formatDVD Video or Digital Video Disc, is a high-capacity optical disc technology developed by a consortium often listed with Toshiba as the first stakeholder. It is fairly unique amongst audio in video formats in the sense that it has always been a collaborative effort between industry players that most often compete rather than collude. Indeed, even the movie studios, the main content providers for the format, were included in the consortium, though whether this was desirable is probably a question consumers would answer in the resounding negative.
DVD owes much to the design of that most formidable of optical disc formats, the ubiquitous Philips Compact Disc (CD). The Audiotek document Notes about Compact Disc discusses the physics of optical discs in general and CDs in particular, and a knowledge of the low-level technical aspects of both of these topics are useful to understand comparable aspects of DVD. Concepts of video explained in Notes about the VHS video format could also be helpful.
While DVD, like CD, can be used as a data medium, this document does not discuss that application in any detail. The main distinction, other than sheer difference in capacity, is that DVD was designed first as a multi-purpose data carrier, wheras CD was designed first as an audio medium.
Although DVDs are physically the same size and shape as CDs, this is where the similarities end. DVD increases the capacity for storing data over a 12 cm diameter CD by a factor of seven (DVD-5) to twenty-five (DVD-18,) by using smaller pit dimensions, more tightly packed tracks and, by neccessity, a shorter wavelength laser. Notice also that, unlike CD, DVD offers a range of capacities, owing to the possibility of a dual-sided, dual-layer configuration for DVD discs.
Where CD uses Eight-to-Fourteen Modulation (EFM) and Cross Interleaved Reed Solomon Code (CIRC) for encoding and error correction respectively, DVD uses an enhanced EFM Plus encoding (which modulates 8 bits to 16 bits instead of 14) and a revised RS-PC error correction scheme to provide a ten-fold increase in decoding reliability over CD.
The increased capacity and corresponding improvement in reliability has made possible an application which CD struggled with—the delivery of high quality video of a feature length. This is DVD's killer application, and is the reason why DVD Video was first designated "Digital Video Disc." Of course, DVD also has vast potential as a data and audio storage format as well, which lead to the backronym "Digital Versatile Disc."
Like CD, besides professionally authored product, DVD is also offered in recordable and rewritable modes for consumer use. Unlike CD[1], there are physically different recording and rewritable mechanisms (two standards and one renegade format) as well as the usual gamut of technical variations of video encoding (NTSC, PAL.) One of the standards covers both DVD-R, the recordable format comparable to CD-R, and DVD-RW, comparable to CD-RW. The other standard covers DVD-RAM, a format optimized for use as a random-access rewritable medium more like a magnetic hard disc. The renegade formats will be mentioned briefly below.
All three flavours of television transmission systems, PAL, SECAM and NTSC, suffer from noise and other disturbing artifacts as a result of their analogue nature and their composite luminance and chrominance encodings. While some consumer formats, notably JVC's SVHS, have attempted to improve on this, none have succeeded like DVD.
DVD offers a fully component oriented digital encoding scheme which individually preserves two crisp chrominance signals and one finely detailled luminance signal with digital clarity to a precision of 8 bits. Like CDs magic 44.1 kHz sample rate, poised on the limits of human auditory perception, the 8-bit precision in each of the signal domains is enough to represent almost all the colours discernable to the human eye that can be emitted by today's video display hardware. That is, until we get better on-screen colours, DVD is at the edge of the visual envelope in terms of colour accuracy.
The resolution of DVD in the spatial domain has been modelled on the physical limits of today's televisions, and in fact almost matches the standard CCIR-601 standard for digital television production. Digitally, DVD encodes 720 by 588 pixels (PAL) or 720 by 488 pixels (NTSC,) translating to an analogue horizontal resolution of 500 lines on a display device with a 4:3 aspect. This is marginally greater than what is often termed "broadcast quality."
Because DVD Video is largely used as a carrier medium for movies, and because movies do not use the interlacing mode of television transmissions, DVD also offers an advantage over other video formats in that it encodes film using a progressive scan mode. This mode improves horizontal resolution while maintaining the correct projection speed. A mode which projects material with an apect of 16:9 the same resolution as 4:3 aspect material further increases the faithfulness of home cinema reproduction of widescreen material.
