question: How do scratches on DVDs screw up the playback?
comment: Data is read from the dvd using light. A scratch can mean that the light is reflected from the disc in a different way and meaning the receiver can not receive the data.
passages: Damaged spots on a LaserDisc can be played through or skipped over, while a DVD will often become unplayable past the damage. Some newer DVD players feature a repair+skip algorithm, which alleviates this problem by continuing to play the disc, filling in unreadable areas of the picture with blank space or a frozen frame of the last readable image and sound. The success of this feature depends upon the amount of damage. LaserDisc players, when working in full analog, recover from such errors faster than DVD players. Direct comparison here is almost impossible due to the sheer size differences between the two media. A scratch on a DVD will probably cause more problems than a scratch on a LaserDisc, but a fingerprint taking up 1% of the area of a DVD would almost certainly cause fewer problems than a similar mark covering 1% of the surface of a LaserDisc.
   An old-fashioned CD player reading subcode correctly sees a missing audio frame and interpolates any missing information that it cannot correct using information from neighbouring frames. Because these missing frames occur at points where the waveform was nearly a straight line anyway, this interpolation is very accurate and generally transparent to the user.
  
  What happens with computer drives is very specific to the hardware and firmware of the drive in question, assuming they have at least seen past the second data session and can play the audio session.
   The depth and width of scratches as well as the direction in which scratches on the bottom run all determine whether or not the data on the optical disc will be readable. Small scratches on the substrate generally have no effect on the readability of a disc as the laser is reading through the substrate to the data layer. If a scratch is deep or wide enough to affect laser focus, error correction is usually possible, but scratches can be too deep and wide or too close together for error correction to be successful. If a scratch runs along a track or is deep enough to scratch the data layer, data may be lost.
   Section::::Damage to optical discs.:Scratches.:Scratches on the top of an optical disc.
   BULLET::::- Analog signals can be degraded in ways that interfere with or confuse some recording devices. For example, Macrovision attempts to defeat recording by VCRs and DVD players by outputting a deliberately distorted signal, crippling the automatic gain control for video, causing the brightness to fluctuate wildly. While this is only supposed to happen to copies, it may, as an inadvertent side-effect, happen when viewing the original video as well. Some vendors claim to have developed equivalent techniques for preventing recording by video capture cards in personal computers. Devices exist, however, to counteract this measure.
   When creating a copy-protected videocassette, the Macrovision-distorted signal is stored on the tape itself by special recording equipment. By contrast, on DVDs there is just a marker asking the player to produce such a distortion during playback. All standard DVD players include this protection and obey the marker, though unofficially many models can be modified or adjusted to disable it.
   In addition, LaserDisc videos sometimes exhibit a problem known as "crosstalk". The issue can arise when the laser optical pickup assembly within the player is out of alignment or because the disc is damaged or excessively warped, but it could also occur even with a properly functioning player and a factory-new disc, depending on electrical and mechanical alignment problems. In these instances, the issue arose due to the fact that CLV discs require subtle changes in rotating speed at various points during playback. During a change in speed, the optical pickup inside the player might read video information from a track adjacent to the intended one, causing data from the two tracks to "cross"; the extra video information picked up from that second track shows up as distortion in the picture which looks reminiscent of swirling "barber poles" or rolling lines of static.
   BULLET::::- The reading and writing process may not happen at a steady speed. CD drives and many early DVD drives stored data with constant linear velocity, so that the data rate remained the same regardless of the position of the optical head. Modern DVD drives use Zoned Constant Linear Velocity with different data rates in different zones.
  
  Section::::Optimal writing speed.
   DVD hardware accesses the additional layer (layer 1) by refocusing the laser through an otherwise normally-placed, semitransparent first layer (layer 0). This laser refocus—and the subsequent time needed to reacquire laser tracking—can cause a noticeable pause in A/V playback, the length of which varies between hardware. Studios began printing a standard message on keep cases explaining that this pause is not an indication of a damaged or defective disc.
   The analog information encoded on LaserDiscs does not include any form of built-in checksum or error correction. Because of this, slight dust and scratches on the disc surface can result in read-errors which cause various video quality problems: glitches, streaks, bursts of static, or momentary picture interruptions. In contrast, the digital MPEG-2 format information used on DVDs has built-in error correction which ensures that the signal from a damaged disc will remain identical to that from a perfect disc right up until the point at which damage to the disc surface is so substantial that it prevents the laser from being able to identify usable data.
   Assuming the player's optical pickup is in proper working order, crosstalk distortion normally does not occur during playback of CAV format LaserDiscs, as the rotational speed never varies. However, if the player calibration is out of order or if the CAV disc is faulty or damaged, other problems affecting tracking accuracy can occur. One such problem is "laser lock", where the player reads the same two fields for a given frame over and over, causing the picture to look frozen as if the movie were paused.
   There are more details about this technique (e.g. the twin-sectors need to be recorded in large extents, the SubQ-channel has to be modified etc.) that were omitted. If the twin sectors are right next to each other as shown, the reader would always read the first one, "Jill"; the twin sectors need to be farther apart on the disc.
  
