MPEG works by looking at a picture and noticing what in the scene and between the scenes might be redundant information and what is always changing. MPEG reduces the amount of data it needs to store by identifying this redundant information. For instance, if the scene was a man talking against a static background (the classic talking head), there is much in the scene that is redundant - a great expanse of wall, the clothes the man is wearing, etc. If the scene on the other hand, was a panning shot of a field of tall grass blowing in the wind, then there might be much in this image that is unique and changing both within the frame and from frame to frame. By using variable bit rate compression, less bits can be applied to the static scene and more to this very complicated scene (see MPEG2 compression, for more information).

One of the issues that will cause the issue of bit rate and length to be re-examined is the implementation of the dual layer and dual sided discs. As these disc configurations and their manufacturing become more commonplace, the ability to have relatively high bit rates and voluminous amounts of content on a disc will make the DVD format even more impressive.

WHAT FORMAT IS THE SOURCE MASTER?

A digital component Source Master on D1 or Digital Betacam is preferred because the compression process itself is also component. In component recordings, the color information is recorded separately from the black and white information. This results in a master where the colors are sharper and the artifacts commonly associated with composite recordings such as "chroma crawl" are absent. Some of these composite artifacts create additional non-redundant information, which causes the wasting of those precious bits used in the encoding process. An analogue component recording such as from BetacamSP is actually preferable to a Digital Composite Master such as D2 for this very reason. This is not to say that one cannot encode from a composite source, but a digital component master is always preferable. If a composite or analogue source is provided, we will create a digital component master from which we will encode.

WILL THE DISC CONTAIN ADDITIONAL MATERIAL SUCH AS LOGOS, TRAILERS, INTERVIEWS, ETC. ?

In determining the overall bit rate, all video material, including FBI warnings, logos, and trailers need to be taken into account. In addition, it is helpful to make sure that all material that needs to be encoded is available at the same time so that encoding can take place efficiently.

HOW MANY AUDIO TRACKS AND IN WHAT CONFIGURATION?

Obviously, the number and type of additional tracks will have a bearing on the overall average bit rates for the disc. In addition, the preparation of language tracks for AC-3 encoding are an important element in the making of a DVD disc. The format supports Dolby AC-3 which can exist in a stereo configuration that can be compatible with ProLogic Surround or 5.1 channel surround sound. This 5.1 channel sound is so named because there are front left, front center, front right, rear left, rear right and a subwoofer channel (the .1 channel in the name). If these tracks already exist for theatrical release, it is possible that they can be used for DVD. However, these tracks may require additional audio post production to optimize them for the DVD format. If 5.1 channel tracks do not exist, then they will need to be prepared. In addition to the lead time necessary to prepare sound tracks, it is important to be able to locate additional foreign language elements and to verify that they exist in the correct format and to the same master to which video is being encoded. One of the challenges in efficient DVD title creation is to ensure that all the elements necessary for authoring - all video, audio, and graphic elements are available and approved so that authoring can proceed.

An interesting specification built into the DVD format is that Linear PCM audio (such as used on Compact Audio Discs) can also be used. The DVD format also supports 96Khz Uncompressed PCM audio which has twice the bandwidth of today's current audio CDs. The fidelity of this sound format is phenomenal although files are quite large and will consume additional "real estate" on the disc if used.

WILL THE DISC CONTAIN SUBTILES, CLOSED CAPTIONING OR OTHER SUBPICTURE INFORMATION ?

The DVD format supports up to 32 channels of subpicture information. These sub-pictures are essentially graphic files that are "bit-mapped" or overlayed onto the pictures one pixel at a time. The color palette for sub pictures consists of 16 different colors and contrast values, 4 colors and 4 different contrast values can be displayed on the screen in one channel at a time. By using the various channels of sub-picture information that can be triggered by time code or button depressions, relatively sophisticated graphics, even simple animation can be put onto the screen. Subpictures can scroll and fade on the screen and can change every field. More basic uses of sub-pictures are captions, sub-titles or other text information.

WILL THE DISC BE RELEASED IN A 4x3 OR 16x9 FORMAT?

The DVD format allows for the specification of an aspect ratio in the display of the disc. The disc itself can be programmed to allow for a 16x9 format transfer to be displayed that way on 16x9 televisions as well to also force the display of a scope or 1:85:1 film in a "letterbox" on a 4x3 television set.

WILL THE DISC BE A PAL OR NTSC?

