SE447524B - SIGNAL KOMPRESSOR - Google Patents

Description

10 15 20 25 30 447 524 2 Typically, the most audible effect is not the direct effect of those very high or very low signals as such, but rather modulation effects on signals between the outer regions of the bandwidth caused by extremely high and low frequency signals not being able to now the expander. In the following discussion, this effect will be mentioned "midband modulation effect".

In broadband companders, an amplitude error at control the frequency of appearing to the same degree in all other parts of the tret. This can be either acceptable or unacceptable. IN "sliding belt type" companders, described below, will the frequencies are at the bandwidth edges, errors at such frequencies to be substantially multiplied by the center frequencies. (If the reverse control the frequencies are at the center frequencies, which is usually the case, errors at the frequency ends will be reduced, which is an advantage with "sliding belt type" companders).

The term "transmission channel" in the present application is intended to include all parts of the plant between a compressor and an expander.

For noise reduction systems that record compressed signals on a medium with relatively small bandwidth, such as low magnetic tape speed, such as compact cartridges, it is especially difficult to achieve correct complementary expansion of the compressed signals.

This is due to the inability of such systems to provide a flat amplitude. tud curve for the signals especially at low and high frequencies. Dy- However, similar problems also occur in professional compressors / / expander system. The fault occurs after the compressor and can be caused by the tape recorder, tape, playback device 'Zler any combination thereof, including bandwidth limiting filters.

Similarly occurs in other systems, such as PM radio and satellite transmission, compression errors in the transmitter, i the medium transmitting the signal, in the receiver or in any combination hence. ' In the compressor / expander systems performed in practice it has been found necessary to introduce steep barrier filters before compression and before the expander. The filters are at least of the low-pass type but preferably also contains high-pass parts. Such band-limiting files limit frequencies at the edges of or outside the system reversible passbands so as not to limit the bandwidth of the system.

Filters of this kind have several functions! 10 15 20 25 30 35 HO 447 524 _ r 51- a) attenuation of subcarrier components and the at broadcasts using the 19 kHz pilot tone; this attenuation has to purpose of eliminating "birdie" interference (whistling) and error tracking processing of the encoding and decoding processing circuits, b) attenuation of the tape recorder's pre-magnetization, which may may leak to the signal circuits; this damping means that one avoids encoder low encoder error tracking. c) attenuation in the input signal of radio frequency or above it audible area lying signal components, which would otherwise be result in audible intermodulation products and / or capabilities netting "birdies", d) attenuation of band noise above the audible range or other transmission channel noise at the decoder input, this in and to avoid encoder / decoder error tracking, (e) a signal bandwidth definition for the promotion of mentality between the curves of encoders / decoders.

In professional applications, it is desirable to use a high frequency bandwidth limiting filter (eg 20-25 kHz) And preferably also a ~ band-limiting filter of 1åšfFekVenStYP 20 Hz).

If an ideal channel exists, the melian encoder and the decoder, (for example. should strictly disconnect the decoder input filter, since its presence in certain signaling situations would entail one some non-complementarity (the encoder signal undergoes a filtering step, unless the decoder signal undergoes two filtering steps).

However, removing the decoder input filter is not practical taking into account points a) toe) above. Even when a good channel exists between the encoder output and the decoder input, there will therefore be of the (for protection purposes) the most necessary in-house filter H05 o ~ a; - - v x the decoder to result in a inherently non-complementary under certain signal conditions. As a result, it may be system to help to consider the input filter of the expander as a transmission part of the channel.

An object of the present invention is to achieve suppression of those of errors in the amplitude curve of the transmission channel caused adverse effects on complementarity 1 noise reduction compander-type system.

Utïry Ut kt in other words is a purpose with pre- sentive “Uu to suppress the harmful effects of complementarity 1 Compander-type noise reduction systems, which are caused by amplitude 10 15 20 25 30 35 HO 447 524 curve error occurring between the compressor and the expander.