Beyond the superb visual attributes of DVD Video, in the area of audio DVD far surpasses anything currently available to consumers of analogue television and video formats. DVD offers a number of variants of discrete multichannel audio schemes with better-than-CD-quality dynamic range and signal to noise, and supports multiple soundtracks accompanying the same video stream. Up to six channels of discrete audio may be individually encoded to accompany the high quality pictures DVD delivers, including four satellite surround speakers, a central dialogue speaker and a low frequency effects cone. Matrixed sound is also supported, delivering from four to seven channels using matrix decoders for Dolby Surround, Dolby ProLogic, DTS ES or Dolby Digital EX.
DVD also offers new features not present on other consumer video media: interactivity via animated menus and still video frames, metadata accompanying video, including foreign language and graphical subtitles, as well as advanced video presentation features such as branching and multiple angles.
DVD Video is limited to standard television resolution, as technically described above. Even if a professionally authored DVD claims that it is produced from a high-definition (HDTV) master, the extra information present in that master over conventional digital television (SDTV) resolution is lost in the transfer to DVD. The primary reason for this end-use limitation is the capacity required to store HDTV data, and the bitrate required to transfer it—both are beyond the physical capabilities of DVD.
This limitation is a addressed by experimental optical disc products including Sony's Blu-Ray format. It is likely that an experimental format such as this will become the next generation of DVD. These new discs will not play on existing players, but it is likely that new equipment that supports such a format will be able to play current DVD Video material.
JVC's D-VHS solution is a currently available way of economically storing HDTV-quality material in the home environment. However, it carries with it all the attendant burdens of cassette-based media, and is also very bulky when compared with optical disc media.
Another limitation of DVD Video is that it does not support uncompressed audio at a sampling rate above 48 kHz. (Some players do support 96 kHz, but this is not mandatory.) DVD Audio does. There is much argument as to whether audio of a quality greater than that offered by DVD Video offers any real benefits to the vast majority of consumers. Certainly the level of adoption of DVD Audio is very low, particularly when sales of DVD Audio discs are compared with sales of professionally authored DVDs or CDs.
All recordable and rewritable DVD formats are presently limited to the single-layer DVD-5 (or double-sided DVD-10) capacity, or 4.7 GB (9.4 GB.) Dual layer DVD-9 discs will arrive in a few years time, but will not be recordable on present equipment as they are not part of the DVD-R version 1.1 standard. We will therefore not consider recordable DVD-9 any further.
To aid consumer understanding of digital video compression and the recording capacity in terms of hours of programming stored, an analogy is drawn with video cassette formats like VHS, leading to four compression levels:
The LP and EP capacities are achieved by sacrificing quality, in a similar, but not technically comparable, way to SVHS and VHS. Indeed, the exact mechanisms for reducing quality may not be consistent from player to player, so that EP quality on one player may be qualitively different from one recorder (or software application) to another. Also, these capacities are merely guideposts on a continuum. Many manufacturers permit capacity to be fine-tuned in increments much less than two hours.
While this may sound all very simple and familiar, there are complexities. Besides the standard DVD-R/RW format, there is one other standard for data applications (though sometimes used for video with very limited compatibility,) DVD-RAM, and a competing set of formats, called VD+ in this document.
Thankfully the foregoing discussion of compression ratios can be applied to all but DVD-RAM media, and this includes DVD-R/RW and VD+ R/RW. VD+ (also referred to as DVD+R or DVD+RW by its proponents) offers additional recording options beyond simply designating Fine/SP/LP/EP, but these will not be covered here. VD+ is not supported by the DVD Forum, the collective responsible for DVD standards, and it is not compatible with playback equipment unless that equipment has been specifically designed to accept it. Standard recorders will also not write to the VD+ format, and it is often the case that VD+ recorders will not write to the standard formats.[2]
Another subtlety is that DVD-R has a variant called DVD-R for (Professional) Authoring which can only be recorded by professional equipment. Standard DVD-R as discussed before is distinguished from this format by using the terminology DVD-R for General (Use.)
Using DVD-R for General Use usually entails many more restrictions than using DVD-R for Authoring, primarily due to the limitations of tools on the former platform, not due to limitations of the format itself. Consumer recorders typically allow a manufacturer-chosen style of interactive menu to be placed on the disc, and may also allow some basic branching functions to be used for editing, but advanced features like multiple soundtracks, angles and subtitles are placed off limits. Typically, only a single Dolby Digital two channel soundtrack is allowed.
DVD-RAM is a format out on its own. It is supported by the DVD Forum as a data carrier medium. It is also used in some consumer equipment as a video format. This format offers very little compatibility with standard players, and is also not well supported by standard or VD+ recorders.
Only professionally authored DVDs are region coded. Please skip this section if you are interested in DVD as a recording medium only.
