Hendrik Knoche

Can small be beautiful?: assessing image resolution requirements for mobile TV

Mobile TV services are now being offered in several countries, but for cost reasons, most of these services offer material directly recoded for mobile consumption (i.e. without additional editing). The experiment reported in this paper, aims to assess the image resolution and bitrate requirements for displaying this type of material on mobile devices. The study, with 128 participants, examined responses to four different image resolutions, seven video encoding bitrates, two audio bitrates and four content types. The results show that acceptability is significantly lower for images smaller than 168x126, regardless of content type. The effect is more pronounced when bandwidth is abundant, and is due to important detail being lost in the smaller screens. In contrast to previous studies, participants are more likely to rate image quality as unacceptable when the audio quality is high.

Enhancing social sharing of videos: fragment, annotate, enrich, and share

Media consumption is an inherently social activity, serving to communicate ideas and emotions across both small- and large-scale communities. The migration of the media experience to personal computers retains social viewing, but typically only via a non-social, strictly personal interface. This paper presents an architecture and implementation for media content selection, content (re)organization, and content sharing within a user community that is heterogeneous in terms of both participants and devices. In addition, our application allows the user to enrich the content as a differentiated personalization activity targeted to his/her peer-group. We describe the goals, architecture and implementation of our system in this paper. In order to validate our results, we also present results from two user studies involving disjoint sets of test participants.

Design requirements for mobile TV

In this paper we examine the interaction and delivery requirements for Mobile TV. By comparing the features of traditional TV with current Mobile TV services we outline the design requirements for a future Mobile TV interface. The proposed design is implemented on an iPAQ and evaluated with end-users in a field trial. Preliminary results of the user study suggest that use of the interface is intuitive and successful in giving a TV-like experience. As a secondary study we investigate the bandwidth requirements for different types of TV content on a 3G phone and a PDA. The results reveal marked differences in bandwidth requirements for different content types. The findings are discussed in the context of future Mobile TV services.

Fragment, tag, enrich, and send: Enhancing social sharing of video

The migration of media consumption to personal computers retains distributed social viewing, but only via nonsocial, strictly personal interfaces. This article presents an architecture, and implementation for media sharing that allows for enhanced social interactions among users. Using a mixed-device model, our work allows targeted, personalized enrichment of content. All recipients see common content, while differentiated content is delivered to individuals via their personal secondary screens. We describe the goals, architecture, and implementation of our system in this article. In order to validate our results, we also present results from two user studies involving disjoint sets of test participants.

Utility curves: mean opinion scores considered biased

In the Coqos project task performance measures and a corresponding framework are suggested and pursued as a novel and more suitable means for determining utility curves. TPM are intended to avoid limits inherent in traditional measures like mean opinion scores. MOS rely merely on subjective ratings rather than on more objective performance in relation to a particular task or application of interest. Informational relevance and its impact on subjects can be measured more effectively by TPM. Inhibiting psychological and cognitive effects like consciousness or nonconsciousness of degradations or individual focusing and perspectives of subjects can be more appropriately evaluated and dealt with by means of TPM. The increasing importance of adaptation, in particular with the advance of MPEG4, as a means for QoS provisioning, both in wireless and wired environments, requires sensible techniques to effectively determine utility curves.

