HCI and Usability Issues of Multimedia Internet broadcasting

1. HCI and Usability Issues of Multimedia Internet broadcasting Lecture 3

2. Introduction • Using Multimedia Internet Broadcasting • Appears simple • Controls like CD or VCR player • But has variable bandwidth - this leads to • loss of service • reduced service parameters • Needs QoS contract

3. QoS and usability • Two sides to the problem (see Lecture 1) • The tools and methods available to the broadcaster to create the options and • The tools and methods available to the viewer to evaluate the options

4. Conflicting Goals • Broadcasters • To offer options that attract viewers • Increase audience size • Reach all likely users • Users • To get the highest QoS with a given budget • To maintain service through a broadcast

5. The Problem • Choice of package determines • audience and • parameters available for choice • Packages allow choice of • compression algorithm • audio compression rates • video compression rates • video frame rates etc

6. Choice example • Given a particular compression algorithm • IF broadcasters choose to deliver • three different frame rates (High. Medium and Low) • three different audio quality levels • three levels of image quality (compression rates) • Gives 27 options - ignoring other parameters

7. Graphical representation

8. Scale Linearity • Human perception of video quality is non-linear • E.g linear scale for video compression • 0%-25%-50%-75%-100% • Does not provide usable choices • More likely to conform to a power law R=KSB R=the observer’s perception of magnitude S= measurable stimulus, K is a constant and B is the compression function used by the observer

9. Video frame rate • Experiments performed by West et al show B=0.68 for video frame rate Magnitude estimation results describing the relationship between frame rate and the perception of smoothness.

10. Content type • Users prefer different QoS settings for different content types • Could be related to • user’s goals or • perceptually-based • or both at the same time

11. Scaling Quality • The scale used to measure QoS are a difficult area • high, medium, low are subjective judgements (but probably the easiest to agree upon) • Scales that use more levels are used differently by different users • QoS agreements need to be made with individuals - who all have different perspectives

12. Presenting the options • Quality levels should be in the scale that the user needs (need to determine high and low bounds) • Content-related effects need to be considered • Quality settings should seem to be equally spaced when viewed • Quality labelling should reflect what is to be expected

13. QoS in the Interface • QoS is dependent on the various parameters of the broadcast • this is not a simple relation • many dimensions used to provide overall QoS • e.g. audio and video compression,frame rate etc • Interface could have control over each parameter • or some particular combinations

14. Comparison of QoS parameters

15. Use of the comparator • Right screen presents preset value - Left screen presents users changed version • The 2 TV model • naturally affords the process of comparison • but still makes it difficult to determine audio parameters • (no easy comparison) • video interference (associated video events interfere) • easier to compare video without sound!

16. Summary • QoS levels can be very subjective • Scales are non-linear but • Users tend to think of them as linear • QoS contracts need to be based on users own perception of what QoS means • This is different for each user!