Gservo
15th May 2002, 10:37 PM
This section is only designed to give a very basic overview of the physical characteristics of DVD, the ones that apply to all the various disc types (DVD-Video, DVD-ROM, etc.), as well as a little bit of history. Please see the rest of the FAQ and other tech links at the right navigation sidebar for more information.
Q: So just what is DVD, anyway? And what does it stand for?
DVD, which stands for Digital Video Disc (or Digital Versatile Disc, as it has been called) is simply the latest audio/video storage medium to hit the market. Looking exactly like a compact disc or a CD-ROM, numerous advances in optical disc technology have made it now possible for hours of high-quality video and audio to now be stored on these 5-inch digital wonders.
To put it another way (and to steal an advertiser's blurb), "DVD does for video what CDs did for music!."
Q: How does DVD actually work?
Although complex, the basics of how a DVD (and CD) works is really rather simple, even comparable to how old vinyl record albums functioned. A DVD disc spins around really fast, and a laser beam "reads" across the surface of the disc, picking up all the information stored on it and sends it to the internal decoding circuity inside a DVD player. (Very similar to how a needle would follow the groove of a vinyl LP as it spun, sending the audio information to the stereo and then to the speakers.)
DVDs can hold so much data because not only does the disc spin very, very fast, but developers were able to pack the grooves the laser follows very tightly together, and very refined and sharp lasers were developed to read them. What the laser is actually "reading" are millions of tiny indentations on the disc surface. The laser light hits these indentations (which are only in two positions, "up" or "down"), and bounces the light back, relaying what position the indentation is. These "up" or "down" positions correspond to 1's and 0's, which is the basis for a digital bit stream. The way the 1's and 0's are arranged in the bit stream forms a series of codes that tell a DVD player what to do. The decoder in the player then converts these encoded digital commands into the images and sounds we can see and hear.
This, obviously, is a very basic explanation. (For a more detailed explanation, please see C-Cube's cool DVD FAQ.)
Who developed DVD, and why? And what is the "DVD Spec"?
Although, technically, DVD was only developed a few years ago, in reality the quest for a high-quality storage medium to offer video on a CD-like disc has been going on for years. Unlike most technological industries, our current television and video systems are hopelessly antiquated: VHS, cable and broadcast television are all still working off the same basic NTSC (National Television Standards Committee) system devised in the 1950's! Also, as the computer revolution took hold and it became apparent that Digital Television (which has been in development for over 20 years) was finally becoming a reality, the electronics industry realized that the world needed a new type of storage medium that could handle these new types of digital data.
So, around the mid 90's, technological advances were bringing us close to finally achieving "movies on CDs," snd the leading video manufacturers got together in a rare show of togetherness, drafting a single digital storage standard called the DVD Specification Guidebook, known more simply as the DVD spec. This is simply a patented set of rules governing products that can bear the DVD logo. All DVD products must be compatible with these specs. (This spec is also being constantly revised, so you often hear of Version 1.0, Version 1.2, etc., of the spec.)
Of course, it wasn't this simple, and took many years of blood, sweat, tears and infighting. But, luckily for us, it all worked out, and we have DVD right here and now, with nearly unanimous support from all current hardware and software manufacturers and content suppliers. (For more information on the DVD spec and who owns it, please see Section 6.1 of Jim Taylor's FAQ.)
What is all this stuff about different DVD "layers," DVD "9's and 10's," and "RSDL"? Is this contagious?
To increase DVD storage capability even further, the format was also developed to have more than one layer of information. A layer is simply another level on the disc with its own complete set of indentations. A DVD is capable of containing four total layers per disc, two on each side. Each layer can roughly hold about 2 1/4 hours of high-quality video, audio and subtitles. So, then, a single DVD could conceivably hold over 8 hours of material.
Of course, not all discs need to hold the same amount of information, so different numbers were assigned to the different types of discs to keep them straight. (Please note that as DVDs store digital data, their capacity is always express in bytes of information.)
