Transcode

Procedura utilizzata per correggere un file DivX danneggiato. Avidemux non era in grado di aprire il file.

Per cominciare con mplayer (che pare essere più bravo a decodificare file danneggiati) si è convertito in yuv4mpeg (vedere l'opzione -vo yuv4mpeg).

Con transcode si converte un file dal formato yuv4mpeg al formato XviD. Prima bisogna eseguire la prima passata (-R 1) durante la quale viene salvato il file divx4.log:

transcode -i stream.yuv -R 1 -y xvid -o /dev/null

Poi si esegue la seconda passata a bitrate variabile (-R 2), video encoder bitrate 6000 and keyframes every 250 frames (-w), formato audio mp3 (-N 0x55) encoder bitrate kBits/s (-b 96). Contestualmente si effettua il deinterlacing con il filtro smartyuv:

transcode -i stream.yuv \
    -J smartyuv=highq=1:diffmode=2:cubic=1:Blend=1:chromathres=4:threshold=8:doChroma=1 \
    -R 2 -w 6000,250 -y xvid \
    -V \
    -N 0x55 -b 96 \
    -o output.avi

Mplayer supports a very large set of video formats, including several closed codecs from the Windows platform. Using the youtube-dl script, we downloaded some clips from YouTube, which are in the Flash Video format.

The first, very simple recipe uses mencoder(1) to convert a Macromedia Flash Video to XviD/MP3 AVI in a single pass, with fixed bitrate (360 kbit/s in this example):

mencoder file.flv -oac copy -ovc xvid -xvidencopts bitrate=360 -of avi -o file.avi

The output audio codec is copy because flv should be already in mp3 format, otherwise we can specify something like -oac mp3lame -lameopts br=96:cbr.

An insanely complicated recipe will use mplayer to extract the video and audio streams, then use transcode to convert the video to XviD (two pass, 400 kbit):

FILE=youtube_clip
mplayer -ao pcm:file=$FILE.wav -vo yuv4mpeg:file=$FILE.yuv $FILE.flv
transcode -i $FILE.yuv -R 1 -w 400 -y xvid -o /dev/null
transcode -i $FILE.yuv -R 2 -w 400 -y xvid -o $FILE.xvid
rm $FILE.yuv divx4.log

Finally we used avidemux to convert audio to mp3 and to mux audio and video into an avi file. How to use transcode for this final step?

Un video .flv con audio in formato nellymoser si ascolta correttamente con mpalyer, ma l'audio estratto con mplayer -dumpaudio oppure con avidemux non viene riconosciuto ed è impossibile ascoltarlo. Per fortuna ffmpeg è in grado di effettuare l'estrazione e la conversione automaticamente:

ffmpeg -i video.flv output_audio.wav

See this paragraph: Ripping DVDs with mencoder - Subtitles.

This is a tool that allow to adjust audio/video syncronization witout re-encode audio and video streams, you must specify time shift in frames. If count is positive, audio starts with audio frame count at the beginning of the AVI-file. If count is negative, audio is prepended count padding frames.

avisync -i input.avi -o output.avi -n -10

AC3 audio (aka Dolby Digital) is a lossy compression algorithm, so converting from and to this format always introduces some more loss. Neverthless I had to edit a DVD audio track with Audacity, so I need to convert it from AC3 into something that Audacity can open. The ac3 audio track was extracted with mplayer and the -dumpaudio option.

transcode -i title1.ac3 -y null,wav -o title1.wav