NL8103122A - CHAIN DEVICE FOR CHANGING THE DYNAMIC AREA. - Google Patents

Description

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Chain device for changing the dynamic area.

The invention generally relates to circuit devices that change the dynamic region of signals, namely compression members that compress the dynamic region and expansion members that expand the dynamic region. The invention is particularly useful for handling audio signals, but is also applicable to other signals.

Compression members and expansion members are normally used together (a compression expansion system) to effect noise reduction; the signal is compressed for transmission or recording and expanded upon reception or playback from the transmission channel. However, it is also possible to use only compression means to reduce the dynamic range, for example to follow the capacity of a transmission channel without an additional expansion when the compressed signal is sufficient for the end purpose. Additionally, only compression means are used with certain products, especially audio products, which are intended only to transmit or record compressed broadcasting or pre-recorded signals. Expansions are used only with certain products, especially audio products, which are only intended to receive or play from already compressed broadcasting or for recorded signals. In certain products, especially audio recording and playback products, a single device is often constructed for a switchable operation as a compression means for recording signals and as an expansion means for playing compressed broadcasting or pre-recorded signals.

30 The degree of compression or expansion can be expressed in dB. For example, a compression of 10 dB means that a dynamic area with an input of N dB is compressed to an output area of (N - 10) dB. Men 8103122% -2 -

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says that in a noise reduction system, 10 dB of compression followed by 10 dB of complementary expansion provides 10 dB of noise reduction.

The invention relates in particular to a circuit arrangement for changing the dynamic range of an input signal and comprises a first circuit with a bi-linear characteristic (wherein "linear" in this context indicates a constant gain) consisting of: 1) a linear part with a low level up to a threshold, 10 2) a non-linear (change of gain) part with an intermediate level above the threshold and up to an end point that provides a predetermined maximum compression ratio or expansion ratio, 3) a linear high level part with a gain that is different from the gain of the low level part.

The characteristic is called a bi-linear characteristic because there are two parts with practically constant gain. In practice, the threshold and end point 20 are not always well-defined "points". The two transition regions where the intermediate level portion terminates in the low level and high level linear regions may each vary in shape from a flat curve to a sharp curve depending on the control characteristics of the compression member 25 and the expansion member. It is also pointed out that circuit devices with bi-linear characteristics are distinguished from two other known classes of chain devices, namely: (a) a logarithmic or non-linear chain device with either a fixed or changing slope and 30 with a non-linear part: the gain changes over the entire dynamic area.

(b) Chain devices with a characteristic and two or more parts, of which only part is linear ("uni-linear").

A chain device with a bi-linear 8103122-3 characteristic has certain advantages and is widely used.

The threshold can be set above the input noise level or transmission channel noise level to exclude the possibility of chain control for noise. The high level portion with a practically constant gain avoids nonlinear handling of high level signals that would otherwise introduce distortion. In addition, in the case of an audio signal for which the circuit must be syllabic, the high level portion provides an area within which shooting is occurring that occurs at a syllabic chain as the signal level suddenly increases. This overshoot is suppressed by cut-off diodes or the like. Only bi-linear characteristics are able to deliver this combination of benefits.

The majority of the known bi-linear characteristic chains in use today in consumer audio products provide 10 dB compression and expansion which is sufficient for many purposes. However, some noise remains audible for some listeners and for the highest quality, more compression and expansion is desirable, for example 20 dB. It is difficult to provide such compression or expansion without encountering issues affecting the quality of the signal.

Chains are known and commercially available that provide 20 dB of compression or expansion, and even more, but these are usually constant slope logarithmic chain devices with constantly changing gain across the entire dynamic range or close to the entire dynamic range. These chains have greater distortion and signal tracking problems at very low and very high signal levels than the bi-linear chains in which the gain change is limited to an intermediate part of the characteristic and overshoot problems are more severe than in bi-linear devices characteristic. Known constant slope eompression expansion devices apply compression ratios 35 in the range 1.5: 1.2: 1 and 3: 1 »but 2: 1 it is 8103 122

• “V

* - ½ - most common.

The compression ratio is defined as the ratio of the incremental input of the dynamic area to the incremental output of the dynamic area.

The expansion ratio of a complementary expansion device is the inverse of the compression ratio. When the compression ratio is 3: 1 then the expansion ratio is 1: 3. It is easy to use the idea of the inverse expansion ratio which for the example 10 given is 3: 1, that is, corresponding to the compression ratio.

For the sake of simplicity, the discussion will be largely limited to the compression ratio, provided that the same considerations apply with appropriate formations to the expansion ratio.

A large compression ratio has a drawback in that it is difficult to ensure complementarity between the compression member and the expansion member; in particular, level errors or errors in the frequency response of the transmission or recording medium give rise to corresponding multiplied errors at the output of the expander. ·

It is known (for example US-PS 2,558-002, US-PS U.061.87½ and Japanese specification 51-2012½) to increase the amount of available compression by cascading a number of compression stages. These known circuits (controlled impedance device, diodes, etc.) multiply the compression ratios of the individual stages so that the result is a large compression ratio with the drawback set forth above. For example, a chain with a compression ratio of 2: 1 and the others with a compression ratio of 3: 1 will give an overall ratio of 6: 1. The resulting expansion ratio of 1: 6 would be a heavy burden with regard to the uniformity of the transmission channel. Another consideration is the requirement placed on the circuit which accomplishes the change of gain needed to effect the compression or expansion characteristic. It is relatively easy to cause a chain to make accurate gain changes over a 10 dB range, but significantly more difficult to ensure that the same chain makes accurate gain changes over a 20 dB range. Thus, it is difficult to establish a controlled, reproducible characteristic for use in an eompression expansion system. Japanese publication 51-2012 * 1 · concludes that a number of compressors (and expansion organ) arranged in series are not suitable as a noise reduction system for high quality display systems.

It is also known (US-PS 3 * 902,131 and US-PS 3,930,208) to cascade a number of compression stages that operate in mutually exclusive frequency ranges.

While such devices need not result in any increase in the compression ratio over that of a single stage, they do not provide any increase in compression.

In light of all these considerations, the object of the invention is to provide increased compression or expansion without an undesirably large increase in the compression ratio and without placing undue demands on some chain to effect a change of the reinforcement.

A further object is an increase in audio compression or expansion without producing an undesirably large increase in overshoot delivered from the transition signal conditions.