DVD discs can be PAL and NTSC. To create a NTSC disc, the video is encoded from a 30 frame 525 line Digital Component or NTSC source master. To create a PAL disc, the video is encoded from a 25 frame 625 line Digital Component or PAL source master. The format also has different audio configurations in its NTSC and PAL versions. In NTSC countries, AC-3 Audio or Linear PCM audio playback capability may be used and MPEG2 audio playback capability is optional. In PAL countries, MPEG audio or Linear PCM audio playback capability may be used and playback AC-3 Audio capability is optional.

This means that in order to have an NTSC and PAL product, the disc needs to be mastered, encoded and authored in both formats.

WHAT WILL THE ON-SCREEN MENUS LOOK LIKE ?

WILL THE DISC CONTAIN CHAPTER STOPS THAT ALLOW A "JUMP" TO A PARTICULAR PART OF THE CONTENT?

WILL THE DISC CONTAIN CONTENT THAT NEEDS TO BE "BRANCHED" BASED ON USER INTERACTION ?

MPEG2 COMPRESSION.

MPEG2 was born out of a continuing desire and demand to achieve the best possible audio and video quality for digital storage applications. It follows on the heels of MPEG1 which took the first steps in bridging the gap between analog and digital technologies to achieve broadcast quality audio and video. 

WHAT IS MPEG?

 

MPEG (Moving Pictures Expert Group) is the ISOIIEC working group formed in 1988, with contributors from all over the world, to ensure that the compression of digital audio and video signals followed a defined standard. Among the contributing companies were Sony, Matshushita, JVC, Toshiba, Thomson, Motorola, C-Cube, LSI Logic, Texas Instruments, Digital Equipment, AT&T and many others -- including Philips.

In 1992, the efforts of this group resulted in a standard for audio and video coding known as MPEG1. MPEG1 was aimed at coding audio and video at a bit rate of about 1.5 Mbits/sec. which corresponds to the bit rate on a CD-ROM. MPEG1 has been successfully implemented in applications such as CD-i and Video CD, and a variety of broadcast applications, including DSS (Digital Satellite System), DAB (Digital Audio Broadcast), DVB (Digital Video Broadcast), and Satellite feeds to cable networks. MPEG-encoded files are available from a growing number of platforms including CD-ROM and the Internet. Among new applications, MPEG1 will also have a place in DVD products.

Since its inception and widespread application, MPEG1 has been well accepted as a boon to digital storage and transmission. MPEG has also become part of many other standards such as the new Digital Audio Visual Council (DAVIC) standard which will be incorporated in numerous digital television decoders; the ITU-R recommendation (BS 1115) for emission, contribution and distribution; the ETSI Standard on Digital Audio Broadcasting (ETS 300401, February 1995); and the ITU-R draft recommendation for Digital Terrestrial Television Broadcast.

THE EMERGENCE OF MPEG2.

 

The compression work for MPEG1 was based on film and other progressive sources, but its bit rate is not suited for standard broadcast interlaced video with good compression. To bridge this gap, the MPEG committee continued in its development of the technology. From their work, MPEG2 was created and became a standard in November 1994. MPEG2 has been presented as the audio/video solution for supporting bit rates beyond 5 Mbits/sec. Today MPEG2 delivers picture and sound quality equal to TV studio standards.

WHAT IS MPEG2 AUDIO?

 

MPEG2 audio is a compatible extension of the MPEG1 audio coding which enables the transfer of mono, stereo, or multichannel audio in a single bitstream. It can operate at data rates from 32 kb/s up to more than 1 Mb/s, and supports sampling rates of 32, 44.1 and 48 kHz. For stereo, a typical application would operate at an average data rate of 128-256 kb/s. A multichannel movie soundtrack requires an average bit rate of 320-640 kb/s, depending on the number of channels (5 to 7, plus a sub woofer channel) and the complexity of the encoded audio.

WHAT ABOUT MPEG2 VIDEO?

 

Where MPEG2 shines is in its higher bit rate and support of interlaced video. The video compression of MPEG2 is much more sophisticated than that of MPEG1. MPEG2 provides 60 fields/s and supports bit rates of 6 - 8 Mbit/s. It specifies the syntax and semantics of a compressed video bit stream (and the parameters such as bit rates, picture sizes and resolutions which may be applied) and how it must be decoded to reconstruct the picture.

MPEG2 fully supports the coding of interlaced video material. This level of performance, combined with new optical disc technology, makes it possible to record a full-length feature film, with digital surround sound, on a single high-density optical disc.

