SE450985B - CIRCUIT FOR MODIFYING THE DYNAMIC AREA OF AN INPUT - Google Patents

Description

10 15 Z0 25 bi UI 40 450 985 2 part provides a predetermined, maximum pressure or expansion ratio; and 3) a high-level linear part with reinforcement other than the strengthening in the low-level part.

This characteristic is termed "bi-linear" because it has two parts with essentially constant reinforcement.

In practice, the threshold and end point are not always defined "points". The two transitional areas where the level part transitions to the low and high level linear parts each can vary in shape from a soft to a sharp curve depending on the power characteristics of the compressor and expander stik.

It should also be emphasized that circuit arrangements with -linear characteristics differ from other classes of circuit arrangement, namely: (a) a logarithmic or nonlinear circuit arrangement with fixed or variable slope and without linear part: the change changes over the whole dynamic range; (b) circuit arrangement having a characteristic having two or several parts, of which only circuit). reinforcing one is linear ("uni-linear" Circuit arrangements with bi-linear characteristics have specific benefits and are widely used. To avoid the risk of the circuit is controlled by noise, the threshold can be set so that it is located above the input signal noise level or the transmission channel noise level.

The high level part with essentially constant non-linear processing of high-level signals is avoided, something like otherwise would introduce distortion. For audio signals, then circuit must be syllabic, in addition to providing high level part an area within which the overshoots can be trades that occur in a syllabic circuit when the signal level increases abruptly. The overshoots are suppressed by cutting diodes or similar bodies. Only bi-linear characteristics Cows are capable of providing all of these benefits.

Most known circuits with bi-linear characteristics currently used in home audio products provides 10 dB compression and expansion, which is adequate for many purposes goal. However, this leaves some noise that can be perceived by some listeners, and for maximum fidelity is greater compression and expansion desirable, say 20 dB. It is difficult to achieve reinforcement means that ~ ngn-_ -f-f -v 10 15 Z0 25 30 35 40 -rzermnvnua-.f M m ", u. ° - 450 98 so much compression or expansion without encountering problems affecting signal quality.

Circuits providing 20 dB compression or expansion, or Empty. even more, are known and commercially available, however they usually consist of logarithmic circuit arrangements with constant curve slope, which thus have a continuous finite amplification over all or almost all dynamics the area. Such circuits have the disadvantage of having larger dis- torsion and major signal tracking problems at very low and very high signal levels than the bi-linear circuits in which the gain change is limited to an intermediate part of characteristics, and in addition the problems of overshoot more severe than in the bi-linear arrangements. Known constant curve slopes use compressors ionic ratios in the range 1.5: 1, 2: 1 and 3: 1, however 2: 1 is most common.

The compression ratio is defined as the ratio between incremental input dynamic range and incremental output gait dynamics area. The expansion ratio for a complementary expander is inverse to the compression ratio. If compressed If the ratio is 3: 1, the expansion ratio is 1: 3. For For the sake of simplicity, it is appropriate to use the term "inverse expansion ratio", as for the example just mentioned is 3: 1 and thus corresponds to the compression ratio. For For the sake of simplicity, the following discussion will be is limited to the compression ratio, which is to be understood that the same conditions, with appropriate amendments, also apply to the pension ratio.

A large compression ratio has the disadvantage that it is difficult to guarantee complementarity between compressions sorn and the expander; especially leads to level errors or errors in the frequency response of the transmission or recording medium to corresponding magnified error in the expander's output signal.

It is known (for example, from US-PS Z 558 002, US-PS 4,061,874 and Japanese Patent Publication 51-20124) to increase the available compression by cascading an number of compressor stages. These known circuits (devices with controlled 'impedance, diodes etc) multiplies the individual steps compression conditions so that a result is obtained high compression, accompanied by the disadvantages explained above. ....-. ~. - ..-, - ,. _ ...__ .V,. _ .. _..-.- 10 15 20 30 40 450 985 4 For example, obtained with a circuit with compression ratio-H the 2: 1 and another circuit with the compression ratio 3: 1 a total compression ratio of 621. The resulting the expansion ratio of 1: 6 would set very large requirements for the uniformity of the transmission channel. Another question that must be taken into account are the requirements placed on the circuit (of which kind it may now be) which provides the necessary the reinforcement change to form the compressor and expander the characteristics. It is relatively easy to persuade one perform correct gain changes within an area about 10 dB but considerably more difficult to persuade the same circuit to perform correct gain changes within a 20 dB range. The is thus associated with difficulties in forming a controlled, re- producible characteristics intended for use in a companion system. In the above-mentioned Japanese publication is drawn the conclusion that a large number of series compressors (and -expanders) are not suitable as noise reduction systems for hi-fi rendering systems.

It is also known (U.S. Pat. No. 3,902,131 and U.S. Pat. No. 3,930,208). to cascade several compressor stages operating within mutually exclusive frequency ranges: Since such arrangements cannot result in any increase in the compression ratio country compared to a single step, increased compression. they thus bring no one In the light of all these considerations, the present finning for the purpose of achieving height compression or expansion without placing too great demands on anyone of the circuits involved in the implementation of change.

Another object is to provide greater audio compression. sion or expansion without generating an undesirably large increase of the oscillations formed under conditions when transient signals appear.

A closer study of bi-linear circuits is at hand that they are not only in possession of the previously enumerated the advantages but in addition exhibits another - namely a possible ability to solve the problem of large compression ratios countries and, in the case of audio circuits, also an option solution of the problems associated with large overshoots.

Note that the overlay of the linear areas does not : zfàsrin fl f-'Imlbaxcuzwa-: n-nrzv-vmanz. ': = f ~ 1 -, - ». ~ .- :, ~ .'-- www ---" ~~' V- '""' “'“ "" "" "" ""' '' '' '"" "" "" ""' '"' '_' 10 15 ZS 30 35 40 S 450 985 increases the compression ratio in these areas; compress the ratio increases only in the limited area there dynamic effect occurs. In accordance with the present invention it has therefore been found possible to separate the areas with dynamic effect in such a way that the required total increase of the compression is obtained without simultaneously changing the total la, the maximum compression or expansion ratio in to any significant degree.

A further feature of this arrangement is that the total the result is bi-linear, with all the associated benefits.

The possibility of dispersed function of bi-linear devices thus represents an additional, hitherto unknown of this type of device.

The above objects are achieved with the present invention, which is characterized by a first circuit having a bi-linear in / out characteristics, followed by one or more more circuits which likewise exhibit bi-linear characteristics at any given frequency within the circuit common to the circuits. same frequency range. The thresholds of the circuits and the dynamic ranges set to different values in order to spread the circuit characteristics the intermediate level parts of the connectors so that a "reinforcement change comes over a larger intermediate level range than for the circuits individually thanks to the spread and at the same time generate greater difference between the reinforcements at low and high signal levels, but with a maximum compression ~ or ex: pension ratio, which is essentially no greater than the maximum compression ratio for a single circuit.