A particular object of the present invention is to non-complementary behavior occurring at very high (low) frequencies mental effects which give rise to audible effects in the the frequency range (in order to reduce the centerband modulation effect).

Another object of the invention is to suppress such adverse effects in plants in which the compressed signal bandwidth exceeds the bandwidth within which the transmission channel amplitude curve is relatively flat.

A further object of the invention is to suppress the equally harmful effects in audio tape recorders using compact cassettes or other bandwidth-restricted media, including audio part of videotapes and discs. Yet another object of the present invention is to reduce the non-complementary modulation of remote level gain frequency signals (for example about 500 Hz - 1 kHz) when these occur simultaneously with signals of extremely high frequency (for example over 10 kHz) in such audio tape system.

A further object is to reduce non-complementary effects. effects introduced by using input filters of the expanders. Yet another object of the invention is to reduce the above effects in "sliding belt systems" of the kind described below.

In bandwidth-limited systems, they approach or exceed the bandwidth of the compressed signals is the usable bandwidth of the recording / playback transmission channel, and thus are systems are particularly prone to errors in the recording / playback frequency time, especially at the upper and lower edges of the frequency band.

The problem has two aspects: 1) the band output of the compressor output signal width may exceed the part of the bandwidth within which the recording / / playback curve is relatively flat in a self-employed audio equipment, and 2) the bandwidth of the compressor output signal of the device used for the completion of pre-recorded tapes or skis may exceed the bandwidth within which the audio device is yield curve is flat.

While errors can theoretically occur anywhere within spectrum, as explained below, the invention is directed to the pressure'of the effect of faults at the plant's band boundaries, in particular on suppression of midband modulation effects caused by dylika fel. C The invention which constitutes the solution to the problem of 10 15 20 25 30 35 h-o 447 524% centerband modulation is rather surprising in its simplicity. Closer In particular, the signals processed by the compressor are subjected to an ruptured high and / or low frequency cases with a cut-off frequency as good 'and is well within the usable passband of the system, and more specifically tuned slightly below (above) the frequency at which the transmission channel and / or the recording / playback curve shows its main ones wrong. The signals processed by the expander are preferably exposed for a complementary increase so that an overall flat frequency curve maintained. If a not totally flat frequency curve can be accepted, the invention can only be included in the compressor system's compressor ~ parties.

In accordance with the present invention, a part (or parts) of the signal spectrum within which the transmission channel or the recording / playback curve shows errors for an attenuation to such level that the subdued party is essentially excluded from governing compression and expansion.

In accordance with the invention, it is exposed by the compressor treated the spectral content of a change or asymmetric operation spectral skewing so that the compressor action is significantly exposed less for the influence of signals outside the abrupt cut-off frequency.

In a preferred embodiment of the invention, one is placed suitable filter (or network) in the signal path to the compressor input (and preferably a complementary filter in the signal path of the expander). derutsignalen). This solution is preferred because it requires the least the amount of circuit components and operates at all signal levels. By one equivalent arrangement used two filters: one in the compressor control circuit path and the other in the signal path of the compressor output signal.

Correspondingly, there is a filter in the expander in the control circuit path and another in the expander output path. Additional options are possible in the case of dual-track compressors and expanders (for example of the kind shown in US-PS 3,8U6,719 and US-PS Re 28 H26) are used: a suitable filter can in this case be located only at the entrance to the sidewall of the processor to both affect the passers-by thereby the signals and control the control circuit of the side track; alternatively, the the valent two-filter arrangement is used, one filter being placed in the side track guide track and the other in the side track exit track.

For example, to reduce band saturation, it is in kcmyander- plants known to provide attenuation at high frequencies in compressor side of the plant and to provide a complementary in the expander side of the plant (US-PS 3 BHS 719, US-PS U 072 91u 10 15 20 25 30 35 40 at these extreme frequencies due to the nature of the human ear} = 447 524 and "Rundfunktechn. Mitteilungen" volume 22 (1978), H. 2, p 53-74). However, the damping is weak and insufficient to maintain high level signals of high (low) frequency within the system unsafe area from affecting the compressor, which, on the other hand, achieved by the present invention.

