Overview
We use many compression techniques in combination to try and reduce the data rate of the screen images. The techniques are deliberately lossless so what you see is exactly what the user saw. All the values given in this document assume that mediaCam is operating with its default settings. By adjusting the capture rate and resolution of the video and screen capture as well as the area of screen capture the rates quoted below can be altered significantly. The figures below represent typical values.
The Audio stream
The audio data rate depends on the type of audio selected. Compressed audio has a fixed data rate of 50KB/min.
Compressed 50KB/min
8khz 1.9MB/min
22khz 2.5MB/min
44khz 5.2MB/min
Video stream
The compression used can vary the output size depending on the
images captured, but there is a fairly good typical data rate
which it does not deviate from by much. Since version 2.1 it
is possible to crop the camera's input thereby reducing the
captured data by half.
Typical data rate ~ 2MB/min
Screen and pointer capture stream
This is the hardest one to predict. We use many compression
techniques but due to their lossless nature the size of captured
data can vary quite dramatically depending on the captured material.
It is worst with dynamically moving 'real' images. For example
take a picture of a horse in a field within your web browser
, wobble it up and down using the scroll bars and you could
produce data rates as high as 16MB/min. Get the BBC news front
page in your browser and read it for a couple of minutes without
scrolling and you're down to 0.5MB/min. Constant scrolling up
and down on this website will produce values of around 4MB/min,
typical use about 2-3MB/min.
Data rate minimum ~ 0.5MB/min
Data rate maximum ~ 16MB/min
Data rate typical ~ 3MB/min
Typical values
Adding all the values together we get:
Four stream data rate minimum ~ 2.55MB/min
Four stream data rate maximum ~ 18MB/min
Four stream data rate typical ~ 5MB/min
So for a 1 hour session we have values of:
Data minimum ~ 153MB
Data maximum ~ 1080MB
Data typical ~ 300MB
Real World Data
A lab of ten machines installed with mediaCam AV was used to
evaluated a new website before it went live. Testers were asked
to perform task on the site while mediaCam AV captured the screen,
cursor and mouse clicks.
29 people were tested, the test lasted just over 1 Hour and
the captures ranged from 21MB to 47MB.
This gives an average data rate of ~ 34MB/Hour - 0.58MB/Min
- 9.6 Kb/Sec.