JPS6339986B2 - - Google Patents

Description

[Detailed Description of the Invention] In a video recorder, it is known that a video signal is recorded on a so-called oblique track that is inclined at an angle of about 6 degrees with respect to the longitudinal direction of the tape by frequency modulating a video carrier wave. be. In this case, one field is preferably recorded in each case along a diagonal track of this type. Recording and scanning
This is done by two heads, each scanning an alternating diagonal track.

In this type of device, the audio signal is recorded and played back by a stationary head on a longitudinal track approximately 1 mm wide, extending parallel to the tape edge. For recording and reproducing audio signals, therefore, the longitudinal velocity of the tape is important, and not the significantly higher relative velocity between the head and the tape's diagonal tracks. The longitudinal speed of the tape is actually limited to a value on the order of 2 cm/s in order to increase the total recording time of the tape. This low relative speed between the tape and the audio head has a detrimental effect on the quality of the recorded audio signal. At such low relative speeds Hifi quality is practically no longer achievable. Moreover, the audio signal recorded is approximately 70Hz to 7-10K.
It only has a bandwidth of Hz. Since the width of the track in the longitudinal direction is small, the signal-to-noise ratio is also relatively poor. The signal-to-noise ratio is even worse when recording two audio signals for stereo playback because the width of the longitudinal track has to be halved.

Analogous to the case of video discs, it is conceivable to record audio signals together with the video carrier on diagonal tracks. However, solutions of this type have not been advantageously realized in practice to date. This is because, on the one hand, the available frequency bands are already fully utilized. Approximately 0~1.3M
The Hz band is occupied by the down-converted and orthogonally modulated color subcarrier, and the remaining frequency band is occupied by the frequency spectrum of the modulated video carrier. It is not practically possible to record the audio carrier in the narrow frequency gap that still exists between the modulated color subcarrier and the frequency spectrum of the video carrier for the following reasons. That is, phase and group delay time distortions occur due to the steep filter characteristics required for the color subcarrier and the video carrier. On the other hand, when audio signals are recorded on diagonal tracks, significant noise is generated during playback due to head alternation. During video playback, head replacement occurs during the vertical blanking period, so it does not interfere. However, when the audio signal is reproduced, head replacement is audible as interference noise. This is because head alternation briefly interrupts the scanning of the audio signal from the tape, for example by modulating the carrier wave, at a cycle of 50 Hz. The resulting needle-shaped interference pulses with a fundamental frequency of 50 Hz and mostly harmonics produce continuous interference noise during audio reproduction.

An object of the present invention is to reduce the noise generated in the audio due to head replacement when an audio signal is recorded on a diagonal track by frequency modulating an audio carrier wave.

This problem can be solved by the present invention as claimed in claim 1.
The problem is solved by the features described in the section. Advantageous embodiments of the invention are described in the subclaims.

That is, in the present invention, the dip in the amplitude of the audio frequency that may occur due to a head change is avoided by causing the selective amplifier provided for amplifying the audio carrier to oscillate during the head change period. . The dip in the amplitude of the audio frequency is then reliably compensated for by a carrier wave whose frequency has a value that also occurs in the modulated audio carrier during normal operation. Since the selective amplifier for the audio carrier is needed only for amplification and need only be periodically boosted by keying pulses for amplification,
The circuit technology costs are low. Increasing the amplification factor in this way is possible, for example, by periodically reducing the normal negative feedback factor present in the amplifier. According to the present invention, as a result of experiments, it was possible to reduce the noise in the audio signal by 20 dB, that is, about 10%.

The curve shape of the amplification-increasing keying pulse is preferably selected to have a continuous course without abrupt changes in amplitude, for example a triangular shape. The amplification of the amplifier is then increased continuously to a maximum value approximately in the middle of the head change.
It is then continuously reduced to normal amplification operation again. By continuously varying the amplification factor in this way, undesired transient oscillations and disturbances of the useful signal can be avoided.

