> On Fri, 25 May 2007 04:16:01 -0700, Dirk
> <Dirk[ at ]discussions.microsoft.com> wrote:
>
> >I have written an application with the Windows Media Encoder SDK to
> >transcode mxf-files to wmv. The format of the mxf-videostream is D10 Video
> >IMX,
> >that is an interlaced mpeg2 with I-frame-only at 50 MBit/s. For mpeg2
> >decoding I use Main Concept DShow filter from OpenCube.
> >I'll have to encode the WMV with a bitrate of 600kBit/s.
>
> So you're going for nearly a 100:1 compression - however MPEG2 is
> already compressed, albeit lightly I assume at 50MBps
>
> It's perfectly reasonable to expect 100:1 for uncompressed content
> when using well matched settings for WMV9, VC-1 or H264. However you
> may be pushing the limits with precompressed material, as the raw data
> rate could be at least 100MBps.
>
> Approximately what dimensions and framerate is the source video ?
>
> >The encoded content has a good quality for sourcefiles that have a low
> >bitrate, i.e. 25 MBit/s or 15 MBit/s.
>
> If all things are equal when capturing the lower bitrate streams, it
> implies something about the level of compressibility of the content -
> if you're saying these files are using unconstrained encodigin
> bitrates, and the 3 sample files happened to end up at those bitrates
> (i.e. usuing solely a quality based encode process)
>
> For example you might expect high motion sports footage to require
> more bitrate to maintain quality, compared to nature footage with
> lower changes between frames.
>
> >But surprisingly the results for mxf-files with higher bitrates (about
> >50MBit/s) are worse quality with visible artifacts.
>
>
> Transcoding won't really change that scenario, so you may have reached
> the limits of what WM compression can be achieved with the source
> content and a fixed bitrate encode.
>
> Is there any improvement if you go to VBR and allow the encoder to
> "mop up" spare bitrate for the high motion parts of the content ?
>
>
> >I have tested this with different profile-settings, deinterlacing and
> >different encoding-codecs. I tried the "Windows Media Video 9 Advanced
> >Profile" (VC-1 codec) with different registry settings that may improve
> >quality, without any usable results. I also tried different settings for the
> >mpeg2-decoder, so as a completely other decoder. The results were still bad
> >in quality.
>
> We don't really know what settings you're using as yet ;-)
> Talking specifics would be a great help there !
>
> >My question to you: what can I do to get a better quality of my wmv-encoded
> >content, without raising the target-bitrate? Does it depend on my hardware
> >(CPU) in any way?
>
> No. For file based encodes, the encoder will take as long as it takes
> to encode to the specified settings, rather than device based content
> which can be CPU constrained and may drop frames or compromise the
> encode to reach the output goal of realtime encoding.
>
>
> HTH
> Cheers - Neil
> ------------------------------------------------
> Digital Media MVP : 2004-2007
>
http://mvp.support.microsoft.com/mvpfaqs>