Perhaps the biggest stumbling block to the widespread acceptance of DVD by consumers is the content producers imposed limitation of region coding. Under this scheme, the world is divided into seven regions, and discs coded to play in subsets of these regions must be securely rejected by all players belonging to those regions outside these subsets.
This is a technological implementation of a legal concept which has been exploited by content producers for decades. The legal concept, underpinned by the Berne Copyright Convention and supported by the majority of developed nations' governments, is that the author of a work (for the purposes of copyright) has the right to impose restrictions on the distribution of their work, and therefore the right to authorise that only particular agents in each juristiction may distribute the work in that juristiction.
The exploitation is evidenced in the way in which movie studios (the authors) control the release times of movies to cinema, video, pay television and free-to-air television for each country. To ensure the revenue of the distributors involved on each stage of release (and hence justify the amount charged for distributors to obtain exclusive rights in that country,) the release times are spaced such that consumers pay multiple times to see the same content at each stage, with little or no competition between stages. Unfortunately for the authors, this monopoly cannot be enforced if:
In this case, the content from one country will compete with the content in the other country, reducing the level of exclusivity, and the amount paid to the author for that exclusivity. Authors therefore wish to prohibit this, and legally it is their right to enforce it, both through legislation implementing the principles of the Berne Convention and also the contracts the authors have with their distribution chain and even the end consumer.
Clearly this a monopolistic situation which benfits authors alone, and as such should be countered by regulations in the consumer interest which promote free trade and competition between distributors, even across international borders. Unfortunately for the consumer, in most countries, such consumer protections do not exist and aren't even being considered.
Despite the law being mostly on their side, the regulations in their favour are not seen as sufficient by the content providers to DVD: largely the DVD Forum member, the Motion Picture Association of America (MPAA.) They required that the DVD Forum develop a format to enforce their monopoly in technology. And as content was the final piece in the DVD puzzle, the consortium were forced to comply. The solution provided was really more (legally if not technically) than was asked for too, and the MPAA have not let consumers forget this, region coding even those back catalogue titles they have no real interest in issuing exclusive rights for overseas in competing modes.
The only solace for consumers prepared to test the Berne Convention and its implementation is that region coding hasn't worked, almost by design. Region coding is not an encryption scheme, and therefore not the slightest bit difficult to break—it is simply a status flag on a DVD. And it is not enforced through complex hardware, but readily alterable modular software. While discs are potentially the weakest link in region coding (just a couple of bits need flipping to disable it,) user duplication of DVD has only become increasingly common in recent times. But perhaps because of that, the little bit of extra effort (read reverse engineering) required to subvert region code in players was made, and as a result, outside the main content zone of the US (Region 1,) region free players are easily obtainable for those with even the slightest inclination. In some countries, manufacturers' agents themselves have been known to make the modifications "under the table," for it is so simple to effect, difficult to trace, not really illegal in itself (and thus ultimately defendable should it come to that,) and potentially offers higher sales.
Not content with controlling distribution of the DVD media, content makers also insisted that DVD be protected aginst copying. Essentially an impossible feature to have in clear digital media (as opposed to a smart card or black box device,) the technical side of the DVD Forum made four gestures to quiet the content providers enough to get DVD into the marketplace:
The technical enforcement of security is an extremely difficult enterprise, particular in consumer equipment which needs to interoperate in order to provide unimpeded service to users. All of the schemes used in protecting DVD content against copying are technically defficient and almost completely ineffective. They keep honest people honest perhaps, but they do nothing to prevent mass digital piracy. This wouldn't be so bad, except that many schemes cause many problems for legitimate usage—backups should be permitted for all digital content in this author's view, and analogue outputs must remain uncorrupted or they do not comply with video standards. Corrupt analogue signals will produce substandard results on high-end consumer and professional equipment alike.
As an example of the technical failure, what actually is provided by CSS is a flimsy encryption scheme which will have no success in preventing bit-for-bit copying and limited success in ristricting successful decoding of the data stream to only those players and software made by manufacturers and developers:
As DeCSS proved, it was easy to reverse engineer one manufacturer's players so that its key to decrypt the encoded stream could be stolen, and the result was the means (almost directly) to decode all DVD discs pressed to that point. But even assuming keys are secure, the scheme itself is vulnerable because through its very application it provides a potentially unlimited source (given the number of DVD discs available and the amount of data comprising them) of encrypted data and corresponding decrypted data obtained using fixed encryption and decryption keys—effectively the holy grail of any cryptanalyst seeking to break a cipher.
