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Optimal Temporal Sampling Aperture for HDTV Varispeed Acquisition By Emil Borissoff
According to the perceptions of the human visual system, optimization of the temporal sampling aperture is required to best convey motion blur during HDTV image capturing. Filtering temporal aliasing artifacts becomes possible with the latest technologies in picture acquisition, and is applicable for advanced tape-based or tapeless recording. This paper presents an analysis of video capturing and recording in HDTV, for the case of variable frame rate (varispeed) acquisition. It proposes an optimization of the exposure duration, so that the duration becomes dependent on the frame rate of the master format. This approach restrains the strobe effect and utilizes motion blur for better rendering of dynamic scenes. The improved overall image quality also benefits the processes of editing, compositing, conversion, and compression. Finally, the research results are integrated into an embedded algorithm to suggest how to implement this strategy.
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ntil recently, varispeed acquisition was used for film cameras only. This mechanism allows film to be transported and picture exposed at a wide range of frame rates. Replaying processed film at the standard rates of 24 frames/sec creates attractive effects of slow motion or accelerated scene object displacement. Motion blur caused by emulsion response and blurring of the background image when tracking a moving object have been noted as a desirable smoothing factor on the time-frame samples, if retained in an optimal range. Today, advances in HDTV varispeed acquisition allow two ways of accomplishing the same task in the video domain, both counting on the camera’s optical sensor integration ability: • Capturing at variable frame rate and recording at the same rate on hard disk drives or optical media. Playback from these devices is at the standard HDTV rate, and therefore the process fully competes with the film procedure. At present, this method is used in some nonbroadcast applications and requires an adaptive sync generator. • Capturing and recording at the highest possible standard frame rate on a VTR. The playback of selected frames from the shot sequence is at the master HDTV rate, thus transforming scene motion proportionally to the ratio of the two frame rates. This method has already been successfully implemented and is well accepted in TV production, post-production, and broadcasting.1 Furthermore, the improved spatial resolution of HDTV leads to an improvement of the temporal resolution, including uses in the varispeed mode. The use of progressively scanned pictures associated with some SMPTE
Journal, April 2004 •
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OPTIMAL TEMPORAL SAMPLING APERTURE FOR HDTV VARISPEED ACQUISITION HDTV formats, and the possibility for enhanced motion rendering, provide another stimulus for renewed exploration of the principles behind frame sampling. From this perspective, Fig. 1 illustrates the motion blur traces in 24 frame/sec acquisition. If the exposure shutter is open for Figure 1. Motion blur, fill factor = 1. the duration of the TV frame, maxizoom that occurs in the rest of the TV frame. The posimum motion blur is recorded at 24 discrete moments tion of the object in the next frame will determine its per second. The human visual system (HVS) perceives interframe displacement and may appear as a jump. To it differently from the natural way of viewing, and only constrain a scene element relocation with regard to the image retention on the retina, together with brain frame frequency, a second condition applies. interpolation, provide a realistic scene reproduction.2 Nominal Condition 2: Object Displacement Film cameras have their shutter open for 1/48 of a sec2.1 The object linear displacement should be less ond in normal shooting, which reduces motion blur, but than half of the screen distance during one frame perialso acts as a partial low-pass temporal filter that od.5 Then Ts should be: reduces possible strobe effects. Zagier3 proposed that Ts