  Section::::Technology.:Data position measurement.
   BULLET::::- DVD+R, DVD+RW and the recordable Blu-ray formats are immune from buffer underrun as these discs contain technology that allows the recorder's write mechanism to precisely locate the end of the recorded track and to seamlessly carry on from where it left off. Many disc authoring utilities disable the buffer underrun protection option when these discs are being written.
   Section::::Compact Discs.
  
  A "skip" or "jump" is when the laser of a Compact Disc player cannot read the faulty groove or block of data. Skips are usually caused by marks blocking the path of the beam to the disc, e.g. a finger mark, hair, dirt in general, or a scratch. Since the read mechanism has very little contact with the disc's surface and the data itself is not on the outer layer of the disc, the blockage is not a physical issue as with a record, but rather reflective.
  
  Section::::Compact Discs.:Basic players.
   Section::::Methods.:Misregistration.
  
  Misregistration is produced by the physical noise of historically analog media such as motion picture film. It includes dirt, scratches, smudges and markings that can distort physical media also impact the playback of digital recordings on media such as CDs and DVDs, as electronic music composer Kim Cascone explained in 2002:
   A player may utilise one or more of these techniques, depending on how faulty the data is. In the case of severe, irrecoverable damage to the data, the player may try to rescan the disc to relocate its position. In this case, the machine may make a series of audible chirping noises as the laser is moving from the faulty block to the data information area and back again.
  
  Section::::Compact Discs.:Later players.
   Section::::Video.:Motion compensation block boundary artifacts.
  
  Block boundary discontinuities can occur at edges of motion compensation prediction blocks. In motion compensated video compression, the current picture is predicted by shifting blocks (macroblocks, partitions, or prediction units) of pixels from previously decoded frames. If two neighboring blocks use different motion vectors, there will be a discontinuity at the edge between the blocks.
  
  Section::::Video.:Mosquito noise.
   However, when trying to edit footage deck to deck, portions of the old recording's video may be between the erase head and video recording heads. This results in a faint rainbow-like noise at and briefly after the point of the cut as the old video recording missed by the fixed erase head is never completely erased as the new recording is printed.
   When the buffering circuitry is in operation, the compact disc is read at a fixed read speed or CAV and the content is buffered (with optional ADPCM compression) and fed to RAM within the player. The audio content is read from RAM, optionally decompressed, and then sent to the digital-to-analog converter. When the disc reading is interrupted, the player momentarily reads the data stored in RAM while the tracking circuitry finds the passage prior to the interruption on the CD.
   Analog Copy Protection (ACP), the encryption technology created by Rovi Corporation (formerly Macrovision, now TiVo), is designed to thwart users' attempts to reproduce content via analog cables. When a DVD is played through an analog video cable and recorded using a VCR, Rovi's ACP technology will distort the copy partially or completely.
  
  The technology works by adding additional lines to the video signal. In the NTSC video standard, blank lines (vertical blanking intervals) that the user cannot see are used for functions like closed captioning. Rovi Corporation uses these blank lines to implement its ACP technology.
   A similar problem can also occur in videotape recordings, causing the picture to jitter slightly, or the top of the picture to wobble. This is known as flagwaving.
  
  Section::::Correction.
   BULLET::::- Some recorders are, in fact, able to stop writing in the middle of a session, and resume writing whenever the buffer is refilled. Recorders with such buffer underrun protection handle the interruption with an extremely small gap in the recorded track on the disc. Since the techniques for protecting against buffer underrun are proprietary and vendor-specific, technical details vary.
   BULLET::::- The patent specification mentions the use of a protective layer(s) or coating to prevent scratching during handling, but the layer does not offer significant benefits, as its task is merely to protect. Fingerprints and scratches will obscure the data read. In a CD, on the other hand, where a focused laser beam is used in conjunction with a protective layer at the reading side of the disc, scratches and fingerprints are out of focus, and thus not detected by the reading spot. As a result, the CD/DVD method offers a great resilience against disc anomalies, offering great playability.
   Section::::Noise.:Wow and flutter.
   Some players have a feature called " detection" or "inverse telecine" which attempts to better handle the artifacts which result from differing film and video rates in conjunction with interlaced scanning of the film. However most line doublers used in these players are not able to achieve the anticipated inverse telecine functionality, refer to line doubler for details.
  
  Progressive scan output can not use connections intended for interlaced video, such as composite video (single RCA terminated cable) and S-Video (Mini-DIN terminated cable). The following connection methods are common for using progressive scan:
  
  BULLET::::- VGA (analog)
  