30 A close examination of bi-linear chains shows that they not only have the aforementioned advantages but also a further advantage, namely a way of solving the problem of the high compression ratio and in the case of audio chains also a way of solve the problem with a high degree of overshoot.

8103 122 - 6 -

It is noted that the superposition of the linear regions does not enhance the compression ratio in these regions; the compression ratio has only increased in the limiting region in which the dynamic operation takes place.

Therefore, it has been found possible to separate the areas of dynamic action in such a way that the required overall increase in compression is obtained while at the same time not significantly changing the overall maximum compression or expansion ratio.

A further feature of this device is that the overall result is bi-linear with all associated advantages. Thus, the possibility of cascading bi-linear devices provides a further hitherto unrecognized advantage of this class of devices.

The objects indicated above are achieved with the invention which is characterized in that a first circuit having a bi-linear input-output characteristic is followed by one or more further chains which also have bi-linear characteristics at any given frequency 20 within a frequency range which is common to the chains. The thresholds and dynamic ranges of the chains are set to different values to stagger the intermediate levels of the characteristics of the chains to provide a change of gain over a wider range of intermediate input levels than for each of the chains individually and to increase provide a difference between the gains at low and high input levels, but with a maximum compression or expansion ratio that is practically no greater than the maximum compression ratio of any single chain by staggering.

In the case of audio chains, when the chains have over-suppression (limiting elements), it is also possible to jump their thresholds along with the jump of the syllabic thresholds. The overshoot of the lower level peaks or stages is correspondingly reduced with a minimum total overshoot of the various stages. This is in contrast to conventional logarithmic compressors in which inherently large overshoots are provided.

Each chain may introduce a change in the spectral content of the signal, for example, a low level triple boost in the case of an eompressor. Thus, each successive stage may be driven by a signal with progressively changing spectral content.

In the case of complex signals, this has the merit of spectrally spreading the chances of an error in the decoding function. For example, in the case of a tape recorder with an uneven frequency response characteristic, the spectral shift tendency reduces the total dynamic and frequency response errors of the decoded result.

Attention will now be paid to the amount of offset required. For the sake of simplicity, a series connection of two compression chains will be considered. The compression ratio of each of the first and second chains 20 will rise from the unit at the respective drentel to a maximum (e.g. 2: 1) and this will be called the rising edge of the compression ratio. The ratio will then drop to unity and this will be called the falling edge. Strictly speaking, the falling edge may approach unit 25 asymptotically, but for all practical purposes it can be considered that it has reached the unit if it has some value different from the unit by any small amount.

The offset of the intermediate level parts of the first and second chains results in the falling edge of one chain overlapping the rising edge of the other chain.

In a first approximation, the difference between the two thresholds can be made at least such that the overlap of the flanks results in a total compression ratio that practically does not exceed the maximum compression ratio of each chain at 8103122.

The threshold of the second chain is preferably lower than that of the first chain (when more than two chains are used, preferably each further chain 5 has a progressively lower threshold), in the case of a compression member and vice versa in the case of an expansion device. In principle, the order can be reversed with the first compression chain having the lower threshold. In the case of more than two chains, the order of the threshold levels can in principle be reversed in them and arranged in any order as long as the intermediate level parts of the chains are correctly staggered.

Thus, the ideal offset is considered to be that which causes the falling edge of one circuit to overlap the rising edge of the other chain in order to limit, as far as possible, the level range in which a dynamic action occurs in the entire series-connected device while at the same time, any practical increase in the maximum compression or expansion attitude to that of a single device is avoided. For example, when the maximum compression ratio of each chain is 2: 1, the compression ratio of the total chain will rise to 2: 1, keep this value at the overlap, and then decrease to the unit. Ideally, therefore, there is no increase at all above the 2: 1 ratio, unlike the prior art devices of cascaded compression stages which multiply the ratios to h: 1.

In practice, it may be difficult to obtain an optimal overlap at all frequencies, but it is possible that, provided a reasonable approximation is made to the ideal, to prevent the total maximum compression ratio from rising excessively above 2: 1 in the given preview. It may rise to 2.5: 1 in a practical chain arrangement. A low maximum compression ratio (e.g. 1.5: 1) allows 8103122 - 9 - an expansion member to more easily follow the compression member to provide good complementarity with signal channels providing somewhat unreliable gain and have frequency responses. However, a low congress ratio spreads dynamic action over a rougher range of levels causing greater sensitivity to noise modulation for a given maximum noise reduction (difference in gain at low and high levels). Thus, there is an exchange between unwanted effects caused by both high and low compression ratios. Accordingly, the ideal compression ratio will depend on the system environment and the design purpose of the system.

The ability to jump bi-linear steps gives the designer an additional way to optimize an entire chain. By doing this, the shapes of the compression characteristics of individual stages can be designed with staggering specifically in mind.

The transition characteristics of the circuits have also been considered, and preferably the opportunity is taken to offset the 20 overshoot suppression thresholds at audio compressors and expansions resulting in a minimum total overshoot.

A known type of a chain, called "sliding band", which can be used for each of the first and second chains is the indicated desirable characteristic for the case of high frequency audio compression or expansion by applying a high frequency boost (for compression) or limiting (for expansion) by means of a high-pass filter with a variable lower angular frequency.

As the signal level in the high-frequency band increases, the filter corner frequency shifts upward to narrow the chased or contained band and exclude the useful signal from chasing or limiting. Examples of such chains can be found in US-PS Re 28Λ26, US-PS 3,757-25 ^, US-PS U.072.91 ^, US-PS 3.93 ^ -. 190 'and Japanese patent application 8103 122-10 - 555.29 / 71.

Thus, each of the first and second chains can be such a "sliding band" chain. In principle, the entering angle frequencies of the two sliding band chains can be different and can be used to provide a compression or expansion rate that is higher in one part of the treated frequency band than in the other. In accordance with an important further development of the invention, however, the angular frequencies are made practically identical. This leads to the advantage of a sharper distinction between the frequency range where chasing if retraction is applied and the area where it is not applied and thus a sharper distinction between the area where noise reduction no longer occurs due to the occurrence of an interest. rich useful signal and the area where noise reduction remains effective.

On the other hand, there are also known circuits in which the frequency spectrum is split into a number of bands by corresponding bandpass filters and the compression or expansion is effected in each band by a gain control (either an auto-appealing diode-type limiter or a controlled limiter -ting) in the case of a compression member having some form of a reciprocal or complementary expansion member circuit. Examples of such chains can be found in US-PS 3.8½.719 · These split band or multiple band chains have the advantage of independent operation in the different frequency bands and when this property is required such chains can be used as the first , second or more stages in the chain device according to the invention.