The amplitude enhancement of the modulated audio carrier caused by increasing the amplification during the head change is:
It is again compensated by the amplitude limiter that is usually provided in the FM demodulator.

Next, the present invention will be explained in detail using illustrated embodiments.

According to FIG. 1, a color subcarrier F having a frequency of 0.63 MHz is recorded on the magnetic tape in the lower frequency range. The chrominance subcarrier is obtained from the orthogonally modulated PAL-chrominance subcarrier by frequency downconversion. The video signal Y is recorded by frequency modulation of a video carrier wave. The frequency deviation of the modulation of the video signal is the frequency 3.8MHz and
4.8MHz, with 3.8MHz corresponding to the black level of the video signal and 4.8MHz corresponding to the white level. 1.3 to 7.3MHz depending on modulation
A frequency spectrum of . At frequencies 1.7 and 1.9MHz within this frequency spectrum,
Two audio carriers 1.2 each FM modulated by one low frequency audio signal are recorded.
These two low frequency audio signals are stereo signals or audio signals of various languages. of this signal,
Recording on the diagonal tracks of a magnetic tape is carried out by means of two video heads, with one field being recorded on each diagonal track. The two video heads thus each act alternately on one field during recording and playback. This head replacement naturally causes noise having a fundamental frequency of 50 Hz in the reproduced audio signal. This noise can also occur when the scanning periods of the two video heads overlap each other and the signal path receives signals from the two video heads during this period. That is, depending on the phase position of the two carrier waves scanned for reproduction from the video head, these carrier waves are added together, and thus the amplitude increases or, in the case of opposite phases, the carrier waves cancel.
A reduction in amplitude may then occur.

In FIG. 2, the signals of FIG. 1, scanned alternately by two video heads, are fed via an amplifier 3 to two frequency selective filters 4,5. The two video heads are symbolically illustrated by only one video head 6. In the filter 4, the modulated chrominance subcarrier F with a frequency of 0.63 MHz is selectively evaluated, converted again in the frequency converter 7 to the original PAL-chrominance subcarrier frequency of 4.43 MHz and sent to the summing stage 8. Supplied. The modulated video carrier over 1.3 to 7.3 MHz is frequency-selectively evaluated in the filter 5 and fed to the FM demodulator 9. The video signal Y thus obtained is likewise supplied to a summing stage 8, from which a color television signal (FBAS) is taken off at a terminal 10 of this summing stage. The filter 5 includes two notch filters, the notch filters being 1.7 and 1.7 where two sound carriers are present.
Remove the 1.9MHz frequency. This avoids noise caused by the audio carriers 1 and 2 in the reproduced video.

The signal from the output side of the amplifier 3 is further passed through an amplitude controllable amplifier 11 with a frequency of 1.7 and
two filters 12 tuned to 1.9MHz,
Reach 13. By means of filters 12, 13 the two modulated sound carriers 1, 2, each with sidebands, are evaluated and fed to an FM demodulator 14, 15. Two low frequency audio signals NF1 and NF2 appear at the outputs of these demodulators.

Also shown in FIG. 2 is a head wheel 16. The speed of this head wheel is adjusted by servo adjustment 17 in a known manner so that the video head precisely scans the magnetic tape along a diagonal track. head wheel 1
6, a headwheel pulse 18 is taken off from the permanent magnet and the associated excitation winding. This headwheel pulse occurs during each head change and thus after each scanning of one diagonal track, ie after each field. This pulse 18 is fed to a pulse shaper 19. A vertical synchronization pulse 21 is further obtained from the video signal at the output side of the filter 5 in a synchronization signal separation stage 20,
This pulse is likewise fed to a pulse shaper 19. The pulses 18 substantially determine the frequency of the keying pulses 22 produced at the output of the pulse shaper based on the rotation of the headwheel and filter 5.
The keying pulse 22 produced at the output of the pulse shaper 19 is a pulse 21 based on the signal at the output of the pulse shaper 19.
Determine the correct phase of

During the scanning of the field, amplifier 11 is used as a selective amplifier for the two modulated audio carriers 1, 2. During a head change, the amplification factor of amplifier 11 is increased by keying pulse 22 such that the amplifier oscillates at its amplitude frequency, ie approximately at the frequency of the modulated audio carrier. This avoids a drop in the amplitude of the audio carrier waves supplied to the filters 12, 13.

