Audio & Video: Assessing Complimentary Quality

Audio and video quality issues are usually assessed as separate entities. However there is substantial evidence that the quality of one medium can have an impact on the users perceived quality of the other (Watson & Sasse, 1996; Hollier & Voelcker, 1997; Rimmel, Hollier & Voelcker, 1998).

Early studies by Neuman illustrated that the end user’s perception of HDTV image quality could be increased when the accompanying audio was improved (cited in Negroponte, 1995).

Other similar work has been conducted by Hollier & Voelcker (1997) where users were presented with several video clips with an accompanying audio commentary; the videos used varied at 6 levels of image quality and the audio had 4 levels of quality. Results showed that the identical audio segment would receive different quality ratings depending on the quality of the video that accompanied it. Also the audio quality was found to have an influence on the perceived quality of the video.

Users perception of quality is also likely to vary with the task (Hollier & Voelcker, 1997). If the users are involved in learning a foreign language then the audio quality may need to be substantially higher for success than if the task is to present a report at a routine meeting (Watson & Sasse, 1996). Video quality is likely to be more important in an intense interview situation than it might be in other relaxed scenarios. User perception of audio and video quality may be directly linked to the level of quality they assume is necessary for the situation.

Watson & Sasse (1998) have recently instigated a new approach in measuring user perception of quality. Users are requested to rate the video quality in a session on a 200mm unlabelled continuous scale that only has a plus and minus sign at either end to signify polarity. So far the results seem promising and the ratings have been remarkably consistent. Observations also indicate that this scale has reduced the tendency of subjects to avoid the end points of the scale.

Audio and Video: Synchronisation

The user perception of quality increases when audio and video are synchronised; this may be attributed to an increase in speech quality when the user is able to "lip-read" the speech if necessary from the accompanying video stream (Reisberg, McLean & Goldfield, 1987; Summerfield, 1992). However, because the co-ordination of conversation requires precise timing between participants, any delay can alter the communicative interaction and dynamics of the conversation (e.g. O’Connail, Whittaker & Wilbur, 1993). In some cases this has been shown to affect user performance (Krauss & Bricker, 1967; O’Malley, Langton, et al, 1996). Other subjective research has shown that the audio and video have to approach a mismatch of approximately 80-100 msecs before the user perceives a lack of synchronisation. (Jardetzky et al.). Watson & Sasse (1996) found that where video is slower than 5 frames per second, the user impression is that audio and video are not synchronised.

Compression techniques also affect synchronisation. The trade-off with compression and decompression is that it involves processing time at both ends of the transmission. The use of compression can add a delay of approximately 200-400msec to any video transmission. To maintain a level of synchrony, a delay is sometimes introduced to the audio channel so that the user’s perception of real time transmission is maintained.

Copyright EMMUS 1999.
Last updated: September 26, 1999.