Videodisc

Comput & Graphics Vol. 7. No. 3~4, pp. 351-353, 1983 Printed in Great Britain.

0097-8493/83 $ 3 . 0 0 - . 0 0 Pergamon Press Ltd.

Glossary

VIDEODISC WOLFGANG STRASSER Technische Hochschule Darmstadt, Fachgebiet Graphisch-Interaktive Systeme, D-6100 Darmstadt, West Germany

(Received 15 July 1983) INTRODUCTION

Many applications in CAD, office communication, broadcasting, and printing industry--to name but a few--demand a synthesis of Computer Graphics methods and Image Processing capabilities in one system. Computer Graphics offers the means for powerful modeling and easy to use graphic man-machine interaction. Image processing capabilities are needed to handle the different representations of information, e.g. vector graphics, raster graphics, video, facsimile, etc. in a way that hides information specific coding from the user. The results of this electronic information processing is a growing demand for online storage capacity to record and retrieve extremely large quantities of data at very high transfer rates; e.g. digital video requires about 100 Mbit/sec and 2 Mbit/frame. In addition, random access to single frames and lines of frames within seconds or less is required. Until recently no storage system was available which met these requirements and in addition offered the capacity for ten thousands of frames or the equivalent of 101° bits or more at reasonable cost. The advent of optical discs promises a breakthrough in the technology of mass storage devices. Existing devices are already capable to fulfill some of the above mentioned requirements. Technical characteristics of these devices, viz. the optical videodisc and the optical disc are presented. OPTICAL VIDEODISC

Optical videodiscs are read-only-memories. The production of a videodisc begins with a mastering process whereby the user's information from videotape is recorded on a master disc (Fig. 1) in video format (costs less than $1000[1]). Frame numbering, special encoding and recording of two additional audio channels take place during this mastering process. From the master, a stamper is formed which is then used to replicate, in volume if needed, inexpensive plastic video discs (~< 1$). Each revolution of the disc, called "track" represents one full TV frame. The 54000 tracks of the disc form a spiral. Through the use of a servo tracking mirror (Fig. 2) in the player (or disc drive) the focused laser beam can be controlled to follow the spiral tracks to produce moving pictures or to follow one particular track again and again to produce a still picture. The unique digitally encoded address number of each frame can be used for search-and-locate random access operations. The entire disc surface of 54000 351

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frames can be scanned in a few seconds ( ~<5 sec) and any selected frame can be displayed using a standard TV set. With the focusing arm stationary the tracking mirror allows for the addressing of 100 tracks. Thus, a very fast access time (~<60#s) results when the addressed track is within the beam deflection range of the mirror. Since the optical system is contactless, there is no wear of the record or player during the playback thus allowing continuous display of a still frame indefinitely. Videodisc players for NTSC video format incorporating programmable microcomputer controller, external sync and standard interfaces like IEEE-488 or RS-232 are sold for less than 30005. With these features the optical videodisc is a very inexpensive medium to realize vide picture bases offering --random access to single frames and preprogrammed frame sequences --immediate display and picture refresh --compatibility to video and new services like video text, cable TV, etc. --reliable technology (hopefully) comparable to TV sets. This random access picture store is attractive for all applications in which pictures have to be composed from information of different sources (Fig. 3) and which need no digital representation of the resulting picture. Of course, the composite video signal of the optical videodisc can be digitized in real time. But this causes a loss of picture resolution and is therefore rejected in most cases. OPTICAL VIDEODISC USED AS DIGITAL READ ONLY MEMORY

Digital data recording on the videodisc can be accomplished within the constraints of the video format. That is, the information is interrupted by line synchronisation signals every 64 #s (Fig. 4).

