JPS5912052B2 - Circuit that changes the dynamic range of a signal - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a main signal path arranged to transmit an input signal from an input terminal to an output terminal with dynamic range linearity, and a main signal path connected to an input terminal or an output terminal of the main signal path. 'AI' containing a 111 limit device with inputs
and a coupling device in the main signal path for receiving the output of the separate path and coupling this output either additively or subtractively to the signal in the main signal path.

Such a circuit is described in British Patent No. 1,253,0βl (Patent No. 833,897). When the combination is additive, the circuit acts as a dynamic range compressor, and when it is subtractive, the circuit acts as a dynamic range expander.

Compressor and decompressor systems are well known devices for noise reduction. It is also known to use separate paths to split the signal into multiple frequency channels, each containing its own limiting device.

This allows compression and expansion to occur selectively on those various channels. The present invention relates to an improvement on these known circuits by providing a separate filter to split the signal into complementary frequency bands.

The invention can be particularly used to separate chrominance and luminance signals from color television signals. The usual technique for doing this separation is to use a bandstop filter with a notch in the chrominance subcarrier to take out the luminance signal (which is slightly influenced by the sidebands of the chrominance signal), and then A bandpass filter with a passband complementary to the notch centered in frequency is used to extract the chrominance signal (which is influenced by the upper sideband of the luminance signal). However, when noise reduction techniques are applied separately to different parts of the frequency spectrum, the relationship between the two separated signals is important because the signals must subsequently be recombined. It is an object of the present invention to be able to provide both of these two separate signals in a precisely defined relationship and to be easy to adjust, compared to circuits that use separate filters to extract the two signals. It is an object of the present invention to provide a signal dynamic range changing circuit with a more stable filter.

A signal dynamic range changing circuit provided according to the present invention includes a main signal path arranged to transmit a main signal component from an input terminal to an output terminal while maintaining dynamic range linearity; } V) a separate signal path including a potency limiting device, and receiving the output of the separate signal path and adding this output to the main signal component; a coupling device in the main signal path that is selectively or subtractively coupled; a first output terminal 11 connected to the output of the current amplifier 14, and a second output terminal 13 connected to the output of the current amplifier 14; a filter connected to the input of said current amplifier via a series combination of two filters at said first and second output terminals;
first and second limiting devices 24 for limiting the two signals, respectively;
25, and the coupling device connects the first and second
combine said two signals at the output terminals of said signal and combine this resultant signal with said main signal component, thereby
The present invention is characterized in that two signals are added to or subtracted from the main signal component. The input of the current amplifier effectively constitutes ground. For this reason, the first
The filter characteristics that can be applied to the signal obtained at the output terminal are:
When the reactive circuit described above is determined by a circuit network serving as a shunt arm, and this reactive circuit is a series tuned circuit, a band cancellation characteristic occurs and the center of the cancellation band becomes the resonant frequency of the tuned circuit. However, the characteristics applicable to the signal obtained at the second output terminal are determined by the circuit network in which the reactive circuit is a series arm, and when this reactive circuit is a series tuned circuit, a bandpass characteristic occurs. The center of the passband becomes the resonant frequency of the tuned circuit. Thus, the invention makes it possible to create complementary bandpass and bandstop characteristics, thereby reconstructing the spectrum of the input signal by adding the signals of the two output terminals. This property is particularly advantageous for separating chrominance and luminance signals. The present invention will be explained below with reference to the drawings.

As explained in the above-mentioned patent specifications, effective noise reduction requires treating different parts of the entire frequency spectrum separately, and this also reduces the signal power to be handled by one carrier. This is also necessary to ensure that this subcarrier does not suppress the noise reduction effect when included.

This is because if the signal we are dealing with contains a subcarrier, such as the color subcarrier of a color television signal, this subcarrier will always appear as a large amplitude signal and any noise reduction in the presence of such a signal will This is because the band limiting device is always active, thereby effectively preventing compression or expansion of the dynamic range. Therefore, the band division method shown in FIG. 1 can be used to handle the frequency spectrum of a color television signal. Figure 1 shows the 4.4
Although shown for the special case of a 3 MHZ color subcarrier frequency, it is clear that similar methods can be used for other schemes using different subcarrier frequencies. The peak passing level is o(TB). The -3αB point of the filter characteristic delimits the various bands. The characteristic shown is a passive filter characteristic.

