|Each time the waveform crosses the zero line -- every time a pit-to-flat or flat-to-pit transition is encountered -- the player writes a 1 to a buffer. All other samples are 0s. Thus from just a part of the original waveform, the player is able to recreate a very close approximation in digital form. However, the newly created horizontal part of the waveform -- the time data of the clock -- isn't a digital signal at all. The clock is a constant-flow analog signal generated by an analog source, a crystal oscillator. Now this stream of 1s and 0s must be converted to analog so we can hear the music.
Several things can and do go wrong during this whole process, from the glass master if generated from DAT tape or CD-R to the glass-mastering
|machine to the translucent quality of the actual polycarbonate, the quality of the stamper to form accurate pits, to the reflective aluminum layer which may or may not be thick enough or suffer from pin holes.
In practice, it is the lack of crisp pit formation in a stamped CD that causes most of the problems. If a clock 'tick' prompts the laser to read the next sample and the wall of that pit isn't interpreted as a wall, the sample isn't misread but read just a little too late. Every transition from pit to flat and flat to pit becomes read as a digital 1. For a purely digital likeness, time delays don't pose any problems since the 1 is properly extracted. During playback, however, you will notice a peculiar harshness often associated with the medium. These offsets in time are called jitter.
If we tie up all of the above -- and there are in fact many more issues that influence CDs - we can conclude that all manner of mechanical and external issues can interfere with our musical pleasure of playing back a CD. In hindsight, you might even wonder why this intermediate medium was ever invented. If we had direct digital access to the master recording on the recording engineer's hard disc, we could skip all the format and domain conversions....
That could become a thing of the near future. For now, we do not have to wait. We simply accept that CD is somewhat overrated while admitting that several small labels are doing their utmost to deliver as good a product as possible. Labels like Mobile Fidelity, Chesky, MA Recordings, Waterlily and many others are examples. Needless to say, it's somewhat problematic that some of these very small labels release merely a couple of hundred CDs while the mega labels push out hundreds of thousand of copies per album.
However, once purchased, we can do something about the often bad stamping quality of a CD. When we first discovered the potential improvements possible by making a CD-R copy from an original, it was a surprise in two ways. One was the musical improvement: The copy wins in transient response, blackness, bass and treble while not much changes in the midrange. The second was the lack of difference in the pure digital domain. A bit-for-bit comparison does not show any differences between copy and original. This proves that the time domain is a factor - the clock is not on the CD and improperly formed pits affect it.
Young German student Andre Wiethoff from Dortmund/Germany was fed up by all the audio grabbers available at the time. Audio grabbers are computer programs designed to 'rip' or copy music from a CD to create compressed MP3 audio files so detested by audiophiles. Andre dove deep into the RedBook specifications for CD and CD drives and came up with a design for the most perfect audio grabber. Most grabbers just transfer bit-for-bit data from CD to computer hard drive which results in a jitter-free copy since a computer's reading of audio data does not involve any clock mechanism other than the computer's own clock which simply prompts the computer CPU to perform an action which, unlike with CD audio, isn't limited to serial processing.
A computer running under Windows or Mac OS or any other operating system shares its CPU not only with its operating system but with any other program that is running simultaneously. While program A is waiting for some data from the hard drive which the operating system is retrieving, the CPU can perform calculations for program B. Once the requested data has been fetched from the disk (which is relatively slow compared to the CPU that works at GHz speeds) and stored in memory, the CPU can stop working on program B and attend program A by using fresh data.
Thus ripping CD data to a file on a computer's hard disc is a parallel computer task. If it were playing back music the same way, that would be terrible. Later we'll see why and how that works a little differently. Andre's studies of the workings of both the RedBook and CD player formats plus his cunning programming skills have resulted in the most accurate audio ripper extant. For music lovers like us, it is sufficient to have access to a program that can extract data from CDs in such a way as to 'overlook' all manner of distortions caused by either flaws during the CD manufacturing process or from prior use like scratches and to create perfect copies to our computer's hard drive.
|For our purposes, we only use a small portion of Exact Audio Copy's extended features which include almost 90% of the old Cool Edit Pro program like wave comparison, frequency analysis etc. EAC only suffers one handicap. It can presently only run under the Windows operating system no Linux or Mac OS. On the other hand, the software is free. Andre only wants a nice postcard with a nice stamp in return. From us, he can even get a nice Blue Moon Award for Outstanding Software Contribution To The World.
After downloading the shareware from Andre's website, EAC installs as a zipped file. With Windows XP, you don't even require an external program like Winzip to unpack the file. However,
|with what follows, we expect a little PC savviness to work things out for yourself. EAC is still in Beta form. At the time of this writing, Andre is at version 0.95 Pre Beta 5 but what's in a name? Unzipping the program installs itself in the folder C:\Program Files\Exact Audio Copy. You now merely need one little additional piece of software -- file WNASPI32.DLL -- to add to the EAC folder. You can download this .DLL files here. This file is essential to create communication between your CD drive and the Windows operating system. Once that DLL file is unzipped and stored in the correct folder, you are ready for the next step - configuring EAC for your environment.
The best results so far have been achieved with a combination of SCSI-based CD reader/writer (ideally set up on a vibration-isolated platform) and a dedicated hard disc, preferably also of the SCSI kind. Now run the EAC software. An install wizard will help you through the setup steps. The program first scans the computer for CD drives and you will select the one to use if more than one are available. Andre supplies the software with a list of known drives for optimal settings. Those can be overruled but for popular drives like a Plextor SCSI, the default settings are perfectly calibrated. Now skip all questions about external decoders we are not interested in compressed files, we want better copies! Answer the next question about beginner or expert with expert. The finish prompt then exits setup.
For more accurate settings, go to the EAC Menu and choose the EAC Options tab. In the Extraction tab, select High Error Recovery Quality. Since we won't create MP3 files, deselect all boxes under the General tab in the Menu. The same goes for the check boxes under Tools. Now proceed to the most important settings by selecting the Drive Options tab from the EAC Menu. Click away the warning and look at the Extraction Method tab. This hides the pearl in Andre's oyster. Make sure to enable Secure Mode. This causes the program to read each track of a CD at least twice and most of the time not even at full speed! In fact, we've observed EAC scan a CD and re-read a specific track more than 20 times. In theory, EAC can read a track up to 82 times to guarantee the best possible data extraction [i.e. overcome what might invoke the error correction protocol if your CD player read that particular passage]. To prevent EAC reading from the drive's own memory cache upon repeat scans, check the box Drive caches audio data so that EAC can switch the caching off by itself. It is this cache in conjunction with the memory used by an audio player program that plays back a CD on your PC. It buffers enough music data to play from buffer memory while the operating system as prompted by the audio player software retrieves additional audio data which is stored in the cache and at a particular memory location. If your CD drive is capable of its own C2 error correction, uncheck that on the screen. For best results, all this should be left to EAC.
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