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| vcd | svcd | minidvd | divx | dv formats | comparison |
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vcd
most vcd players that play pal video resolution on a ntsc monitor correctly will cut 24 lines of resolution from the top and bottom of a pal image. this conversion problem does not exist when a vcr is viewed on a pc monitor and indeed, most modern day tvs and dvd players are multisync and able to play both ntsc + pal format. a dedicated vcd player is designed to play only vcds and is not a general purpose multimedia machine. it can be thought of as a vcr for compact discs. a vcd has the capacity to hold up to 74/80 minutes [on 650mb/700mb media respectively] of full-motion video and stereo sound. mpeg-1 compression technology [iso standard iec 11172] is used for encoding, providing video quality equivalent to vhs vcr tape and near cd-da audio quality. the degree of standardisation is such that a vcd can be played on almost all standalone dvd players, on games platforms – such as playstation, sega and dreamcast – provided they're equipped with the necessary add-ons and on dvd-rom or cd-rom drives with the help of a software based decoder/player. however, some dvd players will not handle vcds on cd-r media [that is, home made discs]. experience has shown that most sony and pioneer models are least likely to have problems, whilst dvd players from other manufacturers may fail to recognise vcds burned to cd-r media. vcd was the first moderately successful digital video standard and the forerunner to the dvd format that was to eventually dominate the digital video consumer market. however, fuelled by the increasing availability of affordable cd rewritable drives and their ever cheaper media, other formats were to emerge – some official 'standards', others not – that provided consumers with alternative solutions for distributing movie content in formats that offered the combination of quality and low cost. supervcd – effectively a vcd that uses mpeg-2 encoding rather than mpeg-1 – was the next such format to emerge." [quote-source] svcd "with over 20 million households owning vcd payers by the late 1990s, it is unsurprising that china was instrumental in the development of a successor to the vcd format. the first of three independent efforts to bring the next-generation vcd standard to the chinese market started in 1997. c-cube microsystems – the market leader manufacturer of vcd systems – was first to market with its cvd [china video disc] product a year later. however, thanks to the intervention of the chinese government it was the 'svcd' standard developed by a consortium that included philips, sony, matsushita and jvc – the companies that created the original white book videocd specification – that was to eventually win out. the 1998 'super vcd' [from the chinese 'chaoji vcd'] format – subsequently standardised as iso iec 62107 – is a natural evolution of the vcd standard. the basic difference is that mpeg-2 is used for the video stream [instead of mpeg-1], which is encoded at higher resolutions and bitrates. it also allows subtitles and variable bitrates. as a consequence, svcd is capable of delivering up to 2 times sharper video images than its predecessor, at the cost of reduced capacity – typically between 35 minutes and 80 minutes depending on the average bit rate used – per cd. most feature length movies, therefore, are distributed on a 2- or 3-cd set. in common with vcd, svcd supports 16:9 [anamorphic wide screen] image aspect ratio. however, unlike its predecessor, some svcd players can signal the tv set to automatically switch to the appropriate mode. the format has extensive support for subtitling and karaoke lyrics colour highlighting, neither of which were possible in vcd 2.0. an svcd video stream can contain up to four independent subtitling channels for different languages. the subtitles are overlaid on the top of the video image in real time, and may be enabled or disabled at will. since the subtitles are stored as bitmap graphics, they are not tied to any particular character set. finally, the svcd standard supports html style hyperlinks, still images, playlists/slideshows, multi-level hierarchical menus and chapters [indexing]. the table below summarises svcd's principal technical characteristics:
since commercially produced svcd titles only be available in far east, it is unlikely that standalone svcd players will become available outside of china and surrounding countries. they are generally compatible with vcd 1.1, vcd 2.0, cd-i and cd-da formats and some are capable of playing mp3-based cd-roms. whilst svcd players can't play dvds, the fact that they both use the same mpeg-2 encoding scheme means that some models can be 'upgraded' to handle the format simply by replacing the cd drive mechanism with a dvd drive. until late 1999 it was only dvd players sold in asia that were capable of playing svcds. nowadays this shouldn't be a problem. if it is, especially since dvd players are based on only a small number of underlying platforms, it will often be possible to get a player to handle the format by means of a firmware hack. xvcd and xsvcd – extended vcd and extended svcd respectively – are unofficial 'tweaks' of the vcd and svcd standards designed to achieve improved image quality, for example by increasing the bit rate in line with modern cd-rom drives' faster data transfer capability. xsvcd works on the principal that since dvd players can handle svcd media and the mpeg-2 encoding scheme is common to both svcd and dvd, it ought to be possible to push the svcd format a little further towards dvd levels of performance. consequently, it increases bitrate limits up to the same level as dvd-video – up to 9.8mbit/s – and supports use of full dvd resolutions in addition to svcd's regular 480*576/480*480 resolutions. since neither xvcd or xsvcd formats are official standards