In principle, one of the first and second chains can be a multi-band chain and the other one can be a sliding band chain. This will be important in a particular situation where it is desirable, for example, to increase the compression or expansion rate in one part of the total frequency band through the sliding band chain and one or more of the band splitting channels about this part of the frequency band.

It is known to construct bi-linear compression and expansion members of so many the sliding band type as the splitting band type using only a single signal sweep. However, it is generally preferred to construct such devices by providing a main signal chain velke is linear with respect to the dynamic region with a combination chain in the main chain and a further chain velvet derives its input from the input or output of the further chain and the output of which is linked to the combination chain. The further circuit 15 includes a limiter (self-explored or controlled) and the bounded further chain signal chases the main chain signal in the combination circuit for the case of compression but bends (bucks) the main chain signal for the case of expansion. The boundary signal from the further veg is smaller than the signal from the main sweep in the upper part of the input dynamic region.

The main and further chains are preferably signal streaks to be identified separately and in the most appropriate manner.

Such known compression members and expansion members are particularly advantageous because they allow the first kind of transfer characteristic to be established in an accurate manner without problems of high level distortion. The low level portion with a practically constant gain is accomplished by thresholding the further veg above the noise level; below 30 this threshold the further veg is linear. The intermediate level portion is formed by the area over which the boundary spring of the further veg becomes partially effective and the high level portion with practically constant gain is created after the limiter has become fully effective so that the signal from the further veg ceases to increase and negligibly, 8103 122 4 'i, - 12 - is compared to the main road signal. In the highest part of the dynamic input region, the output of the circuit arrangement is effectively only the signal passed through the linear major path, ie linearly with respect to the dynamic region. For two-way audio chains, creating overshoot suppression is particularly helpful.

Examples of these known chains can be found in US-PS 3,846,719 US-PS 3-903,485 and US-PS Re 28426.

Analogue circuits are also known which achieve the same results but in which the further road characteristics have reversed limiter characteristics and the further road output presses the main road signal for compression and drives the main road signal for expansion (US-PS 3-828.280 and US-PS 3-875-537).

Each of these known bi-linear chains can thus be used as the first and second chains of the chain device according to the invention in order to obtain the advantages inherent in them and also to provide a good way of achieving the desired degree of offset. . This is done by adjusting the thresholds and dynamic areas of the two further paths appropriately.

As stated above, it is not essential to form the desired shape of a bi-linear characteristic with such "dual path" techniques. Alternatives exist that work with some roads as described in 25 US-PS 3-757-254, US-PS 3-967-219, US-PS 4,072,914, US-PS

3-909-733 and Japanese Patent Application 55529/71, for example. Although these alternative chains are usually unable to provide such good results as two-way or less convenient chains and thereby less economically, they can generally provide similar results. Thus, these known chains can also be used as one or more of the chains of the chain device according to the invention. If desired, one of the first and second chains can be a two-way chain and the other a single-way chain.

The invention will be elucidated on the basis of drawing 8103 122 * * - 13 -.

Figure 1 is an example set of curves showing complementary bi-linear compression and expansion characteristics.

Figure 2 is a block diagram showing the invention in general terms.

Figure 3 is an example of a graphical representation of the areas of dynamic operation and how they can be separated on a compression member or expansion member connected in series.

Figure U is a further simplified form of Figure 3.

Figure 5 is a series of idealized bi-linear characteristic curves showing a general technique for staggering the thresholds of series chains.

Figure 6 is a schematic circuit diagram of a sliding belt compression member of the prior art.

Figure 7 is a schematic circuit diagram of a prior art sliding band expander.

Figure 8 is a schematic chain diagram of a modification of Figures 6 and 7

Figure 9 is a chart record showing the response below the compression threshold of two series-connected compression members and expansion members according to an embodiment of the invention.

Figure 10 is a chart record showing the response below the compression threshold of a compression member and a prior art expansion member in accordance with Figures 6, 7 and 8.

Figure 11 is a map record of the input-output response as a function of the frequency of a compress! e-member with series devices according to an embodiment of the invention.

8103122 »« r - -

Figure 12 is a map record of the input-output response as a function of the frequency of a prior art compression member with a single device.

Figures 13-15 show a series of pilot tone curves indicating the sliding band action of an embodiment of the invention and the circuit of Figures 6 and 8.

Figure 16 shows characteristic curves below the compression threshold of a further embodiment of the invention.

Figure 17 shows characteristic curves similar in type to those of Figure 11 but for a further embodiment of the invention.

Figure 18 shows characteristic curves that are similar in type to those in Figures 11 and 17 but exhibit an outer spur.

As an example, Figure 1 shows bi-linear complementary compression and expansion transfer characteristics (at a given frequency) which (for the compression characteristic) indicate the low-level part with a practically constant gain, the threshold, the part where a dynamic action occurs, the end point and the high level portion with practically constant gain.

Figure 2 shows the invention in general terms: a first bi-linear compression member 2 receives the input information and applies its output to a second bi-linear compression member, which is connected in series and whose output is applied to a noise information-carrying channel. A pair of bi-linear expansions 6 and 8 connected in series receive the input of channel I at expansion member 6 and provide a noise reduction system output at the output of expander 8. The areas of dynamic operation of the series devices are separated. or staggered relative to each other within the frequency range common to the devices. Although the figure shows two devices on each side of the information channel N, two or more may be used, the invention contemplating two or more sets of bi-linear compressors or expansions. In a construction as a complementary noise reduction system, the same numbers of arrays of bi-linear compressors and expansions are provided.

The order of stages with certain characteristics in the compression member is reversed in the expansion member. For example, the final stage of the expansion member is complementary to the first stage of the compression member in all respects - a steady state and time-dependent dynamic response (frequency, phase, and transition response at all signal level and dynamic conditions) .

A graphical representation of the separation or offset for two bi-linear devices is shown as an example in Figure 3 in which the compression ratio is plotted against the input amplitude level (horizontal axis) for a compression member or an expansion member operating at a certain frequency. For clarity, the curves are given in idealized form; as a practical matter, the curves are somewhat asymmetrical in practical embodiments of applicant's A-type and B-type noise reduction systems (US-PS 3.8U6.719 and US-PS Re 28.h26, respectively). Curve 12 refers to the dynamic action of one compression member or expansion member (the high level stage). Curve 10 is that of a further compression member or expansion member (low level stage), with a separate area of dynamic action.