From the above literature sources it is known that placing anti-saturation filters at different locations in the signal path; before or after the compressor / expander and within the compressor / / expander so that they are simultaneously included in the control circuit. Thus there is many possible locations of the anti-saturation filters in the compressor and the signal paths of the expander in order to reduce them to the recording the band carried the high frequency signals. In contrast, it must in circuit performed in accordance with the present invention may be more accurate placed because it is intended to affect the compressor and consequently the expander by changing the frequency spectrum of the signals which the compressor processes. Although the present invention is not directed to mitigating ra mëttningseffekter, it is nevertheless helpful to i to some extent avoid band saturation. However, since the the frequency of the abrupt case is usually located at a frequency frequency which is above (below) the area of saturation begins to occur, tape saturation is preferably treated in other ways, for example as described in "Audio", May 1981, p. 20-26.

Similar to known anti-saturation arrangements, the present invention a reduction in the noise reduction effect in that area where the signals are abruptly attenuated as a function of frequency. However is the cut-off frequency of the abrupt case for the upper ox range of the system at a relatively high frequency in the area where the human ear is significantly less sensitive to noise. With regard to the invention turn at the system's low frequency limit range, the ear is also there less sensitive, and even in this case essentially nothing soundable noise is present at these frequencies in a correctly dimensioned system r even if there is reduced noise reduction. gg- The invention is intended spectral asymmetry only at extremely high and / or low frequencies, because in practical systems these frequency ranges are the only ones within which amplitude curve of the transmission channel shows more significant errors.

Another reason is that a deterioration of the noise reduction can be tolerated - n s ....-. __ In another known system, a bandpass filter is reused 12 dB / octave and with cut-off frequencies of 20 Hz and 10 kHz in the control circuit fr » 10 15 20 25 30 35 447 524 to a broadband compander noise reduction intended for home band players certification system (marketed under the trademark "dbx II"). In this case, however, the same result is not achieved as in the present case invention, since no filter is arranged in the signal path and a high-level high-frequency signal (low-frequency signal) above 10 kHz (resp. below 20 Hz) is amplified in accordance with the level of all nals present within the passband of the control circuit. Such an arrangement thus leads to excessive amplification of high-level high-frequency signals (low frequency signals) (outside the band 20 Hz - 10 kHz) when high amplitude signals within said band do not exist, which starves in override of the transmission channel.

It is also known that in video noise reduction systems (US-PS 3 846 719) set up a variable Q band-blocking filter (notch type) sensitive to the chrominance level and centered at the color subcarrier frequency, in order to prevent chrominance signals from being produced. suppression in the compressor and expander and thus eliminated reduces the noise reduction at frequencies below the color interference the wave. The filter center is thus within the bandwidth edges of system signals and within the predictable frequency range of transmission channel, and moreover is not intended to eliminate recording / playback curve error, which is the purpose of the present invention.

It is further known to set up low pass, band pass and high pass pass filter with 12 dB / octave in the side lanes of double lane compressors and expanders (U.S. Pat. No. 3,846,719; Journal of Audio Engineering) Society ", Vol. 15, No. H, October 1967, pp. 383-388). These filters has, however, a completely different purpose, namely band division of noise reduction effect in separate side banks and has no effect on the compression upper and lower limits of the sorin signals.

Likewise, the previously described have sharp band limiting filters limit frequencies that are intentionally outside the useful range width because they are not intended to affect the upper bandwidth and lower outer frequency ranges.

