The present invention relates to a system for recording into a recording medium or reproducing thereoutof a digital signal produced by modulating original digitized data, which is prepared by the pulse code modulation (PCM) of an analog signal, such that the digital signal has a maximum reversal interval equal to the bit period of the digital data and a minimum reversal interval that is one half the bit period.
In a magnetic data recording and playback system, an analog signal may be digitized by the PCM system and the resulting signal may be further modulated by a certain system to prepare a digital signal, which is capable of self-clocking. As generally accepted in the art, recording and reproducing such a digital signal into and out of a magnetic medium is far more desirable in reducing the influence of distortion of the magnetic medium or that of irregular operation of a drive line on the reproduced signal and thereby promoting high quality signal recording and playback, than simply recording and reproducing an analog signal directly into a magnetic medium. Typical of such digital signals is one which is provided by the frequency modulation (FM) system to reverse itself at a minimum reversal interval, which is one half the bit period of original data prepared by the PCM system, when the original data is (logical) "1" and at a maximum reversal interval equal to the bit period when the original data is "0". This type of digital signal will hereinafter be referred to as "digital FM signal" for convenience.
In the prior art digital signal recording and playback system discussed above, demodulation is effected by determining whether a pulse duration of the reproduced digital signal is the maximum reversal interval or the minimum reversal interval, that is, whether it is a repetitive frequency component of the maximum reversal interval (called frequency component "F" hereinafter) or a repetitive frequency component of the minimum reversal interval (called frequency component "2F" hereinafter). The result is the need for a recording and playback signal transmission system for the magnetic medium which has a band wide enough to transmit even the higher frequency component 2F without substantial attenuation, e.g. FM demodulator circuit. This sets limits to an increase in recording desity available for a magnetic recording and playback device for public use, the signal band of which is designed relatively narrow from the economic standpoint.