FI79428B - NAET FOER DAEMPNING AV MELLANFREKVENSMODULATIONSFENOMEN HOS KOMPRESSORER, EXPANDRAR OCH BRUSDAEMPNINGSSYSTEM. - Google Patents

Description

1 79428

Network for attenuation of medium frequency modulation phenomena in compressors, expanders and noise reduction systems

The present invention relates generally to circuit arrangements that change the dynamic range of signals, namely, compressors that reduce the dynamic range and expanders that extend the dynamic range. The invention is particularly useful in the processing of audio signals but can also be applied to other signals.

Compressors and expanders are usually used together (as a compander system) to provide noise reduction. The dynamic range of the signal is reduced before transmission or recording and expanded from the transmission channel after reception or reproduction. However, the compressors can be used alone to reduce the dynamic range, for example to suit the capacity of the transmission channel, without subsequent expansion when the dynamically reduced signal is sufficient for the intended use. In addition, compressors alone are used in certain products, especially those related to audio reproduction, which are intended solely to transmit or store dynamically reduced transmitted or pre-recorded signals. Expanders are used alone in certain products, especially those related to sound reproduction, which are intended solely to receive or reproduce previously reduced radio or pre-recorded signals. Certain products, especially sound recording and reproducing products, often have a single device designed to be switched-on to act as a compressor for recording signals and as an expander for reproducing dynamically reduced broadcast or pre-recorded signals.

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Transmission channels are known to be frequency dependent. Therefore, the frequency spectrum of the received or reproduced signals changes, and when compressed signals are applied to the transmission channel, the frequency characteristic of the transmission channel degrades the complementary expansion of the signals.

In compander-type noise reduction systems, complementarity presupposes not only that the expander must have an inverse characteristic of the compressor, but also that the transmission channel between the compressor and the expander must maintain relative signal amplitudes at all frequencies in the frequency band of the compressed signals. When received in an expander, the changes in relative levels caused by the transmission channel cannot be distinguished from the signal processing performed by the compressor. Therefore, it follows from the transmission channel errors that the signals expanded by the expander differ from the input signals of the compressor. The differences can be significant and audible depending on the spectral content of the signals. With high compression / expansion ratios, transmission channel errors become more significant. The most easily audible effect is typically not a direct effect on very high frequency or very low frequency signals but a modulation phenomenon on the signals between the ends of the band caused by the fact that very high frequency and very low frequency signals do not reach the expander. For the sake of explanation, the phenomenon is hereinafter referred to as the medium frequency band modulation phenomenon.

In broadband companders, the amplitude error at the control frequency occurs to the same extent in all other parts of the spectrum. This may or may not be acceptable, as the case may be. As will be explained below, if the control frequencies in the sliding band companders are at the ends of the band, the errors occurring at these frequencies will substantially increase at the center frequencies. (If, on the contrary, the control frequencies are at 3 79428 hairs, as they often are, then extreme frequency errors are reduced, which is an advantage for sliding companders.) As used herein, a "transmission channel" is defined to include all parts of the system between a compressor and an expander.

In noise reduction systems that record compressed signals on a relatively narrowband recording medium such as a low speed magnetic tape, e.g., C-cassettes, it is particularly difficult to achieve accurately complementary expansion of the compressed signals. This is due to the inability of such systems to generate a uniform signal amplitude response, especially at low and high frequencies. However, such problems also exist in professional compressor / expander systems. Deficiencies occur after the compressor and may be due to the recording device, tape, playback unit, or combinations thereof, including bandwidth limiting filters. Similarly, in other systems, such as FM or satellite transmissions, errors occur after compression in the transmitter, the transmission medium, the receiver, or combinations thereof.

In practical compressor-expander systems, it has been found necessary to include severely restrictive filters before and after the compressor. The filters are at least low-pass filters, but may advantageously also include high-pass components. The cutoff frequencies of these bandpass filters are at or outside the limits of the usable passband of the system. These filters have several functions: 4 79428 a) Attenuation of subcarrier components and 19 kHz pilot frequency used in FM transmissions to avoid pre-magnetization "Twitter" scrambling results (whistles) and tracking errors in noise reduction coding and decoding processing circuits.

b) Attenuation of pre-magnetization of the recorder that may leak into the signal circuits to avoid encoder and de-encoder tracking errors.

c. Attenuation of radio-frequency or higher-frequency components of the encoder input signal that could otherwise cause audible central modulation results and / or "tweeting" of the pre-magnetization.

d) Attenuation of band noise or other transmission channel noise above audio frequencies to avoid encoder and decoder tracking errors.

e) Signal band limiting to improve the complementarity of encoder and decoder responses.