As will be clear from what will be described later, the filters that define the various bands are feedback loops or feedback loops connected to limit filters. Located within the loop, the characteristics of the filters change in a dynamic manner as in known circuits. The frequency spectrum above about 250KHZ is 4
It is divided into three bands W, X, Y and Z.

The band w from 250 KHZ (example) to 1.6 MHZ can be selected by a conventional band filter.

Ignoring the low frequency cut-off at 1.6 MHZ, characteristic X consists of a passband from 4 MHZ to 4.9 MHZ with a notch 0.9 MHZ wide.

Characteristic Y consists of a pass band from 4 MHZ to 4.9 MHZ, if the steep notch at 4.43 MHZ is ignored, and therefore Y is complementary to X. It is these two bands that are provided by the circuit portion of FIG. 2 which implements the invention for splitting the signal into two bands. 1.6
The low frequency cutoff of band x at MHZ is the high-pass filter C.
2, R3, while the notch in band Y is provided by a sharply tuned subcarrier trap 28.

Finally, band Z is a sharply tuned passband that processes the color subcarrier and its adjacent sidebands. In this way, the band w handles relatively low frequency luminance components, the band Band Z handles the chrominance subcarrier and adjacent chrominance signal sidebands.

This method does not indicate anything about the treatment of the lowest luminance sidebands, but if needed, e.g. 5KHZ to 250
The band up to KHZ is covered by British Patent No. 1,428,963 (
It can be processed as described in Japanese Patent Publication No. 57-24689). FIG. 2 only shows the implementation of the complete circuit for dynamic range modification.

British Patent No. 1,253,031 (Patent No. 833,89
According to Figures 1 and 2 of No. 7), the universal input terminal 1
0 is connected to the input terminal or output terminal of the main path that transmits the main signal component while maintaining dynamic range linearity.
The output terminal 23 of the separate path is connected to a coupling device in the main path.
The combiner either adds the output of the 'Al' to the main signal component to compress the dynamic range, or subtracts the output of the 'Al' from the main signal component to extend the dynamic range. Referring to FIG. 2, input terminal 10 is connected to first output terminal 11 via resistor R1.

This terminal 10 is further connected to a second output terminal 13 by a resistor Rl. It is connected via series tuned circuits Cl, Ll and a current amplifier 14. Ideally, amplifier 14 has zero input impedance and infinite output impedance.

Although in practice these characteristics can only be obtained approximately, the input 15 of the amplifier effectively constitutes ground. This amplifier is a feedback pair amplifier (this is an amplifier consisting of a common emitter transistor followed by an emitter follower transistor, with the output of the emitter follower transistor being fed back to the base of the common emitter transistor). ) or a common base transistor amplifier is possible. Terminal 13 is connected to ground via a resistor R2, which is the output load impedance of amplifier 14. If we put the voltages at terminals 10, 11 and 13 as VIN, VA and VB as shown in FIG. 2, then C and L are the same as C1 and L1 since input point 15 is effectively ground.

In other words, as far as terminal 11 is concerned, this filter is a band-stop filter or notch filter tuned to the frequency ω/2π such that jωL+/°C=0. At this frequency, the tuned circuit Cl,Ll provides a direct short circuit of the input of the amplifier 14 to ground. On the other hand, when the current gain of amplifier 14 is A, it can be seen that the limited D filter for terminal 13 is a bandpass filter tuned to the same frequency as the notch filter.

A is approximately 1 in the case of a common base type current amplifier. Furthermore, if R1=AR2, VA is independent of the values of C and L.
+VB-VIN. This means that the notch in band X has exactly the same width as band Y, which further means that the Q of the band-stop filter and the band filter are the same. Equations 1 and 2 above were written in jω notation to represent the steady state condition at an arbitrary angular frequency ω, but
These equations hold even if jω is replaced by the general operator p to represent not only steady state conditions but also transient conditions.