^ In the case of audio circuits, and whether these circuits are seen with override suppressing (limiting) elements it is possible to also spread their thresholds together with the spread of the syllabic thresholds. Overflow- the lows of the low level circuits or steps are reduced accordingly degree, with minimal total overshoot of said plurality of steps.

This is in contrast to the conditions at conventional logarithmic compressors, which by their nature generate large overshoots.

Each of the circuits can introduce a change of sig- For example, a low-level treble increase in the compressor case. Thus, each subsequent step can may be affected by a signal with gradually changing spectral 10 15 20 25 30 35. 40 450 985 6 .content. In the case of complex signals, this means the gain that spectral spread occurs due to the risks of decoding errors function. When, for example, it concerns a tape recorder with uneven frequency thread, the spectral shift tendency leads to a reduction occurs from the total dynamics and frequency response errors in the coded the result.

The size of the required spread must now be taken taking into account. For the sake of simplicity, reference is made here cases where two compressor circuits are connected in series. Compression the relationship of each of these two (first and second) circuits rise from the value one at each threshold to a maximum. This will be called the compression ratio. its rising flank. The compression ratio then drops again to one, and this will be called the descending one the flank. Strictly speaking, the falling flank can approach value an asymptotic, but from a practical point of view can the flank is considered to have reached the value one when it differs from this value only arbitrarily small.

The spread of the intermediate levels of the first and second circuits parts cause the falling edge of one circuit to overlap the rising flank of the other circuit.-At least according to one first approximation can be the difference between the two thresholds be made so that the overlap of the flanks results in one total compression ratio which does not significantly exceed slides the maximum compression ratio of the circuits var separately. __ The threshold of the second circuit is preferably lower than that first circuit threshold (if more than two circuits are used, preferably the subsequent circuits are given gradually lower thresholds) in the case of a compressor; at an expander is the relationship the opposite. In principle, the scheme can be reversed, thus the first compressor circuit has the lower one the threshold. In the case of more than two circuits, the order between the The principles are in principle made arbitrary provided that the circuits intermediate level parts are correctly spaced.

The ideal spread can thus be considered to be the one that causes the falling edge of one circuit to overlap 'the rising flank of the second circuit in sy te that as far as possible limit the level range within which dynamic effect occurs in the total, series-connected device during the 10 15 20 25 35_ 40 ,, ._- ... ¿.-. of __ -v a-. fl u-m-wf., ..- 7 45Û 985 early avoidance of any significant increase in maximum the compression or expansion ratio compared to what prevailing in a single device. If, for example, the maximum the compression ratio of each circuit is 7: 1 thus comes the compression ratio of the total circuit arrangement that rise to 2: 1, maintain this value over the overlap and then eat fall to the value one. Ideally, then, no one happens any increase in the ratio over 2: 1, this in contrast to the conditions of prior art arrangements of cascading coupled compressor stages where the ratio is multiplied by 4: 1.

In practice, it can be difficult to achieve optimal overlap. at all frequencies, but it is clear that - provided provided that a reasonable approximation is made in relation to grip ideal - the total maximum compression ratio can be prevented from rising too sharply above 2: 1 at the given the example. In a circuit arrangement achieved in practice can the ratio may rise to Z, 5: 1.

A low maximum compression ratio (eg 1.5: l) enables the expander to more easily follow the compressor in and to achieve good complementarity with signal channels which have somewhat unreliable gain and / or frequency times. However, a low compression ratio spreads dynamic effect over a larger level range, which gives greater tendency to noise modulation for a given noise reduction _ (gain difference at low and high input levels). There is a trade-off between undesirable effects caused by high and low compression ratios. The the ideal compression ratio therefore depends on the environment of the system and the objectives set for it the construction. system- The ability to spread bi-linear steps gives the designer another way of optimizing the overall circuit. Hereby can the shape of the compression characteristics of the individual steps determined in mind especially by this spread. Regard also take into account the transient properties of the circuits and the opportunity to spread overshoot the pressure thresholds in audio compressor and expanders so that the result is minimal total overshoots.

A well-known type of circuit called a "slider circuit" can be used --1.;.:.: ..: => '2-_f "' .- u.-.'- ..-...: ~ '.' - -i -......... ~ - 10 15 20 25 30 35 40 - -, - v- un: - ~ _f 450 985 8 for both the first and the second circuit and accomplishes specified desired characteristics of audio compression or expansion at high frequencies by introducing hf-h ning (for compression) or hf-lowering (for expansion) m using high-pass filters with variable lower transition frequencies As the signal level increases within the high frequency band, si moves ("slides") the transition frequency upwards in order to limit raise or lower the band and exclude the useful signal from the increase or decrease. Examples of circuits of this sl can be found in US-PS Re 28,426, US-PS 3,757,254, US-PS 4,072 U.S. Patent 3,934,190 and Japanese Patent Application 55529/71.

Thus, each of the first and second circuits u made by a "sliding band circuit". In principle, they can at rest current transition frequencies of the two sliding bands the circuits may be different, and advantage may be taken from this to achieve a degree of compression or expansion which is st re within one part of the treated frequency band than within the other one. In accordance with an important further development of up the finding, however, is made of the transition frequencies in general identical. This leads to the advantage of being sharper difference arises between the frequency range where increase or decrease occurs and the frequency range where this does not occur and thus sharper distinction between that area where noise reduction no longer occurs as a result of behavior the öj- oath s.

O b the ag 914, t .. 1 .. micro- p- e et of a significant utility signal and the area where noise reduction: the ring continues to operate. _ On the other hand, circuits are well known at which frequency the spectrum is divided into a number of bands by means of the opposite corresponding bandpass filter, whereby compression or expansion performed within each band by means of an associated understanding control device (either an automatic responder) n rk- e diode type limiter or a controlled limitation device) in the compressor and with any kind of reciprocal or complex mental circuit device for the expander. Examples of such circuits are found in U.S. Pat. No. 3,846,719. These are banded circuits or multiband circuits have the advantage of operating independently. depending on the different frequency bands, and on this property desired or needed, circuits of this type can be used in the first, the second or in additional steps in the circuit arrangement according to the invention. .., _. \ n___ f- 15.5. , .¿ ~ e ~, _-___, -_, _ _;, .___ _ ~ -. ; u uu fl aw " l0 15 20 25 35 40 _ .. ~ »; ..'.- 1 ~. =;:. A. ~ 9 450 985 In principle, one of the first and second circuits can be a multi-band circuit and the other a slider band. This may be of interest in special situations when, for example is desirable to emphasize the compression or expansion within a portion of the total frequency band, sounding the sliding band circuit and one or more of the band splitting channels act over this frequency batch.

