Apparatus for multiplex recording and reproduction of audio and video signals

A recording circuit for a video tape recorder which records superposedly a frequency-modulated audio signal with a frequency-modulated luminance signal and a carrier color signal converted to a low frequency band records a carrier wave signal which is frequency-modulated by the luminance signal at a substantially constant recording current within the frequency range corresponding to a range from the sync signal leading edge to the white peak. In an ordinary recording circuit, the extent of erasure of the recorded audio signal varies with the content of the frequency-modulated luminance signal when the luminance signal is recorded on the recording track of the frequency-modulated audio signal, because recording is made by increasing a recording current for a signal having an instantaneous frequency which is low. Therefore, the recorded signal which is frequency-modulated by the audio signal is amplitude-modulated by the luminance signal, and interference with the audio signal due to the change of S/N corresponding to the change of the amplitude occurs. In the recording circuit of the present invention, however, no interference is applied to the audio signal because the extent of erasure of the recorded audio signal by the luminance signal remains substantially constant irrespective of the content of the luminance signal.

BACKGROUND OF THE INVENTION 
This invention relates to an apparatus for multiplex recording and 
reproduction of audio and video signals, and more particularly to a 
recording apparatus suitable for multiplex recording of audio signals and 
color video signals by use of a magnetic recording/reproduction apparatus 
such as video tape recorder. 
In video tape recorders in general, a video signal is recorded by a rotary 
head so as to form an oblique track on a magnetic tape, while an audio 
signal is recorded by a fixed head on a track disposed in the longitudinal 
direction of the tape at a portion separate from the oblique track 
(generally, at the edge portion of the tape), as is well known in the art. 
In a home video tape recorder, a slow running speed is selected for the 
magnetic tape in order to increase the recording density and to facilitate 
long time recording. In an NTSC video tape recorder, for example, the tape 
running speed is prescribed to be about 33 mm/sec in the standard mode and 
about 11 mm/sec in the triple recording mode. This tape running speed is 
lower than the tape running speed of an audio tape recorder using a 
compact cassette. For this reason, the video tape recorder can not provide 
a sufficiently satisfactory frequency range for the recording and 
reproduction of audio signals for which the tone quality is of particular 
importance, particularly in the triple recording mode. 
A method of recording and reproducing audio signals by use of a rotary head 
in the same way as video signals has therefore been proposed to eliminate 
the problem described above. In the home video tape recorder, the color 
television signal is recorded by a common rotary head on the magnetic tape 
as an addition signal of a signal obtained by frequency-modulating a 
suitable carrier by a luminance signal and a carrier color signal which is 
down-converted below the band of the frequency-modulated carrier signal 
described above. Recording of the audio signal in accordance with this 
proposal is made in a superposed manner by the rotary head on the same 
recording track with the video signal as a frequency-modulated signal with 
its band set between the band of the luminance signal generated as the 
frequency-modulated signal and the band of the carrier color signal 
converted to a low frequency band. As one of the means for the multiplex 
recording of the audio and video signals, it has been proposed to use 
rotary heads that are individually disposed for the audio and video 
signals in such an arrangement that these heads can trace the same track 
on the magnetic tape. In this case, the frequency-modulated audio signal 
of the low frequency band is first recorded, and the frequency-modulated 
luminance signal having a high frequency is then recorded. According to 
this method, the signal having a low frequency magnetizes the deep portion 
of the magnetic layer of the magnetic tape, whereas the signal of a high 
frequency magnetizes only the shallow portion of the magnetic layer of the 
tape. Therefore, the shallow portion of the recording magnetization by the 
audio signals having the low frequency (in the form of the 
frequency-modulated signal, of course) will be erased by the subsequent 
recording of the frequency-modulated luminance signals, but the deep 
portion remains unerased. As a result, two kind of signals can be recorded 
superposed on one track at the deep portion of the magnetic layer and at 
its shallow portion close to the surface. 
This multiplex recording method of the video and audio signals is effective 
where no margin for multiplexing them as a frequency multiplex signal 
exists between the frequency band of the frequency-modulated luminance 
signal and that of the carrier color signal converted to the low frequency 
band. When the color video signal is recorded in the VHS system video tape 
recorder, for example, it is stipulated to convert the color video signal 
so as to attain the frequency spectrum such as shown in FIG. 1. In the 
case of the luminance signal of the NTSC signal, the carrier signal is 
frequency-modulated so that the leading edge of a sync signal is 3.4 MHz 
and the white peak level if 4.4. MHz, and a frequency distribution such as 
represented by reference numeral 4 is provided. In the case of the carrier 
color signal, the carrier frequency is converted to about 629 KHz, and a 
frequency distribution having a band width of about 1 MHz, such as 
represented by reference numeral 5 in the drawing, is provided. Thus, 
there is no margin between the distribution 4 of the luminance signal and 
that of the carrier color signal. For the spectrum of the video signals 
for recording, the audio signal converted to the frequency-modulated 
signal is set within the frequency range represented by reference numeral 
6 in the drawing, and is recorded and reproduced by the separate rotary 
head. 