So far, it has not been considered desirable to change to a greater degree _frequency spectrum within the usable system bandwidth. At professional national recording, it is considered inconceivable to place the frequency of an upper band-limiting filter below 20 kHz. At FM- transmissions, an upper band limit of 15 käz is maintained rigorously through all audio signal stages. 447 524 Although the present invention is not limited to use in any particular type of noise reduction compounding system and improves the function of all types of companions, including broadband expanders, the invention is particularly useful for in "sliding belt type" noise reduction systems. Examples of systems of this kind are found in US-PS Re 28 H26 and US-PS 3,757 25H. In dy- equal slider circuits, high frequency audio compression and expansion are achieved. by using high frequency boost (for compression) and high frequency reduction (for expansion) using a high pass filter with variable lower cutoff frequency. When the signal level within the high frequency the band increases, the cut-off frequency of the filter is shifted upwards in order to limit the raised or lowered band and exclude the useful one the signal (utility signal) from raising or lowering.

In sliding device devices, an uncertainty factor in the input / output the performance curve of the resulting modulation of low-level intermediate lane frequency signals when at the compressor input there are high frequency signal within the range of the playback / playback uncertainties. In dub- track sliding devices of the type described in US-PS Re 28 H26 this effect can be suppressed by placing it abrupt the high frequency drop in the side path of the device. While a such a configuration gives the abrupt case only we low and medium sub-high signal levels, it is adequate to substantially fulfill the object of the invention in double track devices both of sliding type (US-PS Re 28 426) as of such fixed band devices (US-PS 3 8U6 719). ' The invention is explained in more detail below in the form of some examples and with reference to the accompanying drawings. Fif. 1-h ï / Å are block diagrams showing alternative locations of spectral symmetry _, - e nets according to the invention. lig. 5 is a frequency curve generally showing the properties partly for a high frequency spectral asymmetry: right partly for a complementary symmetry network, for use in L player. Pig. 6 shows the weighting curve standardized according to CCIR for noise. Pig. 7-10 are representative frequency response curves for typical cassette recorder. Pig. 11-16 are representative curves for facilitating understanding of the invention.

Reference is now made to Figs. 1-U, which show generalized block diagrams to illustrate the different locations where the spectral symmetry networks according to the invention may be located.

Fig. 1, which shows the simplest and preferred embodiment, shape, is the spectral asymmetry network (provided with a low or 10 15 20 25 30 35 40 9 447 524 high-frequency section) or networks 2 (equipped with both low- and high- frequency sections) located in the input path of a compressor H, whose output signal is fed to a transmission channel N. On the reproduction side of the channel N, a complementary expander B acts on the reproduced one the signal and supplies it to one or more optional spectral metering network 8 that has characteristics that are complementary to the network (s) in the compressor inlet path. This location of the network is particularly favorable when the compressor and the expander each comprise two or more series devices as described in "Audio", May 1981, p 20-26.

Fig. 2 shows an equivalent configuration, as in practice is not preferred because it is more complex, requires additional sar and is more expensive. In this equivalent configuration, one or more asymmetry network 10 located in the control circuit of the compressor and one or several additional asymmetry networks 12 in the output signal of the compressor path. If the optionally insertable symmetry is used on the reproduction side, one or more symmetry nets 14 are used in the expander input signal path, and one or more asymmetry networks 10 with the same characteristics that the network (s) 10 in the compressor control circuit are located in the expander's 6 control circuit. The characteristics of the networks H and 12 can differ slightly from the network (s) 2 and from each other in achieving the same overall result as the network (s) 2. This observation also applies to the networks 14 and 16. When both the compressor 4 and the expander 6 contains series devices, as described in the aforementioned article in "Audio", one (or more) asymmetry network 10 is required only_i the control circuit of the first compressor device, and one (or more) networks 12 are arranged only in the first compressor series device output path, and optionally there is one or more networks 1H only in the input the path of the last expander series device with one or more network 16 only in its control circuit.

Pig. 3 and M illustrate the location of the spectral asymmetric the networks in the side paths of double-track compressors and expanders.

Such compressor and expander configurations are known per se and will therefore not be described in detail. For the further the path N (20), however, has two main shapes. One option (Figs. 7 and 8 of U.S. Pat. No. 3,8H6,719) is a filter followed by a controlled limiter, which is caused by a rectified and smoothed control signal perform a progressive constraint as the signal increases. The other the alternative (US-PS Re 28 H25) consists of a high-pass filter of sliding belt type where the passband is made progressively narrower by means of a guide 10 15 20 25 30 35 10 447 524 so that large signal components are excluded from the filter output signal.