DVD 5 - single sided, single layered (4.7 GB total capacity)
DVD 9 - single sided, dual layered (8.5 GB total capacity)
DVD 10 - dual sided, single layered (9.4 GB total capacity)
DVD 18 - dual sided, dual layered (17 GB total capacity)
So, when you hear of DVD-9's or DVD-10's, these are just ways of saying how many layers and sides a DVD has. RSDL is a type of dual-layer disc that contains a set of instructions that tells a DVD player to automatically "jump" to the next layer when the laser gets to the end of the first. This allows for a near-continuous playback of a program....for example, a very long movie would have to be stored across two layers of a DVD, and an RSDL disc would make this possible without interruption of playback by jumping to the next layer as the program is playing. Otherwise, you'd have to manually switch layers via the remote control, or even flip sides (if the two parts of a long program were spread across two sides rather than two layers). What RSDL actually stands for is "Reverse Spiral Dual Layer," which is a technical way to say that a laser reads from the inner ring to the outer ring of the disc, jumps to the next layer, and then reads the back in from the outer to the inner ring again. Actually, quite simple, no?
It is important to state that ALL DVD-players are capable of reading all of the above types of discs. Also, some early titles that appear on the market contained a single film or program broken across two sides of a disc. DVD-9 disc production took a while to perfect, so many of the first releases were DVD-10 instead. (However, the vast majority of discs being produced today utilize dual-layer over double-sided.) Also, in order for the laser in a DVD player to refocus on the next layer, the two layers have to actually be different colors. Hence, the second layer on RSDL discs are hued gold...this is why you often see DVD discs with a golden hue instead of the traditional silver tone (no, these gold discs are not made by Willy Wonka and will not entitle you to a free candy bar!).
Please see the individual FAQ sections for further information on any limitations in regards to layers for each DVD platform. (For real tech stuff, see Imation's great primer on disc layers in their DVD FAQ.)
What about DVD-ROM, DVD-Audio and all these other acronyms I keep hearing about? And what does this have to do with "convergence"?
When DVD was being developed, it was decided it should be a "convergence" product, i.e., one that would bridge home electronics, television, computers and audio products together. A storage medium that could be used in a wide variety of devices, not just on home televisions or computers alone. Therefore, there are different types of DVD-discs and players, better known as disc platforms...and this is were it gets confusing. (To help, think of it is very analogous to CDs and CD-ROMs.) The main flavors of DVD are:
DVD-Video: discs designed to be used in DVD-Video players, mainly as a medium for home entertainment (movies, concerts, etc.)
DVD-ROM: discs designed for computer applications, incorporating high-quality video, audio and multimedia applications
DVD-R, DVD-RW, DVD-RAM: recordable DVD formats designed to work with DVD-ROM capable computers
DVD-Audio: a high-quality successor to the music CD
DIVX: a modified DVD-Video player with the DIVX "feature" that allows for "Pay Per View" viewing
Yes, this seems very confusing, and these multiple types of DVD discs have cause many to throw up their hands and say "forget it"! But don't let all these permutations scare you...all these products are designed to work together and do not make the other "obsolete." A good analogy is, again, CD and CD-ROM. All these features look and act the same (i.e., they are digital and store information), but they have different applications and uses.
The day may soon come when one single player will be able to play all these different types of discs, and you will be able to attach it to either your computer or television. This is definitely possible, and coming! But, right now, different people have different needs, and these products are being developed and introduced for different markets. So, like most computer-type products, you do have to be careful you are buying the right equipment for your needs. Please see the individual FAQ sections for further information on what each format platform can do.
I'm still a bit confused on how DVDs are actually produced. What is MPEG-2, glass masters and DLTs...and can I get one of those to go with bacon?
The DVD production process is, big surprise, quite long and involved. But, there are a few basic stages that have to be done to create a DVD that can be pretty easily explained. First, the source material (movie, TV show, computer program, etc.) must be digitized. Usually, this involves the process of telecine. This is a big contraption that actually "photographs" every single frame of film and stores it as digital data. In other words, you literally have to scan every single frame of film one by one...just like scanning your photographs on computer to send them to Aunt Martha. Except, since film runs at 24 frames a second, those are a lot of scans! (Note that most movies, obviously, are shot on film, but even many television and computer material involving images are as well.)