When the high level stage is the first in the series of 30 compression members (second in the series of expansion members), the curve 12 represents the variations of the compression ratio of the first (compression stage} as a function of the input level to the first stage and curve 10 is the variation of the compression ratio of the second (a compression member) stage 35 as a function of the input level to the first stage. "-----—-- 1» .---- mts 8103 122 - 16 - curves are those of compression members and the lower curves are those of expansion members. In this example, the ranges of activity are separated in response to the input amplitude level. in such a way that the product of the two curves results in an overall characteristic with a compression ratio or expansion ratio not exceeding 2: 1 (1: 2) between the two maximum compression points 10a and 12a (10b and 12b) of the two establishments.

Thus, even with two devices in 10 series, the terminal operating areas are still fixed with the maximum compression and the maximum expansion ratios not increased beyond those of some devices and the advantages of some bi-linear devices retained. Accordingly, any error that occurs within the range of the dynamic operation caused by the devices in series should not affect that of a single device.

Most bi-linear devices determine the constant gain fixed end regions by means of fixed preset circuit elements such as resistors and capacitors that are inherently stable and cannot introduce dynamic errors, waveform distortions and the like. Therefore, only in a transition region of operation between the linear regions of constant gain can any dynamically active part of the circuits introduce signal errors.

It is then noted in the representation of Figure 2 that the dynamic operation of a conventional logarithmic compression member or expansion member becomes a horizontal line; for example, line 11 is the characteristic of a 2: 1 compression member, at 13 is that of a 1: 2 expansion member. It is clear in this analysis that there is no opportunity to separate or stagger the actions of such devices.

For an analysis and to obtain a first approximation of necessary threshold levels to provide an optimal long jump according to the invention it is useful to further idealize 8103122-17 figure 3. Then assume that each compression member (and expansion member) immediately reaches its maximum compression ratio at a threshold and maintains this ratio until it reaches an end point at a higher level where its dynamic action suddenly stops.

Then, a series of compression members and expansion members depicted in the manner as shown in Figure 3 appear as tangential rectangular curves as shown in Figure 4. As an example, three compression members and expansion members with bi-10 linear typically connected in series. The low level device which is preferably the third compression member (first expansion member) has the lowest threshold (T3) indicated at -62 dB with its end point (F3) at -U6 dB which is the threshold (T2) of the middle level staircase. The mid-level stage 15 has its end point (F2) at -30 dB which is the threshold (Tl) of the high level stage. The high level stage has its end point (F1) at -i dB. All levels refer to the total input. It has further been assumed that each stage has a gain of 8 dB and a maximum compression ratio of 2: 1.

Figure 5 indicates idealized characteristic curves (total input against output) for a compression based on the example of figure k (the mirror image expansion curves are omitted for clarity). The drawing shows how the dynamic action of each stage occurs in addition to that of its adjacent stage resulting in a total compression ratio of 2: 1 while obtaining 2h dB of compression.

Based on the observations from Figures k and 5, a single equation gives the relationship between threshold levels 30 (T) s end point (F), maximum compression ratio (C) and gain (G) of each given stage: m = f CG_ TF c - Γ

Using this equation, the threshold levels for each stage can be determined with a fairly close approximation using an iteration method. For example, if a total end point (F1) of - 14 dB with a step gain of 8 dB and a maximum compression ratio of 2 is desired, the equation shows that the high level threshold (T1) - 30 dB is. This value is then used as the end-5 point (F2) of the mid-level.trap to determine that its threshold should be - k6 dB etc. So each stage refers back to the result of the previous stage in this analysis. However, the calculated threshold is the total threshold with reference to the entry of the sequence. To obtain the threshold of a given circuit with reference to its own input, the cumulative signal gain up to that point is taken into account.

For example, the threshold of the lowest level stage of Figure 5 is - U6 dB when reference to the input of this stage.

The equation can also be solved for the end point F, the compression ratio C or the gain G.

So the designer can determine the parameters of his chain based on what his goal is. Such a target may include requiring that the lowest level threshold be above the noise bottom, that the highest level endpoint is low enough to allow the use of overshoot protection, and that the total maximum compression ratio does not exceed a certain value. In chains in practice, the threshold and end points are not always well-defined points as they are in an analysis. As discussed in the introduction, the areas in which. The intermediate level parts of the characteristic end in the low level and high level linear parts are flat or sharply dependent on the characteristics of the circuits controlling the dynamic operation. Thus, in practice, a threshold region of one chain will overlap the endpoint region of another chain.

Considering the above equation in Figure 5 shows that for the given case of a 2: 1 compression ratio, half of the threshold offset is provided by the signal amplifications of the stages that the other 35 should be supplied by a different setting on the 81 03 122 - 19 - control element and / or changed gain of the control amplifier (increased gain for lower threshold). For 1.5: 1 and 3: 1 compression ratios respectively, 1/3 and 2/3 of the stag, respectively, is provided by 5 stage gains and 2/3 and 1/3 of the stag, respectively, must be provided by the control circuit . In Heath Figures 1 and 5, 0 dB is a nominal maximum or reference level. In practice, a headroom of 10 to 20 dB is provided above the 0 dB level.

As mentioned previously, it is usually preferred that the high level stage is the first in a compression member array and the low level stage is the lowest.

However, a reverse arrangement is also possible. Conversely, the first stage control amplifier needs a large gain to achieve the required low threshold. This low threshold then applies signals even in the presence of high-level signals, which in the case of sliding handheld systems generally known in the art usually leads to a poor noise modulation result of the overall system. In this reverse device, each stage must provide sufficient gain to the control amplifier to obtain the threshold required for this stage. In addition, each threshold is essentially defined and independent of the operation of the other stages. This is a consequence of the fact that the signal gain of each previous stage has practically dropped to the unit when the threshold is reached for the corresponding subsequent stage. The calculation of the thresholds required for an optimal long jump in the reverse case is the same as for the preferred case. However, the threshold of each stage with reference to its own entrance becomes the same as the total threshold.