Favorable cut-off frequency values for these variable filters are around 375 Hz at rest but gradually transitions to increasingly narrow pass 1 depending on the control signal. Double track compressors and ex- pandras (single or in series) can also be used for kcnfiguratí0nêT * according to Figures 1 and 2.

In Fig. 3, the asymmetric network (s) 18 are located in the input signal the path to the compressor 22 (and possibly also the expander 2%) of the effluent reduction circuit. Fig. U shows an equivalent configuration in the same belt with a compressor 26 and expander 28 of the "sliding belt type". In the equivalent configuration, one or more asymmetric networks 28 are placed in the sideband control circuit 30 which controls a frequency variable unit. or a sliding band filter circuit 32. An additional asymmetric network 34 is located at the sidewalk exit. Optionally, the same network can 'oeras i expanderbanan. The configurations in Figs. 1 and 2 are to be pull in front of them in Fig. 3 and U because the former work at all signal levels and thus also reduces channel override or bandwidth saturation phenomenon at the bandwidth limit (s). Although a single side web is shown in Figs. 3 and 4, even one several side tracks are used, for example as shown in US-PS 3 BMG 719.

Furthermore, the side paths can be designed so that the compressor side path is equipped with a feedback unit and the side track of the expander with a positive feedback unit, for example as shown in U.S. Pat. No. 3,903 N85.

When using series-connected dual-track devices as described in "Audio", May 1981, pages 20-26, it is most appropriate to use one (or more) asymmetry networks in the first series of the compressor's series coupler device and (optionally) such a net in the last series of the expander coupled device, even if the configuration of the present application Fig. 1 is preferred.

The spectral asymmetry property preferably has the following feature: (a) a cut-off frequency in the area within which the the frequency response of the mission channel or tape recorder - including frequency response of the expander's bandpass filter - is reliable within the league boundaries; i.e. slightly below (above) the frequency at which the the frequency response of the mission channel or tape recorder is uncertain or begins to fall; (b) an abrupt interruption in order to bring about a violent defined limit for the frequency control circuit; 10 15 20 25 30 35 duo 11 447 524 c) a well-defined curve shape, depending on the case just mentioned in the frequency curve, so that it becomes easy to achieve reciprocal characteristics during playback or playback in the event of a maintaining an overall flat frequency response (if desired); d) a form that benefits the most from the human ear low sensitivity to noise and disturbance at low levels, ie. frequency- the curve falls quickly and as sharply as possible without the introduction of an audible increase in noise level using the reciprocal characteristics during playback or playback.

Because the spectralosymmetry properties effectively reduce the reduction of noise reduction above (below) the sharp limit the frequency - if this is above 8 kHz or below 50 Hz - it will elevated noise not to be audible with respect to the human low sensitivity of the ear to low-level noise of both high and low frequency frequency, especially if the noise level is extremely low, as is the case when the present invention is used in connection with tape recorder expanders.

This somewhat surprising aspect of the invention has been verified experimentally. mentally.

Justification of this treatment can also be found in the appearance of the CCIR noise weighting curve, which is shown in Fig. 6. This curve follows the low sensitivity of the human ear to low-level noise.

Note that the sensitivity is weak at low frequencies and that it in addition, it drops very fast above a limit of about 6-7 kHz.

Thus, there is a reduced need from a psycho-acoustic point of view for maintaining greater noise reduction at frequencies above 8-10 kHz.

This is the high frequency equivalent of the observed ability of noise reduction system to provide a subjectively large noise reduction even if the low frequencies are treated insignificantly or not at all. Good sizing can eliminate hum, as in cassette recordings the only low-frequency noise that causes subjectively perceived discomfort.

In professional noise reduction systems, in which low frequency noise reduction is used as a guarantee against unexpected Lrum problems, usually gives little need for noise reduction; during the lowest humming component that is likely to occur (ie 50 Hz).