In professional applications, it is desirable to use a high frequency band limiting filter (e.g. at 20-25 kHz) and most preferably also a low frequency band limiting filter (e.g. at 20 Hz).

Strictly speaking, if there is an ideal channel between the encoder and the decoder, the decoder input filter should be removed, as its addition may cause slight non-complementarity in certain signal situations (encoder signal is affected by one filter stage, decoder signal by two filter stages). However, removing the input filter of the decoder is not practical due to points a) to e) above. Thus, although there is a good channel between the output of the encoder and the input of the decoder, the presence of the necessary decoder input filter (for security purposes) results in an inherently non-complementary system in certain signal states. For this reason, it is useful to think of the expander input filter as part of the transmission channel.

s 79428

It is an object of the present invention to attenuate adverse phenomena affecting the complementarity of compander-type noise attenuation systems due to transmission channel amplitude response errors.

In other words, it is an object of the invention to reduce the detrimental phenomena affecting the complementarity of compander-type noise reduction systems due to amplitude-response errors between the compressor and the expander.

The object of the invention is in particular to reduce the non-complementarity phenomena of very high (low) frequencies, which cause audible effects at medium frequencies (to reduce the modulation phenomena of the medium frequency band).

It is an object of the invention to reduce said detrimental phenomena in systems in which the bandwidth of the compressed signals exceeds the band in which the amplitude response of the transmission channel is relatively uniform.

Yet another object of the invention is to reduce said adverse phenomena in audio signal recording systems using C-cassettes or other bandwidth limited means, including audio signal portions of video cassettes and discs.

It is a further object of the invention to attenuate the non-complementarity modulation of low-level medium frequency (e.g. about 500 Hz to 1 kHz) signals when present in conjunction with very high frequency (e.g., more than 10 kHz) signals in such audio signal recording systems.

It is also intended to reduce the non-complementarity phenomena due to the use of expander input filters.

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Yet another object of the invention is to reduce the above phenomena in sliding systems of the type to be described later.

In band-limited systems, the bandwidth of compressed signals is almost equal to or greater than the usable bandwidth of the recording / playback transmission channel, and systems of this type are thus particularly prone to recording / playback response errors, especially in the high and low frequency band edges.

The problem takes two forms: 1) the output bandwidth of the compressor may exceed the band in which the recording / playback response of that complete audio device is relatively smooth, and 2) the output bandwidth of the device used to make pre-recorded tapes or discs may exceed the band in which the audio response of the audio device is smooth .

Although errors can theoretically occur in any part of the spectrum, as will be explained below, the invention is directed to attenuating the effect of errors at the extremes of the system band, and in particular the center frequency modulation phenomena caused by those errors.

The invention, which solves the medium frequency modulation problem, is quite surprising in its simplicity. More specifically, the signals processed by the compressor are subjected to a sharp drop in the high (and / or low) frequency level, the cut-off frequency of which is clearly within the usable passband of the system and somewhat lower (higher) than the transmission channel or recording / playback response. It is preferable to apply complementary emphasis to the signals processed by the expander in order to keep the overall response constant. If the absence of a uniform overall response 7 79428 is acceptable, the invention can only be incorporated into the compressor parts of a compander system.

Thus, according to the present invention, the portion (s) of the signal spectrum in which there are errors in the transmission / recording response of the transmission channel are attenuated to such a level that the attenuated portion no longer substantially affects the compression and expansion control.

According to the invention, the spectral content of the signals processed by the compressor is changed or skewed ("spectral skew") so that the compression operation becomes substantially insensitive to the effect of the signals behind the steep level drop frequency.

