In a UMA system, content adaptation is the key solution to provide the best possible presentation under constraints of various kinds of terminals and network connections available today. Content adaptation has two aspects: one is modality conversion that converts content from one modality (e.g. video) to different modalities (e.g. image), the other is content scaling that changes the quality (e.g. bit rate) of the contents without converting their modalities.

 Modality conversion is obviously first needed when the terminal cannot support certain modalities. Besides, when some resource constraints (e.g. tbit rate of network connection) are limited, modality conversion (together with content scaling) will be used to reduce the resource requirements of contents. Further when the user prefers or even can hardly perceive (e.g. visually impaired users) some modalities, modality conversion is also necessary.

 So far, most researches on content adaptation have focused on content scaling. Our research focuses on the systematic methods to convert the contentsí» modalities in the adaptation process. The research has resulted in several contributions to MPEG-21 DIA. Currently two important issues are being tackled: modality conversion preference and modality conversion QoS.

  • Modality conversion preference

 Content adaptation process may convert variously the modalities of contents. For each content object, there would be many conversion possibilities, whereas the user may prefer or even can hardly perceive (e.g. blind users) some modalities. The role of modality conversion preference is to let user specify his choices on modality conversion. Without this tool, user has to accept any adaptation solution provided by the provider. This tool should be flexible to support various practical cases and demands from user. Also the user preferences should be smoothly integrated into the adaptation process.

  • Modality conversion QoS

 Intuitively, given some resource constraint of terminal/network, the provider will (down) scale the contents to meet the resource constraint while still providing the best possible quality to the user. However, in some cases, the quality of the scaled contents is unacceptable or not as good as that of a substitute of a different modality. A possible alternative solution for this problem is to convert the contents into other modalities. For example, when the bandwidth is too low, sending a sequence of í░importantí▒ images would be more appropriate than streaming a scaled video with low quality.

 From the QoS point of view, the most important question in modality conversion is í░at which resource constraint should the current modality be converted to another modality?í▒ However, currently modality conversion is often carried out just when the terminal cannot support some modality. Our research tries to answer this question when the terminal can support the modalities but the resource constraint is limited.