Untitled record

(74) Civ. Per Onsager.

(54) Signal compressor or expander.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a signal compressor or expander comprising a main signal path for conveying the right input signal as a substantially unchanged signal, an auxiliary signal path adapted to derive a signal from the main path in a frequency band which is within that conveyed by the main path limit this signal to a fraction of the maximum amplitude of the input signal, and a combinatorial device in the highway additive in the case of a compressor and a substrate in the case of an expander.

Such coupling devices are e.g. used for audio, video and other electrical signals, especially for suppression, ie as noise canceling devices, the compressor being connected in front of the noise source and the expander thereafter. However, the expander can also be used on a signal that is not pre-compressed. There are also · uses where both the expanders and the compressors can be used independently.

Coupling devices of this kind are known and are e.g. described in detail in applicant's British Patent Specification No. 1,120,541. In this patent, a direct signal path or main signal path is described, to whose output signals the output signals from an auxiliary signal path are added if it is a compressor and subtracted if it is an expander. Below, the auxiliary path introduces a gain of, for example, 10 dB, but the size of the auxiliary path's maximum output signal is limited to a small fraction of the amplitude of the input signal. Thus, the amplification of the auxiliary pathway raises (compresses) or lowers (expands) substantially at low signal levels. At higher levels of the input signals, the output signals from the auxiliary path can be disregarded compared to the output signals from the main path, since the output signals from the auxiliary path, as stated above, are limited. The advantage of this solution is described in more detail in the patent specification, since the great freedom of distortion is of particular importance.

According to the invention described in the aforementioned patent, the input signals to the auxiliary signal path are the same as the input signals to the auxiliary path using a compressor and consist of using an expander of the output signals from the subtracting device. Accordingly, the expression for the compression takes the form y - (1 + F ^) x and the expression for the expansion the form z = y - FgZ, i.e. z = y / (l + F₂)₃where x is the input level of the compressor, y is the output level of the compressor, respectively the input level of the expander, z is the output level of the expander and? 1 and P₂ are functions that represent the characteristics of the auxiliary signal paths in compressor and expander, respectively. Both F ^ and F₂ at low signal levels takes the form of a multiplier and decreases at higher signal levels in such a way that the output signal from the auxiliary path becomes largely constant or a decreasing function of x and y respectively. For complementary compressors and expanders, F becomes F₂ = F ^ and hence z = x.

Compressors and expanders which operate in accordance with the above expressions will hereinafter be referred to as type 1 '.' The present invention encompasses this group as well as a further group which will be designated as type 2.

For type 2 expanders, the additional path is supplied with the same input signal as the main path, and the expansion formula takes the form z = (1 - F₂) Y.

In type 2 compressors, the input of the auxiliary path is connected to the output of the compressor, whereby the expression of the compression is changed to y = x + F ^ y or y = x / (l - F ^). If F ^ = F_2> becomes z - x.

In a complete compressor and expander noise suppression device, these are preferably of the same type, ie either of type 1 or of type 2, although this is not necessarily necessary in all cases. Thus, e.g. it is shown that errors arising from the simultaneous use of types 1 and 2 become relatively small when the maximum values ​​for F₁ and F₂ are low. It is also possible to build compressors or expanders which, in turn, contain an expander or an air compressor. compressor in the feedback path for a high gain amplifier.

In all compressors and expanders, the additional path can be processed by the entire signal band, but in most practical applications the additional path will only process the signal in a limited portion of the total signal band. Further, two or more signal paths can be arranged in a known manner to selectively treat different frequency bands, as described in the aforementioned patent.

The main difference in the devices of type 2 compared to those of type 1 consists e.g. in the following: a) In a compressor using a positive feedback path with gain less than 1, the circuit has a very high Nyquist stability; b) in the expander no closed loop is used, and stability problems will be avoided. Thus, it is possible to produce simple and inexpensive silencers, especially for video signals where stability problems are of particular importance due to the large bandwidth. An example of a simplified type 2 video expander, such as can be used in a home television receiver, will be addressed in the following.

With reference to FIG.

This describes how to generate the characteristics of type 1 compressors and expanders, and how these must proceed in order to satisfy the requirements. This also applies to a large extent to type 2 compressors and expanders according to the invention.