A preferred embodiment of the invention is to place a suitable filter (or network) in the signal path of the compressor input (and most preferably a complementary filter in the signal path of the expander output). This approach is advantageous because it requires the least number of circuit components and because it operates at all signal levels. However, an equivalent arrangement is obtained by using two filters: one in the path of the compressor control circuit and the other in the signal path of the compressor output. Correspondingly, in the expander, one filter is placed in the path of the control circuit and the other in the signal path of the input of the expander. In the case of two-way compressors and expander (e.g., those described in U.S. Patent Nos. 3,846,719 and Re 28,426), other alternatives are possible: a suitable filter may be placed either at the processor bypass only to affect both the bypass signals and its control circuit, or uses a corresponding two-filter arrangement in which one filter is placed on the side path control circuit and the other on the side path signal output path.

It is previously known to use high frequency attenuation on the compression side of the system and complementary enhancement on the expander side to reduce, for example, the saturation of the tape (U.S. Pat. Nos. 3,846,719, 4,072,914, Rundfunktechn. Mitteilungen, Jahrg. 22 (1978) H.2, p. -74). However, the attenuation used is mild and not sufficient to prevent high level signals at high (low) frequencies in the uncertain range of the system response from affecting the compressor as provided by the present invention.

From the above references, it is known to place anti-saturation filters at different points in the signal path: in front of and after the compressor / expander and inside the compressor / expander and at the same time in its control circuit. The anti-saturation filters can thus be placed in many possible places on the signal paths of the compressor and the expander so as to reduce the high frequency signals applied to the magnetic tape. In contrast, the circuits of the present invention need to be positioned more precisely because the purpose is to affect the compressor and, consequently, the expander by changing the frequency spectrum of the signals processed by the compressor.

Although it is not an object of the present invention to reduce saturation phenomena, it does, to some extent, help to avoid tape saturation. However, since the cut-off frequency of a steep plane drop is usually located above (below) the area where saturation begins to occur, tape saturation is most preferably treated in other ways as described, for example, in Audio, May, 1981, p. 20-26.

Like the known anti-saturation arrangements, the present invention causes a reduction in the noise attenuation effect in the range where signals are sharply attenuated as a function of frequency. However, the cut-off frequency for a sharp drop in the upper end of the system frequency range is relatively high, in an area where the human ear is insensitive to noise.

9 79428 When the invention is used at the lower end of the system frequencies, the ear is likewise insensitive, and in properly designed systems, these frequencies should also have virtually no audible noise, even with reduced noise attenuation. The invention envisages the use of spectrum skew only at high and low extreme frequencies, because in practical systems there are considerable errors in the amplitude response of the transmission channel only in these frequency ranges. Another reason is that the deterioration in noise attenuation is acceptable at these extremes due to the characteristics of the human ear.

Another known system uses a 12 dB / octave bandpass filter with cutoff frequencies of about 20 Hz and 10 kHz in the control circuit of a compander-type noise reduction system (sold under the trademark "dbx II") of a wideband consumer level voice recorder. However, this arrangement does not achieve the features of the present invention because no filter is placed in the signal path and the high level high frequency (low frequency) signal above 10 kHz (or below 20 Hz) is amplified according to the level of any signals operating within the control band passband. As a result, such a system amplifies too much high-level, high-frequency (low-frequency) signals (outside the 20 Hz to 10 kHz band) when there are no high-amplitude signals within 20 Hz to 10 kHz, resulting in transmission channel overdrive.

It is also previously known to provide a video noise suppression system (U.S. Pat. No. 3,846,719) with a Q-adjustable narrowband bandpass filter controlled by the chrominance signal level below the color subcarrier frequency. The center frequency of the filter 10 79428 is thus within the limits of the signal band of the system and in the range of the predictable response of the system transmission channel, and furthermore it is not used to avoid recording / playback response errors, which is the basis of the present invention.

In addition, it is known to place 12 dB / octave low-pass, band-pass, and high-pass filters in the sidewalks of two-way compressors and expanders (U.S. Patent 3,846,719, Journal of the Audio Engineering Society, Vol. 15, No. 4, October, 1967, p. 383- 388). However, these filters have a completely different purpose, namely to divide the noise reduction operation band into separate by-passes and do not affect the upper and lower extreme frequencies of the compressor input signals.

Similarly, the cutoff frequencies of the steep bandwidth filters described earlier are intentionally located outside the usable band of the system because they are not intended to affect the high and low frequency ends of the band.

