Modality conversion currently emerges as an important issue in universal multimedia access. The decision on modality conversion is affected by various factors, such as terminal capability, user preferences, surrounding environment, etc. Here, modality conversion under the constraint of connection bitrate is considered. Intuitively, when content scaling cannot provide the acceptable QoS, modality conversion may be a good choice to deliver an appropriate quality. From the QoS point of view, two

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... portant questions in modality conversion are: 'at what resource constraint point should a change of modality occur?' and 'what is the destination modality?' That is, knowing the conversion boundaries between modalities is crucial for a seamless modality conversion. In this paper, a systematic approach to help answer these questions is presented.