Especially at the high frequency end of the spectrum can use of a spectralosymmetry network does not replace or prevent use of a total-increasing band-limiting filter, sometimes referred to as "multiplex filter" (MPXÜ). The reasons for this have been given above. As dis- previously used, a band-limiting filter, commonly used as a well during recording and playback, a number of functions that are only 10 15 20 25 30 35 40 447 524 12 related to the functions cited in the said discussions. na. Even with an ideal signal channel, it is therefore desirable to have it both the encoding and the decoding exist both 1) bandwidth limiting filter and 2) spectral asymmetry and symmetry networks.

The aforementioned asymmetry and symmetry networks (2, 18, 28, 34) gives an abrupt ledge, lowering or fall as we 5. The dashed lines are intended to show that the sl the frequency (low frequency) curve does not have to have exactly the same appearance as is indicated by the solid lines.

A suitable form of the spectralosymmetry network (monomets) (2, 10, 12, 18, 28, SU) is a sharp low-pass filter (high-pass filter) with a case of 18 dB / octave and with a cut-off frequency within the rapidly decreasing part of the CCIR noise weighting curve (Fig. 5) and during (over) transmission upper limit frequency of the channel. A cut-off frequency of about 8-10 kHz (50 Hz at the low frequency end) would be suitable for a high quality tires with a useful but uncertain frequency range up to about 15 kk: (or 30-60 Hz at the lower frequency end).

The network could also be in the form of a "ledge network" with one 10 dB "floor" as shown in Fig. 5.

Another suitable form on the net is a band-blocking filter with a center frequency of about 16 kHz (20 Hz) and such a Q-value that a limit frequency of about 8-10 kHz (40 Hz) is obtained and with a depth of about 10 dB. You can also use a double tuned (scattered tuned) band-stop filters, especially in professional applications, this to obtain a wider total dip, whereby the duck gives some fraction octave from the first dip. (eg 1/3 cctave).

A depth of about 10-15 dB has experimentally shown s eliminate mid-band modulation effects even in the most difficult cases. One depths as small as 6 dB have, however, been found to provide a significant improvement with respect to the midhand modulation effect, especially when the case is very abrupt, such as 18 dB / octave.

If a flat total characteristic is desired, true and / or are used complementary networks 8, 1G, 1U, 18, 28 and SH in reproduction or the playback part of the system.

With special reference to the property of home tape players for compact cassettes, and for further elucidation of the invention, Referring now to Figures 7-9, as for several typical compact cassette devices, players show representative, measured high-frequency curves at sufficient low levels to avoid saturation of the belt. Pig. 10 í / R / 'A i e, P00 s QHA “FY 10 15 20 25 30 35 M0 13 447 524 shows representative high frequency curves at several input levels for another typical compact cassette recorder. Note in Fig. 7 that the bandwidth of the tape recorder drops sharply above 10 kHz. I fis. 8 the frequency curve starts starting at around 10 kHz and has an output spoken peak at 17 kHz. The curve in Fig. 9 shows a high frequency peak at 15 kHz with a sharp drop above this frequency. The device shown in FIG. The curve shown for the level -20 dB, where saturation is avoided, is close enough ideal, because it is essentially flat away at 20 kHz. Such a good Friday However, it is rare.

Selecting the cut-off frequency to about 10 kHz for the HF spectral the asymmetry network is thus a good choice for such devices as as the frequency curves in Figs. 7-10 show that at levels below saturation the typical, well-adjusted tape recorder has minor shortcomings in the frequency curve below 10 kHz. The spectral asymmetry network thus guarantees led to the fact that at most levels there will be no level significant discrepancy in the reproduction signal due to uncertainty frequencies at very high frequencies. A cut-off frequency at about 10 kHz is a good choice for such devices, because this frequency is also on the sharply falling part of the CCIR weighting curve (Fig. 6), thus some reduction of The noise reduction can be tolerated by the human ear.