It is known to design bi-linear compressors and expanders - of both sliding belt type and belt splitting type - by using only one signal path. However, it is preferred In general, such devices are designed by means of correction of a main signal circuit, which is linear with respect to the dynamic range and which contains a combination circuit, and an additional circuit which receives its input signal from the the input or output of the circuit and has its output add to the combination circuit. The additional circuit includes holds a limiter (self-acting or controlled) and it further the limited signal of the circuit raises the main circuit signal in the combination circuit, in the case of compression, but lowers or counteracts the main circuit signal in the expansion case.

The limited signal in the additional path is less than the main path signal within the upper part of the input signal dynamics area. The main circuit and the additional circuit are suitable and preferably separately identifiable signal paths.

Known compressors and expanders of this kind are particularly advantageous because they allow an accurate up: correcting the desired transfer characteristics without problems with high-level distortion. The low-level part is included in the main thing constant reinforcement is achieved by the further the path is given a threshold above the noise level; below this threshold the additional path is linear. The average level part is formed by it area over Which the limiting effect of the additional course partly comes into operation, and the high-level part with mainly constant gain occurs after the limiter fully operational so that the signal of the additional path ceases to increase and becomes negligible compared to the main nalen. At the highest part of the dynamic range of the input signal is formed the output signal of the circuit in practice only of the signal through the main linear path, by which is meant linear linear with respect to the dynamic range. In dual-track audio .ga-r _. wvh-apggg, _, _ _-¿,;, -,;., .. a-, 4 -, -. ~. = - ~ a.-1n-a ~ f- = z-aue fl. um.- z - -w - - = ~ = ~ - ~ ~ «-, ._ .., _ .. ._ f - wv ø -... ß m. .'V _.--. ___-__. 10 TS 20 30 40 450 985 w circuits is the creation of overflow suppression especially suitable. _ Examples of known circuits of this kind can be found in US-PS 3,846,719, U.S. Pat. No. 3,903,485 and U.S. Pat. No. Re 28,426 known analog circuits which achieve similar results but in which the additional path has a characteristic that is inverse to the constraint characteristic, and further the path output signal lowers or counteracts the main path signal at compression and raises it during expansion (US-PS 3 S28 280 and U.S. Pat. No. 3,875,537).

Any of these known bi-linear circuits can consequently used as first and second circuits in the circuit the device according to the invention in order to achieve the with associated benefits and also to easily establish the desired degree of spread. This is done by thresholds and dynamic ranges of the two additional paths selected appropriately.

As mentioned earlier, it is not essential that it desired form of bi-linear characteristic is achieved by such "dual track technology". There are alternatives, which work with single lanes, as described, for example, in U.S. Pat. No. 3,757,254, U.S. Pat. No. 3,967,219, U.S. Pat. No. 4,072,914, U.S. Pat. No. 3,909,753 and Japanese Patent Application 55529/71. Although these alternatives circuits are usually not able to produce as good results as dual circuit, or may be less suitable and thus less economical, they can generally give equally good results.

Consequently, these known circuits can also be used as one or more of the circuits in the embodiment according to the invention conducted the circuit arrangement. If desired, one of them can the first and second circuits consist of a dual circuit and the other of a single track crects.

The invention will now be described in more detail in form by way of example and with reference to the accompanying drawing. §Ig_l is an example group of curves showing complex mental compression and expansion characteristics of bi- linear type. §Ig_Z is a block diagram illustrating the present present invention in general form. fig_§ is an example of a graphical representation of the dynamic areas of action and how these can be separated into series-connected compressors and pandrar. Fig. 1 is a simplified form of Fig. 3 : z-: xzzsaz-: rzaszrrr .f: zcacaaïarfztazzarrzff-Lfaeau: e. ~ =; - .. f ..-.- ,, .._ ~. =. A.- a ..., _. .--, - »- 10 15 20 25 b: U1 40 H 4bU 985 series of idealized bi-linear curves illustrating one general method for spreading the thresholds of series circuits. §Ig_§ is a schematic circuit diagram of a known compressor of slider type. fig_Z is a schematic wiring diagram for a known expander of the sliding belt type. §Ig_§ is a schematic wiring diagram for a modification to Figs. 6 and 7. §ig_2 is a graph showing the frequency response during compression the threshold for two series-connected compressors resp. expands according to an embodiment of the invention. Pig 10 is a curve diagram showing the frequency response below the compression threshold for a known compressor and expander according to Figs. 6, 7 and 8. Fig. 11 is a curve diagram which as a function of frequency shows the input / output curves of a compressor with series devices according to an embodiment of the invention. Pig 12 is a graph which as a function of frequency shows the input / output curves of a known compressor equipped with a single device. Pig 13-15 is a series of test tone curves illustrating belt slip function of an embodiment of the invention and of the circuits according to Figs. 6 and 8._Fig_16 show characteristic curves below the compression threshold for a further embodiment of the invention. Pig 17 Fig. 11 but for a further embodiment of the invention.

Pig 18 shows characteristic curves of the same kind as those in Figures 11 and 17 but strongly grouped together. shows curves ~ of the same kind as in Examples of bi-linear complementary compression and expansion transfer curves (at a certain frequency) are displayed iv Fig. 1, which (for the compression characteristic) shows the low level the part with essentially constant reinforcement, the threshold, the part where the dynamic effect is present, the end point and the high level part with essentially constant reinforcement.

Fig. Z shows the present invention in general; one first bi-linear compressor Z is supplied with the input information and supplies its output to a second bi-linear compressor 4, which is connected in series with the first and which has its output un ~ closed to a disturbed information transmission channel N. A pair series-connected bi-linear expanders 6 and 8 are supplied at the expander 6 as an input signal signals from the channel N and outputs at the output of the expander 8 the output signal of the noise reduction system.

The dynamically acting areas of the series-connected devices are separated or scattered relative to each other within the an ~ 45Û 985 12 the devices common frequency range. Although in the figure. two devices are displayed on each side of the information channel N, more than two can also be used, and the invention thus relates to led the use of two or more series-connected bi-linear 5 compressors or expanders. When the noise reduction system is made complementary, the same number of series connected is used bi-linear compressors and expanders.

The order between the steps with special characteristics in the compressor is inverted in the expander. For example, the expander 10 last steps complementary to the first of the compressor step, this in all respects - steady state and time-dependent dynamic response (frequency, phase and transient response in all signal level and dynamic states).