This is why DVDs are often called "software" and not simply "video." They really are more akin to computer programs that are giant "bit buckets" of digital information than they are to traditional analog video storage mediums (VHS, Laserdisc, etc.). So this is where the DLT comes in (though, unlike a BLT, no bacon is involved). DLT stands for Digital Linear Tape, and it is simply the whole entire scanned film or already digitized media in its raw form. However, even with the increased storage capacity of a DVD, there is no way it could store the entire DLT on a single DVD untouched. This is where compression comes in.
MPEG (Moving Pictures Engineering Group) developed a process that looks for redundant information in repeated digital images, and then reduces the amount of storage space needed to store them by substituting simple codes for multiple frames. In plain English, MPEG compression would look at a 10 second image of a skyline and say "gee, that blue area up there with 200 dots never changes...so instead of storing each frame with that information, I'm going to instead store it as one command that says make those 200 dots blue for 10 seconds." Thus, MPEG compression continually looks for ways to store redundant, unchanging information with simple commands (This works for sound, too, by the way.) By the way, the 2 in MPEG-2 comes in because this is the second version of the MPEG compression process that was developed (i.e., it is better than MPEG-1, but not as good as the latest version, MPEG-3, etc.).
So, after transfer your film or video to a DLT and compress it via MPEG-2 (which is no easy task!), you then have to take that digital data and make a glass master. Remember all those indentations I told you about in the way up at the top of this FAQ? Well, something has to tell the big mechanical press that is going to physically burn all the little indentations on the actual disc what indentations go where. The final compressed DVD encoded material (all those one's and zero's) are arraanged in order onto the grooves of this glass master. Thus, the glass master is the actual blueprint for the physical manufacturing of DVD discs.
Discs with with multiple layers and sides are a bit more complicated. Double-sided discs are simply two "regular" DVDs bonded together with gooey materia that, thankfully, quickly dries before making a mess. Dual-layer discs are trickier. Codes have to be imbedded during the compression process telling the player to jump to the next layer and what not. Also, bonding two layers together is trickier than a double-sided disc, resulting in many errors and defective discs that must be discarded. Still, it can and is being done. As you can imagine, making a dual-sided, dual-layer disc is a bit of a nightmare...in fact, it hasn't even been accomplished yet! But, it will happen soon.
This is a very abbreviated version of how DVD discs are made. Please see the FAQ sections here for more on the particulars of how discs are made for each DVD platform. (And if you are really motivated, see DVCC's great How To Make A DVD for the complete how-to. Adobe Acrobat is required to view this FAQ, however.)
What are all these manufacturers, replicators and "authorists"? Are they interchangeable?
All those above terms refer to parts of the DVD production process, and are really very simple. First up is the telecine operator, who as previously stated digitizes film media. The titles compressionist and authorist are often used, somewhat erroneously, in an interchangeable fashion. A compressionist, technically, is someone who strictly compresses already digitized media. That is all they do. An authorist usually does compression work, but often does auxiliary tasks as well...creating menus, doing graphic design, working out multiple audio and video streams, etc. (And this is arguable, but authorists can be said to be doing the creative work, and the compressionist the technical work.)
Replicators are usually those that make the physical reproductions of the disc. They are not involved with the creative aspects at all...just the mechanics of it. Manufacturers are often also used interchangeably with Replicators, but actually most Manufacturers usually offer complete DVD authoring suites, i.e., they will offer to make a DVD for you from start to finish, not just make copies from a glass master.
For more on all this nitty gritty stuff, see Section 5 of Jim Taylor's FAQ. Also, check out the individual FAQ sections here for more on production particulars of the various platforms.
Okay, I've listened to all of this and now I've got a headache now. Really, why do we even need a DVD?