MPEG2 supports virtually every video standard worldwide, including PAL (Phase Alternating Line), SECAM, and NTSC (National Television Standards Committee). 

VARIABLE BIT RATE CODING.

 

In both video and audio, certain sections are more complex than others, e.g. a picture of a tree is more complex than a clear blue sky. As a result, the number of bits needed to faithfully encode varies with the program material. In order to encode in the most effective way, it is advantageous to save bits from the simple sections and use them to code complex ones. This is what variable bit rate coding does. Although the coding of video is completely different from that of audio, the principle works equally well for both. This can be illustrated by movie soundtracks in which love scenes are interrupted by gunfights or a sudden thunder clap which enhances the suspense of a thriller.

BENEFITS OF VARIABLE OVER FIXED BIT RATE ENCODING.

 

Operating at a certain (average) bit rate, a fixed or constant bit rate encoder provides variable quality. For fragments that are simple to code, the fixed bit rate encoder can apply more than enough bits. But it does not have enough bits available for the complex fragments and coding artifacts may become noticeable.

A variable bit rate encoder provides fixed quality. It always applies the number of bits necessary to encode without noticeable artifacts. It has been suggested that a fixed bit rate encoder, constantly operating at the same peak data rate as a variable bit rate encoder, will sound equally good or better. But the variable bit rate encoder needs the high peak data rate for only a small fraction of the total time. Applying a fixed bit rate encoder in this manner is a clear overkill: one could easily add another variable data rate bitstream with, for example, a second language version instead.

MPEG2 IS COMPATIBLE WITH MPEG1.

 

Unlike other competitive systems, MPEG2 has been designed with compatibility in mind. With the ever-growing number of applications of MPEG1, especially in the entertainment, satellite broadcasting (DSS) and multimedia markets, this compatibility will provide the consumer with a cross-platform format to enjoy high-quality audio and video reproduction.

The core of the MPEG2 bitstream is an MPEG1 bitstream. This enables fully compatible decoding with an MPEG1 decoder. An MPEG2 video decoder can decode MPEG1 video and an MPEG2 audio decoder can decode MPEG1 audio. Likewise, an MPEG1 audio decoder can decode MPEG2 multichannel audio, providing a stereo output signal. This compatibility eliminates the need to transfer two separate bit streams (one for stereo and another one for the multichannel audio program). For example, a future upgrade of DSS with multichannel audio will not make existing set top boxes obsolete. The existing boxes will reproduce excellent stereo; the new products will produce high quality multichannel.

MPEG2 APPLICATIONS

 

Today's computer systems currently support MPEG1 audio and video, and many will soon support MPEG2. MPEG2 is the video standard for DVD players and is being touted as the next standard in applications where broadcast quality is essential. MPEG2 is also the preferred DVD audio format for 50Hz countries and an option for the 60Hz countries.

AC-3 HISTORY.

 

While conceived and later chosen for HDTV in the US, AC-3 was actually implemented for the cinema first, making it practical to provide multichannel digital sound with 35 mm prints. In order to retain an analog track so that these prints could play in any cinema, it was decided to place the new digital optical track between the sprocket holes, a key factor in defining its maximum practical bit-rate. It was also well documented that a 5.1-channel format would best satisfy the requirements of theatrical film presentation. Altogether, these needs dovetailed with the HDTV requirements that led to AC-3's conception.

Dolby Digital, the film sound format with AC-3 as its keystone, debuted in cinemas in June of 1992. Within less than two years, more than 50 feature films had been released in the new format, and nearly 600 cinemas in 27 countries had been equipped for playback of the digital track. This experience confirmed that prints with both digital and analog tracks could be manufactured economically, that such prints would play in any cinema, and that the AC-3 coded digital track provided high audio quality with extraordinary resistance to wear and tear.

Just as important, Dolby Digital provides a unique springboard for consumer formats based on AC-3, enabling the accumulation of invaluable experience in mixing, recording, and distributing multichannel digital audio. It is also fostering a library of program material immediately available for consumer release, and has facilitated the development of cost-efficient IC decoder technology. Dolby AC-3 is the only multichannel perceptual coding technology with this kind of real-life experience behind it.

Dolby Digital (Surround AC-3) in the Home.

 

Dolby Digital for the home (aka Dolby Surround Digital or Dolby Surround AC-3) forms the final link from multichannel program producer to home listener. Like the film format, it provides separate channels for left, right, and center speakers at the front; two surround speakers at the sides; and a subwoofer at the listener's option.