An exemplary graphical representation of separation The distribution or scattering of two bi-linear devices is shown in Fig. 3, which shows the compression ratio as a function of the amplitude level of the signal (along the horizontal axis) for a compressor or expander operating at a certain frequency. For for the sake of clarity, the curves are shown in an idealized form; in themselves In this case, the curves are somewhat asymmetrical in practical the Dolby noise reduction system type A and type B. (U.S. Pat. No. 3,846,719 and U.S. Pat. No. Re 28,426). Curve 12 relates to the dynamic action of a compressor or expander (high level step). Curve 10 refers to a further one Compressor or expander (low level stage) with another dynamic range. If the high level step is first in the sequence of compressors (and as second step in the sequence expands), so curve 12 represents the variation of the compression ratio at the first (compressor) stage as a function of input input To this step, while curve 10 represents the compression -variation of the ratio in the second (compressor) stage as function of the input level to the first stage.The upper curves na refers to the compressors, the lower to the expensers.

In this example, the areas of action with respect to the input 35. the amplitude level separated in such a way that the product of the both curves give an overall characteristic with a compression expansion or expansion ratio not exceeding 2: 1 (112) meiian the two points ioa and 12 (1ob and 1zb) there the compression is maximum for the two devices. Thus, even with two devices in series, the functional . _ _, _ f. _.._ .., .. _ _ f, .._., .-- »vr-v - - - ~ --.- f -« ....- -fi- -fi- - n -..- f - q- - f- - f - fairy. -..HRS- -.--,- _ . . - ._- - - ~~ - ~ - 10 20 25 35 40 -PâeYIJ¿ü .-. Äaf -: .'-. Z "-. '~ H; 13 450 985 the outer parts of the area to remain fixed and the maximum the conditions of expansion and expansion do not increase so that they come to exceed the corresponding conditions for single-stage devices, why those associated with simple bi-linear devices the parts are retained. Consequently, misconduct comes within the dynamic range of action of the devices connected in series not to exceed the corresponding error in simple devices.

In most bi-linear devices, the fixed, constant reinforcement having the end regions by means of fixing, preset circuit elements, such as resistors and capacitors, which by their nature are stable and cannot introduce dynamic errors, waveform distortion or the like. Consequently, it is only within the transition area between the linear areas with fixed reinforcement as dynamically active parts of the circuits can be ducera signal error.

In the representation according to Fig. 3 it should be noted that dynamics of a conventional logarithmic compressor or clay expander becomes a horizontal line; for example, line 11 characteristics of a 2: 1 compressor, line 13 characteristics the stick for a 1: 2 expander. Through this analysis, it is clear that there are no possibilities to separate or disseminate effect of such devices.

From an analysis point of view and for obtaining a first approximation of the required threshold levels for formation of optimal spread according to the invention, it is practical further idealize Fig. 3. Therefore, assume that each compression (and expander) immediately reaches its maximum compression condition at a threshold level and maintains this condition they till an end point is reached at a higher level where the compressor _dynamic effect abruptly ceases. In this case, a series of and expanders according to Fig. 3 to appear as adjacent overlapping rectangular curves as shown in Fig. 4.

As an example, it has been assumed that three compressors and expanders with bi-linear characteristics are connected in series. Low level the device, which is preferably the third compression device sorn (the first expander) has the lowest threshold (TS), shown at -62 dB, and its endpoint (F3) at -46 dB, which forms the threshold (TZ) for the intermediate level stage. This intermediate level step has its end point (FZ) at -§O dB, which constitutes the threshold (TI) for the high level step. The end point of the high level stage (P1) is located _.-_-_;:. ff.gzs.'1, zr «:. e; .n> 1: ~ z.-. . : zruwf fl- r. - '~. ~~ "f' -" - f '~ ""' ^ "'i *" “' ..., vV ....__ .. ,, .- \ .w -.-. .f .a ...., --_.-_-.._._, ..-.-._ ~> - 10 20 450 985 ” at -14 dB. All levels are referenced to total signal one. Furthermore, it is assumed that each step has a gain of 8 dB and a maximum compression ratio of 2: 1.

Pig 5 reproduces idealized characteristic curves (total input signal vs. output signal) for compression based on the example according to Fig. 4 (the mirror image-like expansion curves have for the sake of clarity omitted. The figure shows how each step dynamic effect appears as lying next to the boundary step dynamic effect; resulting in a total compression ratio of 2: 1 while obtaining 24 dB compression.

Based on the observations in Figures 4 and 5, a single one is obtained equation for the relationship between the threshold levels (T), final point (F), maximum compression ratio (C) and ratio reinforcement (G) for each individual step: T F - CG / (C-Û By using this equation, the threshold levels for each step is determined by acceptable approximation by one iterative process. If, for example, a total end point (P1) of -14 dB is desired for a step gain of 8 dB and a maximum compression ratio of 2, it appears from the equation that high the level threshold (T1) is -30 dB. This value is then used as end point (FZ) of the intermediate level step to determine that its threshold should be -46 dB, etc. For each step, Thus, in this analysis, reference is made to the results of walking rose. However, the calculated threshold is total the threshold with respect to the input of the sequence. To obtain the threshold for a particular circuit with respect to its own input, the cumulative signal amplification is taken into account. up to this point. For example, the threshold for it 35 the lowest level step in Fig. S -46 dB with respect to this step input.

The equation can also be solved with respect to the end point F, the compression ratio C or the reinforcement G. Design The operator can thus determine its circuit parameters on the basis of the construction goals set by him. In these cases can include requirements that the low-level threshold be located above the noise level, that the end point of the highest level should be sufficient law to allow the use of overflow . _ ^ '¿. -fl- F-XfJT fl- u. - r-u-ue- -.- _ ~ <;, - n- »fy» 4 ÉLFJ w-g-.f-U. 10 ...- Un 35 »Fl y ~ af ~ ¿.

QOU Vöb protection and that the total maximum compression ratio does not shall exceed a certain value.

In practical circuits, the threshold and final point not always as well-defined points as in this analysis. As discussed initially, the areas there may the intermediate level of the characteristic passes to the low and high level, linear parts be soft or sharp depending the properties of the circuits that control the dynamic effect. Thus in practice, the threshold range of a circuit will overlap the endpoint range of another circuit.

Considering the above equation and Fig. S shows that for the special case that the compression ratio is 2: 1, then half of the threshold spread will be achieved by step signal amplifications, while the remaining half must be achieved by changing the bias voltage of the control element and / or changed gain in the control amplifier (more powerful reinforcement for lower threshold). For compression conditions of 1,521 and 3: 1 applies correspondingly that 1/3 resp. 2/3 of the spread is achieved by the step reinforcements and that 2/3 and 1/3 of the spread must be achieved by the the circuits. ~ * '“ In both Figs. 1 and 5, 0 dB is a nominal maximum or iefmænsüvâ. In practice, there is a free space around 10-20 dB above the 0 dB level.

