到音乐。编码的必要性在于:音乐需要按一定的运算法则与记 录时间相结合。 Also, because of how the laser detects ones and zeroes on the CD, there cannot be consecutive ones when storing the digital information. The solution for this problem is called eight-to-fourteen modulation (EFM). At the final preparation of storing the encoded music on the CD, eight bit chunks of information are transformed to fourteen bits. Also, three merging bits of zeros are tacked on to each fourteen bit chunk. This final process ensures that no consecutive ones are present before storage. 同时,因为激光探测“1”和“0”的方式要求在存储数字信息时不可以出现连续的“1”。解 决这个问题的方法称为 8-14 调制(EFM)。在存储数字音频于光盘上的最后阶段,将 8 位一 段的数据转化成 14 位一段的数据。同时在每段的 14 位上再添加了 3 个结合位。这样的处理 就确保了在存储之前数据流中不会出现连续的“1”。 Here are the following steps that are involved in the encoding process of digital music: 以下是有关数字音乐编码处理的步骤: 17/20

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Analog music is converted to a digital signal via an analog-to-digital converter. 模拟音乐通过模拟数字转换器转换为数字信号。 The left and right channels are combined to one digital data string via a multiplexor. 通过多路转换器将左右声道合成一个声道。 The digital data string is encoded with error detection and correction algorithms. The algorithms used is Cross Interleave Read-Solomon Code (CIRC) which accomplishes two things: first it provides error detection and correction for defects on the CD, second it spreads out data (interleave) so that different portions of a frame of music is physically located at different places on the CD. This helps in the correction of data that are damaged due to scratches and fingerprints. 数字数据串按照误码探测和纠正规则进行编码。使用的编码规则被称为 CIRC,它主要 实现两个目的: 首先提供一种方法探测和纠正 CD 上的误码。 第二, 使用交叉交插技术, 使同一帧音乐的不同部分分散存放在光盘的不同位置,这有利于减小由于划伤和指纹 所导致的误码的影响,便于纠正。 The encoded data string now passes through the EFM circuits to prepare it for storage on the CD. At this point, the encoded and modulated music is a string of ones and zeros which contains no consecutive ones. 经编码的数据串经过 EFM 电路处理以备在 CD 上进行记录。在这一步,音乐经编码和调 制形成由“0”和“1”组成的数据串,其中不包含连续的 “1”。

The CD player can be called a decoder because it basically reverses the encoding process. Data read from the CD must be: CD 播放机可以被称为解码器,是因为它主要完成与编码过程相反的过程。从 CD 机中读出的 数据必须经过:
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demodulated 解调制 decoded 解码 demultiplexed 信号分离 converted from digital to analog using a digital-to-analog converter 使用数模转换器将数字信号转换为模拟信号

The CD player can be broken down into five components to accomplish this task: CD 播放机可以按上面的工作任务分成 5 个部分: 1. 2. 3. 4. 5. Disc drive system 盘片驱动系统 Laser pickup system 激光拾取系统 Servo system 伺服系统 Data decoding 数据解码 Control and display system 控制和显示系统

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Pits and common object sizes 坑形与一些物体的尺寸对比
Pits are formed in the polycarbonate disk by an injection molding process. As such, they represent some of the smallest mechanically fabricated objects made by humans. The width of a CD pit is approximately the wavelength of green light. The tracks are separated by approximately three times the wavelength of green light. Diffraction from these features (so very close to the wavelength of light) is what gives CD disks their beautiful colors. 坑形通过注塑工艺在聚碳酸脂盘面形成。 这几乎是人类所能加工出的最小尺寸的物体构造了。 光盘坑形的宽度大约相当于绿色光的波长。轨道之间的距离约相当于 3 倍的绿色光的波长。 由于上述特征形成的衍射,使光盘呈现漂亮的颜色。

CD 坑形:0.5×2μm,指纹:15μm, 灰尘:40μm,人的头发:75μm,棉线:150μm,

A thin layer (50-100 nm) of metal (aluminum, gold or silver) covers the pits. An additional thin layer (10-30 microns) of polymer covers the metal. Finally, a label is silk-screened on the top. Notice that the pits are far closer to the silk screened side of the disk (20 microns) than they are to the read-side of the disk (1.55 mm). Thus, it is easier to permanently damage a disk by scratching the top -- than the bottom! 坑形的上面覆盖了一层薄薄的(50-100nm)金属层(铝、金或银),然后在加上一层盖住金 属层的聚合物薄层(10-30μm)。最后在最上面进行印刷。注意坑形距离丝网印刷层更近, 只有 20μ,而距离读取面有 1.55mm,所以在印刷面的划伤更容易对光盘造成永久损坏。

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Reading the pits 读取坑形
The polycarbonate itself is part of the optical system for reading the pits. The index of refraction of air is 1.0 while the index of refraction of the polycarbonate is 1.55. Laser light incident on the polycarbonate surface will be refracted at a greater angle into the surface. Thus, the original incident spot of around 800 microns (entering the polycarbonate) will be focused down to about 1.7 microns (at the metal surface). This is a major win, as it minimizes the effects of dust and scratches on the surface. 聚碳酸脂本身是用于读取坑形的光学系统的一部分。光在空气中的折射率为 1.0,在聚碳酸 脂中的折射率为 1.55。激光射入聚碳酸脂表面将产生较大的折射角。因此,800μm 的入射光 斑(入射时在聚碳酸脂表面)将聚焦到 1.7μm(在金属层表面)。这样有明显的好处是可以 将灰尘和划伤的影响最小化。

The laser used for the CD player is typically an AlGaAs laser diode with a wavelength in air of 780 nm. (Near infrared -- your vision cuts out at about 720 nm). The wavelength inside the polycarbonate is a factor of n=1.55 smaller -- or about 500 nm. 用于 CD 播放机的激光一般由 AlGaAs 激光二极管发出,在空气中的波长为 780nm(接近红外 线--肉眼可见的极限为 720nm)。在聚碳酸脂中的波长要考虑的因数为 1.55--或者大约 500nm。

(eof)

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