FIG. 3a shows the individual carrier packets 23 for each modulated audio carrier for successive fields each having a duration of 20 ms. The head change causes a drop in amplitude in the audio carrier between the two fields, ie after 20 ms. These amplitude drops result in needle-shaped interference pulses shown in FIG. 3b at the outputs of the demodulators 14, 15. These interfering pulses can be heard as crackles at a fundamental frequency of 50Hz during audio playback.
FIG. 3c shows the keying pulse 22. FIG. This keying pulse causes the amplifier 11
The amplification of is increased until the amplifier oscillates during the amplitude dip 24. In order to avoid disturbances due to very rapid amplification changes, the keying pulse 22 with a triangular wave-like course changes the amplification continuously. Since the amplifier 11 oscillates during the head change and therefore also generates an output signal during this period, there is a continuous carrier wave at the output of the amplifier 11 which no longer has the amplitude dip 24 of FIG. 3a. appears (Fig. 3d). After demodulating the audio carrier wave processed in this way, only the interference signal shown in FIG. 3e is produced at the output of the demodulators 14, 15. The amplitude of this signal is reduced by about 10% compared to the amplitude of the interfering signal in FIG. 3b, corresponding to an improvement in the signal-to-noise ratio of 20 dB. The noise based on the signal of FIG. 3e is practically no longer perceptible.

FIG. 4 shows an example of a circuit configuration for the selection amplifier 11. The frequency modulated audio frequency F _NF is
Supplied to the base of transistor 25. Transistor 25 has an oscillating circuit 26 tuned to the audio carrier frequency and used as a working impedance and a negative feedback resistor 27. The amplified sound carrier F _NF is tapped off at terminal 28 . During normal amplification operation, the transistor 29 connected in parallel with the resistor 27 is turned off during scanning of the diagonal track. Resistor 27 then acts in a known manner as a negative feedback resistor for linearizing and reducing the amplification. Transistor 25 then operates as a linear amplifier for the modulated audio carrier. During a head change, transistor 29 is controlled to conduct by keying pulse 22 and shorts resistor 25. This reduces the negative feedback of the amplifier and the effective amplification actually begins to oscillate due to the always present positive feedback path and during this period from terminal 28.
An unmodulated carrier wave having a constant amplitude and approximately the frequency of the modulated audio carrier wave is provided.

According to one embodiment of the invention, a further improvement can be achieved by increasing the amplitude of the recorded audio carrier by keying pulses during each head change.

This method is based on the following recognition. That is, by increasing the amplitude of the audio carrier recorded during a head change, the noise naturally introduced into the audio signal by a head change can be significantly reduced. This amplitude enhancement is possible because no video is played back during this period and therefore does not cause any disturbance to the played video. The amplitude enhancement can span multiple scan lines, for example 6 to 10 scan lines before the vertical sync pulse. This is because in commercially available black-white and color television receivers, these scan lines are not visible on the screen due to the image overlap required due to tolerances.

The noise that occurs in the audio itself due to head switching is
With the configuration of the present invention, it can be reduced by approximately 20 to 30 dB. No additional circuit technology measures are required during reproduction. This is because it is necessarily improved by the amplitude enhancement of the sound carrier that takes place during recording.

One embodiment of this method will be described with reference to FIGS. 5 to 7. Lower case letters indicate where in FIG. 5 the signals of FIG. 6 occur.

