DV decoders... differences in output?

[sphinx99]

Just want to clarify since reading some of the doom9 and other forums has led me to believe that there are problems here. I'm assuming that while DV encoder quality could vary, decode should be identical, right? If I were to decode DV using multiple decoders, should I or should I not get identical output? Are there variances? If not then no further questions But if so, then how should I (a) identify what DV codec I have installed and (b) which one should I be using?

[JohnnyMalaria]

You are right - encoder quality can vary.

For decoding, certain stages of the process are identical for all decoders. These are the stages that get the compressed data to a point that they have been decompressed but still exist in the YUV colorspace (strictly 4:1:1 for NTSC or 4:2:0 for PAL) and they are still in the "DCT domain".

Up to this point, all the decompression has been done.

From this point on is where decoder quality can vary.

The "YUV DCT" data have to undergo two major additional sets of calculations to end up with the final video frame.

Firstly, the "DCT" has to be reversed - so-call inverse discrete cosine transformation. The important thing here is that it is a complex set of mathematical calculations. The precision of the calculations will vary from decoder to decoder.

Once this is done, the data have to be converted into a rectangular image. If the software using the decoder uses YUV colorspace, this is a minor issue. If the software requires images in RGB, then the YUV values have to be converted to RGB.

This is where a number of decoders show their weaknesses. Either they use the wrong equations or introduce additional errors in precision.

To determine which decoder you are using, first of all you need to know whether your software is using a Video for Windows decoder or a DirectShow decoder.

For Video for Windows, it is relatively easy to find out which decoder since Video for Windows only allows one decoder for any given video format.

For DirectShow, it is more complicated since many DV decoders can exist and the software can either rely on Windows to provide a default one (which can also vary!) or the software can choose to use a specific one.

What software are you using?

[sphinx99]

First of all, thank you so much for the wonderful informed post. It went a long way toward clearing up many points of confusion for me. Thank you thank you!

I am using a variety of tools... sometimes I am using avisynth (avisource), other times I am using virtualdub. Occasionally I'm loading a DV file directly into Windows Media Encoder 9. A few times I've kicked files around in the demo versions of Sony Vegas and an older licensed copy of Procoder (not sure which version). Basically, I have no idea which DV decoders are installed on my PC and which ones are being used for which program...