How DVD gets THX certified


	How DVD gets THX certified


	by Guido Henkel

Some time ago I have been researching the current state of the THX certification program and wrote an article for the trade magazine “Replication News” on the subject, because I feel there is still a lot of confusion around, what THX does and does not do. Here is a reprint, as it appears in this months issue of “Replication News” that you should be able to find at your newsstand - if they carry trades, that is.

The acronym THX is popping up all over the place, first in movie theaters with funny little trailers, then on high-end home theater equipment, usually adding another $1,000 on the list price of already overpriced equipment -- and finally on VHS releases, laserdiscs and now, DVD titles.


	As of October 1999 there were 78 THX-certified DVDs. Many consumers are still unsure what these three letters stand for exactly, while others mistake it for a sound format, a notion mostly created by the bombastic surround sound that accompanies the various THX trailers. So, what exactly does the THX logo actually stand for? What can people expect from it when these ominous letters shine from a DVD case? To understand the value of the THX's work, one must first understand its purpose.


	Its origins go back to 1980, when filmmaker George Lucas set out to develop and implement a set of quality control criteria for theaters. His goal was to ensure that his films, as well as those of other filmmakers, would be accurately reproduced for moviegoers for whom sound was an important part of their overall film enjoyment.Three years later THX, named after Lucas' first feature film, “THX 1138”, had been developed and THX systems were installed in two theaters for the premiere of “Return of the Jedi”. Since then, THX has established a certification program for a number of platforms that is intended to optimize audio and

video performance. In movie theaters it guarantees that the theater has been properly calibrated for the best possible experience and that the playback volume level is according to a guideline. It all is supposed to make the experience more enthralling, and the trailer at the beginning of the movie is intended to send a message to audiences that they are about to see a high quality presentation, guaranteed.

In home theater equipment the same is principally true. THX certifies equipment to make sure it meets certain quality standards so that you can be assured you can achieve an optimal performance in your own home. And the same is ultimately true for the THX certification of video transfers found on VHS, laserdisc and DVD.
In 1993 a THX laserdisc program was created and quality control services were offered to studios for films during laserdisc production. In 1996 the laserdisc program was expanded to include VHS videotape and DVD and the name was changed to THX Digital Mastering Program. Presentations of films on DVD are subject to a large number of variances in picture and sound that are not anticipated and may not be in the best



		This is one of the screening rooms that is used to screen video material during the certification process. There’s some high-end equipment installed that allows the technicians to detect even the smallest problems in the video signal.

interest of the presentation or the filmmaker's original artistic creations. THX addresses this variability by outlining certain standards that have to be met by a transfer in order to qualify for their seal of approval. That way, consumers can associate the well-known THX logo with the most accurate reproduction of the original content within the limitations of any given format, regardless of the presentation format itself. On DVD, particular attention is paid to insuring the correct color balance, black level, white level, detail and video noise level on the picture.

Soundtrack transfers are supervised to provide the correct playback volume level, as it would be in a properly calibrated theater while maintaining the widest possible frequency response.The way this is done is methodically testing and correcting flawed elements. The first step in a THX mastering is the monitoring of all film and audio elements through THX. In this step all film transfers and masters are ensured to be of the best possible quality and that all conversions are kept to a minimum to avoid the



	Rooms stacked with laboratory equipment that is used to test and control the electrical and physical qttributes of video tapes and discs.

introduction of conversion artifacts and generation loss.Images and video segments are tested against reference colors, and luminance and saturation levels in the picture are controlled to avoid over- or under-saturation. Furthermore all transfers and images are reviewed by THX to ensure consistency throughout the presentation.

On a DVD this means that also the menu screens have to meet these reference parameters.Although THX does not directly interfere with aspect ratio decisions, which they feel must lie solely within the filmmakers, the company consults on that aspect if required to make sure the presentation matches the original theatrical presentation.
If marketing needs ask for a full-frame transfer, THX will certify such a transfer, making sure that particular transfer meets the standards lined out. THX also supports anamorphic as well as non-anamorphic DVD transfers for certification for the very same reason. Ultimately this decision has to be made by the filmmakers and the studio, depending on their individual needs at any given time.

However, THX's parameters define that film transfers used must come from the highest possible resolution digital medium available to the client, and stresses that high definition transfers should be used throughout. At the same time, image processing and noise reduction must be minimal. In a word, you need a very good source print, telecined into a high definition video transfer in order to qualify for this program. Even more so as the program requires that transfers used are exact copies of the film print, without prior color correction or level changes, and only allowing noise reduction to a certain extent.

Only D1 videotape masters are allowed for the THX certification program, and if digital footage is brought across directly in digital form, it has to come from a calibrated workstation. Once all these parameters are met, the material is compressed for DVD, and a completely new set of guidelines comes into action.

Hardly surprising, MPEG-1 is not allowed for a THX certification due to its numerous technical limitations, and only MPEG-2 is supported. In a first pass, the material is compressed with computer guidance. In a second pass, general problem areas are then manually corrected, and in the third stage, a continuous process of quality assurance takes place.
THX personnel is examining and identifying compression artifacts on high quality, reference grade monitors and playback devices, which are then eliminated in subsequent runs. This procedure is repeated until the entire video presentation meets the compression standards of the THX reviewers. Only if a film is running out of disc space, or if the data rate is getting too high for consumer players to handle, will THX allow minor compression degradations.


	Then the audio for the disc is examined, which can be quite tricky due to DVD's ability to support multiple audio formats in a non-linear fashion. THX encourages the use of one main language 448kbs Dolby Digital (AC-3) and a complementary Left-Right (AC-3 2.0) track whenever possible. If another audio format, such as PCM or DTS, is chosen, THX will determine a suitable reference set based on the content source material. Interestingly, but hardly surprising, THX recommends to keep the number of language tracks to a minimum. The reason is simple and sensible. Because DVD offers a limited amount of space, additional audio tracks eat up valuable disc space that could be better used to increase the overall video quality. By keeping language tracks to a minimum, THX can ensure that at any time quality prevails over quantity.

At the same time, DVD players can only handle so much data at the same time and this amount of data has to be shared by the audio and the video stream. If either one takes up too much bandwidth, it is inevitably affecting the other. As a result, THX outlines that the audio tracks shall at no time reduce the video stream to less than 5.0Mbps, a threshold value that usually allows for good looking video playback.

To make sure the audio tracks do not only feature impressive technical data, but also sound superior, THX is lending its expertise to the projects by helping to select appropriate audio source materials, prepare and transfer them, but also to re-mix them if need be, and to format them into the required digital audio streams. If the project will not contain a separate stereo presentation, THX will also make sure that the DVD-internal 5.1 channel downmix to two channels sounds good. It also determines that a re-mix is required if dialogue is drowned out by sound effects or music.

Once the master that will be used for replication is prepared, THX evaluates the stampers used at the manufacturing facility to press the DVDs. They also ensure that the discs are physically fit. Pit geometry, track crossing errors, layer separation, beam deflections and many other things are controlled and a thorough compatibility test is run as well. Once the first test discs are returned from the manufacturer, they are compared to the records filed throughout the process to make sure every one of the known defects has been fixed.

The disc is thoroughly checked to see if it is in proper working order and all the elements like menus and subtitles are working correctly. Once the disc passes all these crucial criteria, then, and only then, THX will give the disc its seal of approval.


	This article has originally been published in “Replication News” and is reprinted here by permission.

		December 1, 1998


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