Well, to be honest, we don't. But, then, one could argue we don't need movies, music, books or art to survive, either. But the world sure would be lonely place without all of them, wouldn't it?
http://www.dvdfile.com/site/faq/index.html
Q: So just what is DVD, anyway? And what does it stand for?
DVD, which stands for Digital Video Disc (or Digital Versatile Disc, as it has been called) is simply the latest audio/video storage medium to hit the market. Looking exactly like a compact disc or a CD-ROM, numerous advances in optical disc technology have made it now possible for hours of high-quality video and audio to now be stored on these 5-inch digital wonders.
To put it another way (and to steal an advertiser's blurb), "DVD does for video what CDs did for music!."
Q: How does DVD actually work?
Although complex, the basics of how a DVD (and CD) works is really rather simple, even comparable to how old vinyl record albums functioned. A DVD disc spins around really fast, and a laser beam "reads" across the surface of the disc, picking up all the information stored on it and sends it to the internal decoding circuity inside a DVD player. (Very similar to how a needle would follow the groove of a vinyl LP as it spun, sending the audio information to the stereo and then to the speakers.)
DVDs can hold so much data because not only does the disc spin very, very fast, but developers were able to pack the grooves the laser follows very tightly together, and very refined and sharp lasers were developed to read them. What the laser is actually "reading" are millions of tiny indentations on the disc surface. The laser light hits these indentations (which are only in two positions, "up" or "down"), and bounces the light back, relaying what position the indentation is. These "up" or "down" positions correspond to 1's and 0's, which is the basis for a digital bit stream. The way the 1's and 0's are arranged in the bit stream forms a series of codes that tell a DVD player what to do. The decoder in the player then converts these encoded digital commands into the images and sounds we can see and hear.
This, obviously, is a very basic explanation. (For a more detailed explanation, please see C-Cube's cool DVD FAQ.)
Who developed DVD, and why? And what is the "DVD Spec"?
Although, technically, DVD was only developed a few years ago, in reality the quest for a high-quality storage medium to offer video on a CD-like disc has been going on for years. Unlike most technological industries, our current television and video systems are hopelessly antiquated: VHS, cable and broadcast television are all still working off the same basic NTSC (National Television Standards Committee) system devised in the 1950's! Also, as the computer revolution took hold and it became apparent that Digital Television (which has been in development for over 20 years) was finally becoming a reality, the electronics industry realized that the world needed a new type of storage medium that could handle these new types of digital data.
So, around the mid 90's, technological advances were bringing us close to finally achieving "movies on CDs," snd the leading video manufacturers got together in a rare show of togetherness, drafting a single digital storage standard called the DVD Specification Guidebook, known more simply as the DVD spec. This is simply a patented set of rules governing products that can bear the DVD logo. All DVD products must be compatible with these specs. (This spec is also being constantly revised, so you often hear of Version 1.0, Version 1.2, etc., of the spec.)
Of course, it wasn't this simple, and took many years of blood, sweat, tears and infighting. But, luckily for us, it all worked out, and we have DVD right here and now, with nearly unanimous support from all current hardware and software manufacturers and content suppliers. (For more information on the DVD spec and who owns it, please see Section 6.1 of Jim Taylor's FAQ.)
What is all this stuff about different DVD "layers," DVD "9's and 10's," and "RSDL"? Is this contagious?
To increase DVD storage capability even further, the format was also developed to have more than one layer of information. A layer is simply another level on the disc with its own complete set of indentations. A DVD is capable of containing four total layers per disc, two on each side. Each layer can roughly hold about 2 1/4 hours of high-quality video, audio and subtitles. So, then, a single DVD could conceivably hold over 8 hours of material.
Of course, not all discs need to hold the same amount of information, so different numbers were assigned to the different types of discs to keep them straight. (Please note that as DVDs store digital data, their capacity is always express in bytes of information.)
