Many attempts have been made to achieve higher density magnetic recording. One approach is to employ multiple recording in video applications. For instance, a luminance signal, a chroma signal, an audio signal and other signals are recorded on the same surface of media such as VTR's and video floppy disks for home use. In magnetic recording, different signals can be recorded in two layers, one being on top of the other, and two layered media have been proposed. However, the magnetic field of signals for reproduction (i.e., playback) receives a contribution from the magnetization of both the upper and lower layers, and the two cannot be magnetically differentiated from each other. To overcome this problem, different signal frequency bands are separated with a filter.
If multiple magnetic recording is performed on separated signal frequency bands, the following problem occurs. If the signals to be recorded are two modulation signals having different carrier frequencies, f.sub.1 and f.sub.2 (f.sub.2 &gt;f.sub.1), unwanted components (spurious components) corresponding to the sum of, or difference between, integral multiples of f.sub.1 and f.sub.2 will be reproduced on account of the nonlinear characteristics of distortion including the head and tape. If the recording current i.sub.R is expressed as i.sub.1 sin 2.pi.f.sub.1 t+i.sub.2 sin 2.pi.f.sub.2 t, the reproduced output, ep is proportional to i.sub.R +ki.sub.R.sup.3. Of the term proportional to i.sub.R.sup.3, spurious components that occur in the signal bands are i.sub.1.sup.2 i.sub.2 sin 2.pi.(f.sub.2 -2f.sub.1)t, i.sub.1.sup.3 sin 2.pi.(3f.sub.1)t, etc.
Home use VTR's currently available on the market perform recording by FM modulating a luminance signal with f.sub.2 and by AM modulating a chroma signal with f.sub.1. The frequency profile of the reproduced signal components and the spurious components is shown in FIG. 1. In today's home use VTRs, the recording current for f.sub.1 component is limited to such an extent that the spurious components occurring in the reproduced luminance signal will not cause any substantial deleterious effects. More specifically, the recording level of carrier f.sub.1 in the output of a reproduced amplifier is set at a value such that the level of (f.sub.2 -2f.sub.1) component is not more than -22 dB compared with the output level of carrier f.sub.2. This approach, however, has the disadvantage of a low SN ratio for the chroma signal.
USPTO Backgrounds
USPTO Backgrounds