This review of magnetic recording systems concentrates on developments in two particular areas: the video-tape recorder and the rigid disk drive. The consumer VCR represents the state of the art for analog in-contact recording, while the rigid disk drive exemplifies digital recording with rapid access. Various aspects of the mechanical configurations, track-following techniques, heads and media, and signal processing are discussed. More briefly the paper describes other digital systems on flexible media.

The application of Viterbi detection to Class IV partial response on a magnetic recording channel is discussed. The theoretical performance is analyzed in the presence of noise correlation and quantizing errors. A simplified algorithm is described which was implemented on a transverse-scan magnetic tape recorder operating at a linear density of 1800 bits/mm and a transfer rate of 120 Mbits/s. Following an initial 5 bit analog-to-digital conversion of the ternary waveform, the Viterbi detector is implemented entirely in high-speed digital emitter-coupled logic. An advantage of approximately 1.5 dB in effective signal-to-noise ratio is gained over a simple level detector, a result which agrees well with the theoretical development.

This letter clarifies some of the properties of the Hedeman codes and examines their application to band-limited channels. Attention is drawn to the equivalence of H-2 and H-3 to simple linear modulation schemes. All three codes are dc free by virtue of a charge constraint which additionally ensures a minimum distance two (considered as rate one-half codes). The asymptotic error performance at high signal-to-noise ratio can thus be similar to NRZ or biphase signaling, although the Viterbi algorithm is required to achieve this performance in the case of H-1 code. Like Miller code, both H-1 and H-2 will pass through unequalized low-pass channels with bandwidths little greater than required for NRZ signaling.