As already mentioned, it is usually preferred that high the level step is the first step in the compressor sequence and level step the last. However, it is the reverse arrangement possible, whereby the first stage control amplifier must have high gain to the required low threshold to be achieved. This low threshold is then also valid in the presence of high-level signals, which in known "sliding band System "usually leads to the system as a whole getting poor performance with respect to noise modulation. At this reversed arrangements, each step must provide sufficient amplification of the control amplifiers in order to increase the step in question threshold must be reached. Furthermore, each threshold is essentially fixed and independent of the function of the other steps. This is a consequence of the signal gain of each previous step falling to_ essentially one when the corresponding subsequent step has been reached its threshold. The calculation of the required thresholds for optimal '__ _ fi i V_____ Å _____ ___, _ _ _ V, __. Nyby. ._, ._. . 13.7- ..._. ........... _... _. _ 'mar fmar fi' ' 10 15 20 ZS 35 40 -. »# 221 wa. ... rzfzns-a. - sf, - 450 985 16 spread in this inverse case in the same way as in the preferred case. The threshold for each step referred to its own input becomes the same as the total threshold.

In contrast to what is the case with the reversed case applies to the preferred arrangement (where the high-level step thus lies first in the compressor chain and the low level stage last) that there is a favorable interaction between the steps the reinforcements and thresholds. The thresholds of the "downstream" located steps are determined in part by the sig- nificance of the previous steps. nal reinforcements. In a two-stage system with 10 dB low level reinforcement per step, the requirements for reinforcement are reduced in the second stage control amplifier with 10 dB due to the low level signal gain in the first step. When a high- level signal occurs, this 10 dB gain is eliminated in the first step and the threshold of the lâgnívà step are raised in practice 10 dB. With sliding belt type companders, this system improves performance with respect to noise reduction.

In the preferred embodiment, all are foregoing step reinforcement fully effective up to the threshold of one certain subsequent steps. Unlike what is described above the "reverse" system pulls the preferred arrangement the most benefit from the prevailing signal amplifications of the individual steps.

Thus: 1. Under very low level signal condition (below threshold) the requirements for the reinforcement in control are reduced: the amplifier in each step to a degree equal to the cumulative the signal amplifications in all previous steps. By the example according to Fig. 5, the required reinforcement of the control amplifier in the step corresponding to the lowest level, for that a threshold of -62 dB should be reached, with T6 dB compared to what would be required if the step worked independently or in the previously described inverted configuration. On liknan- In this way, the gain in the control level of the intermediate level is reduced. 8 dB amplifier, thus leading to the most economical miska circuit. IN 2. A signal-dependent, variable threshold effect is achieved, by noise modulation effects are reduced at sliding stage steps.

The effective thresholds in the low-level steps are gradually increased with signal level at a certain frequency. At high signal levels (on the linear high level part of the transfer characteristic) ___ m &, _:, _ x__ ___, _, _ V _ «, _, __ _ _, _ ,,,. _., ._. ..._-.... _ .__. 10 15 ZO 25 35 40 W 450 985 the effective threshold of the low-level stage is raised by a level equal to the gains in all low-level steps [sub-threshold steps) up to this point. In the example of Fig. 5, the threshold of the step is raised corresponding to the lowest level, which threshold is normally -62 dB under low level signal conditions, thus by 16 dB, ie to -46 dB, under high level signal conditions. Similarly the threshold of the intermediate level is raised to -38 dB.

In a first practical embodiment of the invention, there series-connected sliding belt devices are used, they are compressed 2 and the expander 8 in Fig. 2 are substantially conventional B-type sliding devices of the type shown in US-PS Re 28 426, while the compressor 4 and the expander 6 have modified frequency characteristics. With that of cassette tape generated noise, it has been found that useful results held when the second device (in compression mode) is not has only spread response regarding the amplitude level of the input signal without also in addition a cut-off frequency which is two to three octaves lower than standard B-type devices. More particularly the threshold levels of the other device are lowered, both in those syllabic filters / limiters and of the overflow suppressive limiter, in order to achieve dissemination and further, the transition frequency of the fixed filter is lowered two or three octaves.

Details of the B-type circuit are shown in Figs. 6, 7 and 8. which correspond to Figures 4, 5 and 10, respectively, in US-PS Re 28 426, and further details of these circuits as well as their function and theory can be found in this publication. The following description of Figures 6, 7 and 8 are derived from US-PS Re 28 426.

The circuit of Fig. 6 is specially dimensioned to _ in å in the ins elnin fl channel of a home ~ bands elare and two p O I such circuits are required for a large tape recorder. The input signals is applied at a terminal 10 to an emitter follower stage 12, which generates a low impedance signal. This signal is fed in part via a main or direct passageway consisting of a resistor 14 to an output terminal 16 and partly via a further path, whose last element consists of a resistor 18, which is connected to terminal 16. Resistors 14 and 18 add the output signals from the main track and the additional track in order to those of the required compression law.

The further path includes a certain filter 20, a 10 15 20 35 40 450 985 ”g filter 22 with variable cut-off frequency and comprising a field effect transistor 24 (constituting the filter / limiter) and an amplifier 26, the output of which is connected to a double diode limiter or mower 28 and also with resistor 18.

The nonlinear limiter suppresses overshoot signal vid abruptly increasing input signals. Amplifier 26 raises the signal in the further path to such a level that the knee in the characteristics of the limiter or overshoot the printer 28, which consists of silicon diodes, is active at correct signal level during transient state. Overflow the effective threshold of the suppressor is slightly above the threshold of the syllabic filter / limiter. Resistors 14 and 18 are dimensioned so that the required damping the degree of compensation is obtained for the signal in the further banana.

The output of the amplifier 26 is also connected to an amplifier. are 30, the output of which is rectified by a germanium diode 31 and integrated in a smoothing filter 32 in order to achieve of the control voltage of the field effect transistor 24.

Two simple RC filters are used, but also equivalent LC or LCR filters can be used. ”The fixed filter provides one cut-off frequency of 1700 Hz, below which a decreasing compression occurs. The filter 22 consists of a series capacitor 34 and a shunt resistor 36 followed by a series resistor 38 and field the power transistor 24, the emitter-collector path of which is connected as shunt resistance. In idle mode with zero signal on the control of the transistor 24, the transistor is throttled and has i essentially infinite impedance, whereby one can ignore the stand 58. The cut-off frequency of the filter 22 is thus 800 H2, which can be observed to be significantly below the fixed filter 20 limit frequency of the year.

When the signal on the handlebars increases sufficiently to the resistance of the tower 24 should be less than, for example 1 kohm, resistor 38 will shunt resistor 56 so that the cut-off frequency increases and significantly reduces the pass band of the filter.

The increase in the cut-off frequency is of course gradual.

