codecs and filters

[puneetarora]

Hello,

I was wondering what exactly is the difference between a codec and a directshow filter.

As I understand it, a codec is something that codes and decodes audio/video information to and from a compressed / coded version and directshow filter is something that renders decoded information on the screen. If thats indeed the case, I guess there can be one layer of directshow over the codec ie. different codecs for different formats but a single layer of software to render the decoded information. Why is that I come across different filters like Xvid directshow filter etc. hows it different from the xvid codec ?

am I wrong in what I think. I'd be grateful if someone can enlighten me ?

Also, some codes have .acm extension while some have .ax while others may also have .dll. Can someone tell me what exactly is the difference ?

thank you,

regards,
-puneet.

[puneetarora]

oops, I'm sorry for posting this here. You may delete it. Sorry for the trouble.

[redwudz]

Moving to Newbie/General.

And puneetarora, please don't double post. I've locked the duplicate topic. If you want a topic moved, PM a Mod.

[celtic_druid]

Directshow filters aren't always just decoders. The codecs you are talking about (Xvid codec) are VfW (Video for Windows). Also VfW doesn't always mean decoding and encoding. It is just an interface.

I'd suggest that you do a search (say at wikipedia) and read up on VfW, dshow, ACM, etc. if you are interested.

[edDV]

"codec" = encoder + decoder
This implies format conversion.

"filter" is a more general term and can include a codec. It basically means a transfer function, or something that alters video.

"DirectShow filters" are part of DirectX and OS specific to Windows. The main idea behind DirectX is that it can serve as a software layer between applications and graphics/video/audio hardware so that applications can call on DirectX for a function and DirectX can direct that function to hardware or third party software (if supported by drivers) or can implement the function call in software using one of its default software modules.

This standardizes application interface so that application writers don't have to write hardware drivers themselves as was the case in the good old days.

[puneetarora]

Thanks for the replies. I had read wikipedia, and tht's where I got my initial idea from. I read bout acm, but didn't find anything about .ax files. Some sound codec files I have end in .ax but most in .acm. (I see that in Audio Codecs in my device manager)

Also, what would u say is the difference between "Xvid Codec" and "Xvid directshow filter". Directshow is a layer between applications and the hardware .. that's exactly what I understand too. But that means it should be a common layer over all applications. Why do we have something called "xvid directshow filter" then ?

[celtic_druid]

xvid directshow filter is Xvid's directshow wrapper filter for decoding. Most windows players are directshow based.

Xvid Codec is Xvid's VfW wrapper/frontend for encoding/decoding via VfW as used by VirtualDub, etc.

All Xvid's actual work is done in xvidcore.dll.

http://en.wikipedia.org/wiki/Directshow
http://en.wikipedia.org/wiki/Video_for_Windows
http://en.wikipedia.org/wiki/Audio_Compression_Manager