One item in the four pronged security regime has been successful, because it is not an attempt at restriction but an out-and-out prohibition:
DVDs are partitioned into titles, and each title is further partitioned into chapters. Titles may consist of video or still frames, accompanied by audio channels and subtitle metadata. So wheras on a CD player, you can select a track by its number, on DVD you can select a title by number, and then further a chapter by its number. However, it is most common for DVDs to have only one title, and even if there are other titles, these are typically trailers or extra features which supplement the main title. Once a title is playing, you can then select the audio track you desire, the angle you prefer and any subtitles you require. Apart from utilizing chapters to select scenes, a scene can also be selected by its time code, and of course speed searches in both directions are also possible.
But believe it or not, the Forum has even bungled this sound indexing scheme. DVD authors are allowed to disable any and all of the indexing features previosuly mentioned, and players are forced to abide by such ludicrous restrictions. And most unfortunately for consumers, DVD authors actually do disable features, and quite often too. Even Stop can be disabled! Patently annoying, player fixes for these restrictions are not as forthcoming as those for thwarting copy protection and working around region coding, so most consumers will be forced to live with them.
The question of course is "why would they want to?" Well, sadly, it is merely because they can. Forcing consumers to sit through copyright warnings, adopting a "father knows best" attitude which only allows features to be accessed through cumbersome menu systems, and obscuring time code information that might give clues to exactly how short a supplementary feature is are just the kind of masochistic practices you should all expect from the DVD Forum (and its content provider participants in particular) by now.
The other annoying thing about almost all DVDs which inhibit use of the indexing system is that they automatically bootstrap themselves as soon as they are loaded. You then have to explicitly stop them to select a title and chapter or title and time code. Even more annoying is that DVDs do not all bootstrap in the same way—some give a title menu, while others immediately begin the main feature. Others will give you a copyright or trailer tirade you can't even Stop. The best place for these last aberrations is a garbage dump.
Thankfully, most DVDs authored by consumer recorders do not automatically bootstrap, and the indexing scheme can be used as intended.
Even given their vast capacities, DVDs are still not big enough to store uncompressed digital video of television quality. For this reason, DVD uses a lossy digital video compression scheme developed by the Motion Picture Experts Group, and the scheme bears their name: MPEG 2. MPEG 1 was first used in Philips Compact Disc Digital Video (CD-DV) and on personal computers, but it is not a variable bitrate format like MPEG 2 and it does not support DVD's enhancements such as multichannel soundtracks using varied codecs or embedded metadata like subtitles.
MPEG is a lossy compression scheme in that image detail which is not usually discernable to the human eye is discarded, so the resulting image has lost some detail, but the image which remains is not noticably degraded. MPEG succeeds and fails to varying degrees on this score depending on the bitrate: the longer the material you want to store on DVD, the more detailed its images are, the less able MPEG will be able to compress it to fit and still retain quality. The level of quality preferred in a DVD transfer is determined during the mastering process (when the MPEG data stream is generated,) and a DVD which sacrifices too much quality is often said to be "poorly mastered" and will suffer from digital compression artifacts.
Digital compression artifacts can be particularly disturbing, because they are not easily compensated for by the human eye like the types of interference we are used to seeing in television broadcasts. Wheras snow (analogue noise) and ghosting (feint "after-images") are the sorts of defects the brain are used to compensating for in low light conditions and during intervals of changing focus, the brain is less able to cope with the more unnatural artifacts compressed digital video sometimes presents.
