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Technical aspects of rippingRipping or Digital Audio Extraction (DAE) is the first step in the process to enable your original CD's to be played back in a computer based high-end audio environment. In the perspective of high-end audio one should take no compromise to extract an audio CD as accurate as possible. There can be said a lot about DAE. We will only reveal the surface of this interesting subject, if you want to dive into the details which can be find on the internet, see the links section on this page. The 3A-concept is all about enabling computer based audio for different purposes and for different domestically environments (although we emphasize high-end audio). So, if you’re not ‘into’ high-end audio you may skip most of the following text. On the other hand if you’re interested in accurate audio extraction you’re welcome to read along. Some background informationBasically, DAE is the process of extracting the audio from an audio CD by using a PC with a CD-ROM drive. This feature of CD-ROM and DVD drives allows the digital audio data from CDs to be passed to the computer via the drive's IDE or SCSI interface. Before this feature became available, extracting CD tracks required using the analog output of the drive and converting it back to digital again, resulting in inferior sound quality. Note. Many people confuse ripping (a informal synonym for DAE) with both the extraction and the compression of audio to a compression format like MP3. This is wrong. Digital Audio Extraction and Encoding are both distinct processes. In practice however, both processes are done in one pass. Information on a CD is physically stored according the Red Book format (CDDA). The Red Book format specifies how data on an audio disc is organized. Data on an audio disc is organized into frames in order to ensure a constant read rate. The specifications are: uncompressed 44.1KHz, 16-bit, stereo, PCM. Besides the raw audio data each frame consists also of user data, plus synchronization, error correction, and control and display bits. DAE means we take the raw data extracted from the audio track and convert them into a file (WAV) that can be stored on a hard disk. There is nothing special about the data encoded on a CD compared with a WAV file. The main difference between a "raw" 44.1KHz 16-bit stereo WAV file and CD audio is the byte ordering. Furthermore the WAV file carries overhead, 'wasted' space, with information not in the CDDA track (CRC checks for example). Don't get confused by programs (such as the Windows Explorer) that show ".CDA" files. This is just a convenient way to display the audio tracks, not a file format unto itself. Error correctionThere can be differences between WAV files extracted more than once from the same CDDA track if you compare them. This depends on the CDROM-drive/computer/extraction software combinations. The key to minimize these differences is being able to accurately read the audio tracks without losing your place. CD-ROM drive manufacturer Plextor does this by adding some extra circuitry to their drives that is used to generate sync signals when reading audio discs. Once you have an accurate sync signal, it makes the process of accurately positioning very easy. When a drive can accurately seek to a specific point on the CD over and over again, it makes it very easy for the software to read the audio data and not have to do jitter- correction and other fixes. This level of error correction can be far beyond the error correction possibilities of a CD-player during playback. By writing to the hard disk, the DAE is allowed to take more time for error correction than would be needed by a CD player when played directly. What does a CD-ROM drive do when it finds an error? It depends on how big the error is. For a small number of errors, the built-in error correction codes are used to fix the errors. For larger numbers of errors, a number of things can happen. Some drives take a guess (interpolate) on what the values should be based on the previous and following values. Other drives just set the bytes to 0. Some drives do a good job of reporting these conditions, while some others don't report any errors. It is all up to the person who programmed the firmware in the drive. You can see how many drives can give lots of errors in the final wav file but not report any errors to the DAE application. The DAE application may use the reported errors to do some additional error correction routines. What does a CD player do when it finds an error?When a CD player recognizes an audio track, it routes the bit stream to its DAC - digital-to-analogue converter. There the minimal layer of error correction is applied to patch up some errors in a disc that's in good shape (it cannot do much more) and the analogue sound output is generated. Therefore, in theory a digital extracted CD played by your computer hard disk can be of a higher quality than the original CD played by your CD-player! If you want to achieve this you need to know your CD-ROM/DVD DAE drive features. What are DAE drive features?Because All CDROM drives are not created equal you have to know the audio reading capabilities of your CD-ROM drive. These are called the DAE drive features. Knowing these features will help to configure your DAE application to ensure you can get a accurate as possible extraction of your CD tracks. Accurate Stream. This is the ability to avoid jitter. Each CD drive reads audio discs slightly out (a number of samples), if your CD drive supports 'Accurate Stream' it will be a constant value, this should be the same for each particular make and model of CD Drive. Caching. All drives cache but not all cache audio. An audio caching drive will not read audio a second time but instead send the cached first again to a DAE application. A DAE application like Exact Audio Copy (EAC) needs 2 reads to find errors. Like that the DAE application can never find any errors. In the case of EAC, there is a setting which informs it should empty the cache every time it treads. Downside is it costs speed. C2 error info. Drives supporting C2 'know' when an error occurs. As a result your DAE application no longer needs to read everything more than once in order to find errors. It will read once, and just look at the C2 info to know whether there is an error. Only then it will do additional reads (to get the correct data). You see that C2 doubles speed, but many drives have an unreliable C2 so that your extracted audio may still contain slip- errors! Overreading & read offsets. This feature tells whether or not the drive is able to overread into the Lead-In (pregap) and Lead-Out. For more information about offsets and how to correct the read offsets see [users.fulladsl.be/spb2267]. There is a database [daefeatures.co.uk] on the internet where you can find these features for a particular drive. CD drive offsets can also be found on the internet [accuraterip.com]. 3A Recommended hardwareTo get most out of the ripping process, 3A recommends a CD-ROM/DVD drive with DAE features. See [daefeatures.co.uk] for a first check if your drive has DAE features. If you’re not interested in accurate audio extraction you can use any CD/DVD drive of course. 3A Recommended rippersWe have experience with several rippers or DAE applications both on the Windows and Linux platforms. Our criteria for evaluating the DAE application are:
If you’re not interested in accurate audio extraction you can use any ripping software to extract your audio. Exact Audio CopyYou can test your drive DAE features by using the application Exact Audio Copy (EAC). EAC can test for Accurate Stream, caching, C2 error info, overreading & read offsets. To check your CD/DVD drive for DAE quality, check the DYI project ‘DAE quality ‘ at the site of EAC [exactaudiocopy.de]. Advantages
Disadvantages
Links related to Exact Audio Copy
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