there are no guarantees as to whether they will be handled by available standalone playing devices. in practice, whilst most major brand players won't play them, many players manufactured in the far east will." [quote-source] minidvd "if svcd can be viewed as format that seeks to combine the advantages of dvd quality with the cheapness of writable cd media, then minidvd can be viewed in a similar light. the minidvd format goes the whole hog though – it is dvd on cd media. the trade-off with the svcd format is simple, better quality for reduced capacity. in practical terms this means about 15 minutes of dvd quality video on a 650mb cd-r[w] disc, sufficient perhaps for a home video or collection of musical videos. it is possible to reduce quality – by decreasing the resolution/bitrate – in order to increase capacity. originally, few dvd players had a dvd drive that was capable of achieving the cd spin speeds required to cope with dvd's high bitrates. by late 2001 dvd rewriters – both of the dvd-rw and dvd+rw variety – had started to become sufficiently affordable to appeal to a wider consumer market. as this trend continues, the number of compatible dvd players is likely to increase in tandem with format's popularity. minidvd may also be played on dvd-rom drives using suitable video player software, such as windows media player. whilst none of svcd, xsvcd and minidvd are official formats, they are at least based on recognised standards. in early 2000 a new video technology emerged from the computer underground that made no pretence to be anything other than a hack, yet appeared capable of doing for movies what mp3 had done for digital music." [quote-source] divx "according its pseudonymous authors the divx codec – no relation to circuit city's now defunct divx digital video disk player – is based on microsoft's mpeg-4 video technology, with the addition of an mp3 audio stream. since a divx-compressed movie is between 10% and 20% the size of the original dvd – a typical 5gb, 80-90 minute dvd movie occupies about 650mb at a resolution of 640*480 – feature-length hollywood movies can generally be stored on a single cd-rom. viewing is simply a matter of installing a widely available add-on to microsoft's windows media player. by mid-2000, students with access to fast online connections and other internet-savvy consumers were beginning to use the new video format to trade high-quality film trailers and even full-length movies copied directly from dvds. fan sites that explained the technology and included links to new divx-format releases available via the internet soon appeared, just as mp3 sites had in the early days of online digital music. many web sites provide recipes for copying a dvd to the divx format, including various techniques for shrinking the film's file size – such as by reducing the fps rate from 30 to 24. whilst its often possible to detect some pixelisation in complex moving images, and matching the sound track to the video can be a challenge for amateur editors, overall the quality is impressive and more than good enough for the average consumer. at present divx is a long way from reaching mp3's mainstream distribution, and as a hacker's creation, it might never break out of the underground communities. notwithstanding the advances in compression technology, many believe that movies are unlikely to reach the status of online music anytime soon and that having to spend hours to download a film – for what is ultimately less than dvd quality – will deter most people from using the technology to any serious extent. also, the fact that the process can take as much as 18 hours is likely to put off most people thinking of ripping a dvd to divx format themselves. how things will develop on the legal front is unclear and it remains to be see what attitude the motion picture association of america [mpaa] – which has already has launched lawsuits against individuals who provide technology for copying dvds – and microsoft adopt towards divx. by late 2001 an entirely new development of divx was well advanced. the new divx – being developed as an open-source project known as project mayo – is commonly referred to as "opendivx" or "divx for windows/linux/mac .....". it is also differentiated from the original codec by its 4.xx [as opposed to 3.xx] version numbering scheme. unlike the original divx, opendivx has nothing whatsoever to do with microsoft. however, it is technically similar to its predecessor, being also based on the mpeg-4 compression format. its developers claim that it matches the high quality of the original codec whilst improving on its the compression ratio [meaning smaller file sizes]." [quote-source] dv-formats "in the late 1990s a new generation of entirely digital cameras and camcorders emerged, and with it a new video format, digital video [dv]. the dv cassette is a small, metal-oxide tape, which is about three-quarters the size of a dat, and confers the significant advantage of allowing the entire video processing cycle to remain within the digital domain. instead of having to be funnelled through a process of analogue-to-digital conversion by a traditional video capture card, dv footage – already in a compressed digital format – can simply be downloaded to a pc in real-time with no loss of quality. panasonic and sony were the first to use the dv standard on their camcorders and though it wasn't originally intended as a professional format, both companies subsequently announced their own extensions to the standard – panasonic with dvcpro in 1995, and sony with dvcam in 1996. however, in common with just about every other maker of digital camcorders, both manufacturers have stuck to the minidv format for their digital consumer equipment. the dv format uses 1/4in [6.35mm] metal evaporate tapes, capable of recording up to three hours of video in sp [standard play] mode on cassettes which measure 125x78x14.6mm. a major advantage of