There are several types of artifacts arising from the compression of digital video:
| Blocking or Pixellation | This is the most extreme form of artifact, and is thankfully quite rare. Heavily compressed MPEG will cause edges to disappear—something to which our eyes are most sensitive—and the edges of the square blocks of solid hue which replace them will instead become the focus. |
| Motion Blocking or Pixellation | During rapid motion, the MPEG codec may be more susceptible to generating blocking or pixellation type artifacts as it tries to cope with the compression of a rapidly changing scene (representing a large amount of visual information that needs to be conveyed to the viewer.) Usually a variable bitrate codec like MPEG2 is able to cope with this by spiking the bitrate to cope with the higher demand, but if the codec is already using its bit budget to provide a high-quality image, there may be no headroom lrft to prevent degradation of the image during the period of high activity. |
| Motion Artifacts | More generally, motion artifacts occur when the motion of an object is not reflected in the motion of corresponding features and textures. This typically occurs during very slow panning or tilting motions of the camera, or when objects are moving very slow with respect to the background. In the worst case, parts of the moving object can appear to remain static, which is very confusing to the vision centres in the brain—we naturally expect the details and texture in a shape to follow it as it moves. Human vision is most used to interpreting three-dimensional scenes, and it expects to see changes in shading as an object moves with respect to a light source, but this too can be undermined by MPEG compression. |
| Mosquito Noise | Mosquito noise occurs around sharp edges in the image, and is most common on computer-generated and hand drawn animation or titles. It appears as random patterns of dots which buzz around the edges of objects within the scene, but it will not be present in large, slowly-graded areas with little detail. |
| Digital Noise | Noise present away from the edges of an image in smooth or slowly-graded areas is something all video formats have in common. However, digital noise can be more disturbing because it tends to move more slowly, and in discrete time intervals. |
| Posterization | If the colour transition between two areas is too rapid, at higher compression ratios an artificial edge may be generated. In the worst case, this edge may also suffer from mosquito and other noise artifacts. |
| Quilting | This is an effect where discontinuities are visible in a venetian or vertical blind pattern, and is most noticable at straight diagonal edges which are slightly angled from the vertical or horizontal. Its appearance in a video stream has little to do with bitrate, and is an undesirable result of compression of some images with some codecs. |
MPEG encoding works by periodically taking a still frame (usually sampled in 24-bit RGB colour at a resolution of around 700 by 500 pixels,) called a key frame, and compressing it by breaking it into small square blocks and then analysing the frequency components of the two chroma and one luma channel in the horizontal and vertical directions for each block. The chroma and luma for a pixel can be derived from its RGB value using a simple matematical transformation, and doing this processing results in data which corresponds better to the way our eye sees detail and colour.
MPEG is effectively using the same technique as used in the Joint Photographic Experts Group (JPEG) scheme for still image compression to compress its key frames. The frequency components will be mostly insignificant, and so can be safely discarded, leaving only pertinent image information with little redundancy. Also, the luma channel is stored with twice the spatial resolution of the chroma channels because the human eye is less able to resolve colour with spatial accuracy. Together, these two optimizations reduce the amount of digital data taken to store a visually equivalent image.
Once a key frame is created, it would be wasteful to store neigbouring frames in the same way, because there are not only spatial patterns within frames (compressed in the key frame,) but temporal patterns between frames. So MPEG encoders generate these neigbouring frames using a different technique which describes how the image has changed in that frame rather than describing the image from scratch. Of course, after a few frames, the similarity may be disappearing. The similarity may also be lost due to a cut between shots. As both these cases are realised by the encoding software, a new key frame is generated and neigbouring frames are then derived from that until a new key frame is needed, and so on.
In a similar way to video, most audio on DVD is compressed using lossy compression schemes, and is embedded alongside blocks of video data in the main MPEG 2 stream. All these schemes work in a similar way to the video encoder, by performing frequency component analysis on brief temporal sections of the audio, discarding the inaudible components and storing only the pertinent components. However, DVD can also store uncompressed audio if this is preferred, and it does this using a format similar to CD, though often with a slightly higher sample rate and dynamic range.
The common compressed audio schemes on DVD are MPEG multichannel, which is defined as part of the MPEG 2 scheme, and Dolby Digital Acoustic Coding 3 (AC3,) a scheme developed by Dolby Laboratories. One other, Digital Theatre Systems Coherent Acoustics (DTS,) is often preferred because it is typically stored at a higher bitrate, and is therefore more faithful to the original source. AC3 is often preferred over MPEG multichannel (though for political rather than technical reasons,) so much so that MPEG multichannel is now becoming rarer than DTS. The only uncompressed scheme in use is PCM (Pulse Coded Modulation,) commonly at a 48 kHz sampling rate, and is available only on some music oriented titles.
The bitrate of the audio formats is not typically variable like it is for the video component of the MPEG stream. This usually means that the quality of audio is usually fairly high and consistent, and it is rare to hear that audio on a DVD has been poorly mastered—more often the criticism is that the source material is poor (for example, is muffled, lacks bass or hisses—not typically digitally induced problems) or the sound mix (including the distribution between channels and relative levels of dialogue, music and various effects) is poor.
As a guide, 384 kbs-1 is a baseline bitrate commonly used for Dolby Digital titles, 448 kb/s is a higher bitrate sometimes used, and 1411 kbs-1 is the typical DTS bitrate, though this is sometimes halved to include better quality video or include bonus materials to supplement the main title. As a benchmark for the sampling rates, 48 kHz is a common option, with 96 kHz being used only in high-end applications. Dynamic range is typically 20 bits.