For comparison with the circuit of FIG. 2, FIG. 3 shows a conventional circuit for separating luminance and color components, again designated VA and VB.

Separate trap filters 17 and bandpass filters 18 are used, preceded by a buffer 19. These two filters have different phase delays and the trap filter must be followed by a delay equalizer 20. This known circuit is considerably more complex than the inventive circuit of FIG. 2 and has inferior performance. It should be noted that the circuit of FIG. 3 is equivalent only to the portions of the circuit of FIG. 2 that have been described so far.

Returning to FIG. 2, the input color television signal at terminal 10 is passed through notch filters Rl, Cl, Ll.
and 1.6MHZ high-pass filters C2 and R3
and into a band Y by band filters Rl, Cl, Ll, R2 and subcarrier trap filter 28.

Trap filter 28 following buffer amplifier 21
is of a bridge-T configuration, in which two series capacitors C3 and C4 are bridged by an inductor L2 and the connection point of C3 and C4 is grounded by a resistor R4. This resistor R4 is connected to the trap frequency (4.43MHZ
) can be adjusted to cause complete rejection. still,
The various ground points shown in Figure 2 are essentially AC grounds (grounds from an AC perspective), and in specific circuit designs, some of these AC grounds may have different DC levels. It is convenient to have. The subcarrier trap 28 is followed by a buffer amplifier 29 and a high-pass filter C7.
, R7, which do not serve to define band Y, but provide a phase lead to compensate for the phase lead introduced by C2, R3 in the X channel. Band W is 250K
Selected by HZ high pass filters C5, R5 and 1.6MHZ low pass filters R6, C6.

The signals in bands W, X and Y are recombined by buffer and combiner circuit 22 to produce an output at terminal 23.

Each of these channels includes an instantaneous diode limiter to limit the range of variation of signals of either polarity. Diode 24 in the X channel and Y
The diodes 25 in the channels return to fixed bias points X and y, which establish a limiting operation that begins to operate at very low levels within the dynamic range of the signals processed by those channels. Diode 26 in the w channel has a similar low limit level, but
These diodes are not returned to a fixed bias point.
Instead, the diodes are returned to the output of the inverting amplifier 27, which is responsive to the input signal. As this human input signal increases, the bias on the diodes 26 decreases and the threshold at which the diodes 26 conduct decreases. The signal in the W channel is therefore more strongly restricted than the signal in the X and Y channels. Diode 30 prevents the signal from amplifier 27 from becoming large enough to forward bias diode 26.

[Brief explanation of drawings]

FIG. 1 illustrates the manner in which the frequency bands of a color television signal are divided for noise reduction purposes. FIG. 2 is a circuit diagram of an embodiment of the present invention for obtaining the passband shown in FIG. 1. FIG. 3 shows a known circuit for comparison with the circuit of FIG. Explanation of symbols, 10: Input terminal, 14
: current amplifier, 21: buffer amplifier, 22: buffer and coupling device, 23: output terminal.

Claims (1)

[Claims] 1 a main signal path arranged to send the main signal component from the input terminal to the output terminal with dynamic range linearity;
a separate signal path having an input connected to the input terminal or the output terminal and including a limiting device; and a separate signal path included in the main signal path receiving the output of the separate signal path and transmitting the output to the In a circuit for changing the dynamic range of a signal, the circuit includes: a coupling device that couples main signal components additively or subtractively; 10, a first output terminal 11 connected to the input terminal via a resistive impedance R1, and a second output terminal 13 connected to the output of the current amplifier 14, and the input terminal 10 is the resistive impedance R1 and the reactive circuit C1, L
b) first and second limiting devices 24 for limiting the two signals at the first and second output terminals, respectively; 25, and said combining device combines said two signals at said first and second output terminals and combines the resulting signal with said main signal component so that said two signals A circuit for changing the dynamic range of a signal, characterized in that the circuit adds to or subtracts from a main signal component.