Codes based on runlength-limited sequences have been the state of the art corner stone of current disc recorders whether their nature is magnetic or optical.

The length of time usually expressed in channel bits between consecutive transitions is known as the runlength. For instance, the runlengths in the word '0111100111000000'are of length 1, 4, 2 3, and 6. Runlength-limited (RLL) sequences are characterized by two parameters, (d+1) and (k+1), which stipulate the minimum (with the exception of the very first and last runlength) and maximum runlength, respectively, that may occur in the sequence. The parameter d controls the highest transition frequency and thus has a bearing on intersymbol interference when the sequence is transmitted over a bandwidth-limited channel. In the transmission of binary data it is generally desirable that the received signal is self-synchronizing or self-clocking. Timing is commonly recovered with a phase-locked loop, which adjusts the phase of the detection instant according to observed transitions of the received waveform. The maximum runlength parameter k ensures adequate frequency of transitions for synchronization of the read clock. The grounds on which d and k values are selected, in turn, depend on various factors such as the channel response, the desired data rate (or information density), and the jitter and noise characteristics.

Recording codes that are based on RLL sequences have found almost universal application in disc recording practice. In consumer electronics, we have the Eight-to-Fourteen Modulation or EFM code (rate = 8/17, d=2, k=10), which is employed in the Compact Disc (CD), and the EFMPlus code (rate = 8/16, d=2, k=10) used in the DVD.

See also