How low can you go? The effect of low resolutions on shot types in mobile TV

The advent of mobile TV which is often viewed on small screens with low resolution has made TV content producers think about refraining from using shots that depict subjects from a great distance. Shot types where the object of interest fills the screen are deemed to be more appropriate for mobile devices. This paper reports a study on how shot types used in regular broadcast television are affected when shown on mobile devices at reduced levels of resolution. Seventy-two native speakers judged the acceptability of four different content types at four resolutions (240u2009×u2009180, 208u2009×u2009156, 168u2009×u2009126, 120u2009×u200990). The results show that acceptability of shot types depends on the content and the resolution. Extreme long shots of football content were only less acceptable than other shot types at resolutions smaller than 240u2009×u2009180. The medium shot which portrays the upper half of a subject’s body was the most acceptable for news content but for football content was judged worse than shot types that showed less detail. Our results suggest that for a young audience extreme long shots may be used with no detrimental effect for resolutions of 240u2009×u2009180 and higher. At lower resolutions and for content with a high degree of dynamism both the medium shot and the extreme long shot might render poorly for the audience. Service providers are well advised to include the results at hand to customize content in terms of shot type use for their audience that will watch the content at very low resolutions. Further research should assess older audiences and the effectiveness of cropping schemes that zoom in on part of the content for low target resolutions.

The kindest cut: enhancing the user experience of mobile tv through adequate zooming

The growing market of Mobile TV requires automated adaptation of standard TV footage to small size displays. Especially extreme long shots (XLS) depicting distant objects can spoil the user experience, e.g. in soccer content. Automated zooming schemes can improve the visual experience if the resulting footage meets user expectations in terms of the visual detail and quality but does not omit valuable context information. Current zooming schemes are ignorant of beneficial zoom ranges for a given target size when applied to standard definition TV footage. In two experiments 84 participants were able to switch between original and zoom enhanced soccer footage at three sizes - from 320x240 (QVGA) down to 176x144 (QCIF). Eye tracking and subjective ratings showed that zoom factors between 1.14 and 1.33 were preferred for all sizes. Interviews revealed that a zoom factor of 1.6 was too high for QVGA content due to low perceived video quality, but beneficial for QCIF size. The optimal zoom depended on the target display size. We include a function to compute the optimal zoom for XLS depending on the target device size. It can be applied in automatic content adaptation schemes and should stimulate further research on the requirements of different shot types in video coding.

The big picture on small screens delivering acceptable video quality in mobile TV

Mobile TV viewers can change the viewing distance and (on some devices) scale the picture to their preferred viewing ratio, trading off size for angular resolution. We investigated optimal trade-offs between size and resolution through a series of studies. Participants selected their preferred size and rated the acceptability of the visual experience on a 200ppi device at a 4:3 aspect ratio. They preferred viewing ratios similar to living room TV setups regardless of the much lower resolution: at a minimum 14 pixels per degree. While traveling on trains people required videos with a height larger than 35mm.

Quantitative QoS-Mapping: A Unifying Approach

Distributed multimedia (MM) systems have to provide users with the ability to specify their performance requirements. Quality of service (QoS) paramexad ters represent an adequate measure for the specification of time-dependent MM-data like audio or video streams. In order to guarantee the fulfillment of application requirements, a mapping onto the involved network and operatxad ing system resources has to be performed. This paper shows how QoS transxad lation can be performed in distributed MM-systems. Parameter translations between abstraction layers including a terminology and the interdependencies between the parameters are presented. Furthermore, mapping stimuli that imxad ply a modification of QoS parameters are identified and their respective effects are described.

The sweet spot: how people trade off size and definition on mobile devices

Mobile TV can deliver up-to-date content to users on the move. But it is currently unclear how to best adapt higher resolution TV content. In this paper, we describe a laboratory study with 35 participants who watched short clips of different content and shot types on a 200ppi PDA display at a resolution of either 120x90 or 168x128. Participants selected their preferred size and rated the acceptability of the visual experience. The preferred viewing ratio depended on the resolution and had to be at least 9.8H. The minimal angular resolution people required and which limited the up-scaling factor was 14 pixels per degree. Extreme long shots were best when depicted actors were at least 0.7° high. A second study researched the ecological validity of previous lab results by comparing them to results from the field. Image size yielded more value for users in the field than was apparent from lab results. In conclusion, current prediction models based on preferred viewing distances for TV and large displays do not predict viewing preferences on mobile devices. Our results will help to further the understanding of multimedia perception and service designers to deliver both economically viable and enjoyable experiences.