The use of a field effect transistor is favorable after- such as this within a suitably limited signal amplitude range acts mainly as a linear resistor (for signaling of either polarity), the value of which is determined by the control voltage i ._ ta. w .a- 5, - W ... ,,, ¿A = -;, --- '-mw-wra-s .. -, _; _ 1 -. ,,, - - =. æ \ .- V -, «a .-» ....-..- «~ .-.,. enter-w- .n-wv-.auu-h- -va-ua- -. . »-.. ---- .. -. -. - ..__......_._ .. ._. ». vanan ..- -_-_. _aø .. _ ..._. _. .-. . -. n - - - Ui 10 15 (s) UI 40 19 450 985 on the board.

The resistor 36 and the field effect transistor are connected an adjustable terminal 46 of a voltage divider containing a temperature compensating germanium diode 48. Socket 46 enables adjustment of the compression threshold of the filter 22.

Amplifier 26 contains complementary transistors such as provides high input impedance and low output impedance. Since the the amplifier operates the diode limiter 28) a finite output is required walking impedance, which is achieved with the coupling resistor S0.

As already mentioned, the diodes are 28 silicon diodes with a sharp knees around 1/2 volt.

The signal over the limiter and thus over the resistor 18 can be shorted to ground by means of a switch 53 when it the compressor is required to be deactivated.

Amplifier 30 is an NPN transistor with an emitter timing constant network 52 which increases the gain at high frequencies.

Strong high frequencies (eg cymbal beats) will therefore be lead to rapid reduction of the band within which compression This is done to avoid signal distortion.

The amplifier is connected to the smoothing filter 32 via rectifier diode 31. The filter consists of “a series resistor 54 and a shunt capacitor 56. The resistor 54 is shunted by one silicon diode 58, which enables fast charging of the capacitor 56 for rapid entry into force combined with good smoothing below stationary state. The voltage across capacitor 56 is applied directly on the transistor's 24 board.

A complete wiring diagram for the complementary the expander is shown in Fig. 7. A complete description is not necessary because the circuit is essentially identical to the circuit then in Fig. 6, and for the same reason is mostly not stated either component values in Fig. 7.

The differences between 6 and 7 are as follows.

In Fig. 7, the further path receives its input signal from the output terminal 16a, the amplifier 26a are inverting and they of the combined signals 14 and 18 resistors are fed to the input the aisle (base) of the emitter follower 12, the output of which (the emitter) is connected to the terminal lóa. To ensure low propulsion impedance, the input terminal 10a is connected to the resistor 14 through an emitter follower 60. Appropriate measures must be taken to prevent biases from entering the expander. 10 15 20 25 30 40 450 985 2 ” The amplifier 26a is made inverting by the output signal ' taken from the emitter of the other transistor (of the PNP type) in let for collector. This change also includes relocation of the 10 kohm resistor 62 (Fig. 6) from the collector to emitter (Fig. 6), which automatically provides a suitable output impedance for driving the limiter. The resistor 50 is therefore omitted in Fig. 7.

For proper adjustment of a complete noise reduction systems, it should be noted that it is important that the same levels are present at the emitters of the transistors 12 in both the compressor and the expander. Measuring terminals M are shown connected to just mentioned emitters.

Pig 8 shows a preferred circuit for replacement of the circuit between points A, B and C in Figs. 6 and 7. When the field effect the transistor 24 is throttled, the second RC network 22 is inoperative, and the first RC network 20 is in this case determining the frequency time in the additional path. The improved circuit combines the existing benefits with respect to the phase _with having only one RC section at rest with damping 12 dB / octave of a two-section RC network during signaling state. - ~ --r In a circuit performed in practice using MPF 104 as field effect transistors, the resistor 36a must be 39 kohm is to provide a finite source impedance for fat effect trans- tower. In this way, the compression ratio is maintained at all frequencies and levels maximum at Z. The resistance Sóa has the same compression limiting function in the improved circuit which the resistor 36 has in the circuit of Fig. 6 or 7. In addition this resistor provides a low frequency path for sig- nalen. _ In the first embodiment of the present invention is used in the compressor 4 and the expander 6 according to Fig. Z as previously mentioned devices of the type shown in Figs. 6, 7 and 8 with modified characteristics. The amended the frequency and the lowered threshold are achieved by modification of the characteristics of the fixed filter (filter 20 in Fig. 6) and also by changing the gain of the control amplifier by changing the frequency response of the amplifier (emitter time constant the network 52 of the amplifier 30 in Fig. 6). Overflow sub- the threshold of the printer is lowered by pressing the appropriate 10 15 20 25 35 40 21 450 985 bias voltage (in the forward direction) across the diodes 28. The impedances in the variable filter (variable filter 22 in Figs. 6 and 8) left largely untouched in order to maintain appropriate adaptation to the characteristics of available voltage controlled, variable circuit elements. An appropriate modification of it shown in Figs. 6-8, the B-type sliding circuit is to change it fix the filter 20 resistance of 3.3 kohm to 18 kohm in and for lowering the cut-off frequency by two to three octaves. For raising the gain of the capacitor increases with the gain of the control amplifier in the time constant network 52 of the amplifier 30 from 0.15, uF to 0.60, uF (or from 0.1 pF to 0.4 p? If proposed the value 0.1 pF is used). Pre-voltages of about 1/4 volt in the forward direction, pressure is applied across the silicon diodes 28, whereby reduction by several dB is due to overflow suppression level.

The variable filter 22 exhibits all-pass characteristics such as response to the rest control voltage, and thus the total filter is lowered cut-off frequency two to three octaves. increase in capacitor the value in the emitter network of the amplifier 30 increases the latter gain at a given frequency. As explained above and in US-PS Re 28 426 comes the limit frequency for the variable RC filter 22 to increase when the control voltage (from the amplifier 30, the rectifier 31 and the smoothing filter 32) increase. Down big capacitance values in the network 52 thus come the variable the filter to be moved upwards in frequency from its rest value in response to low-level signals, by means of spreading level the answer or threshold from what applies in an unmodified B-circuit.

The level answer can be spread in countless ways in addition to change of the control amplifier's emitter network. These opportunities include there is a change in the bias voltage on the control element, another change of the gain of the control amplifier, change of the relative the signal levels in the filter path and the control signal generating path, etc.

Some details in the circuits of Figs. 6, 7 and 8 have further developed over the years and modern circuit types has been published and is well known among those skilled in the art. Reference to Us-Ps Re zs 426 has been made to simplify presentation nen.

Fig. 9 shows an actually recorded recording curve below 10 15 20 25 40 .-. '“~ 5?“ Å -'? J5. ».... '“ -'Z.2 -' «7.- 'lf-IJZIC .Vi 450 985 22 compression threshold for two series-connected compressors, of ' which the first is modified as above; in addition is displayed expander's frequency response. Compare this with Fig. 10 (corresponding Fig. 12 of US-PS Re 28 426), which shows the actual recording curve below the compression threshold for a single compressor or expander according to Figs. 6-8.

