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# lecrec
# Lecrec: A Very Simple Lecture Recorder Based on FFmpeg
**Caveat:**
some of the information given below is work in progress
and has not been completely tested in practice yet.
The program `record` records one audio stream and one or more video streams.
Each video stream is combined with the audio and copied into a file,
called a *primary file*.
Two additional files are created, the *secondary files*:
one that has all the video streams in it as separate video tracks,
and one that has all the video streams in it in one video track
arranged in a horizontal stack.
Typically, there will be one or two video streams.
In case of two video streams,
one typically will be a recorded video of a lecturer writing on a blackboard or whiteboard,
and the other will be a computer-based presentation (slides or hacking demonstrations).
Suggestions on how to get the streams into the computer:
* Audio: use a USB soundcard with XLR input and a microphone with XLR output.
Example: [Focusrite Scarlett 2i2](https://focusrite.com/de/audio-interface/scarlett/scarlett-2i2)
* Video of lecturer: a good USB camera.
Example: [Marshall CV610-U3-V2](http://www.marshall-usa.com/cameras/CV610-U3-V2/)
* Video from computer: an HDMI splitter combined with an HDMI grabber.
There are a lot of cheap devices out there that are supposed to do these tasks,
but many of them have mixed reviews.
I suggest to use more expensive hardware,
for example: [Blackmagic Atem Mini](https://www.blackmagicdesign.com/de/products/atemmini)
## Usage
* `-m`: run `mpv` on each primary file during recording to act as a monitor.
The monitor process is started once the primary file has a size of more than 100000 byte.
It seems that `mpv` misbehaves when started on a file that is too short:
`mpv` will not show anything but also not exit with an error,
so this condition is difficult to detect programmatically.
The minimum number of bytes given above seems to work and was determined by experiment.
* `-p`: create the secondary files while recording;
otherwise they will be created after recording based on the primary files.
If your computer can handle it, then use `-p`.
Otherwise you will have to wait a long time after recording.
* `-P`: FFmpeg preset to use for the secondary files, defaults to `veryfast`.
* `-r`: video resolution to use for all video streams, defaults to `1280x720`.
* `-s`: suffix for all output files, defaults to `mkv`.
This determines the output format.
* `-a`: name of audio device to capture from, as per `devices.yaml` (see below).
* `-v`: name of video device to capture from, as per `devices.yaml` (see below).
Can be given multiple times to add multiple video sources.
Devices are configured via a file `devices.yaml` in the current directory.
For each device, give the following:
* `name`: the device is known by this name on the command line.
* `system`: how the device is known to your operating system.
For audio, use the name given by `pacmd list-sources`.
For video, use the basename of the corresponding file in `/dev/video/by-id`.
* `offset`: temporal offset in the sense of FFmpeg's `-itsoffset`.
## License
Public Domain
Written in September and October 2020 by Lasse Kliemann <<lasse@lassekliemann.de>>.
To the extent possible under law,
the author(s) have dedicated all copyright and related and neighboring rights
to this software to the public domain worldwide.
This software is distributed without any warranty.
You should have received a copy of the CC0 Public Domain Dedication along with this software.
If not, see http://creativecommons.org/publicdomain/zero/1.0/.
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