transcoding quality lower than ffmpeg encoding.

[weeraman]

Hi,

i was testing transcoding of a MPEG PS with mpeg 4 codec with VLC and FFMPEG separately.
Though i select the encoder as FFMPEG with VLC, the quality is very poor comparing with the FFMPEG encoded clip.
also i'm very much bandwidth restricted and that's why i'm keeping 200kbps/18 fps, i simply cannot afford bandwidth.

VLC transocde code: (under windows, only the sout is shown here)

Code: Select all

:no-sout-audio :sout=#transcode{venc=ffmpeg,vcodec=mp4v,width=320,height=240,vb=200,fps=18}:display

FFMPEG encoding of the same clip

Code: Select all

ffmpeg -i music.mpg -vcodec mpeg4 -vb 200000 -r 18 -s 320x240 -an -f mp4 music.mp4

i want to use VLC for this purpose, but i cannot match the quality of ffmpeg encoding (for same encoding parameters)

any ideas how to improve the quality with mpeg4 transcoding in VLC ? Since VLC only allow few ffmpeg options, i cannot use ffmpeg solely.

( i played with the advanced ffmpeg options mentioned in the wiki, but still it couldn't match the original ffmpeg encoder it seems)

[weeraman]

no idea about this ?

Just want to know why there is a difference in quality when both VLC and FFMPEG use same encoding parameters ?

appreciate any feedback on this.

thanks.

[GEFORCEXTREME]

Were you streaming across a network?

Or were you simply doing transcoding using the sout (by streaming to a file)?

Did you have a fast enough computer for the transcoding?


IMHO, I think transcoding in VLC are done in a some what different way compared to other converters/transcoders. For example, if you rip a DVD and convert it to a MPEG-4 AVI for example, the trancoding rate varies with your computer specs but that is not a problem since we will only use the video file after it has completed converting/transcoding.

Whereares in VLC, transcoding is done while streaming, it has to be done fast enough and (in real-time maybe?) because the video will be stream to a client which will be watching the stream. So any of the factors (slow network, slow computer) might play a part to the quality of the transcoded stream.

I don't know much about VLC, but that is what I think. Correct me if I am wrong.

[weeraman]

hi,
i assume it uses 'libavcodec' provided by ffmpeg, to transcode from mpeg2 to mpeg4.

i have done the testing on the same machine, same input file, same encoding parameters (bit rate/framre rate/size) but both produced drastically different clips.
I just need to know why there is such a difference, since the core of the transcoding is the same.

[GEFORCEXTREME]

Ya, it may use the ffmpeg's libraries (libavformat & libavcodec) but the implementation is not the same.

ffmpeg main program (ffmpeg.exe in Windows or simply ffmpeg in Linux) is just a program that uses its own libraries. And in ffmpeg, you're doing offline transcoding.
In VLC, you're doing online transcoding (realtime encoding?) using ffmpeg's libraries (NOT like GUI front-end for some encoders as in Windows), so if you've a slow computer, or a poor network, the result you get varies.

Notice the difference? In offline transcoding, you wait for the whole process to finish, but in VLC's case, the transcoding part is actually part of the sout (stream-out) module, so it can't wait (if you've a slow computer, instead it will drop frames or something like that).

Again, I'm no expert in VLC, but that is what I believe what it is doing.

[GEFORCEXTREME]

Others are having the same problem, look here.

https://vlmc.org/viewtopic.php?f=4&t=74 ... 42#p248696

[Lotesdelere]

The default values used by VLC for MPEG-4 ASP and MPEG-2 are meant for streaming mainly and not for conversion transcoding. For instance by default no B-frames are used, no trellis, etc...
This is the reason why at the same given bitrate the quality is lower than ffmpeg which is using many optimisations by default. To achieve the same quality you need to edit the settings of the ffmpeg encoder in the Preferences:
Tools -> Preferences (All) -> Input / Codecs -> Video codecs -> FFmpeg

[weeraman]

Thanks for the tips you have provided.

Lotesdelere: do you have an idea of what ffmpeg parameters to be adjusted in preferences...