Some of the inventors of the present invention previously proposed an 
apparatus for superposedly recording the audio signal on the track of the 
video signal in U.S. patent application Ser. No. 575,665 dated Jan. 31, 
1984. The previous application proposes the arrangement of each head in 
order to minimize the interference due to cross talk from the adjacent 
tracks and the interference between the video signal and the audio signal. 
In the home video tape recorders in general, the optimum recording current 
at which the reproduction output becomes maxium within a practically 
suitable range tends to decrease with a higher signal frequency. 
Therefore, predetermined frequency characteristics are set to a recording 
amplifier. In the VHS system, for example, the characteristics are set so 
that a 1 MHz recording current is about 6 dB when the recording current of 
a 3.4 MHz signal is 0 dB. 
Therefore, the carrier corresponding to the leading edge of the sync signal 
becomes greater than the carrier corresponding to the white peak in the 
recording current of the video signal. As a result, the 
frequency-modulated audio signal, that has been recorded before the video 
signal, is not uniformly erased by recording of the subsequent 
frequency-modulated luminance signal when the former is somewhat erased by 
the latter, so that the extent of the erasure varies depending upon the 
content of the video signal. In other words, the extent of the erasure 
becomes maxium at the sync signal and minimum at the white peak. 
Therefore, if the luminance signal of the video signal, such as shown in 
FIG. 2a, is recorded, the amplitude of the reproduction output of the 
frequency-modulated audio signal is amplitude-modulated corresponding 
thereto, as shown in FIG. 2b. FIG. 2b shows the envelope of the 
reproduction output of the signal which is frequency-modulated by the 
audio signal, and the amplitude is lowest at the portion 20 corresponding 
to the sync signal and greatest at the portion 21 corresponding to the 
white peak. Thus, though frequency-modulated, the origial audio signal can 
be obtained by demodulation because it is fundamentally a 
frequency-modulated signal. The influence of the amplitude modulation 
generated by the erasing action of the recording of the luminance signal 
is not so great as to cause distortion in the demodulated signal waveform. 
In the case of the reproduction signal shown in FIG. 2b, however, the 
carrier-to-noise ratio C/N of the portion 20 corresponding to the sync 
signal is deteriorated more than the C/N of the portion 21 corresponding 
to the white peak. (Since the audio signal in this case is one obtained by 
frequency-modulating the carrier, the ratio C/N is used so as to 
distinguish it from the signal-to-noise ratio S/N.) As a result, when this 
signal is demodulated, the noise of the portion corresponding to the 
portion 20 relatively increases much more than that of the portion 21 and 
eventually, the S/N of the portion 20 becomes worse than that of the 
portion 21. This means that in the demodulated audio signal, noise 
increases periodically in synchronism with the sync signal of the video 
signal. Particularly, the increase of the noise, that appears in 
synchronism with the vertical sync signal, is very noticable to the 
listener and is unpleasant to the ear, even though it is not great, 
because the fundamental repeating frequency is as low as 60 Hz, for 
example, and because it is the noise that did not exist originally. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a recording 
apparatus which eliminates the problem described above, and prevents the 
frequency-modulated audio signal that is to be recorded superposedly on 
the same track as the video signal from being amplitude-modulated by the 
video signal. 
In order to accomplish the object described above, the present invention 
operates to make a recording current at a sync signal portion equal to 
that of a white peak portion and to make the recording current constant in 
a recording system of a frequency-modulated luminance signal. This can be 
realized, for example, by inserting a trap circuit which traps a signal 
close to the frequency corresponding to the instantaneous frequency of the 
carrier modulated by the sync signal, in a recording circuit including an 
amplification circuit which amplifies and applies a modulated carrier 
signal to a magnetic head.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 3 is a block diagram of one embodiment of the present invention, and 
shows the principal portions of a recording circuit. A reproduction 
circuit is not shown in the drawing. A color video signal applied to a 
terminal 10 is filtered by a low-pass filter (LPF) 12 to extract a 
luminance signal, while a band-pass filter (BPF) 14 extracts a carrier 
color signal. The extracted luminance signal is applied to a modulator 13 
for frequency-modulating a suitable carrier. In the VHS system for 
recording and reproducing the NTSC signal, for example, modulation is 
effected so that the carrier frequency is 3.4 MHz at the leading edge of 
the sync signal and is 4.4 MHz at the white peak. The carrier frequency of 
the extracted carrier color signal in the VHS system is down-converted 
from 3.5 MHz to about 629 KHz by a frequency conversion circuit 15. The 
frequency-modulated luminance signal and the down-converted carrier color 
signal have the frequency spectra such as shown already in FIG. 1. 
