Transcode_Wiki: Command Examples/Pass-Through Modes

Introduction

This section is devoted to the pass-through modes available, which maybe useful for some, not so common situations as outlined below.

Pass-through mode is activated using the -P option of transcode with a parameter, which can get the one of the values

0 = Off (default)
1 = Video
2 = Audio
3 = Audio+Video

Creating Video Clips

Suppose you have mastered a couple of DV clips, concatenated to a well defined playtime clip1.avi that accidently correspond to the length of your favorite music clip clip2.mp3 in MP3 format. We want to keep the quality of the video and simply replace the PCM audio track. This is done by the following command

 transcode -i clip.avi -p clip.mp3 \
           -P 1 \
           -o new_clip.avi -y raw

NOTE: The MP3 stream must be identified by tcprobe. If this fails, we need to supply the import module options -x dv,mp3 to make sure, the audio is decoded properly. The -y raw export module simply writes the video chunks as is together with the re-encoded audio.

Audio Recompression

The following situation is not uncommon. Suppose your single AVI-file movie128.avi is about 2x700MB+15MB, i.e., too large to fit on 2 CD's. Fortunately, the audio is MP3 with a bitrate of 128kbps, which is the default. We recompress the audio to 96kbps to reduce the filesize below 2x700MB. Let's also double the volume of the sound, if possible, or use the recommended value given by tcscan.

 transcode -i movie128.avi \
           -P 1 -b 96 -s 2.0 \
           -o movie96.avi -y raw

This is reasonable fast since video is only passed through and the audio quality is still ok. After you are done, use avisplit to split the file into 2 chunks.

Fixing Truncated AVI Files

The AVI file header is updated (written to disk), whenever video/audio parameter are set. If transcode or hardware crashes, the truncated file is in most cases playable for advanced players. Repairing the AVI-file index is also possible via pass-through option -P3.

 transcode -i crashed.avi -o new.avi -P3 -u X

Option -u X with X>>10 enhances pass-through speed.