Zum Ende der Metadaten springen
Zum Anfang der Metadaten

 

 

If a video is to be uploaded to OpenOLAT, it is crucial to not only consider video format, but also file size (due to network traffic upon download), as network traffic generated by videos is generally quite high. A calculation example can be found here.


File size

The file size of a video depends on a number of parameters. 

Resolution
 Number of pixels on the x and y...
Number of pixels on the x and y axis. Common resolutions are e.g. 480x320, 800x600, 1024x567, 1280x720, 1920x1080. Starting with the resolution of 1280x720, these are considered HD resolutions.
The higher the resolution, the more memory is required.
Frame rate (frames per second) 
 A frame rate of more than 16 pictures per...

A frame rate of more than 16 pictures per second is perceived as a movie, depending greatly though on the intensity of movement. Cinema movies traditionally use a 24 frames per second. The european PAL television format uses 25 frames per second, while the american NTSC uses 30 frames per second.

The framerate can be set either as variable or constant. For a movie made from powerpoint slides for example, it is advisable to use a variable framerate, as some frames may be visible for several seconds without any changes to it.

The more frames (images) per second, the more memory needed. 

Interlacing 
 Video frequency can be doubled with...

Video frequency can be doubled with a simple trick without doubling the memory requirements. Only every second line of the image is alternately transferred and replaced on the screen. 

This results in a slightly less sharp image because the lines in the images do not match every other frame. Due to movement in the movie this is negligible in most cases though.

If used at a constant frame rate, Interlacing reduces the amount of memory required.

 Codec 
 A codec is a compression algorithm...

A codec is a compression algorithm used to compress video data to the least possible size. As a rule, this compression is lossy, otherwise the amount of data would be much too large. Codecs contain a variety of optimization parameters. Depending on the settings, the duration of the compression varies. Improvements with regard to image quality, file size and compression time can be made.

The more compressed, the less memory is required.

Various devices / browsers have different screen resolutions and support different codecs. A transcoding uses special software to produce copies of a high-quality video or audio source. These copies will be compressed with different resolutions, bit rates and codecs.

Transcoding is the conversion of an audio or video file to a different audio format or video format.
 The following versions of a video are sensible, depending on intended utilisation:
  • mp4 / h264 high resolution and high bitrate: Desktop application
  • mp4 / h264 medium resolution and medium bitrate: tablet or desktop with poor internet connection
  • mp4 / h264 small resolution and low bitrate for mobile devices
  • WebM / VP9 medium resolution and medium bitrate for devices not supporting mp4 / h264
  • Ogg / Theora medium resolution and medium bitrate for devices not supporting mp4 / h264

WebM and Ogg are no longer absolutely necessary as Google continues to support mp4 and Firefox now does too. WebM is a container used by Google, and Ogg a container used by Firefox.

For using videos within OpenOLAT, it would be best to choose a small to medium resolution with a low to medium bitrate

Bitrate (kbps) 
 The bitrate defines the amount of...

The bitrate defines the amount of bits created per second of video material. Bitrate can be set either as constant or variable. A maximum will be defined in the case of a variable bitrate. The bitrate defines the file size of a movie and affects picture quality significantly. In order to maintain a consistent picture quality with an increased resolution, the bitrate needs to be increased at the same rate.

The higher the bitrate at a specific resolution, the better the movie quality, but the file size of the movie increases accordingly.


Aspects such as colour space or anamorphic also affect the file size of a movie.


The following table shows the average bitrate as well as expected bitrates for other resoltions with conventional settings. Bitrates vary heavily depending on the type of video, resolution, codec and compression. These are merely benchmarks, and were calculated based on a MP4 file encoded with H.264.

Bitrate /  Resolution

320x240

480x2701024x5761280x7201920x1080
kbps4007001'5002'5004'000
mbps0.40.71.52.54
MB/S0.10.10.20.30.5
MB/Min35.311.318.830

A movie with an average duration of 10 minutes and a 480x270 resolution therefore approximately requires about 53 MB memory. The same movie with a resolution of 720p HD though requires about 180 MB. 


Calculation example: Network traffic

If 50 users watch this 10 minute movie, this will create network traffic of 2.7 GB with a 480x270 resolution. If the movie is watched with a 720p HD resolution, expect 9 GB network traffic.

If those 50 users watch the movie at the same time, the network needs to be able to handle a bitrate of 35 Mb/s (4.4 MB/s). If the movie is watched simultaneously with the 720p HD resolution, expect a network load of about 125 Mb/s (15.6 MB/s)

100 concurrent views doubles the aforementioned numbers accordingly.

For comparison: a common 802.11n wireless access point allows for an effective transfer of about 40-50Mb/s. Special hardware makes a capacity of up to 100 Mb/s feasible, but user equipment also needs to be able to handle this kind of output. 50 users watching an educational video in the same room can therefore prove to be a problem for a network.

HandBrake Settings

If you have to create or provide videos for OpenOLAT yourself, a special transcoding software can help you transform files into the correct file format or the desired resolution. We recommend  HandBrake, a free and open source software. More information on HandBrake can be found on the wikipedia HandBrake page as well as their commercial site https://handbrake.fr/.   

1. Open HandBrake, and select the file to be converted. Starting formats are e.g. M4V or MOV files.

2. Select the "Normal" profile, and a check the "Web optimized" checkbox.

It would be best to leave the preset parameters untouched.

3. Select the desired size / resolution by opening the "Picture Settings".


  • Small: 320 und 480
  • Medium: 1024
  • Large (HD): 1280 and 1920Y
  • Keine Stichwörter