In determining the appropriate cut-off frequency for a spectral asymmetric network, the designer can, based on the parameters of his systems select approximate frequencies that differ from 10 kHz.

For example, a higher cutoff frequency may be acceptable in a high quality lithative transmission channel. In the case of compact cassette drives the kind whose characteristic is shown in fia. 7-10, can acceptable upper cut-off frequencies range from 8 kïz to 11 or 12 kñz. 45 .. __ While a filter of about 18 dB / ek off is desirable -_; to ensure that high level signals of high (low) frequency do not control compressor, however, as little as 12 dB / cc may be sufficient to fulfill the object of the invention for most signals. Damping significantly higher than 18 dB / octave causes problems by provide complementary symmetry recovery and are therefore more expensive.

Required filter case depends on sensitivity to signals above the filter cut-off frequency. Regard some part on the compressor for example the double-track slide belt compressor according to US-PS Fe 28 u26. In this device, high frequency boost is used in the compressor control circuit so that when a signal according to Fig. 11 is applied to the such a signal can be generated by a broadband percussion or UALIFTIVE 01 _10 15 20 25 30 35 14 447 524 shock sound), then the said increase of the control circuit will cause that the signal has an energy spectrum according to Fig. 12. The treble height the signal spectrum has a peak. After alignment, this peak gives it DC control signal which controls the band-shifting action of the compressor.

Pig. 13 shows the uncertain frequency curves of tape recorder the channel for four different, representative compact cassette recorders a, b, c and d. The effect of the spectrum according to Fig. 12 is to four different spectra appear in the control circuit of the expander (decoder), whichever results give four different DC control signals. Obviously the result this in errors during decoding.

In such a case, a5nSka¿ characteristic of the spectralosymmetry network that the expander (decoder) generates the same DC control signal in each of the cases of Fig. 15. A network characteristics with a cut-off frequency of about 10 kHz, as shown in Fig. 5, is suitable. the displacement of the belt, without this only being reduced However, the displacement (or compression in the system according to US-PS 3,846,719) is now recoverable under Note that the network does not eliminate the frequency slightly. band splitting the rendering.

It is obvious that the primary object of the invention is apparent from Fig. 15 - namely to ensure that the same peak of the AC control the spectrum of the signal is present at the rectification point both in the as in the expander.

In the case of sliding belt systems of the kind just mentioned, is the spectralosymmetry network is particularly useful for suppressing center frequency modulation caused by high pressure occurring in the compressor level high frequency signals that are not recovered and fed This modulation effect, which occurs very rarely in to the expander. real mu- sources, refers to the basic function of a sliding band device with imperfect signal channels; a dominant in fact controls the sliding band frequency characteristics and can hoof to audible effects when the signal is of high frequency and recovered during playback. When this dominant signal is located at high frequency, well above the frequency of a low level medium. frequency signal, the effect of the center frequency modulation is audible the frequency signal is intermittent as is the case with shock noise, e.g. wise sounding cymbals, and not reproduced with the same level by the tape recorder. if high- In this case, the center frequency signals are amplitude modulated even after the coding, because the high frequency signal slows down the sliding band frequency response to vary without corresponding complementary expansion in the liríñññm " luuAL ITY 10 20 25 15 447 524 the decoder. It should be noted that this effect is basically not a band saturation effect; it can be caused by the wrong fortune ~ and frequency equalization or of gap losses, poor angular setting or the like. However, it is obvious that the effect will be worse if there is also saturation in the control frequency range.

This problem regarding the effects of low rivet frequency modulaticn can be more easily understood with the help of fig. 16, which shows the results when a sequence of test tone curves in the form of a 15 kHz signal at levels from 0 to -GO dB and below, together with a sweeping low level test signal at -65 dB is recorded and played back on a slider laying.

Note that an increase in the dominant 15 kHz signal in amplitude from -60 dE to for example -SO dB means that the compressor- the output signal changes by about 2 dB. This 2 dB change must be observed during the recording process due to the mid-frequency dynamics due to this dominant signal (a change in the around 10 dB is caused at the range 1 kHz). Any change during recording the control in the control frequency range is thus multiplied considerably in the the frequency range. .