DVD 5 - single sided, single layered (4.7 GB total capacity)
DVD 9 - single sided, dual layered (8.5 GB total capacity)
DVD 10 - dual sided, single layered (9.4 GB total capacity)
DVD 18 - dual sided, dual layered (17 GB total capacity)
So, when you hear of DVD-9's or DVD-10's, these are just ways of saying how many layers and sides a DVD has. RSDL is a type of dual-layer disc that contains a set of instructions that tells a DVD player to automatically "jump" to the next layer when the laser gets to the end of the first. This allows for a near-continuous playback of a program....for example, a very long movie would have to be stored across two layers of a DVD, and an RSDL disc would make this possible without interruption of playback by jumping to the next layer as the program is playing. Otherwise, you'd have to manually switch layers via the remote control, or even flip sides (if the two parts of a long program were spread across two sides rather than two layers). What RSDL actually stands for is "Reverse Spiral Dual Layer," which is a technical way to say that a laser reads from the inner ring to the outer ring of the disc, jumps to the next layer, and then reads the back in from the outer to the inner ring again. Actually, quite simple, no?
It is important to state that ALL DVD-players are capable of reading all of the above types of discs. Also, some early titles that appear on the market contained a single film or program broken across two sides of a disc. DVD-9 disc production took a while to perfect, so many of the first releases were DVD-10 instead. (However, the vast majority of discs being produced today utilize dual-layer over double-sided.) Also, in order for the laser in a DVD player to refocus on the next layer, the two layers have to actually be different colors. Hence, the second layer on RSDL discs are hued gold...this is why you often see DVD discs with a golden hue instead of the traditional silver tone (no, these gold discs are not made by Willy Wonka and will not entitle you to a free candy bar!).
Please see the individual FAQ sections for further information on any limitations in regards to layers for each DVD platform. (For real tech stuff, see Imation's great primer on disc layers in their DVD FAQ.)
What about DVD-ROM, DVD-Audio and all these other acronyms I keep hearing about? And what does this have to do with "convergence"?
When DVD was being developed, it was decided it should be a "convergence" product, i.e., one that would bridge home electronics, television, computers and audio products together. A storage medium that could be used in a wide variety of devices, not just on home televisions or computers alone. Therefore, there are different types of DVD-discs and players, better known as disc platforms...and this is were it gets confusing. (To help, think of it is very analogous to CDs and CD-ROMs.) The main flavors of DVD are:
DVD-Video: discs designed to be used in DVD-Video players, mainly as a medium for home entertainment (movies, concerts, etc.)
DVD-ROM: discs designed for computer applications, incorporating high-quality video, audio and multimedia applications
DVD-R, DVD-RW, DVD-RAM: recordable DVD formats designed to work with DVD-ROM capable computers
DVD-Audio: a high-quality successor to the music CD
DIVX: a modified DVD-Video player with the DIVX "feature" that allows for "Pay Per View" viewing
Yes, this seems very confusing, and these multiple types of DVD discs have cause many to throw up their hands and say "forget it"! But don't let all these permutations scare you...all these products are designed to work together and do not make the other "obsolete." A good analogy is, again, CD and CD-ROM. All these features look and act the same (i.e., they are digital and store information), but they have different applications and uses.
The day may soon come when one single player will be able to play all these different types of discs, and you will be able to attach it to either your computer or television. This is definitely possible, and coming! But, right now, different people have different needs, and these products are being developed and introduced for different markets. So, like most computer-type products, you do have to be careful you are buying the right equipment for your needs. Please see the individual FAQ sections for further information on what each format platform can do.
I'm still a bit confused on how DVDs are actually produced. What is MPEG-2, glass masters and DLTs...and can I get one of those to go with bacon?
The DVD production process is, big surprise, quite long and involved. But, there are a few basic stages that have to be done to create a DVD that can be pretty easily explained. First, the source material (movie, TV show, computer program, etc.) must be digitized. Usually, this involves the process of telecine. This is a big contraption that actually "photographs" every single frame of film and stores it as digital data. In other words, you literally have to scan every single frame of film one by one...just like scanning your photographs on computer to send them to Aunt Martha. Except, since film runs at 24 frames a second, those are a lot of scans! (Note that most movies, obviously, are shot on film, but even many television and computer material involving images are as well.)
