Immink, Hoogendijk and Kahlman: Digital Audio Modulation DIGITAL AUDIO MODULATION IN THE PAL AND NTSC LASERVISION VIDEO DISC CODING FORMATS Kees A. Schouhamer Immink, Ad. H. Hoogendijk and Joost A. Kahlman Philips Research Laboratories P.O.B. 80.000,5600 JA Eindhoven, The Netherlands Abstract We present an extension of the current LaserVision video disc format that includes a digital audio signal. We show the feasibility of a combined digi- tal audio signal according to the Compact Disc Digital Audio format and the current analog audio signals in the NTSC video format, enabling the realiza- tion of a compatible system. For the PAL and SECAM video formats we show the feasibility of digital audio, but unfortu- nately it cannot be combined with the analog audio carriers. O. Introduction In the current LaserVision (LV) video disc coding formats (NTSC, PAL and SECAM), the ana- log audio stereo channels are frequency-modulated and added by means of pulse-width modula- tion of the frequency-modulated video signal (refs. 1,2,3). The maximum audio signal-to- -noise ratio of the LV 525 lines NTSC format at present attainable is approximately 70 dB, which includes 15 dB im- provement by the CX noise re- duction system (ref. 3). In the 625 lines PAL and SECAM formats of the LV the audio signal is approximately 10 dB superior to the NTSC LV format. An improvement in audio quality seems possible only by going digital. For example the 16 bits linear quantization used in the Compact Disc Digital Audio System format (refs. 4,5) achieves a signal- Manuscript received September 19, 1983 Contributed Paper 0098-3068/83/C -to-noise ratio of 96 dB. Furthermore the powerful error correction system of this digi- tal format has a beneficial in- fluence on the effect of drop- -outs. In this paper we report on experiments and simulations to show the feasibility of adding a digital audio signal according to the Compact Disc Digital Audio format in the current LV video disc formats. The bandwidth of the digital audio signal spectrum (approximately 1.5 MHz) and its modulation index on the main carrier are of great importan- ce. The disturbance of the digital audio signal in the video picture plays an impor- tant role in the overall design. A low modulation index of the digital audio signal results in a poor signal-to-noise ratio, giving rise to a high bit error rate. Many experiments have been done to arrive at a com- promise on these conflicting parameters. In Section 1 we briefly des- cribe the LV video formats and derive requirements which a di- gital audio modulation system should meet. We describe the simulations used to find quan- titatively the constraints for the digital audio modulation within the LV coding format. In Section 2 we describe the expe- rimental results obtained with actual discs. 3543-0551$01.00 © 1983 IEEE 543