JPS5914809B2 - emphasis circuit - Google Patents

Description

Detailed Description of the Invention For example, in a VTR (magnetic recording/reproducing device), a luminance signal is converted into an FM signal for recording and reproduction, but when demodulating the original luminance signal from the FM signal during reproduction, This results in noise whose level increases in proportion to frequency, so-called conformal noise.

Therefore, in VTRs, pre-emphasis is performed to emphasize the high range of the luminance signal during recording, and de-emphasis is performed to attenuate the high range during playback to reduce conformal noise. Therefore, increasing the amount of pre-emphasis will improve the S/N during playback, but increasing the amount of pre-emphasis will increase the frequency shift of the FM signal in the high range of the luminance signal when converting it to an FM signal. The band becomes so wide that it becomes impossible to record and play back on a VTR.

The present invention aims to solve these problems and to provide stable operation.

Let's explain one example below.

In FIG. 1, during recording, the luminance signal is input to the input terminal 1.
is supplied to a pre-emphasis circuit 3 through a clamp circuit 2.

This pre-emphasis circuit 3 has a transistor Q1, the base of which is supplied with the luminance signal from the clamp circuit 2, the emitter of which is grounded through a resistor R4, and the collector of which is connected through a series circuit of resistor R2 and coils L1 and L2. It is connected to the power supply terminal T.

Furthermore, a resonance capacitor C1 and a Q-dump resistor R3 are connected in parallel to the coil L0, and diodes D3 and D2 having opposite polarities are connected in parallel through a DC cut capacitor C2. Also coil L2
, a resistor R4 having a -Q dump angle is connected in parallel, and diodes D3 and D4 having opposite polarities are connected in parallel through a DC cut capacitor C3. The luminance signal obtained at the collector of transistor Q1 (which has been pre-emphasized as described below) is then extracted by emitter follower transistor Q2. This luminance signal is then supplied to the FM modulation circuit 4 to be converted into an FM signal, and this FM signal is sent to the recording amplifier 5.
The FM signal is further supplied to the rotary magnetic head 7 through the recording side contact R of the recording/reproduction changeover switch 6, and one field of this FM signal is recorded on the magnetic tape 8 as, for example, one diagonal magnetic track.

On the other hand, during reproduction, an FM signal is reproduced from the tape 8 by the head 7, and this FM signal is supplied to the demodulation circuit 13 through the reproduction side contact P of the switch 6, the reproduction amplifier 11 and the limiter 12, and the luminance signal is demodulated. is,
This luminance signal is supplied to the de-emphasis circuit 14.

This de-emphasis circuit 14 has complementary characteristics to the pre-emphasis circuit 3, and is provided with a transistor Q3, to whose base a luminance signal is supplied from the demodulation circuit 13, and whose collector is connected to a power supply terminal T1 through a resistor R.
and its emitter is connected to resistor R6 and coil L3
, L4 to ground through a series circuit.

Further, a resonance capacitor C4 and a Q-dump resistor R7 are connected in parallel to the coil L3, and diodes D5 and D6 having opposite polarities are connected in parallel through a DC cut capacitor C5. Also coil L4
A Q-dump resistor R8 is connected in parallel, and diodes D7 and D8 of opposite compatibility are connected in parallel through a DC cut capacitor C6. Furthermore, an emitter follower transistor Q4 is connected to the collector of the transistor Q3'0). A de-emphasized luminance signal is taken out from the transistor α, and is sent to the output terminal 16 through the buffer amplifier 15.
It is taken out.

According to such a configuration, the following operations are performed in the pre-emphasis circuit 3.

That is, for the sake of simplicity, it is assumed that only elements R2 and L2 are connected to the collector of transistor Q, as shown by the solid line in FIG.
For example, suppose that a luminance signal as shown in FIG. 3A is supplied to the display. Then, the impedance of the coil L2 is small in the low range, but becomes large in the high range, so there is a level proportional to the high range level of the luminance signal at both ends of the coil L2, as shown in Figure 3B. High-frequency components of the differential waveform can be obtained. Therefore, a pre-emphasized luminance signal is obtained at the collector of transistor Q1, as shown in FIG. 3C. However, in this case, as also shown in FIGS. 3A and 3B, the ratio between the luminance signal and the high frequency component, that is, the amount of pre-emphasis for the high frequency range is constant regardless of the level of the high frequency component of the luminance signal. However, in the pre-emphasis circuit 3 according to the present invention, diodes D3 and D4 are connected as shown by broken lines in FIG.