This is why DVDs are often called "software" and not simply "video." They really are more akin to computer programs that are giant "bit buckets" of digital information than they are to traditional analog video storage mediums (VHS, Laserdisc, etc.). So this is where the DLT comes in (though, unlike a BLT, no bacon is involved). DLT stands for Digital Linear Tape, and it is simply the whole entire scanned film or already digitized media in its raw form. However, even with the increased storage capacity of a DVD, there is no way it could store the entire DLT on a single DVD untouched. This is where compression comes in.
MPEG (Moving Pictures Engineering Group) developed a process that looks for redundant information in repeated digital images, and then reduces the amount of storage space needed to store them by substituting simple codes for multiple frames. In plain English, MPEG compression would look at a 10 second image of a skyline and say "gee, that blue area up there with 200 dots never changes...so instead of storing each frame with that information, I'm going to instead store it as one command that says make those 200 dots blue for 10 seconds." Thus, MPEG compression continually looks for ways to store redundant, unchanging information with simple commands (This works for sound, too, by the way.) By the way, the 2 in MPEG-2 comes in because this is the second version of the MPEG compression process that was developed (i.e., it is better than MPEG-1, but not as good as the latest version, MPEG-3, etc.).
So, after transfer your film or video to a DLT and compress it via MPEG-2 (which is no easy task!), you then have to take that digital data and make a glass master. Remember all those indentations I told you about in the way up at the top of this FAQ? Well, something has to tell the big mechanical press that is going to physically burn all the little indentations on the actual disc what indentations go where. The final compressed DVD encoded material (all those one's and zero's) are arraanged in order onto the grooves of this glass master. Thus, the glass master is the actual blueprint for the physical manufacturing of DVD discs.
Discs with with multiple layers and sides are a bit more complicated. Double-sided discs are simply two "regular" DVDs bonded together with gooey materia that, thankfully, quickly dries before making a mess. Dual-layer discs are trickier. Codes have to be imbedded during the compression process telling the player to jump to the next layer and what not. Also, bonding two layers together is trickier than a double-sided disc, resulting in many errors and defective discs that must be discarded. Still, it can and is being done. As you can imagine, making a dual-sided, dual-layer disc is a bit of a nightmare...in fact, it hasn't even been accomplished yet! But, it will happen soon.
This is a very abbreviated version of how DVD discs are made. Please see the FAQ sections here for more on the particulars of how discs are made for each DVD platform. (And if you are really motivated, see DVCC's great How To Make A DVD for the complete how-to. Adobe Acrobat is required to view this FAQ, however.)
What are all these manufacturers, replicators and "authorists"? Are they interchangeable?
All those above terms refer to parts of the DVD production process, and are really very simple. First up is the telecine operator, who as previously stated digitizes film media. The titles compressionist and authorist are often used, somewhat erroneously, in an interchangeable fashion. A compressionist, technically, is someone who strictly compresses already digitized media. That is all they do. An authorist usually does compression work, but often does auxiliary tasks as well...creating menus, doing graphic design, working out multiple audio and video streams, etc. (And this is arguable, but authorists can be said to be doing the creative work, and the compressionist the technical work.)
Replicators are usually those that make the physical reproductions of the disc. They are not involved with the creative aspects at all...just the mechanics of it. Manufacturers are often also used interchangeably with Replicators, but actually most Manufacturers usually offer complete DVD authoring suites, i.e., they will offer to make a DVD for you from start to finish, not just make copies from a glass master.
For more on all this nitty gritty stuff, see Section 5 of Jim Taylor's FAQ. Also, check out the individual FAQ sections here for more on production particulars of the various platforms.
Okay, I've listened to all of this and now I've got a headache now. Really, why do we even need a DVD?
Well, to be honest, we don't. But, then, one could argue we don't need movies, music, books or art to survive, either. But the world sure would be lonely place without all of them, wouldn't it?
http://www.dvdfile.com/site/faq/index.html