Selection of 3Φ(1,7) code for improving packing density on optical disc recorders

Selection of 3q5(1,7) code for improving packing density on optical disc recordersR.J. TarzaiskiRCA Advanced Technology Laboratories, Bldg. 10 -8, Camden, NJ 08102AbstractA run -length -limited code, designated 3(1,7), has been applied to optical disc recordersfor the purpose of increasing the information capacity of a disc. The 34)(1,7) code wasselected by an organized search of an extensive collection of NRZ -like, phase, and run- length-limited codes. Theoretical examination indicated that greater than a 20% linear packingdensity increase could be expected under the conditions that the recorder was forming thesmallest pits possible. The factors which led to this conclusion and the properties of the3¢(1,7) code are discussed.Details of an experimental verification of the conclusions reached herein are presented ina companion paper, "Implementation of 3q(1,7) code for improving packing density on opticaldisc recorders," by P. Hess and J. Waring.IntroductionData encoding facilitates effective usage of the capabilities of an optical disc datarecording medium and system. In particular, careful code selection permits maximizing theinformation packing density on the disc subject to the system constraints. Since the eco-nomic consequences of maximizing packing density are considerable in terms of reducing thenumber of discs required to contain a given amount of information, there is a great incentivetoward choosing an optimum code.The theoretical studies and experimental verification discussed here show that a particu-lar run - length -- limited code designated 34)(1,7)3 can offer a considerable increase in packingdensity relative to delay modulation code which is a proven good performer in both magnetictape and optical disc systems. The 340(1,7) code requires no restriction on the makeup ofthe user data. Demonstration with a specially constructed CODEC set capable of operatingat rates to 66 Mb /s showed that better than a 25% density improvement, based on equivalentbit error rate performance, could be achieved at a user data rate of 42 Mb /s.Code StudyBinary codes are established procedures for transforming user data, usually originatingin NRZ format, into symbol streams possessing characteristics which match the channel con-veying the information. Of special importance are the system frequency response and immunityto intersymbol interference (ISI). Channel frequency response limits both the symbol rateand the permissible duration of concatenated sequences of similar symbols. Channel -relatedISI limits the symbol packing density.A search of coding alternatives was conducted to establish the achievable linear packingdensity (LPD) of each in a real optical disc operating environment. NRZ -like, phase, andrun -length (RLL) codes were considered during a parametric analysis. It was determined thatRLL codes in general possessed properties which especially suited them for use with opticaldisc systems. Established performance of several systems employing delay modulation, anRLL code, had already shown particular examples of this fact.The code study defined effective values for density -related parameters of each of thecodes examined. These included code rate R = M /N, where M is the number of user bits mappedinto N code symbols; maximum window detection time Tye _ (M /N) Td, where Td is the width ofthe user data bit interval; and code density ratio DR which is the number of user bits con-tained in the minimum distance between successive transitions (minimum run length).The window detection time is the interval during which a decision must be made as to thevalue (assertion or negation) of each code symbol. A criterion for decodability is thateach and every symbol must be detected without error. Since an optical disc system containsnon -perfect record /playback subsystems which record on a non -perfect medium, timing errorsare produced during its operation which degrade code symbol detection capability on play-