Editing and converting videos with Avidemux

Repackaged

Among Avidemux's specialties are its recompressing and conversion into other file formats. The program reads and writes the popular formats AVI, MPEG, OGM, and MKV (Matroska), as well as Flash and Windows ASF videos. It generates PS files for DVDs and TS streaming files and MP4 files for mobile devices, and it reads analog Nuppelvideo format files from the MythTV video recorder software and numbered sequences of single-image frames.

Avidemux also supports all codecs from FFmpeg (MPEG-1/2, HuffYUV, FFV1, LavC MPEG4). Unfortunately, it doesn't use the pre-installed FFmpeg version, so neither license-determined editing nor updates can be applied. It also integrates the external libraries x264 and Xvid, which provide full functionality. Audio codecs provided include Vorbis, FAAC, MP2, AC3, MP3, and uncompressed Wave audio.

Video files tend to be so large that the benefit of compressing plays a role even with the current capacity of hard drives. Thus, DVDs still coded with the MPEG-2 process dating from the 1990s get reduced to half their original size with roughly the same picture quality after recompressing with H264. The matching dialog is hidden in Avidemux under the video track option MPEG-4 AVC . You can select this option instead of the usual Copy option active at startup. Clicking Configure under the codec selections opens the settings with quite a daunting set of tabs at first glance.

Fortunately, the General tab encompasses the few really important settings. All the others require some expertise in compression algorithms. You can save the settings in a profile that the software displays at the top of the dialog.

Double-Stitched

At first, you need to decide on an Encoding Mode . Two methods are available for adjusting the balance between quality and file size (Figure 5). The first method works with the bit rate per second. The size of the compressed file is determined simply by the bit rate times the duration. However, the audio track is missing from this calculation. Moreover, the bit rate is only an approximation; the codec can overrate or underrate it on the basis of the level of detail or noise in the material.

Figure 5: Avidemux is ideal for compressing DVD video material with the H264 codec, which saves about 50 percent of the space normally used with comparable optics.

Alternatively, you can control the quality when compressing using so-called quantization, an abstract measure of compression strength. It relates less to size than to the constancy of the optical quality. Factors of 25 are the norm for today's standards.

A value of less than 20 causes the file size to increase dramatically, but you won't get a result with a better quality. Factors more than 30 usually result in block artifacts. With factors more than 40, the quality degrades to the point that you can't distinguish between human beings.

Apart from bitrate and quanticized compression, the coding mode also differs after the number of passes. Processes with two passes take almost twice as long but provide a significantly better ratio of file size to quality. For two passes, the H.264 codec offers the choice of bitrate-based compression or direct input of the desired file size.

However, this has no effect on the audio track and is, therefore, not included in the calculation. Some help comes from the calculator in the Tools menu (or press F7) that calculates the size of the audio track and subtracts it from the desired file size.