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PRODUCING
DIGITAL VIDEO DISKS
DVD TAKES OVER FROM VHS
By Danny Whybrow, Managing Director,
The Machine Room
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 For
all its flaws, VHS has until now remained
the only universally acceptable option
for widespread distribution of video
to the consumer since the '70s. Despite
the ever-improving quality of professional
tape formats and disk storage, the
end result was often viewed from a
VHS cassette, and the quality of the
resulting picture was heavily dependent
on the quality of the replay machine.
It was frustrating to video professionals
to create D1 masters direct from telecine,
only to see the all-important picture
quality greatly diminished upon transfer
to VHS.
Digital Versatile Disc (DVD) is the
first real physical media capable
of delivering top-quality video to
the consumer. Not only that - it can
offer interactivity, multiple surround
audio tracks, multiple subtitles,
multiple angles - essentially everything
required to deliver the message as
intended and much, much more. The
most obvious and widely employed use
for DVD is currently feature film
release. UK sales figures show that
DVD is selling in a big way - there
are now more than 3 million British
households enjoying the unparalleled
picture and sound quality of DVD video
and over 58 million discs sold since
DVD-Video first became available.
But, movies aside, more and more people
are becoming aware of the potential
of the format and we look set to see
a huge increase in the popularity
of DVD in myriad areas. Education,
training, reference, advertising -
all areas of modern life - have potential
uses for DVD as a means of delivering,
cataloguing or storing information.
In order to appreciate the benefits
of DVD, it helps to understand the
processes involved in the production
of a title, which although more involved
than the creation of a VHS will produce
an immeasurably better result. They
are listed below.
Planning
The planning stage is an essential
part of any DVD project. It is here
that decisions are made on the purpose,
content, overall "look",
level of interactivity and complexity
of the disc. Having a clear plan from
the outset will enable anyone involved
with the project to concentrate on
their particular task, knowing exactly
how their work will combine with the
other processes involved in the production
of a finished DVD. The most common
method of planning the structure of
the disk is by means of a flow chart
depicting all menu choices and destinations.
It is important that the authoring
house is involved as much as possible
in the planning process to have a
full understanding of the nature of
the project and to give advice on
content, structure and extra features
where required.
Gathering assets
Having established the content of
the disc, the next stage is to gather
all necessary assets. When you consider
that the DVD format permits the use
of multiple video, audio and subtitle
streams, this may consist of video
elements, audio elements, existing
graphics and subtitles - many of which
will need to be sourced from storage
either in the UK or abroad if foreign
language material is required. There
is always the possibility that assets
may be unavailable or unusable, in
this case it is wise to have a backup
plan for re-mastering, repairing or
re-sourcing assets. It is always advisable
to have assets quality-checked before
use to ensure that they will make
full use of DVD's potential. Audio
elements will also need to be quality-checked
and referenced to video to ensure
they are in sync with picture.
Graphic design & creation
This stage is an integral part of
the production process. It is here
that the design of the front-end user
interface occurs, determining the
look and feel of the disc. This includes
the creation of menu screens for selecting
particular chapters of a programme
and for setting the language or subtitling
preferences. Other screens may provide
access to behind the scenes documentaries
and other special features. A DVD
player's remote control allows the
user to tab between options using
a directional keypad, so the menu
design stage must involve practical
navigational as well as aesthetic
considerations.
Pre-processing
At this stage any essential re-mastering
work required to enhance the quality
of the materials is undertaken. This
may involve noise-reduction, colour
grading, de-spotting, de-hissing audio
and many more potential types of remedial
work. Essentially, the better quality
the material to be encoded, the more
efficient the compression (see Encoding
/ Compression) and therefore the better
the DVD will look.
Encoding and compression
All video for DVD must conform to
the M-PEG 2 compression standard.
This is a system of converting video
streams into data form, and compressing
this video to reduce the resultant
file size as much as possible, thus
creating more space on the DVD for
additional material or features.
