Compatibility Forum

White Paper

1. Introduction

In today's consumer marketplace, DVD Video has become so widespread and accepted that consumers are quickly transitioning from VHS to DVD. Set-top DVD players are quite affordable, and content, most notably Hollywood movies, is widely available. Moreover, DVD versions of most movies include superior audio and video quality alongside additional, or bonus, content. In fact, DVD Video has been so successful that the expectation for high quality has been firmly established in the mind of the consumer. Indeed, the expectation is that any DVD Video disc can be played in any DVD player. While there are still fringe cases where this isn't true, in the main, this is clearly the typical end user experience.

But as happens with any emerging technology, DVD hasn't been standing still. Over the last few years we've seen the introduction of DVD recorders and recordable DVD media from a wide variety of vendors. Initially the price was too high, and the products too complex for widespread consumer adoption. As a result, vendors focused primarily on professional video producers who had large budgets and plenty of time and incentive to master this new technology. As has happened with other digital media technologies, time has brought with it lower prices and easier to use software such that today a consumer can choose from an assortment of different DVD recording technologies from a number of different vendors.

Unfortunately, this technological evolution has upset the user expectation of playing any DVD disc in any player. This disruption can be especially frustrating for the consumer because the connection between a specific DVD recording technology and a specific DVD player isn't obvious. They are also confronted with a confusing assortment of technologies to choose from: DVD-RW, DVD-R, DVD+RW, DVD+R, and DVD-RAM. Many of these formats also have variants that can significantly affect their compatibility on a given DVD set-top player. Couple this with the fact that each manufacturer will often build a number of different set-top DVD players where some of the players will support certain media types while others, from the same manufacturer, may not. Add in the confusion raised by the competing claims from the different format proponents plus the large installed base of old DVD players and a user can be easily dissuaded from jumping into these waters too quickly.

Lamentably, even many technical individuals in the optical media industry fail to grasp the basic reasons why this is so. This document is intended to provide an overview of the major factors standing in the way of the seamless, universal playback experience that we should expect.

2. Potential Problems

Fundamentally, the problem consumers face today is that many of the discs created with the current DVD recording technology will not play back in some percentage of the current set-top DVD players. The situation is improving with succeeding generations of hardware and software; nonetheless consumers will still face a situation where things may not come together the way they expect. When this happens to an end user, they're likely to express the simple question of, why not? Unfortunately, this simply question doesn't have a simple answer.

There are a number of different reasons why a specific consumer recorded DVD disc will not play back in certain players. Each combination of media, content, recorder, and player will typically have a specific, unique reason why problem-free play back doesn't happen. In the more general case, there are a number of factors that can influence playback. These factors can be thought of as a set of concentric circular barriers, where each ring represents one of the technological barriers to overcome. Starting on the outside and moving inward, each ring, or barrier, must be properly addressed before access to the succeeding inner ring is allowed. Only when all the rings are properly addressed will consumers have true problem free playback.

Applying this model to the DVD scenario, the player must first be able to physically recognize and read the media, it must then be able to find the appropriate files, it must be able to open and extract the content within the files, it must be able to navigate through this content, and it must finally be able to render the content to a TV and/or a stereo system. If any these operations fail, the video will not be shown.

Looking at things in a more general sense, these barriers fall into the areas of Media & Recorder, File Systems, Video Management, and Content.

It should be noted that while this document refers almost exclusively to 120mm DVD media, the specifications also allow 80mm variants as well. To date, this smaller media has been relatively rare and isn't discussed further in this document.

2.A Media & Recorder

The primary barrier to compatibility today is media. As outlined earlier, there are many different formats to choose from, with each format's proponents asserting that their format is superior to all others. Additionally, there are a number of companies making media for each of these formats, and the quality of this media can have an effect on compatibility. But despite all their differences, each format must be readable in the target device to overcome the initial barrier of disc recognition and reading.

Ideally, a consumer recorded DVD should appear to a DVD player the same as a pressed (or pre-recorded), DVD, and the DVD Video specification outlines the specific rules required for this to happen. If a disc fails to conform to these rules, many players will fail to recognize the disc and may generate some sort of error. Some will recognize a non-conforming disc and handle it anyway, but this isn't a requirement for set-top box manufacturers (it's more like a bonus for the user.)

The principal influence on a disc's conformance to the DVD Video specification is a combination of media formulation and recorder technology. There are two principal types of technology, write-once and rewritable.