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Fig. 3. Optical videodisc used as image base in graphics systems[2]. The useful time slot for informa'don is only 53 #s per line. Thus the recorder interface as well as the playback interface will require at least 11 #s buffering to accomodate continuous information. Considering the bandwidth of the video channel, about 375 bits in N R Z coding can be stored per line. The 495 lines per frame provide then a capacity of 185625 bits/track. Utilizing all 54000 tracks yields a total storage capacity of 10 t° bits per disc. Access to given data locations is accomplished by means of the digitally recorded track address inherent on all optical videodiscs. It is interesting to note that a band of 100 tracks, i.e. the range accessible without movement at the readback head, contains about 18.5Mbits. This equals the capacity of typical magnetic discs. In[l] the properties of digital ROM based on the optical videodisc are quoted as follows: --storage capacity of 101° bits or 1250 MBytes --54000 tracks of 1.85 x 105 bits each ---corrected error rate 10 - 9

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- - 7 Mbit/sec data rate --local track access time of 60 #s/track --average track access time 5 sec ----cost per bit 10-Scents. These features support a variety of interesting applications. For instance video pictures and related digital information, e.g. display file, non-graphical data, can be stored in neighbouring tracks, thus assuring fast access. In the case that picture display and data access is simultaneously needed, multiple videodiscs are a very reasonable and cost effective solution. Synchronisation between several discs is no problem because of the sync signals inherent on each disc. Data transfer rates can therefore be increased by using parallel discs. High resolution pictures of raster graphics systems, e.g. 1000 x 1000 x 24 bits or 24 Mbits will require about 120 frames. They have to be loaded into the image memory of the gqaphics system for display. The read-only feature and the time consuming mastering process are serious drawbacks of the optical videodisc. Also the mastering and replication process for videodisc production does not allow dynamic error identification. While small errors may be tolerable for visual or aural presentations, it is absolutely untolerable for digital data. These drawbacks are not present in optical discs. OPTICAL DISCS

The optical disc has a write-once capability, i.e. records of information cannot be overwritten or erased. The recorder is based on the videodisc technology but different recording materials are used. Each side of the disc contains 40000 tracks, with every track containing 32 sectors of 15200 bits each. Each sector has a unique address and 8192 bits of protected data, followed by a secondary error-checking code. The storage capacity per disc side is 1.9 x I0 ]° bits

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unformatted and 1.05 x 10 '° bits formatted. A primary and backup error-encoding systems corrects the error rate to less than 10-'°[3]. Recording is performed with 10 Mbits/sec. With these characteristics the optical disc is an ideal complement of the videodisc. The write-once nature is not a severe constraint considering the total storage capacity and the relative low cost (less than $10) per disc. Information updates are just written on new sectors. Of course the data base management system has to care for consistent sector addressing. For interactive graphics systems we have then the storage hierarchy depicted in Fig. 4. Interactive image manipulation utilizes the refresh memory, which can be loaded initially from optical disc or videodisc. Completed images are stored on an optical disc or videotape for further processing. The videodisc serves mainly for image archives with random access to individual frames. The expected performance development of several storage technologies shown in Fig. 5 demonstrates the advantages of the optical disc.

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/~/'~ Opt icol (~/ d,sc I 1 l I I I 10.8 IO-e I0-4 IO-2 I IO2 Access ~Qme ( sec ) Fig. 6. Prognosis of access time vs price for various storage technologies [3]. The advent of optical storage devices like videodisc and optical disc promises to satisfy the needs of most applications. This expectation is based on the heavy interest that both consumer market and computer industry show for this new technology. REFERENCES

CONCLUSION

The application of interactive graphics systems in electronic information processing and communication exhibits a constant growth due to the rapid advance of semiconductor technology. Especially the picture processing part of these applications demands for large on-line storage capacities to store and retrieve images.

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1. G. Kenney, Special purpose applications of the PhillipsMCA videodisc system. IEEE Workshop on Picture Data Description and Management, University of Illinois, Chicago (1977). 2. W. Strasser, Use of optical videodiscs in graphics sytems. IFIP CAD/CAM-Conf. Proc. Sad Paulo (1981). 3. G. Kenney et al. An optical disc replaces 25 mag tapes. IEEE Spectrum (Feb. 1979).