In contrast to the reverse situation, there is the preferred arrangement (in which the high-level stage is first in the compression chain and the low-level stage last), a useful interaction between the stage gains and the 8103122-20 thresholds. The thresholds of the downstream stages are partly determined by the signal amplifications of the previous stages. In a two-stage system with 10 dB of the low level gain per stage, the requirement of the gain of the second stage control gain is reduced to 10 dB due to the low level signal gain of the first stage. When a high level signal occurs, the 10 dB gain of the first stage is eliminated and the threshold of the low level stage is effectively raised by 10 dB. With sliding band compression expanders, this improves the noise modulation result of the noise reduction operation.

According to the preferred arrangement, the gains of all previous stages are fully effective up to the threshold of any given subsequent stage. Thus, in contrast to the above-described sequence in reverse order, the preferred arrangement has the most advantage of the predominant signal gains of the individual stages. Namely: 1, At signal conditions with very low level 20 (sub-threshold), the requirement for the amplification of the control ampli

each stage is reduced to an amount equal to the cumulative signal gains of all previous stages. Thus, in the example of Figure 5, the gain of the control amplifier required for the lowest level stage 25 to obtain a threshold of - 62 dB is decreased by 16 dB

with respect to what is needed if this stage were to operate independently or in the previously described reverse construction.

Likewise, the gain of the mid-level stage control amplifier is reduced by 8 dB, thus resulting in the most economical circuit.

2. A variable threshold effect dependent on the signal is obtained, with noise modulation effects being reduced with sliding band stages. The effective thresholds of the low level stages are increased progressively with an increasing signal level at a given frequency.

8103 122 - 21 -

At high screed levels (on the high level linear part of the transfer characteristic), the effective threshold of the lowest level stage is raised by a level equal to all low level stage gains (sub-threshold) up to that point.

In the example of Figure 5, the threshold of the lowest level stage, normally - 62 dB and the low level signal conditions, is thus increased by 16 dB, to - k6 dB, under high level signal conditions. Similarly, the threshold of the mid-level stage is raised to -38 dB.

In a first practical embodiment according to the invention using series of sliding belt devices, the compression member 2 and the expansion member 8 of Figure 2 are essentially standard B-type sliding belt devices as shown in US-PS Be 28.U26, while 15 the compression member h and the expansion member 6 have altered response characteristics. It has been found that with the noise generated in cassette tapes a workable result is obtained when the second device (in the compression mode) not only has a staggered input amplitude level response but also a cutoff equation that is some two to three. octaves lower than that of a standard B-type device. More specifically, the threshold levels of the second device are lowered both of the syllabic filter / restrictors and of the overshoot suppression limiter to effect hopping where the angular frequency of the captured filter is lowered by two to three octaves.

Details of the B-type chain are shown in Figures 6, J or 8 which are the same as Figures 1 *, 5 and 10 from US-PS Re 28Λ26 respectively, and further details of the 30 chains in their operation and theory are presented. The following description of Figures 6, 7 and 8 is taken from US-PS Re 28,26.

The circuit of Figure 6 is specifically designed to be included in the recording channel of a consumer tape recorder requiring two such circuits 81 03 122 - 22 - for a stero recorder. The input signal is applied to terminal 10 at an emitter follower stage 12 which provides a low impedance signal. This signal is first applied via a main straight through path formed by a resistor b to an output terminal 16 and secondly via a further path whose last element a resistor 18 is also connected to the terminal 16. The resistors and 18 add the outputs of the major and further roads to provide the desired compression law.

A further route consists of a fixed filter 20, 10, a variable cut-off filter 22 with a FET 2b (these form the filter / limiter) and an amplifier 26, the output of which is coupled to a double diode limiter or cut-off device 28 and with the resistor 18. The non-linear limiter suppresses overshoots of the output signal with suddenly increasing input signals. Amplifier 26 further increases the signal to a level such that the knee in the characteristic of the limiter or overshoot suppressor 28 containing silicon diodes is effective at the appropriate signal level under transition conditions. The effective threshold of the overshoot suppressor is slightly above that of the syllabic filter / limiter. Resistors 14 and 18 are dimensioned such that the required damping compensation degree is then provided for the signal in the further distance.

The output of amplifier 26 is also coupled to an amplifier 30 whose output is rectified by a germanium diode 31 and integrated by a topcoat filter 32 to provide the control voltage for the FET 2b.

Two simple RC filters are used although equivalent LC or LOR filters could be used. The fixed filter 20 provides a cutoff frequency of 1700

Hz, below which decreasing compression takes place.

The filter 22 includes a series capacitor 330 and shunt resistor 36 followed by a series resistor 38 and the FET 2b whose source drain path is connected as a shunt resistor. At zero signal setting conditions on the gate of the FET 2b, the FET is pinched and has a practically infinite impedance; the presence of the resistor 38 can then be ignored. The cut-off frequency of the filter 22 is thus 800 Hz, which, as noted, is considerably below the cut-off frequency of the fixed filter 20.

When the signal at the gate increases enough for the resistance of the FET to drop to less than its predetermined 1 K, the resistor 38 effectively shunts the resistor 36 and increases the cutoff frequency, typically narrowing the filter pass band . The rise in the cutoff frequency is of course a progressive effect.

The operation of a FET is convenient because, within a suitably limited range of signal amplitudes, such a device operates practically as a linear resistor (for a signal of one of the polarities), the value of which is determined by the control voltage on the gate.

Resistor 36 and FET are returned to an adjustable tap h6 in a potential divider containing a temperature compensating germanium diode k8. The branch k6 allows the compression threshold of the filter 22 to be set.

The amplifier 26 includes complementary transistors that give high input impedances and low output impedances. Since the amplifier drives the diode limiter 28, a finite output impedance is required and is provided by a coupling resistor 50. The diodes 28, as already indicated, are silicon diodes and have a sharp knee at 1/2 volt.

The signal on the limiter and thus on the resistor 18 can be connected to ground by a switch 53 when it is necessary to disable the compression.

The amplifier 30 is a UPI resistor with an emitter time constant network 52 that provides increased amplification at high frequencies. Strong high frequencies (eg a cymbal stroke) will therefore lead to a rapid narrowing of the tooth in which compression takes place 8103122 - 2k - · * to avoid signal distortion.

The amplifier is coupled to the smoothing filter 32 through the rectifying diode 31. The filter "includes a series-5 resistor 5 ^ and shunt capacitor 56. The resistor 5 ^ · is shunted by a silicon diode 58 which allows fast charging of the capacitor 56 for fast driving coupled with good smoothing "at steady state conditions. The voltage on capacitor 56 is applied directly to the gate of EET 2h.

