Subject:
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Re: Some questions about MPEG 1 standards (long)
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Newsgroups:
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lugnet.cad
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Date:
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Wed, 14 Nov 2001 20:37:18 GMT
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Viewed:
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375 times
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In lugnet.cad, Eduardo Vazquez Harte writes:
> Well for now I decided to continue with this great format since is
> compatible with a lot of platforms for "Atari ST to Sega Dreamcast"
> including PC's "unix to mac".
> So I just found a way to get a lot of people to view my anymations without
> problems of compatiblity isues or speed isues but what I whould like to know • is
> what type of quality are these bit rate settings?
>
> Data rate:
> ----------
>
> Profile nmae : Byte rate : Bit rate :
> --------------:------------:-----------:
> MPEG-1 : 140 kbytes : 1125 kbits:
> --------------:------------:-----------:
> 28Kbits modem : 3 kbytes : 28 kbits:
> --------------:------------:-----------:
> 300Kbytes CDx2: 300 kbytes : 2400 kbits:
> ----------------------------------------
> 600Kbytes CDx4: 600 kbytes : 4800 kbits:
> ----------------------------------------
>
> now I need to have an idea of quality so tell me what bit rate is near vhs, • dvd.
>
> I hope some understands what I'm asking.
Eduardo,
Unfortunately, it is not as easy as that. Bit rate is a poor measure of quality
because it doesn't actually determine quality all by itself. I worked in a
educational laboratory where we studied video over the web issues and I learned
a lot about quality and standards.
One thing you should know is that the higher the bit rate, the better the
quality, but this only works in general. I will try to provide an example. Bit
mapped files or BMP allocates one byte per pixel of each image. In order to
make a large image you need the same number of bytes as there are pixels. This
compression is poor because every pixel on the screen is represented by some
data. Several image compression schemes have been created that eliminate the
need to represent every pixel with a byte because some of the bytes are repeats
of the pixels around them and so can be eliminated by indicating how many
repeats there are rather than representing each pixel. The most common
compression schemes are now as Graphic Interchange Format (GIF) and Joint
Potographic Experts Group (JPG). (But you probably already know this, so why am
I telling you this?) I turns out that these compression schemes have been the
foundation for video data.
Both GIF and JPG throw out information in an effort to save room and lower the
file size. Video is handled much the same way. In some compression processes
the entire frame is not coded, only the parts of the frame that move or change
from one frame to the next (a reason transitions add a lot to the file size of
a movie). Before I get to deep into compression schemes though you need to
understand the two factors that effect video quality.
In video the most important things to consider is the resolution and
compression. Resoulution is the size of the video picture, i.e. 320x240,
640x480 and 720x480. There are other resolutions but these are the most popular
with the last one being mini-DV resolution. Typically the higher the resolution
the larger the file size and the larger the bit rate will have to be inorder to
transmit that much data without dropping frames. A dropped frame is one that
was not delivered in time for the stream to play it, i.e. it arrived after its
cue to be on stage.
Compression is the next most important consideration to make. By compression I
refer to the CODEC, the compression and decompression standards that enable the
video file to be made smaller for transmission but then be able to represent
the data in a way that doesn't suffer in the decompression.
Currently, there are many options: (1) MPEG, (2) Quicktime, (3) Real, (4)
Microsoft Media Player. There are others but I will limit the discussion to
these. In my humble opinion, MPEG is the best over all three becuase of its
ability to be used in multiple environments, web and TV mediums. It is a little
large for file delivery for anything longer than 5 minutes, but one of the MPEG
Levels, #2, is the standard that DVD is based upon. But for a project that
needs both Web and or CD-Rom based implementation with TV media options (such
as VHS, DVD, or Satellite) then MPEG is clearly the best choice.
As for the other CODECs, Apple is clearly the winner. Real is great if your not
interested in the visual aspects of the presentation. Select Audio as a
priority and who cares if the talking head arrives. If your trying to see the
visual content though then you really need Quicktime. As evidence of what I am
saying, what are movie trailers typically delivered in? Quicktime!(A quick note
about Quicktime. To do it right you need to send everything through Media
Cleaner Pro. Failure to do so leaves your files way to large. I am not exactly
certain how Media Cleaner improves on the compression of the Quicktime CODEC,
but anyone that has made video in both formats, .mov without MCP and .mov with
MCP, knows what I am talking about.)