The invention aims to provide additional path output signals which decrease after an initial increase as a function of the input signal level. This characteristic is referred to as a downward-bending characteristic. Such characteristics are important because the output signals from the auxiliary path at higher signal levels at these characteristics become so low that the output signal from the compressor BB or the expander essentially corresponds to the main path signal.

The object of the invention is to provide an improved circuit with a compressor and / or an expander. .

The circuit according to the invention is characterized in that the auxiliary path contains a filter circuit to which at least one limiter diode is connected, and by a control circuit which can respond to the input signal of the auxiliary path to reduce the threshold of the limiter diode when this input signal exceeds a defined level;

In the following, embodiments of the coupling according to the invention will be described with reference to the drawing.

Fig. 1 shows a limiter for the additional path for a type 1 or 2 device for a video system.

Fig. 2 shows a discriminator circuit for forming the limiter characteristic in an additional path.

In video devices, the aforementioned downward bending limiter characteristics are advantageous mainly because they reduce or remove overshoot at the compressor output. When a composite video signal is passed through a compressor using a limiter with the aforementioned characteristic, e.g. neither at the output nor at the input the synchronization pulses are oscillated. Thus, a compressed video signal becomes comparable to a normal signal, thereby greatly simplifying transmission. Furthermore, a compressed image is largely comparable to a normal image. When the overshoot is limited only to the lowest signal levels, a sharp and hard image is obtained, which otherwise does not deviate from a normal image.

Key circuits are often used for video line suppression circuits. Such circuits can be used to produce a downwardly bending limiting characteristic in the auxiliary path of an image compressor or expander.

In FIG. 1, there is shown a simple limiter which can be used in the auxiliary way of a type 1 or 2 compressor or expander. The limiter is set to the desired frequency by means of an RC filter, and diodes 30 with suitable bias cause a bipolar limitation.

It is possible to imagine a switch placed parallel to the resistor of FIG. 1. For low level signals, the switch is open. For higher amplitude input signals, the diodes work. For large amplitude jumps, the switch closes and stays closed for a short time (about 0.1 / sec.) After the end of the jump. The switch then opens again and the circuit resumes its normal noise canceling function.

The key suppression, which is what is practically accomplished by the electronic method by which to achieve the process described above in idealized form, can also be considered as a threshold regulation where a portion of the incoming signal is applied in the extension circuit in such a way that there is a lowering of the restriction threshold. A downwardly extending characteristic is then produced, as is the case with the low-frequency devices described in the aforementioned patent.

Fig. 2 shows a suitable form of key suppression coupling. Parallel to the signal limiter, a further limiting filter circuit 40 is connected to produce suppression pulses. The output pulses from the suppressor pulse limiter are applied to a full-wave rectifier 42 and the rectified pulses are applied to an amplitude discriminator 44, during which only pulses of amplitude above a certain level are passed through. The suppression pulses are formed by a low impedance driver 46 and supplied with a balanced secondary winding pulse transformer 48 connected to the bias source of the signal limiter diodes.

For low level signals, output signals from the suppression pulse circuit are suppressed by the amplitude discriminator, the fixed biasing of the diodes ensuring normal limiter function. When the signal amplitude exceeds a certain level, the suppression pulse chain is energized, thereby giving the diodes bias in the retransmitting direction. The transients that would normally occur at the output of the signal limiter are thus suppressed.

In a similar embodiment, one makes use of the fact that in any situation of occurring leaps it is only necessary that one or the other diode is conductive. Utgangssignalei¹from the suppression pulse limiter can therefore be used to supply bias directly to the signal limiting diodes via an inverting amplifier with low output impedance. In this case, the transformer of FIG. 2 the loop and the two bias sources directly connected to the output of the low impedance amplifier.

The above method can be improved by using a pulse forming network between the suppression pulse limiter and the suppression diodes. A sli '··; networks may cause amplitude discrimination, pulse formation, pulse length control and control of the output amplitudes in accordance with one or the other; different in the proper function of the amplitudes of the pulse from wind_t (* () ·. ··; --- pu "sbegrenseren.