A complete circuit diagram of the complementary expansion device is shown in Figure 7 but a full "description is not necessary as the circuit is identical to Figure 6, and component values are therefore not shown for the most part in Figure 7 ·

The differences between Figures 6 and 7 are as follows.

In Figure 7, the further way diverts its input from the output terminal 16a, the amplifier 26a is inverted, and the signals combined by the resistors k and 18 are applied to the input ("base") of the emitter follower 12, the output (emitter) coupled to terminal 16a To ensure low drive impedance, input terminal 10a is coupled to the resistor through an emitter follower 60. Appropriate steps must be taken to avoid setting in the expander coming.

The amplifier 26a is inverted by taking the output from the emitter instead of the collector of the second (PIP) transistor. This change involves shifting the 10 K resistor 62 (Figure 6) from the collector to the emitter (Figure 6), automatically providing a suitable output impedance for driving the limiter. The resistor 50 is therefore omitted in Figure 7.

It is noted that it is important in aligning a complete noise reduction system 8103122-25 to have equal signal levels on the emitters of the transistors 12 at both the compression member and the expansion member. The measuring terminals M are shown connected to these emitters.

Figure 8 shows a preferred chain for replacing the chain between points A, B and C in Figures 6 and 7 · When the FET 2k is pinched off, the second RC network 22 will not work, and the first RC network 20 then determines the response from further away. The improved circuit combines the 10 phase advantages of having only a single RC portion around the setting conditions with the 12 dB per octave attenuation characteristics of a two-piece RC filter under signal conditions.

In the practical circuit, using MPF 10U FETs, the 39K resistor 36a is required to provide a finite source impedance to operate in the FET. In this way, the compression ratio at all frequencies and levels is kept at a maximum of about two. The 39K resistor 36a serves the same compression ratio limiting function in the enhanced circuit as the resistor 36 in the circuit of Figure 6 or Figure J. Additionally, this resistance provides a low-frequency path for the signal.

Changes in Figures 6, 7 and 8.

As mentioned above, in the first practical embodiment of the invention, the compression member U and 25 utilize the expansion member 6 of Figure 2, devices of the kind shown in Figures 6, 7 and 8 with modified characteristics. The changed cutoff frequency and lowered threshold are respectively obtained by changing the characteristics of the fixed filter (fixed filter 20 of Figure 6) and also the gain of the control amplifier by changing its pre-emphasis characteristics (emitter time constant network 52 of the amplifier 30 in Figure 6). The overshoot suppressor threshold is lowered by applying appropriate DC (forward) settings to the 35 diodes 28. The impedances of the variable filter (variable 8103122 3-26 filter 22 in Figures 6 and 8) are generally left unchanged to maintain an appropriate adaptation to the characteristics of available voltage controllable variable chain elements. Appropriate changes of the B-type sliding 5 band chain shown in Figures 6, 7 and 8 are to change the values of 3.3 K resistor in the fixed filter 20 to a value of 18 K to leave its cutoff frequency by two or three octaves. For an increase in the gain of the control amplifier, the value of the capacitor in the amplifier 10 emitter time constant network 52 is increased from 0.15 to 0.60 µf (or from 0.1 to 0.1 µf when the proposed 0 , 1 ^ uf value is used). Settings of about + and minus 1 / U volts in the forward direction are applied to silicon diodes 28, reducing the transmit suppression level by several decibels.

The variable filter 22 has an all-transmissive frequency characteristic in response to the bias control voltage, thus decreasing the total filter cutoff by two or three octaves. Increasing the capacitor value in the emitter network of the control amplifier 30 increases the gain of the amplifier at any given frequency.

As explained above and in US-PS Re 28 Λ2β, the cutoff frequency of the variable RC filter 22 increases as the control voltage (of amplifier 30, rectifier 31 and smoothing filter 32) increases. Thus, at larger values of the capacitance in the network 52, the variable filter responds by moving up in frequency from its set value due to lower level signals, thus shifting the level response or threshold relative to that in the unaltered B-30 type chain.

The level response can jump in numerous ways in addition to changing the control amplifier's emitter network, Other possibilities include changing the setting on the control element that would otherwise change the gain of the control gain, changing the relative gain of the relative gain. signal levels between the filter path and the control signal derivation path, etc.

Certain details of the chain of Figures 6, 7 and 8 have evolved over the years and more modern forms of the chain have been published and are "known in the art.

Reference to the specific chain in ÜS-PS Ee 28Λ26 has been made for easy representation.

Figure 9 shows an actual map recording plot of the response "below the compression threshold of the two series-connected compression members, the first of which has been modified as described above; the response of the expansion member has also been shown. Compare with Figure 10 (which is Figure 12 from US-PS Re 28.k26) showing an actual map recording plot of response below the compression threshold 15 of a single compression member or expansion member of Figures 6, 7 and 8. Figure 11 is a map record of the input / output response of the series compression members as a function of frequency Inspection of the response plots shows the two dynamic areas for the curves indicating the two staggered operating areas While the observability of the dynamic areas in these curves it is useful to demonstrate the staggered operation of the devices, in practice it is preferred that the curves be so smooth ak as possible to walk without distinguishing dynamic areas or "punches". The parallel lines A and B are drawn through the threshold areas: line A refers to the standard chain and line B to the modified chain. Compare these curves with Figure 12 (which is US-PS Re 28Λ26) which indicates equal response curves for a single unaltered B-type sliding band compression member 30. Figure 11 shows that the compression device with series devices delivers essentially twice as much compression which is distributed over a greater frequency and level area.

The variable band operation of the staggered series devices can be seen in Figures 13 and 1U showing a card recording test tone response of the compressors connected in series. Compare with Figure 15 (which is Figure 15 in US-PS Re 28.U26) which is an actual map registration obtained from the chain of Figure 6 with Figure 8.

The variable band operation is shown by plotting the compression organ frequency response by means of a low level proof tone (whose level is below the threshold of the compression element) in the presence of a high level signal; the test is detected at the output of the compression member by means of a tracking filter. The high level signal causes the circuit of the compression member to operate, the graph showing the effect on the filter's reverse frequency.

Figure 13 shows the response for a sample tone at -65 dB and 200 Hz signal tones at levels ranging from 15-28 dB and down to + 10 dB. Figure 1U is for a 500 Hz signal tone at levels ranging from - 3U dB and down to + 10 dB.