the minidv format is that since the tapes are very small – 1/12th the size of a standard vhs tape at 66x48x1.2mm – the cameras that use it can be incredibly small too. minidv can record an hour in standard format or up to 90 minutes of lower quality output in lp [long play] mode at horizontal resolutions of up to 500 lines. technically speaking, dv is the summit of the industry's research into video compression and, in particular, complex discrete cosine transformation [dct] codes. it is an intraframe rather than progressive compression technique, using a three-stage process compress data – each frame being compressed on an individual basis rather than being compared to adjacent frames. the first stage uses dct compression, a lossless technique which strips away information that cannot be seen by the human eye. it then separates the information from each pixel into brightness and colour and then samples this, favouring brightness over colour, which gives a colour representation that's acceptable to the human eye but cuts down the data by a third. this is achieved by converting the rgb colour information for each pixel into a yuv colour space – y for brightness, and u and v for colour. the y value is sampled four times, the u and v twice, this formula being described as yuv 4:2:2. the video then gets further reduced as the dv codec optimises the formula to yuv 4:2:0, bunching colour information from adjacent pixels in 4x4 blocks. again, it's a trade-off, but the human eye finds subtle variations in colour hard to detect, so in well-lit natural surroundings the difference is imperceptible. finally, the hardware compression system on the camera compresses the video down further using an algorithm similar to m-jpeg. dv differs by being able to compress different parts of each frame to different ratios. so, the blue sky in an image backdrop can be brought down to, say, 25:1, while the complex forest in the foreground, which needs more detail, is reduced to only 7:1. in this way dv can optimise its video stream frame by frame. m-jpeg, by contrast, has to have a fixed compression rate for the whole video and can't intelligently balance the compression of each image, resulting in more artefacts. it also employs a technique known as adaptive interfield compression, which results in a pair of interlaced fields of a frame [as used by pal, for example] being compressed together if little difference between them is detected. in theory this means that scenes with less movement are handled better than fast action scenes, although in practice it's difficult to observe any perceivable difference. the dv standard also supports pcm [pulse code modulation] stereo, thereby supporting cd-quality 16-bit audio. alternatively, 12-bit mode can be used to record two pairs of audio tracks – one for stereo sound recorded at the time of the video and one for music or narration added later. the net result is that dv video information is carried in a nominal 25 mbit/s data stream – which increases to 36 mbit/s when audio and the various control and error correction data is taken into account. dv's principal problem is that, unlike mpeg-2, it isn't scalable. it was designed for recording to tape with a fixed 25 mbit/s data rate. this, and its limited colour capacity [4:2:0 or 4:1:1, meaning that there's half as much colour information as brightness], mean many consider it unsuitable for professional post-production. for nle, the data rate is too high for off-line editing and too low for high-end effects and graphics-heavy work. the launch of panasonic's dvcpro50 in 1998 – which doubled the data rate to 50 mbit/s and expands the colour depth to a professional 4:2:2 – extended dv's application to the higher end. jvc's digital-s [or d9] format records an identical 50 mbit/s dv bitstream to vhs-sized cassettes. the quality of both formats has been compared to digital betacam, yet at the time they were less than half the price. at the start of the new millennium a split had appeared in broadcasting, with the dv formats and mpeg-2 sitting on opposite sides. however, it appeared that a resolution was on the horizon. dv and prompeg are very similar to dct-based, i-frame-only schemes and it was only a matter of time before someone built silicon to support them all. c-cube and matrox were the first to oblige, launching a codec chip in its digituite dtv video card which supports dv25 and 50 as well as mpeg-2. however, dct isn't the last word in compression, just a standard whose early development and suitability for real-time codecs chips attracted attention at the right time. other currently under-developed technologies do promise better pictures at lower data rates. they include wavelet and fractal algorithms. the former has already been implemented in silicon and has the important advantage of both being moderately more efficient than m-jpeg and degrading more naturally, with images appearing grainy rather than blocky at higher compression levels. these alternatives are unlikely to overthrow dct in broadcasting and consumer electronics. but in the broadband network delivery systems which are likely to replace traditional broadcasting over the next decade, it's a different story. with increasingly powerful cpus becoming commonplace, appropriate real-time decoder software can be delivered with the content. many companies are beginning to offer ip-based streaming solutions for video-on-demand across company intranets. although mpeg is still the dominant technology, some suppliers have demonstrated other techniques that can stream vhs-quality video in as little as 512 kbit/s – more than within the scope of the cable and adsl broadband solutions expect to emerge over the coming years." [quote-source] format comparison the principal technical characteristics of the digital video formats discussed above are summarised in the following table:
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