2.A.a Write-Once Media

Write-once media is constructed using an organic dye layer that is written using a high power laser, but is read with a low power laser. This type of write-once technology is really the evolution of write-once CD recording, and, similar to CD recording, write-once technology offers the best compatibility with DVD players largely because it has better reflectivity (the amount of laser light that reflects back from the disc) than rewritable discs. There are two distinct write-once camps, one called DVD+R ("DVD plus R") and the other DVD-R ("DVD dash"). Both use similar technology and, typically, both offer similar playback compatibility.

The DVD-R specification is further divided into DVD-R for Authoring and DVD-R for General use. DVD-R for authoring is intended for higher-end systems and for users who require access to key areas of the disc not available in the General format. This would be largely for professional content producers. In addition to the standard DVD-R 4.7GB format, DVD-R is also available in a 3.95GB format. This lower capacity format was actually the first DVD recordable format introduced, and is also accepted as the most compatible format.

The DVD+R format is the latest arrival on the DVD recording scene, and is the write-once counterpart to DVD+RW. Like DVD-R, DVD+R discs have a capacity of 4.7GB, but do not offer a 3.95GB variant. DVD+R (and DVD+RW) differ from their DVD-R and DVD-RW counterparts in the way that the blank areas on discs are addressed. Without going into the gory technical details, suffice it to say that DVD+R and DVD+RW claim to offer superior addressing. Theoretically, this superior addressing should yield superior write quality and performance, however this hasn't necessarily been borne out in real world testing. When comparing DVD-R and DVD+R media of similar quality, the compatibility should be very similar.

2.A.b Rewritable Media

Media that can be written and re-written is called rewritable. As is the case with write-once technology, rewritable discs can be considered the evolution of rewritable CD technology. Rewritable DVD uses phase change technology whereby the reflectivity of a mark on the disc can be changed based on how much power is used to write it. A higher power level yields a non-reflective (amorphous) mark while a lower power level yields a reflective (crystalline) mark. The advantage is that the reflectivity of the mark can be changed by simply rewriting it with a different power level (each area can be changed approximately 1000 times). The disadvantage is that rewritable technology has a lower reflectivity than write-once formats. Even though the DVD Video specification has provisions for lower reflectivity, the lower reflectivity is intended for dual layer media rather than single layer, low reflectivity media. As a consequence, many older set-top DVD players can get confused when they see low reflectivity on a single layer disc and may have erratic playback behavior if it plays back at all. Also similar to write-once, the rewritable technology falls into DVD-RW ("DVD dash RW") and DVD+RW ("DVD plus RW"), but adds yet a third group called DVD-RAM. As mentioned previously, DVD-RAM has a different physical construction that requires unique support on the playback side.

Unlike DVD-R, DVD-RW does not offer a 3.95GB version, but it does offer a number of different writing modes: disc or session-at-once, packet writing, and restricted overwrite. In disc or session-at-once mode, the data must be written to the disc in one continual stream and the disc contents cannot be modified without erasing the entire disc. Restricted overwrite mode provides a mechanism where specific blocks of data can be selectively changed without erasing the entire disc. Unfortunately, because of the way it is performed, restricted overwrite mode can slightly reduce the error correction capabilities of the disc, but, in practice, this typically isn't a problem unless the DVD player only marginally supports DVD-RW anyway.

On the DVD+RW side, there is really only one writing mode which is closer to the restricted overwrite mode of DVD-RW than either disc or track-at-once. Recall, however, from our DVD+R discussion above the notion of improved addressing. In the case of DVD+RW, this improved addressing enables the drive to selectively update sections of the disc with much greater precision. The net result is that, theoretically, DVD+RW should have fewer errors when content is changed than DVD-RW in restricted overwrite mode.

When comparing DVD-RW in restricted overwrite mode against DVD+RW in its native mode, it should be noted that this really only applies to content that is changed on a regular basis. This is likely to happen in a set-top player/recorder or in a software editing application that offers on disc editing. In the theoretical worst-case scenario, a rewritable disc that is frequently updated may have a higher likelihood of errors with DVD-RW than DVD+RW, but this can only be verified by extensive testing.

2.A.c Media Manufacturers

Regardless of which format is used, another key factor in DVD player compatibility is the quality of the media. In a variety of testing scenarios, it has been empirically shown that the compatibility can vary considerably by media manufacturer. In most of the media test cases, discs are created using the same recorder, content, and player with only the media changing. Tests performed by DV magazine showed compatibility variations ranging from 45 percent to 80 percent. Today, the greatest variations are with DVD-R media, but that's probably only because it has been around the longest so it has the greatest number of media manufacturers. It would be safe to assume that as time goes by, this phenomenon will spread to other media types as well.

