Well, you said you ask the questions to indicate your lack of
knowledge, but it tells me that you are almost there.
Here are my answers to your questions.
First, MPEG stands for the motion picture experts group. They make a
variety of video (and occasionally audio) standards. Like you stated
mpeg 1 is vcd, mpeg 2 is dvd (and sometimes svcd depending on a few
things), and mpeg 4 is the *basis* for what divx started out as,
though divx is now something entirely different.
Mpeg is always a standard. Mpeg 1 by definition is compressed in the
same way all the time, and can play in a variety of players, for
example. VCD is a standard within Mpeg 1 that defines what resolution
and bitrates may be used with the mpeg 1 standard to be vcd compliant.
Mpeg 2 is similar, and is a video compression standard. Any mpeg 2
player should play any properly encoded mpeg 2 file.
Within the mpeg standard there is also audio compression. The mpeg
standard dictates what parts of the data file are to contain the audio
data. The Mp3's you doubtlessly have heard of is a short name for
"Mpeg 1, layer 3", which is to say, a way to encode audio into an mpeg
1 file. A vcd, for its audio, uses mpeg 1, layer 2, which is less
efficient than layer 3, but requires less processing to decode, which
is why it was used in the past.
To answer your second question, yes there are different ways a
compression standard can be implemented. For example, .asf files were
an early implementation of mpeg 4 made by microsoft. Divx originally
was created as a hacked version of microsoft's asf format in order to
allow it to contain a higher quality audio stream (mpeg 1 layer 3, or
mp3 for short) and slightly improved video. Divx, however, as it has
several versions (now up to version 5)has evovled into an entirely
different way to encode video than mpeg 4. Xvid, similarly, is a video
compression that works similarly to divx, but is encoded using a
different algorithm.
Also, mpeg can be implemented in several ways. Mpeg 2, for instance
can be implemented as DVD format video, which dictates using a
specific resolution, bitrate, allowable audio formats, the file
organization expected on the disc, encryption / copy protection
standards, and so on. SVCD is another standard that uses the same
video compression algorithm that DVD does, but expects a different
file organization, different resolution and bitrate, different audio
formats, and so on.
The reason for these differences is that different formats (dvd discs
versus cds) and target audiences (internet video users vs home
consumers) have different expectations, limitations, and requirements.
Since cds can hold less,
formats that were designed for them have lower resolutions and lower
quality overall. Formats such as divx that were designed to run on a
computer and be transferred using the internet, can be more
computationally demanding to display, in order to accomidate a lower
available bitrate.
Your next question, how do video standards relate, is best answered by
explaining the avi format. AVI stands for Audio/Video Interleaved, and
is simply a standard that specifies how a video file is to explain to
windows how its audio and video streams are organized. AVI is a
non-specific format, that is, any multitude of audio and video
compression formats can be combined together, and the file may still
be an .avi file. .mov is similar in nature, but those files are
usually only playable by apple's quicktime, as .mov is a proprietary
format. Mpeg, on the otherhand, is a very specific format, and there
are only a small number of available audio and video compression
formats that may be used for a file to be a valid mpeg file. You can
even rename a .mpg file to .avi and it will still play just fine in
most players, since .avi is really a catch all format. So long as you
have the right codec (encoder/decoder program) on your computer, your
media players should play any avi file encoded with the same codecs.
Divx and Xvid are examples of codecs.
To answer your next question, yes a different audio format gets rolled
into the divx. The divx encoder dictates which audio formats are
allowed and how they are to be inserted into the file in order to be a
valid divx file. As with any avi file, there may be audio codecs that
are customarily used to encode the audio, but so long as both the
creator of the video and the person playing the video both have the
same audio codec, just about any audio format will work. Mp3 is a
common format that is used.
For your next question, customarily, .mpg files are mpeg 1 files,
however, this is not required. mpeg 2 or mpeg 4 files may be labeled
.mpg, but they will not play in players that only play mpeg 1 files.
Sometimes mpeg 2 files are labeled .m2v, but more often than not, all
mpeg files, no matter what version, have the file extension of either
.mpg or .mpeg.
Lastly, some video editing programs will allow you to remove the audio
from an mpeg file, giving you two files, a file normally with an
extension of .mp2 or .mpa and another file with an extension of either
.mv1 or .mpv.
The dvdrhelp glossary may help with some of this (
http://www.dvdrhelp.com/glossary )
Search strategy:
Poke around at Vcdhelp.com and dvdrhelp.com |
Clarification of Answer by
funkywizard-ga
on
23 May 2003 06:51 PDT
Firstly, yes, exactly.
for the second part, this is not always the case. A few different
audio schemes may be used in conjunction with mpeg video 1, 2, or 4
and still have it be considered a video/audio combination in mpeg
format. There are a few types of audio compression, however, that are
usually associated with these types (layer 3 is usually used with
mpeg4, layer 2 usually used with mpeg 1, and so on), but any of the
standard mpeg audio types may be mixed with the standard video types.
for the third part, technically, mp3 is "layer 3" but depending on
which video codec it is used for, it can indeed stand for "mpeg 4,
layer 3" instead of "mpeg 1, layer 3". It was originally associated
with mpeg 1, even though it is not the dominant audio format for that
video type. Heh, because of all this confusion, it is known as "mp3
audio" almost always.
The fourth part is a little trickier and to answer it, I'll take
another look at dvdrhelp.com. Their glossary faq entry for DVD
(http://www.dvdrhelp.com/glossary#DVD) leads me to a page entitled
"DVD Frequently Asked Questions (and Answers)"
(http://www.dvddemystified.com/dvdfaq.html), which explains this in
more detail.
In the question entitled "Do all videos use Dolby Digital (AC-3)? Do
they all have 5.1 channels?"
(http://www.dvddemystified.com/dvdfaq.html#1.23), it states "Most
DVD-Video discs contain Dolby Digital soundtracks. However, it's not
required...A Dolby Digital soundtrack can be mono, dual mono, stereo,
Dolby Surround stereo, etc. For example, Blazing Saddles and
Caddyshack are mono movies, so the Dolby Digital soundtrack on these
DVDs has only one channel."
So, in short, DVD's typically use AC3 dolby digital audio, which can
be one of several configurations of channels, but it does not have to
be this way. It can use PCM (your basic audio cd format), mpeg audio,
or even a format called DTS.
For the fifth part, the fact that Divx is no longer closly tied to
mpeg 4 actually makes it more likely to survive, since the original
version of divx was encoded with a video format that it was not
licensed to use. Because the new version of divx is an entirely new
compression algorithm, anything that occurs with mpeg 4 (apart from
mpeg 4 becoming a much better video format than divx) will be fairly
irrelevant to the continuance of Divx. As far as mpeg 4 being a
standard, it already is, though I am uncertain as to wether or not the
standard is finalized. Like the other mpeg standards, it does contain
some flexibility, so undoubtedly there will be a number of video
formats that take advantage of the mpeg 4 standard, similarly to how
there are at least two formats (DVD and SVCD) that take advantage of
mpeg 2.
I hope this sheds some light on your questions and you find the answer
satisfactory
-funykwizard
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