Pig ll shows as a function of the frequency in / out response of the series-connected compressors. When considering these curves show the two dynamic ranges indicating the two scattered functional areas. While the possibility of The dynamic ranges of these curves are useful for demonstrating distribution of the diffuse function of the devices, it is in practice one to prefer that the curves are as smooth as possible without observable dynamic ranges or "elevations". Parallel lines A and B are drawn through the threshold areas, line A refers to the standard circuit and line B to the modified circuit rade circuit. Compare these curves with Fig. 12 (which is US-PS Re 28 426) which similarly shows the frequency curves for a simple B-type sliding belt compressor. Fig. 11 illustrates that the compressor provided with series-connected devices gives essentially twice as much compression distributed over larger frequency and level ranges.

The effect of the variable band on the scattered effects the devices can be observed in Figs. 13 and 14, which show the frequency response during test tone recording with said series charge compressors. Compare with Fig. 15 (which is Fig. Lš in US-PS Re 28 426), which shows an actual recording curve obtained with the circuit of Fig. 6 including the circuit of Fig. 8.

The effect of the variable band is shown by recording compression the response rate at a low level test tone (with a level equal to below the compressor threshold) in the presence of a high level nal; the test signal is detected at the compressor output by means of a filter The high level signal causes the compressor circuit to function, and the curves show the effect on the filter frequency.

Fig. 13 shows the frequency response of a test tone at -65 dB and the presence of 200 Hz signal tones at levels between -28 dB and_lower up to f10 dB. Pig 14 applies to a 500 Hz sig- naltons at levels from -34 dB and lower to +10 dB.

In a further embodiment of the invention with : Igwüg «'- ;; ~ __ ,, 1..v, = ~ _-¿;; - 'e: - ..: f- -..... <. “~ _-" -._-- ._ _. _ - -.- - 10 15 25 40 : e-'fe-w-wy-ff f '~ g.- -;., ~ _ .. (,. | ..-; ~ -. f .-. WW. v; 23 450 985 better performance is both the compressor Z and the expander 8 i Fig. 2 modifications of standard B-type devices. Bathe the series devices have their transition frequencies lowered by two octaves to thereby give a sharply rising frequency wave at low levels. Dissemination of the dynamic effect is achieved by lowering the thresholds (both syllabic and overflow suppression] of the second device (in the case of compressor Courage). ' An advantageous feature of the present invention is that the frequency responses of the individual circuits are complex. If as sharply rising noise reduction curve as desired, this is achieved by using circuits with the same low level (vilo-J frequency response.

In the improved embodiment, the choice of the identical filter curves so that they are around 2 octaves below what occurs with standard B devices thus that a characteristic is obtained which rises rapidly over about 300 Hz. Thus, the system acquires the ability to essentially noise reduction in the critical range between 300 Hz and 2 kHz, an area within which band noise is noticeable after that that noise above 2 kHz has been reduced. The noise contribution from the band is negligible below 300 Hz. Because the system only has minimal noise reduction effect below 300 Hz is avoided manipulation takes place with the fundamental signal frequencies, in addition to the complementarity of the system is improved in practical future tape recorders, which may, for example, have frequency errors due to head shocks or the like. By avoiding folding compression of low frequency signals is also improved system compatibility as an increase in low frequency signals would result in troublesome rumble and hum when encoded bands are reproduced in systems that do not have the complementary pandrar.

Reference is now made again to Figures 6 and 8, where the two series connected the devices in the practical under discussion embodiment both have the resistor in the fixed filter 20 end from 3.3 kohm, which causes the total cut-off frequency for filters 20 and 22, about two octaves are lowered to about 375 Hz. In the second device, the capacitance value in the control the emitter network 52 of the amplifier 30 by a factor of four or around, as in the previously discussed embodiment. 10 15 20 ZS 35- 40 iQ-.at-.Lv v. .sa-s 43U 753 M This results in a spread of the threshold levels around 10-15 dB (depending on signal level and frequency). Suitable for- voltage is introduced into the diode limiter circuit 28 for lowering of the level of overflow suppression.

In a modification of the latest embodiment, the the satin 34 in the filter 22 is raised to 0.01 pF in order to promote correspondence between the characteristics of different units and for to improve the noise modulation properties. As a result of the in substantially equal to the time constants of the fixed filter 20 and the variable filter ZZ, in this case the arrangement will be be equivalent to a single-pole variable filter, and that fix the filter can be eliminated. In this case, the resistor is placed 36a (which has the value 47 kohm in modern types of B-circuits) which shunt to the emitter-collector path of the field effect transistor 24 in order to provide a rest transition frequency of about 375 Hz.

However, it is desirable that the fixed filter be maintained in the high-level circuit so that the circuit can be switched to independently ' work as a standard B-circuit.

In practice, a product comes with the aforementioned improved systems to be compatible with existing ones okoaaa or B-koaad software (rexlï fi äna and Fmraaiosänaning- are). The improved systems contain a standard device of the B-type and can thus for the attainment of full compatibility switched to work as a B-type device. When on others page_recorded tapes become available encoded according to the improved system, can now existing consumer products of B-type will provide excessive high frequency information or treble richness, which can be remedied by changing the setting of the treble control in the same way as is done today at playback of B-coded software on equipment without special circuits.

The standard B circuit described in US-PS Re 28,426 has one maximum compression ratio of about 2: 1. This holding has proven to be a good practical choice for cassette tape compander system for private use. In the series cup- circuits in the above-described embodiments are maintained. gives each individual circuit a maximum compression ratio of about 2: 1, and in addition, maximum compression ratios the country for the total combination of series circuits likewise around 2: 1 at most input signal levels and frequencies. In practice .._-- ~ un ~ .-. a ~ = .m ~ <-, ~. -................ -f f-- ' U1 10 15 20 25 40 25 450 985 however, it is difficult to avoid any major relationship, for example 2.5: 1, within a smaller level and frequency range. This can be tolerated if the compression ratio is not greater than 2.5: 1 (or about 1.25 times the compression ratio of each circuit). and on the level and frequency range in which this occurs. more is not too big.

Another particular embodiment of that in Fig. 2 in general The present invention is to perform a compressor and a expander as a band-splitting device, as described in U.S. Pat. No. 3,846,719 and U.S. Pat. No. 3,903,485, and the second compressor and the second expander as sliding device. A suitable split band device, multiband device, is described in "Journal of the Audio Engineering Society", Vol. 15, no. 4, Oct. 1967, pp. 383-388. Banded devices according to with the parameters given therein have become generally known as A-type devices.

In a practical embodiment, a compressor is supplied A-type a flat input signal and emits its output signal to a specific or sliding belt device adapted thereto. It is most beneficial and to position the A-device so that it is fed with an traded input signal, because it is designed to process a flat input signal. Placement of the sliding device first would cause the disadvantage that the flat input signal was changed to a less suitable form for input into an A device. On the reproducing side, the sliding device is fed with the signal from the N-channel, processes this signal and supplies it to an A-type expander.