Unlike analog Dolby Surround with its single band-limited surround channel (usually played over two speakers), Dolby Digital features two completely independent surround channels, each offering the same full range fidelity as the three front channels. As a result, true stereo surround effects can be achieved for an expanded sense of depth, localization, and overall realism. And because Dolby Digital maintains complete separation of the audio channels, it is as suited to music-only recordings and broadcasts as it is to video formats. Thus it has the potential to open up new worlds of multichannel sound reproduction.

That isn't all that Dolby Digital can do. While Dolby Digital is heard in cinemas with a full complement of loudspeaker channels, a standardized playback level, and full dynamic range capabilities, home listening circumstances vary markedly. Therefore, for Dolby Digital consumer formats, AC-3 has been designed to satisfy many diverse requirements.

At the outset, at least, while some listeners will have multichannel systems, most will be listening in mono or conventional stereo. Those with Dolby Surround systems will want a two channel matrix encoded output from their decoders. Many listeners may prefer a restricted dynamic range, but others will wish to experience the full dynamic range of the original signal. Techniques to satisfy these and other needs have been designed in from the beginning:

HOW DOLBY AC-3 WORKS.

The digital audio coding used on Compact Discs (16-bit PCM) yields a total range of 96 dB from the loudest sound to the noise floor. This is achieved by taking 16-bit samples 44,100 times per second for each channel, an amount of data often too immense to store or transmit economically, especially when multiple channels are required. As a result, new forms of digital audio coding--often known as "perceptual coding"--have been developed to allow the use of lower data rates with a minimum of perceived degradation of sound quality.

Dolby AC-3 is the first perceptual coding designed specifically to code multichannel digital audio. It is also the only one to benefit from the development of two other successful perceptual coding systems, Dolby AC-1 and AC-2, and from the development of what are in essence analog perceptual coding systems: the full gamut of Dolby professional and consumer noise reduction systems. Indeed, Dolby Laboratories' unique experience with audio noise reduction is essential to AC-3's effective data rate reduction: the fewer the bits used to describe an audio signal, the greater the noise.

Dolby noise reduction works by lowering the noise when no audio signal is present, while allowing strong audio signals to cover or mask the noise at other times. Thus it takes advantage of the psycho acoustic phenomenon known as auditory masking. Even when audio signals are present in some parts of the spectrum, Dolby NR reduces the noise in the other parts so the noise remains imperceptible. This is because audio signals can only mask noise that occurs at nearby frequencies.

AC-3 has been designed to take maximum advantage of human auditory masking. It divides the audio spectrum of each channel into narrow frequency bands of different sizes optimized with respect to the frequency selectivity of human hearing. This makes it possible to sharply filter coding noise so that it is forced to stay very close in frequency to the frequency components of the audio signal being coded. By reducing or eliminating coding noise wherever there are no audio signals to mask it, the sound quality of the original signal can be subjectively preserved. In this key respect, a perceptual coding system like AC-3 is essentially a form of very selective and powerful noise reduction.

In Dolby AC-3, bits are distributed among the filter bands as needed by the particular frequency spectrum or dynamic nature of the program. A built-in model of auditory masking allows the coder to alter its frequency selectivity (as well as time resolution) to make sure that a sufficient number of bits are used to describe the audio signal in each band, thus ensuring noise is fully masked. AC-3 also decides how the bits are distributed among the various channels from a common bit pool. This technique allows channels with greater frequency content to demand more data than sparsely occupied channels, for example, or strong sounds in one channel to provide masking for noise in other channels.

Dolby AC-3's sophisticated masking model and shared bit pool arrangement are key factors in its extraordinary spectrum efficiency. Furthermore, where other coding systems have to use considerable (and precious) data to carry instructions for their decoders, AC-3 can use proportionally more of the transmitted data to represent audio, which means higher sound quality.

Technically speaking, AC-3 can process at least 20-bit dynamic range digital audio signals over a frequency range from 20 Hz to 20kHz x 0.5dB (-3dB at 3Hz and 20.3 kHz). The bass effects channel covers 20 to 120 Hz x0.5 dB (-3 dB at 3 and 121 Hz). Sampling rates of 32, 44.1, and 48 kHz are supported. Data rates range from as low as 32 kb/s for a single mono channel to as high as 640 kb/s, thereby covering a wide range of requirements. Typical applications include 384 kb/s for 5.1-channel Dolby Surround Digital consumer formats, and 192 kb/s for two-channel audio distribution.