2.A.d Disc Signatures

One more key differentiator between DVD+RW and DVD-RW (and DVD+R and DVD-R as well) is a unique signature (specific bit-fields in the lead-in) on the disc that identifies the media format. Since they are different formats, the signatures are necessarily different.

Unfortunately, many set-top DVD players look only for specific signatures and when those signatures aren't found, the disc is rejected - even if the player would have handled the disc without problems. In some cases, it's possible to set the signature so that the media appears to be a different type and this will work in most cases. However, there are those cases where an incorrect signature can cause a DVD player to treat the disc as if it were another media type, which results in other errors.

2.B File System

Assuming that the player has passed the hurdle of recognizing and reading the media, it must then go about the task of finding the content. In the case of DVD Video, the data is initially organized according to the rules outlined in the Universal Disc Format (UDF) specification. As with most file systems, UDF specifies that certain data structures reside at certain places on the disc. Starting with these initial data structures, the player can then follow links within those data structures to find the appropriate files and folders, which contain the video management structures and then the actual content itself. Fortunately, the UDF specification is fairly restrictive when dealing with DVD, and while possible, the likelihood of problems is fairly low.

2.C Video Management Structures

Once a player is able to recognize media, read data blocks from that media, and find files, it must then open the appropriate files in order to render the content. Unfortunately, this is another of those areas where a number of potential pitfalls can exist. To better explain why, it's important to understand a little about how a DVD movie actually works.

2.C.a Programming a DVD

Unlike older technologies like VHS or audio CD where the playback experience is linear once the content is created, a DVD Video disc was designed from the outset with flexibility and interactivity in mind. The designers wanted to provide content developers with the tools they would need to create not only rich content, but also a mechanism to interact with that content. Basic menus were clearly a requirement, but they also wanted to provide features like multiple camera angles and audio tracks, subtitles, seamless branching, and more. In order to provide this functionality, the designers created a type of programming language whereby a content developer can enable a very simple, uninterrupted playback experience or have menus, secondary audio tracks, or different endings each time a disc is played. Looking at things in a slightly different way, DVD provides a number of assets (video and audio content) and a series of rules for dealing with those assets. The player's job is to correctly follow the rules with the assets specified. It is this level of functionality that has enabled the creation of basic videos and completely interactive games all from a single disc playing back in a set-top player.

However, there's a downside to all this flexibility. As has been demonstrated with human language for millennia, there's always the possibility of misinterpreting any given language and the same holds true for DVD. While not nearly as prevalent today as in the early days of DVD, there are cases where a certain disc will behave strangely when inserted into a player, but when that same disc is put into a different player, everything works great. The disc clearly hasn't changed, but the interpretation of the content changes from player to player. Fortunately, with the advent of more and more discs and ever newer players, these types of problems have diminished considerably. What's not clear is whether this is due to better compatibility among the players or better "programming" by the content producers or a combination of the two. But in the same way that DVD recordable media can disrupt the playback model somewhat, consumer DVD recording can also disrupt the interpretation of the content, although for completely different reasons. To better understand the reasons behind this, let's look at things from both the professional and consumer perspective.

When dealing with professionally produced content, a single master disc (or tape image) is created which is used to create the many thousands of duplicates. Beginning with high quality studio content and some very expensive software tools running on very high end computers, the professional developer sets out to create an entire user experience where the content is merged with different things like director's commentaries, behind the scenes footage, special audio and visual effects. All of these elements are merged together and a test disc is created. The test disc is then run through a series of tests with a variety of different players and devices to ensure the proper playback experience. If things aren't quite right, the developer goes back and tweaks or adjusts things until everything is perfect. Only when the disc is perfect will it be duplicated and distributed.

On the consumer side, the situation is markedly different. There are still a wide variety of tools available and clearly the user has a certain player (probably sitting in the family or living room) where they want to play back content, but unlike the professional, the consumer is very reluctant to create numerous discs where each one is adjusted ever so slightly to ensure adequate playback. The typical end-user is also likely to be less interested in every single bell and whistle afforded by the DVD format and is probably more interested in seeing their priceless family videos showing on the TV with a minimum amount of fuss. It is for this reason that professional video editing software and the consumer counterpart need to be vastly different in their focus and experience. Consumer DVD creation software must necessarily be easier to use than their professional counterparts. Additionally, it must also produce discs that will run on almost any device (given the compatibilities outlined above) without extensive user intervention.