In FIG. 5, the color television signal (FBAS) is selectively transmitted to the amplifier 31 by
A modulated PAL with a frequency of 4.43 MHz is separated into a color subcarrier F and a luminance signal Y. The color subcarrier is down-converted to a frequency of 0.63 MHz in a frequency converter 32 and fed to a summing stage 33.
The luminance signal Y generates a picture carrier wave in an FM modulator 34, which picture carrier wave is also supplied via a filter 35 to an addition stage 33. The pass characteristic curve of the filter 35 lies above the frequency converted color subcarrier and has two, 1.7MHz and
It has two cut-off points located at 1.9MHz.
Two audio signals NF1 and NF2 representing stereo signals or audio signals in different languages are:
Two FM modulators 56, 57 are fed, each generating a frequency modulated audio carrier. The audio carriers at 1.3 and 1.8 MHz are also fed to the summing stage 33 via controllable amplifiers 38, 39. Therefore, this addition stage 33
NF
NF2 provides an audio carrier having a frequency of 1.7 MHz, modulated by NF2, and an audio carrier having a frequency of 1.9 MHz, modulated by NF2. This composite signal appearing at the output of the summing stage 33 is recorded by a video head 40 on diagonal tracks of a magnetic tape 41. Actually video head 4
0 consists of two rotating video heads using head drums that record signals on diagonal tracks of magnetic tape 41 alternately from feed to feed.

Control of the amplitude of the audio carrier wave by the modulators 56 and 57 will be explained based on FIG. 6. FIG. 6a shows carrier wave packets for a frequency modulated signal which is scanned from tape 41 during playback for successive fields each having a duration of 20 ms. It can be seen that an amplitude drop 42 with a repetition frequency of 50 Hz occurs in each case in the signal between the two fields. These drops cause 50Hz needle-like noise in the demodulated latter voice channel. This type of noise can be heard as crackles in the audio. In FIG. 5, a pulse generator 43 is additionally provided. This pulse generator is connected on the one hand to a head wheel 44;
and on the other hand by pulses of the vertical synchronization frequency derived from the Y-signal using a separation stage 45. At its output, the generator 43 has a 640 .mu.s time period, as shown in FIG.
A pulse 46 having a duration of . These pulses reach the control inputs of amplifiers 38, 39 and control their amplitude to double the value. This results in the sound carrier shown in FIG. It is elevated. The audio carrier of FIG. 6c is then used for recording. By increasing the amplitude as shown in FIG. 6c during the amplitude drop 42, the noise naturally occurring during audio reproduction is significantly reduced.

FIG. 7 shows the time position of pulse 46 relative to the composite sync pulse of the video signal.
In this case, the horizontal synchronization pulse Z, the previous equalization pulse A _V ,
The vertical sync pulse and subsequent equalization pulse A _H are shown. The keying pulse 46 is at the start point t of the vertical synchronization pulse V, indicated by the dashed line.
It starts approximately 11.5 scan lines before 2. The center of the keying pulse is at a time t approximately 6.5 scan lines before the start time t2 of the first vertical sync pulse.
Located at 1. Time t1 is the time of head change, at which time, for example, the scanned signal is switched from one video head to the other and the above-mentioned noise may occur in the audio. The beginning and end of the keying pulse 46 are symmetrical with respect to time t1. The total duration of keying pulse 46 is 10 scan lines, ie, 5 scan lines (320 μm) before t1 and 5 scan lines after t1. The duration of the keying pulse 46 can also be of other values, for example on the order of 300-500 μm. The keying pulse 46, which starts already before the start of generation of the vertical synchronization pulse V, can be generated as follows. At the beginning of the field with number n, a pulse is generated from a pulse derived from the head wheel and a pulse derived from the vertical sync pulse, as shown in FIG. This pulse is not initially in the desired position as shown in FIG. 7, but begins a little later. This pulse is shaped by a pulse shaping stage or delay element to a duration somewhat shorter than one field, e.g.
delayed by 300 scan lines and field n+
1 is used as the keying pulse 46 in FIG. The time position of the pulse 46 is determined at the time t1 at which, even with the permissible deviations in the duration of this pulse, the above-mentioned needle-like pulses, which cause noise in the audio, may appear in the demodulated audio signal during reproduction. is selected to span the time domain around .