In basic terms, M-PEG 2 is a form
of compression that compares adjacent
frames of video and discards redundant
information, such as a static background,
from successive frames. The M-PEG
2 decoder built into every DVD-Video
player then reads this information
and reconstructs each frame based
on the comparisons made by the encoder
between surrounding frames. This is
why it is important to remove as much
random dirt, noise and scratches from
the video source as possible - otherwise
the encoder will treat it as essential
information and it will take up space
on the DVD unnecessarily. Instability
on the original source will also prevent
the encoder from efficiently comparing
information between frames.
The art of the compressionist is
to reduce the resultant file size
as much as possible without sacrificing
quality. This is achieved by adjusting
the bitrate, which is the amount of
data assigned to a particular amount
of video, usually expressed in terms
of millions of bits per second (a
feature film DVD will have an average
bitrate of 6 million bits per second).
When using professional compression
equipment, the bitrate can be constantly
varied to apportion more data to fast-moving
or highly detailed scenes and less
data to static or less detailed scenes
without producing visible artefacts.
Audio for DVD must be treated in
the same way. DVD is able to handle
up to 8 separate multi-channel audio
tracks. Each of these tracks can make
use of up to 8 channels of audio,
so surround sound capability is a
major feature. Audio must also be
encoded, but unlike video there are
three methods in use.
The first two, Dolby Digital (AC-3)
and DTS, are audio compression systems
which filter the audio stream and
discard redundant information, leaving
only those sounds audible to the human
ear - so if encoded correctly one
should hear no compression artefacts
whatsoever.
The other method, PCM, is a system
of encoding which is uncompressed
and can employ a sampling rate of
up to 96 kHz, twice that of a standard
Compact Disc. However, the PCM system
is not practical for multichannel
audio as resultant file sizes are
far greater than for DTS or AC-3 and
therefore is rarely found on feature
film DVDs.
Authoring
Once the encoding of the audio and
video streams is complete, the next
step is authoring. This is where the
disc really starts to take shape.
In order to create any DVD title,
the structure of the disc must be
defined to connect all of the elements
on the DVD, and the disc's behaviour
in certain situations must be programmed.
For example: what will the disc do
when it is put into a DVD player?
Will it go straight to the main menu?
Will it play a video clip or a copyright
notice? Links from menus to video,
audio or subtitle streams must also
be programmed. All of these instructions
must be programmed and tested until
a finished disc image is created.
The disc image is what will be written
to the final DVD and read by the DVD
player.
Emulation & checking
Once the disc image is completed,
final tests need to be run before
the finished discs are created. Links
are checked, all video, audio and
subtitle elements are re-checked and
the structure of the disc is checked
to ensure conformity with the DVD
specification. The requirement of
any necessary changes must be discovered
here, as once the disc image is replicated
it is an extremely costly and time-consuming
exercise to make even the smallest
amendment.
Replication
Once the completed disc image is
deemed acceptable, it is either burnt
to DVD-R for very small runs or transferred
to DLT for large-scale replication
at a specialist plant. If created
by a large-scale replicator, this
will be the disc that will end up
in the retail outlets. Dependent upon
the amount of content the disc size
can vary between 5 Gigabyte and 18
Gigabyte. Disc types include the single
layer formats (5Gb single layer single
sided, 10 Gb single layer double sided)
and double layer formats (9Gb double
layer single sided, 18Gb double layer
double sided). Double sided discs
use layers of different reflectivity
to allow the laser to re-focus on
the second layer of a particular side,
thus removing the necessity to turn
a disc over.
As with the traditional post-production
route, the surest way to complete
a DVD project from start to finish
in reasonable time is to discuss your
plans with the DVD production facility
at the earliest stage possible. It
also helps to establish the quality
of assets at an early stage in order
to plan for any remedial work to be
done within the allotted time, and
from the outset to provide the facility
with as much of the required material
as possible. Following these guidelines
should allow the creation of a DVD
title that will exploit the full capabilities
of the format.
Danny Whybrow, Managing Director,
The Machine Room
First published, Spring, 2000
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The Machine Room
54/58 Wardour Street
London W1D 4JQ
Tel: 020 7734 3433
Web site: www.themachineroom.co.uk
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