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发表于 2009-11-18 22:18:32
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厄，你说得对，数据CD有完全的纠错码，可以保证数据绝对不错，音频没有；但是CD是有简单的纠错的，全部内建在拾取机制里面了。Redbook CD的纠错码是CIRC。
选择这种编码是因为当时的电子技术不发达，实时的完全纠错不可能实现。据说当时的CIRC电路就要50美元，成本很高。
以下，看得懂得自己看吧，IEEE的文章。
Shannon-Nyquist sampling theorem
The Shannon-Nyquist sampling theorem dictates that in order to achieve lossless sampling, the signal should be sampled with a frequency at least twice the signal’s bandwidth. So for a bandwidth of 20 kHz a sampling frequency of at least 40 kHz is required. A large number of people, especially young people, are perfectly capable of hearing sounds at frequencies well above 20 kHz. That is, in theory, all we can say. In 1978, each and every piece of digital audio equipment used its own ‘well-chosen’ sampling frequency ranging from 32 to 50 kHz. Modern digital audio equipment accepts many different sampling rates, but the CD task force opted for only one frequency, namely 44.1 kHz. This sampling frequency was chosen mainly for logistics reasons as will be discussed later, once we have explained the state-of-the-art of digital audio recording in 1979.
Towards the end of the 1970s, ‘PCM adapters’ were developed in Japan, which used ordinary analog video tape recorders as a means of storing digital audio data, since these were the only widely available recording devices with sufficient bandwidth. The best commonly-available video recording format at the time was the 3/4" U-Matic.
The presence of the PCM video-based adaptors explains the choice of sampling frequency for the CD, as the number of video lines, frame rate, and bits per line end up dictating the sampling frequency one can achieve for storing stereo audio. The sampling frequencies of 44.1 and 44.056 kHz were the direct result of a need for compatibility with the NTSC and PAL video formats. Essentially, since there were no other reliable recording products available at that time that offered other options in sampling rates, the Sony/Philips task force could only choose between 44.1 or 44.056 KHz and 16 bits resolution (or less).
During the fourth meeting held in Tokyo from March 18-19, 1980, Philips accepted, and thus followed Sony’s original proposal, the 16-bit resolution and the 44.1 kHz sampling rate. 44.1 kHz as opposed to 44.056 kHz was chosen for the simple reason that it was easier to remember. Philips dropped their wish to use 14 bits resolution: they had no technical rationale as the wish for the 14 bits was in fact only based on the availability of their 14-bit digital-analog converter. In other words, the Compact Disc sound quality equals the sound quality of Sony’s PCM-1600 adaptor.
Thus, quite remarkably, in recording practice, an audio CD starts life as a PCM master tape, recorded on a U-Matic videotape cassette, where the audio data is converted to digital information superimposed within a standard television signal. The industry standard hardware to do this was the Sony PCM-1600, the first commercial video-based 16-bit recorder, followed by the PCM-1610 or PCM-1630 adaptors. Until the 1990s, only video cassettes could be used as a means for exchanging digital sound from the studios to the CD mastering houses. Later, Exabyte computer tapes, CD-Rs and memory sticks have been used as a transport vehicle.
Coding systems
Coding techniques form the basis of modern digital transmission and storage systems. There had been previous practical applications of coding, especially in space communications, but the Compact Disc was the first mass-market electronics product equipped with fully-fledged error correction and channel coding systems. To gain an idea of the types of errors, random versus burst errors, burst length distribution and so on, we made discs that contained known coded sequences. Burst error length distributions were measured for virgin, scratched, or dusty discs. The error measurement was relatively simple, but scratching or fingerprinting a disc in such a way that it can still be played is far from easy. How do you get a disc with the right kind of sticky dust? During playing, most of the dust fell off the disc into the player, and the optics engineers responsible for the player were obviously far from happy with our dust experiments. The experimental discs we used were handmade, and not pressed as commercial mass-produced polycarbonate discs. In retrospect, I think that the channel characterization was a far from adequate instrument for the design of the error correction control (ECC).
There were only two competing ECC proposals to be studied. Experiments in Tokyo and Eindhoven -Japanese dust was not the same as Dutch dust- were conducted to verify the performance of the two proposed ECCs. Sony proposed a byte-oriented, rate 3/4, Cross lnterleaved Reed-Solomon code (CIRC) [6]. Vries of Philips designed an interleaved convolutional, rate 2/3, code having a basic unit of information of 3-bit characters [9]. CIRC uses two short RS codes, namely (32, 28, 5) and (28, 24, 5) RS codes using a Ramsey-type of interleaver. If a major burst error occurs and the ECC is overloaded, it is possible to obtain an approximation of an audio sample by interpolating the neighboring audio samples, so concealing uncorrectable samples in the audio signal. CIRC has various nice features to make error concealment possible, so extending the player's operation range [10]. CIRC showed both a much higher performance and code rate (and thus playing time), although extremely complicated to cast into silicon at the time. Sony used a 16 kByte RAM for data interleaving, which, then, cost around $50, and added significantly to the sales price of the player. During the fifth meeting in Eindhoven, May 1980, the partners agreed on the CIRC error correction code since our experiments had shown its great resilience against mixtures of random and burst errors [11]. The fully correctable burst length is about 4.000 bits (around 1.5 mm missing data on the disc). The length of errors that can be concealed is about 12.000 bits (around 7.5 mm). However, the largest error burst we ever measured during the many long days of disc channel characterization was 0.1 mm.