If your interested in Media Player from Microsoft, go for it. It really doesn't
do anything better than the others, but supposedly more people have the player
than Real or Quicktime. I guess that's becuase they ship Media Player with the
operating system which uses it for everything from MP3 playing to video. Media
Player also has the least trouble continuing to run a stream. I am not certain
why this is but Real seems to freak out when things are not right. Media Player
just keeps standing by until content arrives. The only times Media Player has
failed for me happened when the operating system froze. As for Quicktime, the
only problem I have ever had is when it needed a different version of the
player than the one I had.
A side note about MPEG. The new MPEG-4 is out and has been for some time. In
the next year or so you will begin seeing products, software and hardware, that
work with MPEG-4. This compression is better than MPEG-2 and will allow larger
files over smaller bandwidth. It will allow video to be a practical option of
content delivery over the Internet. It will be really key for handheld video on
small palm pilots and things.
As for video bit rate, these compression techniques provide different bit rates
based upon the resolution chosen and the type of video. The standard in
Intranets (note the "a") is 500kbps or 500 kilobits per second (yes bites,
transmission rates are measure in bits not bytes, and yes that means your
getting 8 times less than you thought). Typically the in house ethernet connect
is 100 megabites per second (if your company isn't dust off the resume). A 0.5
Mb/sec video load on the network is typically not going to crash the system (if
it does think switches not hubs) and that video quality is MPEG-1 or MPEG-2.
Compare that to the transmission speed of a 26 Kbps (yes kilobits) modem and
you begin to undestand the difficulties of good video over a modem. I have
worked with some VBrick products that do multicast IP video and its amazing. IP
selectable video, multicast by a server, sending numerous Video selections all
over a 100Mb Intranet Network at less than 500 Kbps speeds and send full
screen, full 30 frame per second video. It uses Media Player which introduces
some artifacting seen on larger monitors with larger resolutions.
I connect from home using a DSL modem and my speeds exceed 1.5 Mb/sec and I can
usually view video with very little download time or streaming difficulties.
Friends of mine are not so lucky with their cable modems speeds that are
advertized at 1 mbps. The reason is simple, cable modems are hubs and DSL
modems are switches. If you don't know the difference it can be boiled down to
this, hubs split the bandwidth coming in among the users. A 100 MB hub with 10
connections provides 10 MB per connection. If you should need 11MB tough luck.
A switch provides the available transmission speeds to the users on demand. If
there are 10 connections on a switch not everyone needs all 10 MB of their
connection at anyone time, so you can get 50MB when no one else is using it.
The cable modem is a shared connection amongst you neighbors. While the TV
transmission is on a different part of the RF spectrum from the Data
transmission you still share the capacity with others. As more people get
online in your neighborhood and begin to exploit their "High Bandwidth"
Connection, your connection slows down. A friend with a performance metter has
found speeds after dinner can drop below 56 Kbps modem speeds.
As for DSL, the connection is a little more independent. I do share the
connection at the level of the phone switch in the routing box in my
neighborhood, but from the box to my house it is all me in the data. Besides
the increased security, I enjoy nearly 1.5 MB sustained data rates as can be
seen by my performance measures.
I haven't tried to access a 500kbps stream of video content yet, but this is in
the not too distant future for testing purposes.
So to conclude a lengthy response. Bit rates are important becuase different
CODECS can support different bit rates as a streaming video. Unfortunately, the
key factors to decide are resolution and compression. These issues are usually
determined by the intended audience and means of delivery. If your have to
deliver over the Internet, then you need small. If your deliverying in house o
n a WAN, LAN or VPN then you can bump the size and quality up just make certain
your IT and network people have no blocks on file size (past experience).
I would love to offer a dicotomous test for you to delineate the "organism" of
delivery, but it would be lengthly and ultimately incomplete. The best advice I
can give is set a standard and then adhere to it after extensive testing proves
the standard is adequate for your needs.
Hope tis helps!
Todd
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 | | Some questions about MPEG 1 standards
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| Well for now I decided to continue with this great format since is compatible with a lot of platforms for "Atari ST to Sega Dreamcast" including PC's "unix to mac". So I just found a way to get a lot of people to view my anymations without problems (...) (23 years ago, 14-Nov-01, to lugnet.cad)
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