CD Response to Defects缺陷对CD响应的影响
The signal from a compact disc is relatively insensitive to the presence of small defects such as dust or fine scratches on the bottom surface of the CD because the laser beam is fairly large at that point, about 0.8 mm. As illustrated below, typical dust particles are much smaller than that. As the laser is further focused down to about 1.7 micrometers at the depth of the pits, any shadow from the small defects is blurred and indistinct and does not cause a read error. Larger defects are handled by error-correcting codes in the handling of the digital data. CD上的信号对底部表面的微小缺陷，如灰尘或细小划痕相对不敏感，因为此时激光束相当大，约为0.8毫米。如下面所示，典型的灰尘颗粒比这要小得多。当激光进一步聚焦到坑的深度约1.7微米时，任何来自小缺陷的阴影都会变得模糊不清，不会引起读取错误。较大的缺陷则通过错误校正码在数字数据处理中得到处理。
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Index CD concepts Sound reproduction concepts Reference Rossing Physics Teacher, Dec. 87 索引 CD 概念 声音重放概念 参考文献：Rossing，Physics Teacher，Dec. 87 | ||
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Error-Correction of CD SignalsCD信号纠错
The data on a compact disc is encoded in such a way that some well- developed error-correction schemes can be used. A sophisticated error- correction code known as CIRC (cross interleave Reed-Solomon code) is used to deal with both burst errors from dirt and scratches and random errors from inaccurate cutting of the disc. The data on the disc are formatted in frames which contain 408 bits of audio data and another 180 bits of data which include parity and sync bits and a subcode. A given frame can contain information from other frames and the correlation between frames can be used to minimize errors. Errors on the disc could lead to some output frequencies above 22kHz (half the sampling frequency of 44.1 kHz) which could cause serious problems by "aliasing" down to audible frequencies. A technique called oversampling is used to reduce such noise. Using a digital filter to sample four times and average provides a 6-decibel improvement in signal-to-noise ratio. For more details, see the references. 紧凑盘上的数据以一种方式编码，使得可以使用一些已发展完善的错误校正方案。一种称为CIRC（交叉交织雷德-所莫诺夫码）的复杂错误校正码用于处理来自划痕和污渍的突发错误，以及由于刻录不准确导致的随机错误。盘上的数据以帧格式存储，每个帧包含408位音频数据和另外180位数据，其中包括奇偶校验位、同步位和子码。一个给定的帧可能包含来自其他帧的信息，帧之间的相关性可以用来最小化错误。盘上的错误可能导致某些输出频率超过22kHz（半采样频率44.1kHz），这可能会通过“混叠”降频到可听频率，造成严重问题。一种称为过采样的技术用于减少此类噪声。使用数字滤波器采样四次并取平均值，可使信噪比提高6分贝。更多详情，请参见参考文献。 |
Index CD concepts Sound reproduction concepts References Rossing Physics Teacher, Dec. 87 Myaoka 索引 CD 概念 声音再现概念 参考文献 Rossing 物理学教师，1987年12月 Myaoka | ||
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Data Encoding on Compact Discs紧凑型光盘上的数据编码
When the laser in a compact disc player sweeps over the track of pits which represents the data, a transition from a flat area to a pit area or vice versa is interpreted as a binary 1, and the absence of a transition in a time interval called a clock cycle is interpreted as a binary 0. This kind of detection is called an NRZI code. The particular NRZI code used with compact discs is EFM (eight-to-fourteen modulation) in which eight bits of data are represented by fourteen channel bits. In addition to the actual digital sound data, parity and sync bits and a subcode are also recorded on the disc in "frames" . In a given frame, 408 bits of audio data are recorded with another 180 bits of data which permit a sophisticated error-correction code to be used. A given frame can contain information from other frames and the correlation between frames can be used to minimize errors. In addition to detection, a significant amount of computation must be done to decode the signal and prepare it for conversion back to analog form with a DAC.
当激光在紧凑盘播放机中扫过记录数据的沟槽时，从平坦区域到沟槽区域或反之的过渡被解释为二进制1，而在一个称为时钟周期的时间间隔内没有过渡则被解释为二进制0。这种检测方式称为NRZI码。用于紧凑盘的特定NRZI码是EFM（八到十四调制），其中八位数据由十四位通道位表示。除了实际的数字音频数据外，还记录了奇偶校验位、同步位和子码信息，这些信息以“帧”形式存储。在每一帧中，记录了408位音频数据，另有180位数据可用于使用复杂的错误校正码。某一帧可能包含来自其他帧的信息，帧之间的相关性可用于最小化错误。除了检测之外，还需要进行大量计算以解码信号并将其转换回模拟形式，以便通过DAC进行转换。 |
Index CD concepts Sound reproduction concepts Reference Rossing Physics Teacher, Dec. 87 索引 CD 概念 声音重放概念 参考文献：Rossing，Physics Teacher，Dec. 87 | ||
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Detection of Compact Disc Data紧凑磁盘数据的检测
The pits which encode the digital data on a compact disc are tracked by a laser. The reflected light from the pits is out of phase with that from the surrounding area, so the reflected light intensity drops when the laser moves over a pit area. The nature of a photodiode is such that it can be used as the sensing element in a light-activated switch. It conducts an electric current which is proportional to the light falling on it. The photodiode and switch can be adjusted so that a transition to a pit area will switch it off, and a transition from a pit area will switch it on. Either transition is interpreted as a binary 1, while the absence of a transition in a given clock cycle is interpreted as a binary zero. The data on the disc is encoded in a sophisticated way, so that decoding is necessary before sending the digital signal representing the sound to a digital-to-analog converter (DAC) for reconversion to analog form.
CD上的数字数据是通过激光追踪坑道来编码的。当激光扫描坑道区域时，坑道反射的光与周围区域的光相位不同，因此反射光的强度会降低。光敏二极管的特性使其可以作为光控开关的传感元件。它传导的电流与照射在其上的光成正比。光敏二极管和开关可以调节，使得进入坑道区域时会关闭，离开坑道区域时会开启。无论是进入还是离开坑道区域的转换都被解释为二进制的1，而给定时钟周期内没有转换则被解释为二进制的0。CD上的数据以复杂的方式编码，因此在将数字信号发送到数字-模拟转换器（DAC）进行模拟转换之前需要进行解码。 |
Index CD concepts Sound reproduction concepts Reference Rossing Physics Teacher, Dec. 87 索引 CD 概念 声音重放概念 参考文献：Rossing，Physics Teacher，Dec. 87 | ||
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