2.C.b Real-Time Recording

Consumer DVD mastering software must also address an issue that the professionals will rarely, if ever, encounter: real-time DVD recording. While most set-top DVD recorders are still a little too expensive for the average consumer, their prices are expected to drop with time. Eventually, many expect the price of these devices to drop to the point where they will offer a reasonable replacement for the currently ubiquitous VHS recorder. Generally, these players produce discs that are in one of two different formats: DVD Video VR format and legacy DVD Video format. The former, DVD VR format, is relatively new and has limited support in the installed base of DVD players. Those that produce the later, or "legacy" DVD Video discs, actually "bend" some of the DVD Video rules in order to accomplish the task of real-time recording. Clearly it would be foolish to think that these discs will not be brought into a desktop DVD recording application. The question then arises for the mastering application developer, should he continue to bend the rules or try to conform to the "official" specifications? The answer will likely depend on where the disc is going to be ultimately used. If the disc is to be put back into a real-time player/recorder, it probably makes sense to maintain the format used for its original creation. If, on the other hand, the disc is intended to be used in a player that doesn't support the real-time format then considerable changes may be required to make the disc fully compliant. Clearly, the right approach really depends on where the disc is going to be used. Unfortunately, it's impossible to determine where a disc is going to go from within an application.

2.D Content Format

If the DVD player has successfully surmounted the issues outlined above, it is now ready to display the video and play the audio. This should be relatively straightforward since the video and audio streams are pretty well documented, but like anything else, there are potential issues here as well.

2.D.a Video Encoding

The video content that ends up on a DVD disc can come from almost any source, but to be DVD Video compliant, it must ultimately be compressed into the MPEG-2 format. This means that anything else, whether it comes from a camcorder or a high-end video production system, must be converted. This conversion process works by finding information in the video stream that is imperceptible to the human eye and throwing it away. Because it discards this data, MPEG is considered a lossy compression technology. In other words, you lose a little visual detail every time something is encoded.

Depending on a wide variety of different factors, the amount of data discarded, and the perceptibility of that loss can vary widely. When the amount of data discarded becomes too high, the video signal begins to degrade and the user is presented with blotchy colors, shimmering backgrounds, pixilation, blurriness, or just a generally poor image. Starting with either higher quality source material or using a better encoder will generally yield superior output. If, on the other hand, you begin with poor quality source material, or have a bad encoder, the resulting video is going to look bad, or may even cause problems with the player.

There are also variations that have less to do with source content and encoding, and more to do with where the disc is being created and played. Today, there are a variety of different video formats. These are generally divided into NTSC and PAL/SECAM. Each of these formats requires the video signal be output in a slightly different format and taking content from one format to another can be problematic. Fortunately, most DVD discs aren't world travelers. NTSC discs tend to be played only on NTSC TVs, PAL discs are played on PAL TVs and so on.

2.D.b Audio Encoding

Similar to video encoding, audio on a DVD disc must also be in one of a handful of different formats and anything else must be converted. A DVD disc can contain AC3 (also called Dolby Digital), Pulse Code Modulated (PCM) audio, MPEG-2 audio, and Digital Theater Systems (DTS) audio. All of these formats except PCM are compressed using lossy compression, similar to video, where imperceptible data is discarded. In most cases, these different audio formats yield good results, but problems are possible when using poor source material, or when content has been repeatedly decoded and re-encoded. It's also possible to find cases where the transcoding, or conversion, of audio from one format (like MP3) to one of the DVD approved formats can result in some audio distortion. When this happens, the audio may sound hollow, muddy, or may possible have some strange echoing. In most cases, these imperfections will probably be too subtle except in the most severe cases where the sound will be degraded enough to be annoying. Additionally, if the audio isn't properly formatted, the sound may have stutters or clicks, or may not play back at all.

2.D.c Audio & Video Synchronization

What's not generally understood by most consumers is that the audio and video are stored separately on a DVD disc. They're both in the same file, but are divided into packets of audio and video that are then merged together at playback time. If these packets aren't merged together correctly, for a variety of reasons, it's possible that the audio and video will not synch. When this happens, the sound for a particular action might happen either before or after the action in question. Depending on how far out of synch the sound and video are, this can be quite jarring for the consumer.

3. The Consumer Impact

For the typical consumer, much of the above can be marginally interesting, but what really counts is creating a disc in their recorder that will play back in their set-top player. So what must the hardware and software companies do to attain this goal?

3.A Hardware

Looking first at the hardware side, it is relatively clear that the set-top DVD player vendors must consider DVD recordable technology into their planning and development process. DVD recording is here to stay; supporting it may not be easy, but it will certainly be required moving forward. Any good DVD player should support all the various recordable formats without problems. This may sound overly optimistic, but there are players available today that play back everything and do a great job of it. Remember that in the early days of CD recording technology we had a very similar experience where certain discs were supported while others weren't. Using the audio CD as an example, the early CD players had problems with CD-R discs and most wouldn't even play CD-RW. Today, virtually all CD players support CD-R & CD-RW without a problem. With time, we should expect the same with DVD recording.