A further practical embodiment of the invention which gives an improved result are the compression member 2 and the expansion member 8 of figure 2 with the modifications of the.

standard B-type establishments.

In both series devices, the angular frequencies have been lowered by two octaves to provide a sharply rising low-level response characteristic. A dynamic jump action is provided by lowering the thresholds (both syllabic and overshoot suppression) of the second (in the compressor means) device.

A feature and useful advantage of the invention is that the frequency responses of the individual chains are compounded. When the sharpest rise noise reduction characteristic is desired, this is achieved by using chains with the same low level (adjustment) frequency response characteristics.

In the improved device, the selection of identical filter characteristics at about two octaves 81 03 122 - 29 - below that of the standard B-type devices thus results in a characteristic that rises rapidly above about 300 Hz. Thus, the system can achieve a significant noise reduction in the critical range of 300 Hz to 2 kHz, an area where noise can be distinguished in band-5 once a noise of above 2 kHz is reduced. There is negligible audible noise contribution from the band below about 300 Hz. By providing only a minimal noise reduction operation below 300 Hz, the system avoids manipulation of fundamental signal frequencies and improves the system's complementarity in practical tape recorders that have, for example, frequency response errors due to head shocks and the like. In addition, avoiding the compression of low-frequency signals improves the compatibility of the system because chasing low-frequency signals would result in annoying rumble and bass enhancement when playing encoded bands on systems that do not have complementary expansions.

Referring again to Figures 6 and 8, at 20, the two practical arrangement series to be discussed, the resistance in the fixed filter has changed from 3.3 K to 13 K, causing the overall lower cutoff frequency of filters 20 and 22 shift by about two octaves lower to about 375 Hz. In the second device, the capacitor in the emitter network 52 of the control amplifier 30 has increased in value by a factor of about U as in the embodiment discussed above. This results in a staggering of threshold levels from roughly 10 to 15 dB (depending on the signal level and the signal frequency). An appropriate setting 30 has been introduced into the diode limiting circuit 28 to decrease the shot suppression level. In a modification of the last described practical embodiment, the capacitor 3 ^ in the filter 22 can be increased in value to 0.01 ^ uf to promote consistency in the unit-to-unit characteristics and to improve the noise modulation characteristics.

8103 122 - 30 -

In that case, due to the practically equal time constant of the fixed filter 20 and variable filter 22, the device is equal to a single pole variable filter and the fixed filter can be eliminated. In that case, the resistor 36a 5 (which has a value of 1 + TK in modern forms of the B-type chain) is shunted with the source-drain path of FET 2b to provide an adjustment angle frequency of about 375 Hz. However, it is desirable to retain the fixed filter in the high level chain so that the chain can be switched to operate by itself as a standard B-type chain.

As a practical matter, a consumer product containing the improved systems just described will be compatible with existing unencrypted and B-type encoded software (e.g. tapes and FM broadcasters). The improved systems 15 contain a standard B-type device and thus can be switched to operate as a B-type device for full compatibility. On the other hand, when registered bands become available encoded with the improved system, existing B-type consumer systems may perceive an excessively high frequency information or "brightness" which can be treated by setting the high frequency tone control in the same manner as not provisioned consumer systems are currently handling B-type encrypted software.

The standard B-type chain described in US-PS Re 25 28.1) -26 has a maximum compression ratio of about 2: 1.

This compression ratio has proven to be a good practical choice for consumer cassette tape compression and expansion sets. In the series-connected chains of the above-described embodiments, each chain maintains a maximum compression ratio of about 2: 1 and the maximum compression ratio of the total combination of series chains is about 2: 1 at most input signal levels and frequencies.

In practical embodiments, however, it is difficult to avoid slightly larger ratios such as, for example, 2.5: 1 »35 in a narrow range of levels and frequencies. This may be added when the compression ratio is no more than about 2.5 '1 (r about 1 - 1 / U times that of each chain) and when the level range and frequency range in which it occurs is not wide. .

Another specific embodiment of the invention generally indicated in Figure 2 is to construct one compression member and expansion member as a split belt device as described in US-PS 3.8U6.719 and US-PS 3903Λ85 » and the other compression member and expansion member as a sliding belt device. A suitable split tape or multiband device is described in the Journal of the Audio Engineering Society, Vol. 15, No. U, October 1967, pp. 383-388.

Split band devices in accordance with the parameters in this publication are known as A type in directions.

In a practical embodiment, an A-type compression member receives a flat input signal and applies its output to a specially made sliding tape device. It is most advantageous to position the A-type device so that it receives an unprocessed input signal because it is designed to process a flat input signal. Placing the sliding tape device first would have the drawback that the flat input signal is changed to a shape less suitable for the A-type device input. On the weather side, the sliding band expander receives the channel Sf signal, processes it and applies it to the A-type expander.

Figure 16 shows curves similar to that of Figure 30-9 for the low signal level response of an A-type compression member only, the sliding band compression member alone and the response of the combined compression member. The expansion response curves are complementary in the manner of Figure 9. The A-type device provides 10 dB compression up to about 5 kHz, above which increase in level slowly rises to 15 81 03 122 t.

- 32 - dB "at 15 kHz. Benefit is drawn from this increasing response of the A-type characteristic to desensitize the sliding band characteristic at high frequencies (see the high-frequency part of the" sliding band ", in Figure 16). This is advantageous in reducing the effects of high frequency channel response uncertainties which will be described in more detail below, so the combined response curve slowly increases to 20 dB where it remains essentially level to about 1U kHz where it descends. tape device 10 is designed to have operating thresholds and resulting areas of dynamic action that are clearly free from those of the A-type chain.

Figure 17 shows a series of response curves at different levels for the Series A type and sliding band 15 compressors. These curves have the same type of information as Figure 11. The hatched area C generally indicates the dynamic areas resulting from an operation of the A-type device; the shaded area D of the operation of the sliding belt device. This arrangement results in a maximum compression ratio which does not exceed about 2: 1 at any level or frequency and is therefore relatively free of error amplification effects in practical tape recording channels.

It will be appreciated that as an example, a standard 25 A type device is connected in series with a special sliding band device. In principle, however, the A-type device can be modified to shift its areas of dynamic operation to best suit the operating areas of the sliding belt device.