The catalogue of Audiotek prerecorded DVDs is the ATKX catalogue. The catalogue includes region 4 (Australia,) region 1 (US,) region 2 (UK) and non-region-coded material, and the catalogue lists the region code for each title (with 0 meaning non-region-coded.) The catalogue of Audiotek mastered DVD-R/RW acetates is the ATMU catalogue. Numbers in both catalogues consecutively starting at 001 (ending at 999 for ATKX and at 899 for ATKU.) Discs may be freely deleted from either catalogue, but only numbers in the ATMU catalogue may be reallocated.
The ATKX catalogue lists either the chapters (for a feature film, music compendium or TV special) or titles (for a special features disc or collection of TV episodes) which appear on each disc. The structure is flat, and is therefore unlike the programme/subprogramme two-tier hierarchy of videotape catalogues (including ATKV, ATKW and ATMO.) If a music compendium contains bonus titles, these are listed after the chapters as if they were chapters of the main title.
For each title or chapter, the following metainformation is captured:
The soundtracks and subtitles for a particular language (field 7) are listed separated by commas, with music or novelty soundtracks assigned to the count of main language tracks for the disc:
| Code | Meaning |
| [l](DD|M|DTS|PCM)n[,m] | [l] main audio track(s) available using specified codec/channels |
| [l]C(DD|M|DTS|PCM)n[,m] | [l] commentaries available using specified codec/channels |
| [l]S | [l] subtitles available |
For each disc, the following metainformation is captured:
The main language soundtracks (field 9) are listed separated by commas:
| Code | Meaning |
| DDn[,m] | Dolby Digital with n channels used [and m channels used] |
| Mn[,m] | MPEG multichannel with n channels used [and m channels used] |
| DTSn[,m] | DTS with n channels used [and m channels used] |
| PCMn | PCM using 2 channels with a sampling rate of n kHz |
The presentation formats (programme field 10, or disc field 9,) in order from most desirable to least desirable (in terms of faithfulness to the shape of the original work and the fullest possible use of available display without distortion or cropping) are:
| Presentation | Description of digitized image as stored on disc |
| Spherical | Source image fills 4:3 aspect display with no distortion or matting. |
| Anamorphic | Source image fills 4:3 aspect display using distortion (if intended aspect is 1.85:1) or distortion and matting (if intended aspect is 2.35:1.) |
| Matte | Source image fills 4:3 aspect display with matting but no distortion. |
| Open Matte | Source image fills 4:3 aspect display by removing matting. |
| P&S | Source image fills 4:3 aspect display with cropping achieved through panning and scanning. |
Spherical presentations fill a standard 4:3 display device, and are preferable on those devices because they use the full resolution with no distortion, cropping, panning, scanning or matting. Anamorphic presentations can be converted to Matte presentations by the DVD player, and are to be preferred on 16:9 display devices or 4:3 display devices with a 16:9 compression mode because they use the full resolution of the device with no distortion. Matte presentations preserve aspect without distortion, but not at the full available resolution of the display device. Open Mattes present an alternatively framed presentation to fit your screen, and are a compromise which let you see more image than intended. P&S presentations present a cropped, constantly panning and scanning presentation which while filling your screen are a serious compromise.
[1] Technically CD does have at least two variations of rewritable, but CD-WO is exceedingly rare and was never a serious competitor to the more advanced CD-RW format developed later.
[2] Unlike cassette or tape-based media, the physical format of the media needs to be laid down before video information can be laced into it. While in magnetic disc media, the physical format can be changed, in optical media it cannot—it must be laid down during manufacture. The effect of this is that DVD-R and DVD-RW recorders, while using a very similar media, cannot be used with VD+ media and vice versa.
[3] Some televisions are adversely affected by Macrovision's phantom signal in the vertical blanking interval because the alternating peak white and black signal it consists of is rendered on scan lines hidden behind a masking area at the border of the tube. In poorly calibrated sets, the different horizontal position of clusters of white pixels (intense negatively charge areas) and clusters black pixels (uncharged areas) occupying the same temporal location in adjacent scan lines can cause a "bleeding" into subsequent scan lines, where these too are also rendered at different horizontal positions because the electron stream is diverted by the charge rather than magnetism. The shape of the picture is thereby distorted, and the picture curls at the top. Macrovision Colourstripe and Level 2 cause even more problems!
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Author and editor: Kade "Archer" Hansson; e-mail: archer@kaserver5.orgLast updated: Friday 31st October 2003