3.B Software

The recording software has likewise evolved from very technical, professional programs only suitable for serious technicians to today where the leading applications are extremely easy to use and suitable for the average consumer. Moreover, when creating software for the consumer, the software developer must provide tools that allow consumers to create discs that have the greatest degree of compatibility possible; remember that the consumer user isn't concerned with the exercise of every nuance of the DVD Video specification. They want discs that play back in their set-top player. Typically, this means taking a more conservative approach to the authoring process, but this doesn't require the sacrifice of rich user functionality. Fortunately for the software developer, there are a number of fine software tools available from companies like MEI, Interra, and Philips to verify the validity of a DVD Video disc and these tools should be used extensively. Beyond basic specification compliance, it is critical that the discs be tested in a real-world scenario: playback on real, live DVD players. An extensive testing laboratory is a big plus here, but, like software test tools, there are a number of companies that offer testing of real discs on real players in an effective, impartial environment. Unquestionably, any company that expects to support more than a handful of customers must take advantage of any and all test tools at their disposal.

3.C Roxio - A Real World Example

In the same way "a dwarf standing on the shoulders of a giant may see farther than a giant himself" , today's technology is evolving and building on that which has come before it. Roxio's initial entry into DVD recording happened a number of years ago when there was only one consumer DVD recorder available (that is, as long as that consumer was willing to spend $17,000.) This application was specifically targeted at the entry level professional/high end consumer and provided an abundance of options and features that would allow the target customer to accomplish their chosen task. Of course, all these options and features made it very easy for the novice to create real problems for themselves. At the same time MGI (later acquired by Roxio) was working on sophisticated video editing software and made their initial forays into DVD recording. As the DVD recording market began to mature, it became obvious that more consumer-friendly applications would be required so the existing applications were refined and new consumer focused applications were created. These applications leveraged what had been learned, but, as happens with any new application, there were still things that needed to be addressed.

As these applications became functional, one of the first things to be done was to feed a variety of different types of content into the application and actually produce discs. The resulting discs were then run through the MEI, Philips, & Interra test suites which checked for DVD specification compliance. Understandably, these initial tests indicated a number of potential issues and warnings, but, due to the experience gained previously, the issues were understood and easily resolved. During a short iterative period, changes were made and bugs were fixed until all the major issues were resolved and only a few minor issues remained outstanding. This process was repeated with a wide mix of content, arranged in a number of different ways to fully exercise the applications. Each time a new disc was created, it was passed through the test suite and if any errors were found, they were resolved. At that point in time, we were pretty confident that the discs being created were compliant with the DVD specification.

But, as outlined above, DVD specification compliance isn't the only factor to DVD playback compatibility. In order to have true playback compatibility, the discs were then run through a wide variety of DVD players in our QA labs. In each case, it was noted what worked and what didn't and the engineering team then focused on those cases where things didn't work as planned.

One of the interesting things found was an issue with the video encoding. Remember that according to the compliance tests, everything was within specification, but we still ran into a situation where the video was erratic on certain players. The video stream was further analyzed and found to be fully MPEG2 compliant, but it still didn't playback correctly. Fortunately, after some serious analysis, it was determined that some players were sensitive to the number of headers in the MPEG stream (part of the "programming" portion outlined previously) and increasing the number of headers resolved the problems on the players that were previously having problems.

Once we had addressed the outstanding specification issues and were comfortable with what was being created, we created a number of discs and sent them to Intellikey for compatibility testing. Intellikey performed a variety of tests on the discs across a wide range of players and reported back the results. We then took the results and addressed the areas of concern. With the major issues resolved, a new set of discs were then created and re-submitted. Happily, in testing on over 170 of the most popular set-top DVD players, the results were positive and indicated a high degree of compatibility in the areas where we had control. Unfortunately, the issue of certain players not recognizing certain types of media persists, but hopefully that will be resolved by the player and media manufacturers with time.

4. Summary

It is clear that a number of barriers confront the DVD recording enthusiast, but it is also clear that those barriers can, and will be, overcome. Recorders are rapidly evolving, the media is getting better, and the players are becoming much more tolerant. The software is also much better than what was available just a few years ago and it will continue to improve as well. Looking at all these factors in this light, it becomes clear that while we may be in the early stages of DVD recording, the future looks very bright and promising.