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a frequency spectrum for recording individual signals on a magnetic tape, FIG. 2 is a block circuit diagram for playback of an embodiment of the video recorder of the present invention, and FIG. FIG. 3b shows a needle-shaped disturbance pulse in relation to FIG. 3a; FIG. 3c shows a keying pulse also Figure 3a is shown in conjunction with Figure 3a; Figure 3d is a diagram showing a continuous carrier wave with no drop in amplitude;
FIG. 4 is a circuit diagram showing the construction of an example of a selective amplifier; FIG. 5 is a recording of an embodiment of a video recorder according to the invention; FIG. FIG. 6a shows the carrier packets for successive fields; FIG. 6b shows the keying pulses temporally related to FIG. 6a; FIG. 6c shows the higher amplitude FIG.
FIGS. 7a and 7b are diagrams showing a pulse train of a composite synchronizing signal, and FIG. 7c is a diagram of a keying pulse showing the time position relative to the pulses in FIGS. 7a and 7b. 1, 2...Audio frequency, 11...Selection amplifier,
16,44...Head wheel, 18...Head replacement pulse, 21,V...Vertical synchronization pulse, 2
2, 46...Keying pulse, t1...Keying pulse, t2...Start point of vertical synchronization pulse.

Claims (1)

[Claims] 1. A video carrier wave FM modulated by a video signal and an audio carrier wave 1, 2 modulated by a low frequency audio signal are recorded along diagonal tracks of a magnetic tape, and the scanning is alternating with each other. In a video recorder with improved audio reproduction, which is carried out using a plurality of heads, during reproduction the scanned audio carriers 1, 2 are guided through a selective amplifier 11, the amplification of which depends on the duration of the head change. A video recorder with improved audio reproduction, characterized in that a medium keying pulse 22 causes the amplifier 11 to be raised to oscillate at its resonant frequency. 2. The video recorder according to claim 1, wherein the curve shape of the keying pulse 22 has a continuous course without abrupt changes in amplitude. 3. The video recorder according to claim 2, wherein the keying pulse 22 has a triangular curved shape. 4 The duration of keying pulse 22 is approximately
10. The video recorder according to claim 1, wherein the recording time is 100 μs. 5. The video recorder of claim 1, wherein the keying pulse 22 begins before the head change and ends after the head change. 6 The keying pulse 22 is the head wheel 1
2. A video recorder according to claim 1, wherein the video recorder is formed using pulses 18 derived from 6. 7. The video recorder according to claim 1, wherein the keying pulse 22 is formed using a vertical synchronization pulse 21 derived from a video signal. 8. Video recorder according to claim 1, wherein the amplitude of the audio carrier is increased by keying pulses 46 each time during a head change. 9. A video recorder according to claim 8, wherein the amplitude is increased approximately by a factor of two. 10. The video recorder according to claim 8, wherein the keying pulse 46 controls the amplification degree of the amplifiers 38 and 39 provided in the path of the audio carrier wave. 11. A video recorder as claimed in claim 8, in which the keying pulses 46 are obtained from a head alternating pulse derived from the headwheel 44 and a pulse derived from the vertical synchronizing pulse V. 12. The video recorder of claim 8, wherein the keying pulses 46 are obtained during a head change period by delaying pulses generated during a temporally preceding head change period. 13. The video recorder of claim 8, wherein the keying pulses 46 have a duration of about 400-600 μs. 14. The video recorder of claim 8, wherein the keying pulse 46 temporally precedes the vertical synchronization pulse of the video signal. 15. The video recorder according to claim 8, wherein the starting and ending ends of the keying pulses 46 are positioned symmetrically with respect to the time t1 of head replacement. 16 The middle of the duration of the keying pulse 46 is approximately from the starting point t2 of the first vertical synchronization pulse.
6.5 The first claim in front of the scan line
The video recorder according to item 5.