If a noise reduction system is designed for the treatment of flames frequencies, a corresponding effect occurs at low frequencies.

Extremely low frequency hum of the compressor input signal can affect the compression the circuit. If the playback device does not reproduce these hum components, signal modulation effects at the expander output will be obvious only. ÅPOOR I auAuTv

Claims (16)

16 447 524 Signal compressor for use in a signal transmission system, in which a compressed signal is supplied to a transmission channel, which in turn is connected to an expander, the relative signal amplitude curve in the transmission channel between the compressor and the expander being uncertain in the area around the the high and / or low frequency outer regions of the usable bandwidth of the signals supplied to the compressor, which compressor has its compression controlled depending on the frequency and amplitude level characteristics of the applied signals, and which compressor comprises a bandpass filter whose transition frequency and filter slope are such that said high and / or low frequency outer regions are substantially excluded from the control of the compressor, characterized by a circuit arrangement for attenuating said high and / or low frequency outer regions of the compressed signal in addition to substantially excluding these outer regions from the control of the compressor. comprre ssorn. 2. A signal compressor according to claim 1, characterized in that it comprises at least one band-blocking network with at least one steep drop with a cut-off or transition frequency located so as to attenuate said high and / or low frequency outer regions. 3. A signal compressor according to claim 2, characterized in that it comprises at least one band-blocking network with one or more cases of between 12 d / octave and 18 dB / octave. HRS. A signal compressor according to claim 2 or 3, characterized in that it comprises at least one network located in the input signal path of the compressor. Signal compressor according to claim 2 or 3 and comprising a control circuit responsive to the frequency and amplitude of the signals supplied for compression, characterized in that it comprises at least one band barrier network located in the control circuit and at least one further band barrier network located in the output path from kcmpressor. 6. A signal compressor according to any one of claims 2-5, characterized in that said at least one band barrier network consists of a low-pass filter within the high-frequency outer region and / or of a high-pass filter within the low-frequency outer region. A signal compressor according to any one of claims 2, H, 5 or 8, characterized in that said at least one band-block network comprises a band-type network block network within the high and / or low-frequency outer region. Føøwøpøøøñøæøøræ fi v. PÛÛR QU UTY, I? A signal compressor according to any one of claims 2-6, characterized in that said at least one band barrier network comprises a "shelf network" in the high and / or low frequency surface areas. h z: 9. A signal compressor according to any one of claims 2-8, characterized in that the belt locking network or networks have a depth of 5-10 dE or more within said high and / or low frequency outer regions. A signal compressor according to any one of claims 1-9 in combination with an expander for expanding the dynamic range of the signals compressed by the compressor and received via the transmission channel, which expander comprises a circuit arrangement for raising the attenuated high and / or low frequency outer regions in and to achieve a substantially flat frequency curve. A signal compressor according to any one of claims 1-3, wherein the compressor 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 of the main path. or output and its output with the combination circuit, which further path at the combination circuit emits a signal which raises the signal of the main path but which is limited so that it is smaller than the main path signal, characterized in that said circuit arrangement is located only in said further path of the compressor. 12. A signal compressor as claimed in claim 11, characterized in that at least one network is located in the input path of the further path. Signal compressor according to claim 11 with a control circuit located in the further path which, for compression control, reacts to the frequency and amplitude of the applied signals, characterized by at least one band barrier network in said control circuit and at least one further band barrier network located in the output path of said further path. 1H. A signal compressor according to claim 12 or 13, characterized in that said at least one network comprises a low-pass filter in the high-frequency outer region and / or a high-pass filter in the low-frequency outer region. 15. A signal compressor according to claim 12 or 13, characterized in that said at least one network comprises a band-blocking filter in the high and / or low-frequency outer areas. se pooR "'" QUALITY 19 447 524 A signal compressor according to claim 12 or 13, characterized in that said at least one network comprises a "landing network" in the high and / or low frequency outer areas.