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Codecs & players (beginner questions)

Last response: in Graphics & Displays
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Anonymous
March 13, 2005 5:44:42 AM

Archived from groups: rec.video.desktop (More info?)

Hi

I have been reading lots of places trying to grasp the mysteries of
digital video. Not easy!

So here are my beginner's questions :

I have a video file ( .mpg)
AVIcodec says that
-the fourcc for the video codec is MPG1
-the id for the audio codec is 0x51

1)
AVIcodec says that the both codecs are 'supported', I suppose it means
that the codecs are present on my system.

However in the Codecs pane, they are not in the list.
For video, I have cvid, divx, iv31, iv32, etc.... no MPG1 !
For audio I have 0x0001, 0x0002, ... no 0x0051 !

The clip plays fine though... how is that possible?
Is there a secret mapping that uses one codec for another?

2)
I used VirtualDub to reencode the clip with Cinepak (cvid).
When I play the file in DivX Player, the file plays upside down and
has lots of compression artifacts (I think that is the term for those
ugly random squares).

When I play the file in Core Media Player, BSPlayer, WinAmp, it plays
perfectly.

How is that possible? I thought these "players" merely read the file
and handed the bytes over to the codec. So the same codec should yield
the same rendering.

Apparently, that is not quite the way it works?

Can anybody enlighten me?

Thanks
Fred
Anonymous
March 13, 2005 1:11:53 PM

Archived from groups: rec.video.desktop (More info?)

On Sun, 13 Mar 2005 02:44:42 +0100, Fred Paris <nono@nono.invalid>
wrote:

>I have a video file ( .mpg)
>AVIcodec says that
>-the fourcc for the video codec is MPG1
>-the id for the audio codec is 0x51
>
>1)
>AVIcodec says that the both codecs are 'supported', I suppose it means
>that the codecs are present on my system.
>
>However in the Codecs pane, they are not in the list.
>For video, I have cvid, divx, iv31, iv32, etc.... no MPG1 !

What you see in that Codecs pane, are meant for AVI files.
Theoretically, there could be an mpeg-1 codec, and you should be able
to have an .avi encoded as mpeg-1. In practice, there isn't an mpeg-1
codec, and mpeg-1 encoding is reserved for mpeg (.mpg) files. Mpeg
files also have encoded audio and video, and are similar or equivalent
to Avi files, but are actually a different file format. As for mpeg-2,
it has additional features that would not fit well into an .avi, but
other than that, it could also be encapsulated within an Avi. In
practice, there is no mpeg-2 codec, either.

There is a codec, Ympeg, which encodes as mpeg, and if you install it,
you shall find it within the list of available codecs. But when you
use it, the generated .avi is a dummy one, and it generates an
additional .mpg file, which is the good one.

(BTW: Why nobody has ever done a true mpeg-1 Vfw, valid for Avi,
codec? Because of the most astounding lack of vision in the history of
video edition, if you want to know my opinion. Something capable of
converting from .avi to .mpg, (and the other way round) without
re-encoding, would have provided the vast amount of Avi-oriented
software for the edition of mpeg material, specially if that software
were to give a better support for non-KeyframeOnly codec edition.).

>For audio I have 0x0001, 0x0002, ... no 0x0051 !
>
>The clip plays fine though... how is that possible?
>Is there a secret mapping that uses one codec for another?

I'm not sure what 0x0051 means, but it may well be "mpeg audio". Once
more, it shall not appear in the list of installed codecs.

It should appear, though, on the list of DirectShow filters. Those
filters can perform video and audio decoding (even encoding, in some
cases), so they are similar in function to Codecs. When you play an
..mpg, the player uses those filters to perform the decoding. But these
filters do not appear on the Codecs pane, and you'll need something
like RadLight Filter Manager, DirectShow Filter Manager, or DXMan
(http://www.videohelp.com/tools?s=38#38) to get the full list.

>2)
>I used VirtualDub to reencode the clip with Cinepak (cvid).
>When I play the file in DivX Player, the file plays upside down and
>has lots of compression artifacts (I think that is the term for those
>ugly random squares).
>
>When I play the file in Core Media Player, BSPlayer, WinAmp, it plays
>perfectly.
>
>How is that possible? I thought these "players" merely read the file
>and handed the bytes over to the codec. So the same codec should yield
>the same rendering.

There may be a couple of additional steps, like colorspace
conversions, which may affect the result. For instance, if you find
your DivX files look like 256 colour video, try renaming msyuvv.dll,
and you'll find they start looking allright. Now, msyuv.dll is not the
DivX codec, but is used (with horrible results) in the process of DivX
decoding. If you remove the msyuv DLL, then the whole process is done
by the DivX codec itself, or by other DLLs, which can perform the task
rightly (Why msyuv.dll behaves so bad with DivX? Well, it's a DLL from
Microsoft, and DivX is competition. Coincidence?)
Anonymous
March 13, 2005 2:21:07 PM

Archived from groups: rec.video.desktop (More info?)

Bariloche,

Thank you for you most detailed and informative reply.

How did you learn all that? Is there a book I should read?

I find the info on the net to be too fragmented (even on videohelp.com
which is one of the best sources) & I need to learn the basics. Any
recommendations?



On Sun, 13 Mar 2005 10:11:53 +0100, Bariloche
<bariloche@bariloche.com> wrote:

> [...]
!