We also had to decide on the channel code. This is a vital component as it has a considerable impact on both the playing time and the quality of ‘disc handling’ or 'playability'. Servo systems follow the track of alternating pits and lands in three dimensions, namely radial, focal, and rotational speed. Everyday handling damage, such as dust, fingerprints, and tiny scratches, not only affects retrieved data, but also disrupts the servo functions. In worst cases, the servos may skip tracks or get stuck, and error correction systems become utterly worthless. A product with such devastating weaknesses would remain a laboratory toy. A well-designed channel code will make it possible to remove the major barriers related to these playability issues.
The system designer should find a good trade-off between long playing time and playability. Both partners proposed some form of (d, k) runlength-limited (RLL) codes, where d is the minimum number and k is the maximum number of zeros between consecutive ones. The differences between the various proposals were the code rate, runlength parameters d and k, and the spectral content. The spectral content has a direct bearing on the playability. In their prototype, Philips used the propriety M3 channel code, a rate ½, d=1, k=5 code, with a well-suppressed spectral content [1]. M3 is a variation on the M2 code, which was developed in the 1970s by Ampex Inc. for their digital video tape recorder [5]. Sony started with a rate 1/3, d=5, RLL code, but since that did not work, they changed horses halfway, and proposed a propriety rate ½, d=2, k=7 code, a type of code that had been used in magnetic disk data storage. Both Sony codes did not have spectral suppression, and the engineers had opposing views on how the servo and synchronization issue could be solved. In May 1980, the choice of the channel code therefore remained open, and ‘more study was needed’. Before continuing with the coding cliffhanger, we take a musical break.
Playing time and Beethoven’s Ninth by Wilhelm Furtwängler
Playing time and disc diameter are probably the parameters most visible for consumers. Clearly, these two are related: a 5% increase in disc diameter yields 10% more disc area, and thus an increase in playing time of 10%. The Philips’ top made the proposal regarding the disc diameter. They argued 'The Compact Audio Cassette was a great success', and, 'we don't think CD should be much larger'. The cross diameter of the Compact Audio Cassette, very popular at that time and also developed by Philips, is 115 mm. The Philips prototype audio disc and player were based on this idea, and the Philips team of engineers restated this view in the list of preferred main parameters. Sony, no doubt with portable players in mind, initially preferred a smaller 100 mm disc.
During the May 1980 meeting something remarkable happened. The minutes of the May 1980 meeting in Eindhoven literally reads:
disc diameter: 120 mm,
playing time: 75 minutes,
track pitch: 1.45 µm,
can be achieved with the Philips M3 channel code. However, the negative points are: large numerical aperture needed which entails smaller (production) margins, and the Philips’ M3 code might infringe on Ampex M2.
Both disc diameter and playing time differ significantly from the preferred values listed during the Tokyo meeting in December 1979. So what happened during the six months? The minutes of the meetings do not give any clue as to why the changes to playing time and disc diameter were made. According to the Philips’ website with the ‘official’ history: "The playing time was determined posthumously by Beethoven". The wife of Sony's vice-president, Norio Ohga, decided that she wanted the composer's Ninth Symphony to fit on a CD. It was, Sony’s website explains, Mrs. Ohga's favorite piece of music. The Philips’ website proceeds:
“The performance by the Berlin Philharmonic, conducted by Herbert von Karajan, lasted for 66 minutes. Just to be quite sure, a check was made with Philips’ subsidiary, Polygram, to ascertain what other recordings there were. The longest known performance lasted 74 minutes. This was a mono recording made during the Bayreuther Festspiele in 1951 and conducted by Wilhelm Furtwängler. This therefore became the maximum playing time of a CD. A diameter of 120 mm was required for this playing time”.
Everyday practice is less romantic than the pen of a public relations guru, as at that time, Philips’ subsidiary Polygram –one of the world's largest distributors of music– had set up a CD disc plant in Hanover, Germany. This could produce large quantities CDs with of course, a diameter of 115mm. Sony did not have such a facility yet. If Sony had agreed on the 115mm disc, Philips would have had a significant competitive edge in the music market. Sony was aware of that, did not like it, and something had to be done. It was not about Mrs. Ohga’s great passion for music, but the money and competition in the market of the two partners. The decision regarding diameter/playing time was taken outside of the group of experts responsible for the CD format. So I, a former member of that group, can only guess what happened at the upper floor. But something unforeseen happened: at the last minute we changed the code.
