Digital movies are typically expected to be large data structures with poorer quality reproduction than analog tape or video-disk. However, for computer demonstrations this is not the case, and movies may be made that are accurate in reproducing the details of the computer screen while occupying little disk space. This section details some practice in experience in producing such a movie with a resolution of 512 by 342 pixels, high-quality sound, playing for 8 minutes and occupying only 11MBytes. Figure 3.1 shows a frame from the movie being played in Apple's MoviePlayer. It shows interaction in a word processor with a document that itself contains an embedded QuickTime movie. The size and resolution of the demonstration are apparent in the quality of the text and images shown.
Figure 3.1 Tutorial demonstration as a high-resolution QuickTime movie
The movie demonstrates KWrite, an open-architecture, multi-media, active document production system, being used to develop, publish and use an active document in which the diagrams are knowledge structures in the form of semantic networks. They can be edited from within the document and interrogated by other applications as the knowledge base of an expert system. The document shown in the movie is an example of the actual application of KWrite. It was published as camera-ready copy by Cambridge University Press in December 1992 in the Proceedings of the British Computer Society Expert Systems Conference (Gaines and Shaw, 1992). It was also issued for FTP on Internet and on CD-ROM as a parallel publication of an active document that could be used for solving room allocation problems.
The primary tool used was Camerman from Vision Software, which provides live screen capture from Macintosh computers. Draft sound overlays were added using CamerManEdit which allows one to add sound to a captured movie while it is playing. This is a good technique for generating a spontaneous commentary, but not for getting smooth voicing and good sound quality. Hence, the scripts generated this way were transcribed into text and re-recorded separately in a quiet room using a Casio DA-R100 DAT recorder.
The R100 is a high-quality machine, small enough to slip into a pocket, offering two 16 bit 34Kbytes CD-ROM quality audio channels with 90db signal/noise ratio and no cross-talk between the stereo channels. It is useful for conventional movie capture to supplement a camcorder giving three channels of sound in the field, with one microphone on the camcorder and two on the R100. The peak-holding LCD level meters on the R100 span 80 db and enable one to see the room noise accurately as well as setting microphone levels to avoid clipping.
The sound was digitized at a Macintosh Quadra 900 through a dbx 1 sound compressor. This is usually necessary with camcorder input from the field where sound levels may fluctuate quite widely. The dbx 1 with its extensive level controls is also a useful front end to the computer since the line inputs are somewhat insensitive and result in unnecessarily low sound levels from standard line levels.
The compression was not all that important in this application because the scripts were recorded under controlled conditions. However, a dbx compressor is a valuable tool in attaining good sound quality in QuickTime digitization. With 8-bit sound it is worthwhile conditioning the signal on the audio side to be at maximum peak level before digitization. When 16-bit audio becomes common much of this conditioning can be done post-digitization.
The audio was digitized at 22KHz using Adobe Premiere's audio input, and the data rate was reduced to 11KHz in the compression phase without noticeable loss of quality. Premiere was used to synchronize the audio and visual tracks, primarily by adjusting the speed of play of the movie segments.
There is an interesting stylistic problem in putting voice over animation as to whether the voice is the primary focus of attention and introduces the visuals--in which case vision should lag sound--or whether the movie is the primary focus, with the sound a commentary on the images--in which case sound should lag vision. There is no universal rule and it is important to adjust the time relation until it feels right, with vision leading sound in most cases since this leads to the most rapid attention switches.
The synchronization problems of the current QuickTime implementation can override these subtle considerations, and it is worthwhile to try and detach the commentary somewhat from the visual changes. On slower machines the sound can gradually come to lag several seconds behind the video. To compensate for this the movie was divided into 6 segments and a silent period of some 5 seconds left in the sound track between sections. This allows QuickTime to resynchronize the sound and movie between sections.
The six segments are: an introduction, an explanation of the KWrite document architecture, a demonstration of the knowledge elicitation tool KSS0, a demonstration of the knowledge editing tools KDraw, a demonstration of KWrite representing the knowledge embedded in a document, and a demonstration of a HyperCard stack being used to solve a room allocation problem by interrogating the knowledge base in the KWrite document.
The four demonstration sequences were captured by CamerMan. The two initial animation sequences were developed as sequences of PICTs in Canvas, loaded into Premiere and adjusted in length to synchronize with the commentary.
The six segments were pasted together in MoviePlayer and then compressed into a single fork movie with a maximum 90KBytes/sec data rate in George Cossey's MovieShop. The single-fork `flattening' is important in allowing it to be played under Microsoft Windows on PC platforms. It is important in using MovieShop to compress animations to set the `similarity level' at which it discards frames higher than for camera video so as to avoiding losing small changes that are significant to the animation. The default is 240, and 252 was appropriate for the animations in this movie. MoviePlayer and MovieShop were issued by Apple as part of the original QuickTime distribution. Premiere 4.0 now has both data rate limitation and flattening capabilities.
The movie was transferred to CD-ROM using a Philips CDD521 recorder and OMI QuickTOPiX software. It was also put up for FTP on the Internet.
The movie demonstrates that tutorial demonstrations are readily created and disseminated as digital movies. Production is neither complex nor expensive--CamerMan alone at under $100 could be used to produce virtually all of this movie.