Therefore, the level of the high range of the luminance signal increases and the coil L
Even if the levels of the high-frequency components at both ends of 2 try to increase,
Since the diodes D3 and D4 act as limiters, their high frequency components are amplitude limited, as shown in FIG. 3D, and pre-emphasis is performed by the amplitude limited high frequency components. Therefore, the ratio of the luminance signal to the high frequency component, that is, the amount of pre-emphasis for the high frequency range, becomes smaller as the level of the high frequency range of the luminance signal increases, as shown in FIG. In the present invention, as shown in FIG. 1, elements Ll and Cl are also connected, so by selecting their resonance frequencies in advance, pre- ence facilitation is achieved even for the middle range of the luminance signal. Since the diodes Dl and D2 are connected, the amount of pre-emphasis for the middle range becomes smaller as the level of the middle range of the luminance signal increases, as shown in FIG. That is, in the pre-emphasis circuit 3 of FIG. 1, pre-emphasis is performed independently on the high range and middle range of the luminance signal.

The amount of pre-emphasis becomes smaller as the level of the high range and the middle range becomes larger. In other words, the amount of pre-emphasis becomes larger as the level of the high range and the middle range becomes smaller. On the other hand, in the de-emphasis circuit 14, since each element is connected to the emitter of the transistor Q3, the pass characteristics are complementary to the characteristics of the pre-emphasis circuit 3, and therefore de-emphasis is performed. Thus, in the present invention, the fourth
Pre-emphasis is performed with the characteristics shown in Fig. 5 and Fig. 5. Therefore, when the level of the luminance signal is large, the pre-emphasis amount is about the same as before, but when the level of the luminance signal is small, the pre-emphasis is Since the amount can be increased, reproduction with a sufficiently high S/N ratio can be performed.

At this time, even if the pre-emphasis amount is increased when the luminance signal level is low, the pre-emphasis amount will be small when the luminance signal level is high, so when converted to an FM signal in the modulation circuit 4, overmodulation may occur. There is no problem, and recording and playback is possible. In addition, since emphasis is applied independently to the high and mid frequencies, optimal emphasis can be performed according to the frequency distribution (energy distribution) of the luminance signal, and from this point of view as well, reproduction with good S/N can be achieved. . Further, in the de-emphasis circuit 14, the de-emphasis element is connected to the emitter of the transistor Q to perform de-emphasis, so that no oscillation occurs due to a phase shift in the circuit 14, and stable operation is performed. Furthermore, since there is no oscillation, there is a large degree of freedom in design, and appropriate characteristics can be obtained. In addition, if coils Lll, Ll2, capacitor Cl2, and resistor R3 are connected as shown in FIG. 6 instead of elements Ll, Cl, and R3 in FIG. 1, the pre-emphasis frequency band for the middle range in FIG. It is possible to widen the slope and sharpen the characteristics of the sloped portion.

However, in that case, the resonant frequencies of the elements Lll and Cll and the resonant frequencies of the elements Ll2 and Cl2 are set as the upper and lower cutoff frequencies of the frequency band. The same can be done for the de-emphasis circuit 14 as well. Furthermore, element Ll
, Cl, C2, R3, Dl, D2 and L3, C4, C5
, R7, D5, and D6 may be further provided to perform emphasis on other bands.

[Brief explanation of the drawing]

FIG. 1 is a connection diagram of one example of the present invention, FIGS. 2 to 5 are diagrams for explaining the same, and FIG. 6 is a partial connection diagram of another example of the present invention. 3 is a pre-emphasis circuit, and 14 is a de-emphasis circuit.

Claims (1)

[Claims] 1 A transistor is provided, the collector or emitter of which is connected to one end of a power supply through a series circuit of a resistor and at least first and second inductance elements, and two diodes are connected to the first inductance element and connected to each other. A capacitor is connected in parallel to the second inductance element, and two other diodes are connected in parallel to each other with opposite polarities, and the emitter or collector of the transistor is connected to the other end of the power supply. an emphasis circuit connected to the transistor, an input signal being supplied to the base of the transistor, and an emphasized output signal being taken out from the collector or emitter of the transistor.