Popular literature, as exemplified in Philips’ website mentioned above, states that the disc diameter is a direct result of the requested playing time. And that the extra 14 minutes playing time for Furtwängler’s Ninth subsequently required the change from 115mm to a 120 mm disc. It suggests that there are no other factors affecting playing time. Note that in May 1980, when disc diameter and playing time were agreed, the channel code, a key factor affecting playing time, was not yet settled. In the minutes of the May 1980 meeting, it was remarked that the above (diameter, playing time, and track pitch) could be achieved with Philips' M3 channel code. In the mean time, but not mentioned in the minutes of the May meeting, the author was experimenting with a new channel code, later coined EFM [3]. EFM, a rate 8/17, d=2, code made it possible to achieve a 30 percent higher information density than the Philips' M3. Due to its good spectral suppression, EFM also showed a good resilience against disc handling damage such as fingerprints, dust, and scratches. Note that 30 percent efficiency improvement is highly attractive, since, for example, the disc diameter increase from 115 to 120 mm only offers a mere10 percent increase in playing time.
A month later, in June 1980, we could not choose the channel code, and again more study and experiments were needed. Although experiments had shown the greater information density that could be obtained with EFM, it was at first merely rejected. At the end of the discussion, which at times was heated, the Sony people were specifically opposing the complexity of the EFM decoder, which then required 256 gates. My remark that the CIRC decoder needed at least half a million gates and that the extra 256 gates for EFM were irrelevant was jeered at. Then suddenly, during the meeting, we received a phone call from the presidents of Sony and Philips, who were meeting in Tokyo. We were running out of time, they said, and one week for an extra, final, meeting in Tokyo was all the lads could get. Sony stated that if the EFM hardware would be less than 80 gates, they would accept it. I had a week to reduce the gate count. I used the first Apple II computer in the lab, which was much handier for such an interactive design using trial and error than the IBM mainframe, especially as I had to walk to the IBM computer center for every job. I succeeded in bringing the gate count down to just 52 gates, and on June 19, 1980 in Tokyo, Sony agreed to EFM. The 30 percent extra information density offered by EFM could have been used to reduce the diameter to 115mm or even 100mm, (with, of course, the requested 74 minutes and 33 seconds for playing Mrs. Ohga’s favorite Ninth). However such a change was not considered to be politically feasible, as the powers to be had decided 120mm. The option to increase the playing time to 97 minutes was not even considered. We decided to improve the production margins of player and disc by lowering the information density by 30 percent: the disc diameter remained 120mm, the track pitch was increased from 1.45 to 1.6µm, and the user bit length was increased from 0.5 to 0.6µm. By increasing the bit size in two dimensions, in a similar vein to large letters being easier to read, the disc was easier to read, and could be introduced without too many technical complications.
The maximum playing time of the CD was settled at 74 minutes and 33 seconds, but in practice, however, the maximum playing time was determined by the playing time of the U-Matic video recorder, which was 72 minutes. Therefore, rather sadly, Mrs. Ohga’s favorite Ninth by Furtwängler could not be recorded in full on a single CD till 1988, when alternative digital transport media became available. On a slightly different note, Jimi Hendrix's Electric Ladyland featuring a playing time of 75 minutes was originally released as a 2 CD set in the early 1980s, but has been on a single CD since 1997.
The inventor of the CD
The Sony/Philips task force stood on the shoulders of the Philips’ engineers who created the laser videodisc technology in the 1970s. Given the videodisc technology, the task force made choices regarding various mechanical parameters such as disc diameter, pit dimensions, and audio parameters such as sampling rate and resolution. In addition, two basic patents were filed related to error correction, CIRC, and channel code, EFM. CIRC, the Reed-Solomon ECC format, was completely engineered and developed by Sony engineers, and EFM was completely created and developed by the author.
Let us take a look at the numbers. The size of the task force varied per meeting, and the average number of attendees listed on the minutes of the joint meetings is twelve. If the persons carrying hierarchical responsibility of the CD project are excluded then we find a very small group of engineers who carried the technical responsibility of the Compact Disc ‘Red Book’ standard.
Philips' corporate public relations department, see The Inventor of the CD on Philips' website [7], states that the CD was "too complex to be invented by a single individual", and the "Compact Disc was invented collectively by a large group of people working as a team". It persuades us to believe that progress is the product of institutions, not individuals. Evidently, there were battalions of very capable engineers, who further developed and marketed the product, and success in the market depended on many other innovations. For example, the solid-state physicists, who developed an inexpensive laser diode, a primary enabling technology, made CD possible in practice. Credit should also be given to the persons who designed the transparent Compact Disc storage case, the ' jewel box', made a clever contribution to the visual appeal of the CD.
Philips and Sony agreed in a memorandum dated June 1980, that their contributions to channel and error correction codes are equal. Sony’s website with their 'official' history is entitled 'Our contributions are equal' [8]. The website proceeds, “We avoid such comments as, ‘We developed this part and that part’ and to emphasize that the disc's development was a joint effort by saying, ‘Our contributions are equal’. The leaders of the task force convinced the engineers to put their companies before individual achievements.” The myth building even went so far that the patent applications for both CIRC and EFM were filed with joint Sony/Philips inventors.
[ 本帖最后由 pig2man 于 2009-11-18 22:23 编辑 ] |
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发表于 2009-11-18 22:19:27