The exact amount of jump or shift needed in this and other constructions set forth herein will depend on the parameters of the signal processing devices used. The purpose of staggered areas of dynamic action is to minimize stickout effects in the 35 response curves. The protrusion is an indication of a large compression or expansion ratio. See, for example, Figure 18 which exhibits excessive bulging; that is, at some frequencies and levels, a 10 dB change in the input level results in a 2-1 / 2 5 dB change in the output - a U: 1 ratio. Optimally, with a suitable offset, a 2: 1 ratio is never significantly exceeded. With a cassette compression expansion system above most level and frequency ranges. In other types of transmission systems, higher eompression ratios may be acceptable.

8103 122

Claims (20)

1. Chain device for changing the dynamic region of an input signal and having a first circuit with a hi-linear characteristic composed of a low-level part with a practically constant gain up to a threshold, an intermediate level part above the threshold with changing gain provides a maximum compression or expansion ratio, and a high level portion with a practically constant gain different from the gain of the low level portion, characterized in that the first chain is followed by at least a second chain which also has a bi-linear characteristic a frequency area common to the chains, wherein the intermediate level parts of the characteristics of the chains are staggered within a frequency range common to the chains, such that a change in gain is provided over a wider range of intermediate input levels than for any of afzo the chains attractive and an increased difference between the gains at low and high input levels but with a maximum compression or expansion ratio that is practically no greater than that of any single chain due to the offset. Chain arrangement according to claim 1, characterized in that the chains are compression members and the thresholds are set to different values to offset the intermediate level parts of the characteristics of the chains. Chain arrangement according to claim 1, characterized in that the chains are expansions and the thresholds are set to different values to offset the intermediate level parts of the characteristics of the chains. The chain device according to any one of claims 1, 2 or 3, characterized in that the falling edge of the compression or expansion ratio of each of the chains is the rising edge of the compression or expansion ratio of another of the 8103122 - 35 chains overlap to such an extent that the total compression or expansion ratio in the overlapping region does not significantly exceed the maximum compression or expansion ratio of the adjacent chains. Chain device according to claim 4, characterized in that each of the chains has a maximum compression or inverse expansion ratio of practically 2: 1 and does not significantly exceed the maximum ratio of the full chain device. Chain arrangement according to any one of claims 1 to 5j, characterized in that at least one of the chains comprises a variable filter which provides a boost or constraint in a high or low frequency region of the signal band and responsive to signals in this region to cause that the filter angle frequency shifts in the direction that narrows the hunted or restricted area. Chain device according to claim 6, for audio signals, characterized in that the or each variable filter has a rectifying, smoothing and amplifying control circuit which supplies a control signal to a controlled impedance device of the filter to shift achieve the filter corner frequency. Chain device according to claim 7, characterized in that each chain contains a variable filter. Chain device according to claim 8, characterized in that the control chains of each variable filter have a different gain such that the different thresholds of the chains are established. Chain arrangement according to one of Claims 30 or 9s, characterized in that the entering angle frequencies of the variable filters are practically the same. Chain arrangement according to claim 10, characterized in that the chasing or limiting is provided in a high-frequency range, characterized in that the angular frequencies of the variable filters are in the range 8103 122 - 36 - from 300 to 400 Hz. Chain arrangement according to any one of claims 8, 9, 10 or 11, characterized in that in at least one chain the variable filter is in series with a fixed filter having a narrow pass band than the variable filters in the setting state. Chain arrangement according to claim 12, characterized in that at least two chains have a variable filter in series with a fixed filter and that the angular frequencies of the fixed filters are practically the same. 1U. Chain device according to claim 13, wherein the chasing or limiting in a high-frequency range is provided, characterized in that the fixed filters have an angular frequency in the range from 300 to HOO Hz. A noise reduction device with a compression device according to claim 2 or any one of claims i + - 1¾ and an expansion device according to claim 3 and one of claims 1+ - 1 ^, characterized in that the threshold of each successive compression chain is lower than the threshold of the preceding chain and that the threshold of each successive expansion chain is higher than the threshold of the preceding chain. Noise reduction device with a compression device according to claim 2 or one of claims k - 1i *, and an expansion device according to claim 3 and one of the thresholds claims 4 - 14, characterized in that / each successive compression chain is higher than the threshold of the preceding chain and that the threshold of each successive expansion chain is lower than the threshold of the preceding chain. Chain arrangement according to any one of claims 30 1 - 16, wherein at least one of the chains is a two-way chain with a main road that is linear with respect to the dynamic area, a combination road in the main road and a further road whose entrance is connected to the input and output of the further road and the output of which is connected to the combination circuit, the further road providing a signal that drives the main road signal at least in an upper part of the frequency band at least in a higher part of the frequency band means of the combination circuit but so limited that in the upper part of the input dynamic region the signal from the further road is smaller than the signal from the main road. 18. Chain device as claimed in any of the claims 6-17 and 17 »wherein at least one of the chains is a two-way chain with the variable filter in the further way thereof. Chain device according to claim 1, 10, characterized in that the approximate threshold level T of a CG given stage is determined by the relationship T = F - - ^, where F is the end point of the stage, C is the maximum compression ratio of the stairs and G the reinforcement of the stairs. The chain device according to any one of claims 1 to 1U, wherein the chain device is for changing the dynamic range of audio signals and wherein each chain contains an overshoot suppressor with a threshold level, characterized in that the threshold levels are offset below the chains to provide a reduction in the overshoots of the total chain arrangement. 21. Chain arrangement according to claims 15 and 20, characterized in that the overshoot suppressor threshold of each successive compression chain is lower than that of the preceding chain and that the overshoot suppressor threshold of each successive expansion chain is higher than that of the preceding chain. 22. Chain arrangement according to claims 16 and 20, characterized in that the overshoot suppressor threshold of each successive compression chain is higher than that of the preceding chain and that the overshoot suppressor threshold of each successive expansion chain is lower than that of the successive compression chain. preceding chain. Chain arrangement according to claim 11, characterized in that there are two bi-linear chains, each with a low level gain of practically 10 dB. 8103 122 -38- 2k. Chain device according to claims 1, 2, 3, 15 or 16, characterized in that there are three bi-linear chains, each with a low level gain of practically 8 dB. Chain device according to claim 6, wherein one of the chains contains the variable filter, characterized in that there is a further chain containing a number of bandpass flashes each in combination with a limiting device. The chain device according to any one of claims h or 5S, characterized in that the maximum compression or inverse expansion ratio for the full chain device does not exceed about 1 -1A times the maximum ratio of one of the chains. 27. Chain device substantially as described in the description and / or shown in the drawing. 8103122