Fig. 16 shows curves corresponding to those in Fig. 9 for low level the response of a single A-compressor and for a single sliding compressor and the answer for the combined compressor.

The expansion curves are complementary in accordance with the holdings of Fig. 9. The A-type device provides 10 dB of compression up to about 5 kHz, and above that the level rises slightly about 15 dB at 15 kHz. Benefit from this rise frequency response of the A-characteristic to make the sliding characteristic less sensitive at high frequencies (see high the frequency portion of the "slipband" curve in Fig. 16); this is beneficial and to reduce the impact of uncertainties in the event of high frequency of the channel response, which is described in more detail below.

The combined frequency response curve thus rises smoothly 10 15 20 ZS 35- 4DU YÖÖ N to 20 dB where it flattens out and remains mainly at this level to 14 kHz where it drops again. The sliding device is dimensioned so that its work thresholds and resulting dynamic action areas are well beyond what applies to the A-circuit.

Pig 17 shows a group of frequency response curves for different levels for compressors with series-connected A-ngr and sliding device nings. These curves represent the same type of information as Fig. 11. The shaded area C generally indicates the dynamic ranges resulting from the action of the A-device, while it the shaded area D derives from the action of the sliding device.

This arrangement results in a maximum compression ratio. not exceeding at any level or frequency about 2: 1 and is therefore relatively free of reinforcement errors in practice tape recorder channels.

It will be appreciated that the series connection of a standard A device with a special sliding device refers to an exemplary case. In principle, however, the A-device can be modified for shift of their dynamic ranges so that it best fits together with the working areas of the sliding device.

Exact amount required spreading or shifting at this and other configurations described above depend on the use the parameters of the signal processing devices. the purpose with spread of the dynamically active areas is to be minimized grouping effects in the frequency response curves. Such a hop-- grouping is a sign of large compression and expansion conditions. See, for example, Fig. 18, which illustrates one cases with very strong aggregation; at certain frequencies and levels, a 10 dB input signal level change results in an end fring of the output signal of Z 1/2 dB - a ratio of 4: 1. Op- timalt comes with appropriate spread a ratio of 2: 1 should never be exceeded to a greater extent in cassette expanders. systems over most of the level and frequency range. At others types of transmission systems with higher compression ratios be acceptable.

Claims (14)

450 985 27 "Patent claims A circuit arrangement for modifying the dynamic range of an input signal and comprising a first circuit having a bi-linear characteristic consisting of a low level portion with substantially constant gain up to a threshold, an intermediate level portion lying above the threshold with variable gain, which provides a compression or expansion ratio, which varies between one at the ends of the intermediate level and a maximum value between these ends, and a high level part with substantially constant gain, different from the reinforcement of the low level part, the first circuit, in the case of compression for radio transmitters, if desired in front of it have a broadband compressor or limiter, characterized in that the first circuit is followed by at least a second circuit, which also has bi-linear characteristics within a frequency range common to the circuits, the intermediate level parts of the characteristics of the circuits being spread within a for the circuits common frequency range in and for åstadk a gain change over a larger range of intermediate input levels than for each circuit separately, as well as a greater difference between the gain at low and high input levels, but with a total maximum compression or expansion ratio, which due to the spread is essentially not larger than for a single circuit, _and a total characteristic, which is also bi-linear; Circuit arrangement according to claim 1, characterized in that the maximum compression or expansion ratio in total does not exceed approximately 1.25 times the maximum ratio for any of the circuits. 3. A circuit arrangement according to claim 1 or Z, characterized in that the portion of the intermediate portion adjacent to the high level portion of the bi-linear characteristic of one of the circuits overlaps the portion of the intermediate portion adjacent to the low level portion of the bi-linear portion. linear characteristics of another circuit to such an extent that the total compression or expansion ratio within the overlap area does not significantly exceed the maximum compression or expansion ratio for adjacent circuits. Circuit arrangement according to one of Claims 1 to 3, characterized in that each of the circuits has a maximum compression or expansion ratio of substantially 450 985 28 2: 1 and that the maximum ratio in its entirety does not substantially exceed 2: 1. Circuit arrangement according to one of Claims 1 to 4, characterized in that the approximate threshold level T for a given step is determined by the relationship _ _'E r_1 = (H, where F is the end point of the step, C the maximum compression of the step ratio and G step amplification. A circuit arrangement comprising both a compressor and an expander, according to any one of the preceding claims, characterized in that in the compressor the threshold of each of said second circuits is lower than the threshold of the preceding circuit and in the expander threshold of each of said second circuits is higher than the threshold of the previous circuit. A circuit arrangement according to any one of claims 1 to 6 and arranged for modifying the dynamic range of audio signals, each circuit comprising an overshoot suppressor having a threshold level, characterized in that the threshold levels are spread among the circuits in order to effect a reduction. total circuit device overshoots. Circuit arrangement according to one of Claims 6 and 7, characterized in that the thresholds of the surge suppressors in the individual circuits of the compressor and of the expander, respectively, are spread in the same sense as the thresholds of the successive circuits. 9. A circuit arrangement according to any one of claims 1-8, characterized in that it comprises bi-linear circuits, each of which has a low level gain of substantially 10 dB. Circuit arrangement according to any one of claims 1-9, wherein at least one of the circuits comprises a variable filter which provides an increase or decrease within a high or low frequency range of the signal band and which responds to signals within this range, so that the filter transition frequency is caused to slide in such a way that the raised or lowered area becomes narrower, preferably each circuit comprises a rectifying, smoothing and amplifying control circuit for outputting a control signal to a controlled impedance device of the filter to effect the sliding of the filter transition. frequency, characterized in that each circuit includes a variable filter. 450 98g5 29 11. A circuit arrangement according to claim 10, characterized in that the control circuit of each variable filter has a special gain for forming different thresholds for the circuits. 12. A circuit arrangement according to claim 10 or 11, characterized in that the rest-transition frequencies of the variable filters are substantially the same. Circuit arrangement according to claim 10, 11 or 12, wherein the increase or decrease is in a high frequency range, characterized in that the resting transition frequencies of the variable filters are in the range 300 to 400 Hz. Circuit arrangement according to any one of claims 1-13, wherein at least one of the circuits consists of a double path circuit, which comprises a main signal path which is linear with respect to the dynamic range, a combination circuit located in the main path and a further path having its input connected to the input or output of the further path and its output connected to the combining circuit, the further path emitting a signal which, at least in an upper part of the frequency band, raises or lowers the main path signal by means of the combining circuit but which is so limited, that it is smaller than the main path signal within the upper part of the input signal dynamic range, characterized in that each of the circuits is a double path circuit.