Conventionally, they are added and are then recorded through a recording 
amplifier. In accordance with the first embodiment of the present 
invention, however, these two signals are not directly added as shown in 
FIG. 3, but a carrier color signal converted to a low frequency range is 
added by an adder 16 to a frequency-modulated luminance signal which has 
passed through a trap circuit 22 disposed in the path of the 
frequency-modulated luminance signal. After being added, these signals are 
amplified to a predetermined magnitude by the recording amplifier 17, are 
applied to two rotary magnetic heads 8a and 8b, and are thereafter 
recorded on the magnetic tape 3. This embodiment shows an example when the 
invention is applied to a video tape recorder of a two-head helical scan 
system. 
The audio signal that is to be superimposed on the recording track of the 
video signal that has been recorded in the manner described above is 
applied to a terminal 11. This audio signal is applied to a modulator 18 
having a carrier frequency different from that of the modulator 13 for the 
luminance signal. This modulator 18 generates a frequency-modulated audio 
signal having a frequency band 6 above the band of the carrier color 
signal 5 converted to a low frequency range at the low frequency portion 
of the frequency-modulated luminance signal as shown in FIG. 1. If the 
audio signals applied to the terminal 11 are stereo audio signals, 1.3 MHz 
and 1.7 MHz, for example, are set as the center frequency of the carriers 
for the right and left channel signals. This frequency-modulated audio 
signal is applied to the two rotary magnetic heads 9a and 9b through the 
recording amplifier 19, and is then recorded on the magnetic tape 3 with 
the video signal. 
FIG. 4 shows a definite example of the trap circuit 22. FIG. 5 shows an 
example of the frequency characteristics of this circuit. The trap circuit 
22 comprises the combination of an LC series resonance circuit with a 
transistor using the former as the emitter follower. An ordinary trap 
circuit can be used as the trap circuit 22. Is is most optimal to set the 
trap frequency within the range of from 2.7 to 3.5 MHz for the VHS system 
of NTSC. The overall frequency characteristics of the recording current of 
the frequency-modulated luminance signal are such as represented by solid 
line in FIG. 6, for example, when this trap circuit is used. In this 
manner, it is possible to make constant the recording signal current 
within the frequency range corresponding the range of the leading edge 
(3.4 MHz) of the sync signal of the frequency-modulated luminance signal 
to the white peak (4.4 MHz). In the drawing, the two-dot chain line 
represents the frequency characteristics of the conventional recording 
signal current. Since pre-emphasis is applied, the instantaneous carrier 
frequency of the frequency-modulated luminance signal is primarily 
distributed within the range of from 3 to 5 MHz; hence, the energy within 
this range accounts for the major proportion. It is therefore preferred to 
have the signal current within this range as constant as possible in order 
to prevent any interference to the frequency-modulated audio signal that 
is superposedly recorded. 
If the signal current of 3 to 5 MHz is made substantially constant in the 
manner described above, the frequency characteristics deviate somewhat 
from the characteristics of the conventional, optimum recording current 
(shown by the two-dot chain line in FIG. 6) that are set so as to obtain 
the highest reproduction output level. However, the change of the 
reproduction output level with the change in the quantity of the recording 
current is not very critical, and an error of from about .+-.1 to about 
.+-.1.5 dB from the center value is pemissible without any problem at all 
when setting the current value. Therefore, when the present invention is 
practised, deterioration of picture quality due to the drop of the 
reproduction output of the video signal does not occur. 
FIG. 7 shows the reproduction output envelope of the frequency-modulated 
audio signal and the waveform of the video signal when the video signal 
and the audio signal are recorded so as to be superimposed in accordance 
with the embodiment of the invention. FIG. 7a shows the waveform of the 
video signal and FIG. 7b is the envelope waveform of the 
frequency-modulated audio signal in the same way as in FIG. 2. As shown in 
the drawing, the reproduced frequency-modulated audio signal is not 
amplitude-modulated by the video signal because there is no difference in 
the extent of erasure due to the signal content of the frequency-modulated 
luminance signal of the video signal. Therefore, the problem of the 
carrier-to-noise ratio (C/N) of the audio signal periodically 
deteriorating at the sync signal portion can be eliminated, and recording 
and reproduction of excellent tone quality can be obtained. 
The carrier color signal converted to a low frequency range that is 
simultaneously recorded with the frequency-modulated luminance signal is 
set to a level lower by 10 to 12 dB than the current of the 
frequency-modulated luminance signal and is recorded. Therefore, it is 
substantially possible to neglect the amplitude-modulation of the 
frequency-modulated audio signal, that has been recorded previously, by 
the carrier color signal, and there is no problem with the effect of the 
color signal. 
In the embodiment shown in FIG. 3, the trap circuit 22 is shown disposed 
before the adder 16, but this position is not particularly critical so 
long as the frequency characteristics of the current of the 
frequency-modulated luminance signal to be recorded eventually become 
those shown in FIG. 6. It is also possible to obtain such frequency 
characteristics by other means, that stress a specific range, in place of 
the trap circuit. It is therefore obvious that the effect of the present 
invention can likewise be obtained so long as the overall frequency 
characteristics are such as those shown in FIG. 6.