CD Ripping Terminology
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22-02-2011, 09:34 AM
CD Ripping Terminology
From LinnDocsConfusion often arises when terms such as “Jitter” and “Error Correction” are used when referringto digital audio. The terminology used in CD ripping is particularly confusing as there appears to bea lack of consistency (and in some cases, accuracy) between the various software providers anduser forums on the Internet. This document is an attempt to clarify some of these terms and showhow they relate to the fidelity of ripped audio.
Jitter can have many meanings in digital audio but it generally refers to a timing error of some sort. Someforms of jitter can have a considerable effect on audio quality, others are benign as long as they are below acertain level. It is worth describing three forms of jitter so that the differences between them can be seen :
1. EFM Jitter
3.Read Offset Jitter
EFM (Eight-to-Fourteen Modulation) is the final part of the encoding scheme used to produce the spiral ofmicroscopic bumps on a CD. When a CD player reads this spiral pattern, bumps will be interpreted as onelogic level (e.g. ‘1’) and the spaces between the bumps as another (e.g. ‘0’). This stream of 1’s and 0’s iscalled the EFM signal and in an ideal case it should be identical to the original signal used to record the disc.Unfortunately, because the CD mastering and replication processes are not perfect, errors occur in the shapeand position of the bumps. These physical errors translate into timing errors in the recovered EFM signal andthis is what is termed EFM Jitter. Because EFM jitter is always present, the CD reading process is designedto be immune to it – up to a point.The EFM data stream is ‘self-clocking’, meaning that the original timing of individual ones and zeros can berecovered using a phase-locked loop (PLL). The PLL generates a bit clock that locks on to the EFM signalsuch that, on average, the edges of both signals coincide. The EFM signal is then re-sampled using this clockand the resulting data fed into an elastic buffer. A second fixed-frequency clock controls the rate at whichdata exits this elastic buffer and the disc speed is controlled to keep the average buffer occupancy at 50%.This 2-stage re-timing process completely decouples the output (i.e. audio) clock from the jittery EFMsignal.Excessive high-frequency EFM jitter levels may result in bit errors (e.g. a ‘1’ being read as a ‘0’), but thesecan generally be handled by subsequent error correction processes. Excessive low-frequency jitter (as maybe encountered with an eccentric, or out-of-balance, disc) may exceed the capacity of the elastic buffer, andthis can cause dropouts in the audio stream. One advantage of ROM drives is that they typically have largebuffers that can tolerate large amounts of low-frequency EFM jitter.EFM jitter can be measured and reported by a number of modern CD and DVD ROM drives. Plextor’s‘PlexTools Professional’ software, for example, includes a ‘Beta/Jitter’ test that measures EFM jitter (via aPlextor drive). Nero’s ‘CD-DVD Speed’ toolkit can also reportedly measure EFM jitter from a select numberof drives as part of its ‘Disc Quality’ test.
In digital audio, each sample represents the audio signal amplitude at a single point in time. The sample ratethen defines the time interval between each sample. In order to recreate the signal in the analog domain,both the amplitude and the sampling interval must be recreated with sufficient accuracy. An error in eitheramplitude or sampling interval will result in distortion of the original signal.All digital-to-analog converters (DACs) use some form of oscillator to generate a clock at the requiredsample rate. This clock may be either free-running, or locked to some incoming digital audio stream (e.g.SPDIF). In either case, the period of the clock signal will not always be exactly the same as the originalsampling period. The error between the two is termed Sampling Jitter.Sampling Jitter can have a serious effect on audio quality and most audio equipment manufacturers go togreat lengths to minimise it. It is important to note, however, that Sampling Jitter is only important at thepoint of conversion between the digital and analog domains. Whilst the signal is in the digital domain, thesample period is just a number, and as such has no jitter. It is impossible, therefore, for Sampling Jitter to begenerated by any lossless CD ripping process. It is, however, possible for Sampling Jitter to be generated bylossy processes such as sample rate conversion.
Read Offset Jitter
The term ‘Read Offset’ requires some explanation before Read Offset Jitter can be described.The information stored on an audio CD consists of a number of data channels. The main channel carries theaudio data but there are also a number of low-bandwidth ‘subcode’ channels that carry information such asdisc time and track number. The original purpose of these subcode channels was to enable simple navigationand track/time display. Because an audio CD player doesn’t need to navigate to high degree of accuracy, theresolution of these channels was limited to 1/75th of a second. This means that when audio data is requestedfrom a CD ROM drive, it can only be accessed with an accuracy of 1/75th of a second.The separation of audio and subcode channels introduces further uncertainty to the navigation process.Because the audio data channel undergoes more processing (error correction, buffering, etc.) than thesubcode channels, there is generally an offset (in samples) between where the player thinks it is and where itactually is. In CD ripping terminology this is termed the Read Offset.A consistent Read Offset is important for audio ripping as data is typically transferred as a sequence of smallblocks rather than as one continuous data stream. If the Read Offset is consistent then all the data in theblocks will line up perfectly. If the Read Offset is inconsistent then the data in some blocks may overlap, orthere may be data missing between blocks - this is Read Offset Jitter.A ROM drive that has a consistent Read Offset is sometimes termed ‘Accurate Stream’ capable. This justmeans that it is capable of accessing audio data repeatably to within a single sample. Some ripping softwarecan perform tests to determine whether a drive has a consistent Read Offset.A drive that has an inconsistent Read Offset can still be used with certain ripping applicationswhich attempt to get round the problem by requesting overlapping blocks of data and performing arealignment process in software. Some ripping applications refer to this process as ‘Jitter Rejection’.As an aside, CD-ROM discs have additional synchronisation patterns and sector headers embedded in thedata channel which enable absolute navigational accuracy. It is therefore quite possible for a ROM drive tofunction perfectly when reading ROM discs, but still exhibit Read Offset Jitter when reading audio discs
Error correction is a vital part of all CD playback systems and without it CD’s would be unusable. Asmentioned earlier, the CD replication process is far from perfect, and single bit errors (e.g. a ‘1’ being read asa ‘0’) occur quite frequently. Noise and other imperfections in the read channel will also contribute to the biterror rate. The microscopic nature of the medium means that scratches, fingerprints, and other surfacedefects can result in prolonged bursts of erroneous bits.The error correction system chosen for CD is called Cross-Interleaved Reed-Solomon Code (CIRC). Thissystem adds additional bytes to the original data which allow almost all errors to be detected and for mosterrors to be corrected. A special interleaving technique is used to distribute the data on the disc to minimisethe impact of burst errors. Burst errors of up to 4000 data bits (equivalent to approximately 2.5mm tracklength on a disc) can be completely corrected. CD-ROM discs have an additional layer of error correctionembedded in the data stream which improves data integrity beyond that achievable with the basic CDformat.On playback, the data stream from the disc passes through a CIRC decoder which attempts to detect andcorrect both random bit errors and large burst errors. The CIRC decoder is typically split into two layerstermed C1 and C2. The C1 decoder uses the inner layer of error correction coding to correct random errorsand detect burst errors. The C2 decoder then uses the outer layer, together with information passed from theC1 decoder, to correct burst errors and random errors that the C1 decoder was unable to correct. If the C2decoder encounters errors that cannot be corrected it will flag them for subsequent concealment (i.e.interpolation or muting).
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