Using FFMPEG to Process downloaded USENET Movies

Common Display Resolutions Chart

Quality curves based on bit rate for the different HD Classes

Constant Bitrate (CBR) Vs. Variable Bitrate (VBR)

Exactly as it sounds, a constant bitrate spits out data at the same rate all throughout the video, and in streaming this means that viewers will receive data at the same rate. The problem with constant bitrates is that, unless the stream is just a static image, video segments will change. Some segments will have more detail and some have less detail. This translates to lower quality during complex segments and unused bandwidth on simple segments. Some situations will still use constant bitrates, but these instances are rare.

Variable Bitrate (VBR)

Variable bitrates are the data transfer process of choice for the majority of circumstances, because encoders will adapt to the detail demand of the segment. It essentially allocates more data during complex segments, and less data during simple segments. For instance, if your stream has a segment of you standing in front of a plain white background, the encoder will determine that the video needs less data and transfer less. If you streamed at a concert, the encoder will see varying details and transfer more data when needed.

If size of the final output is less important than its quality, use variable bitrate.

Be aware that the numbers and parameters for setting VBR are not standard and vary from codec to codec. Read the documentation for the CODEC you plan to use.

The best VBR encoders as of this writing are the x264 implementation (for 8-bit video), and the x265 implementation (for 10-bit video), both of which are included in FFmpeg.

Audio Desync

Some older DVDs, especially from the 90s, don't honor the correct signaling info when you convert to MP4. Blurays don't have this problem. Audio desync can also happen when you're trying to remove unwanted audio streams using the -map  parameter. When in Handbrake you check the "Align A/V Start" checkbox, Handbrake will set certain FFMPEG parameters to force the physical alignment of the audio & video streams. How Handbrake does this is still under investigation. Unfortunately as of this writing, even with the use of Handbrake some stubborn DVDs still cannot be reencoded without audio desync; the audio and video streams for these have to be copied to a new container. VLC using the File | Convert Stream... option often works.

Decrypting Downloaded DVD Images

If what you've downloaded off the USENET is not a video file but a copy protected VIDEO_TS type DVD folder, you can use makemkv to convert the folder into an MKV file.

Video File Concatenation Explained

FFmpeg has three concatenation methods.

1. concat video filter

$ ffmpeg -i opening.mkv -i episode.mkv -i ending.mkv \   
         -filter_complex "[0:v] [0:a] [1:v] [1:a] [2:v] [2:a] concat=n=3:v=1:a=1 [v] [a]" \   
         -map "[v]" -map "[a]" output.mkv

Note that this method performs a re-encode. Recommended when your input files have differing properties.

2. concat demuxer

$ for i in *.mp4; do printf "file '$i'\n" >>a.txt; done && ffmpeg -f concat -i a.txt -c copy out.mp4 && rm 'a.txt'

This is a one-line, very fast way to concatenate multiple files without re-encoding.

3. concat protocol

$ ffmpeg -i "concat:input1|input2" -codec copy output

This method does not work for many formats, including MP4, due to the nature of these formats and the simplistic concatenation performed by this method.

Which one to use

• concat filter: Use if your inputs do not have the same parameters (width, height, etc), or are not the same formats/codecs, or if you want to perform any filtering. (You could re-encode just the inputs that don't match so they share the same codec and other parameters, then use the concat demuxer to avoid re-encoding the other inputs).
• concat demuxer: Use when you want to avoid a re-encode and your format does not support file level concatenation (most files used by general users do not support file level concatenation).
• concat protocol: Use with formats that support file level concatenation (MPEG-1, MPEG-2 PS, DV). Do not use with MP4.

If in doubt try the concat demuxer.

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