Until now, · a substantial change in the frequency spectrum within the usable bandwidth of the system has been considered undesirable. For example, in professional recording, it has been considered impossible to place the cut-off frequency of any upper end band-limiting filter below 20 kHz. Similarly, in FM broadcasting technology, the upper limit of the passband is regularly 15 kHz at all audio frequency signal levels.

Although the present invention is not limited to the use of any particular type of noise reduction compander system, and although the invention improves the performance of all types of companders, it is particularly useful when used in connection with sliding noise reduction systems. Examples of such systems are disclosed in U.S. Patent Nos. Re 28,426 and 3,757,254. The lower limit frequency. As the signal level in the high frequency range increases, the cutoff frequency of the filter slides upward, narrowing the highlighted or attenuated band and removing the useful signal from the highlight or attenuation circuit.

In sliding band devices, one effect of the recording / playback response uncertainties is the resulting modulation of low-level mid-frequency signals when the compressor input has a high-frequency signal in the range of recording / playback response uncertainties. For example, in sliding two-way devices of the type disclosed in U.S. Patent No. Re 28,426, the phenomenon can be attenuated by causing a sharp drop in the high frequency level to the side path of the device. Although such an implementation provides a steep level drop only at medium and low signal levels, it is substantially sufficient to accomplish the objectives of the present invention in both sliding band (U.S. Patent No. Re 4,426) and fixed band (U.S. Patent No. 3,846,719) dual path devices.

The invention will be explained in more detail below by means of an example with reference to the accompanying drawings, in which:

Figs. , Figures 7-10 are curves representing the responses of typical cassette recorders / players, i2 79428 Figures 11-16 are exemplary curves useful in understanding the invention.

Figures 1-4 show generalized block diagrams illustrating various possible locations of the spectrum skew networks according to the invention.

In Fig. 1, which shows the simplest and preferred embodiment, a spectrum skew network (having either a low or high frequency portion) or networks (having both low and high frequency portions) 2 is placed in the input signal path of the compressor 4. The compressor output is fed to the transmission channel N. The signal reproduced on the repetition side of the channel N is affected by a complementary expander 6 which supplies the signal to the desired spectralization network (s) 8, which has a complementary characteristic (complementary characteristics) compared to the compressor input network (s). curves). This network location is particularly advantageous when both the compressor and the expander include two or more serial devices as in Audio, May, 1981, p. 20-26 are shown.

Figure 2 shows a similar implementation, which is not the most advantageous in practice because it is more complex, requires more circuits and is more expensive. In this equivalent implementation, a spectral skew network (s) 10 is placed in the compressor control circuit and a second spectral skew network (s) 12 is placed in the compressor output signal path. (n) 10, where (i) 11a has the same characteristic curve as the network (s) of the compressor control circuit 10. The characteristic curves of the networks 10 and 12 may differ somewhat from the network (s) 2 13 79428 and from each other to achieve the same overall results as the network (with networks) 2. This statement also applies to networks 14 and 10. When both compressor 4 and expander 6 include serial equipment as explained in the Audio article of said publication, the spectrum skew network (s) 10 is only needed in the control circuit of the first compressor device and the network (s) 1 2 is placed only on the output path of the first series compressor device and (if desired) the network (s) 14 is placed only on the input path of the last series expander device and the network (networks) 16 is placed only on its control path.

Figures 3 and 4 illustrate the placement of spectrum skew networks on the side paths of two-way compressors and expanders. Compressor and expander implementations of this kind are known per se and will therefore not be explained in detail. However, with respect to the additional path N (20), there are two main forms. Another alternative (Figures 7 and 8 of U.S. Patent 3,846,719) is a filter followed by a controlled limiter that is controlled to progressively limit as the signal level increases by means of a rectified and filtered control signal. Another alternative (U.S. Pat. No. Re 28,426) is a sliding band overpass filter, the passband of which is progressively narrowed by a control signal to exclude large signal components from the output of the filter. The cut-off frequencies of the controllable filters are preferably about 375 Hz at rest, but the high-pass band narrows progressively under the influence of the control signal. Two-way compressors and expanders (alone or in series) can also be used in the implementations of Figures 1 and 2.

In Figure 3, the spectral skew network (s) 18 are located in the input signal path of the noise attenuation side circuit circuits 20 of the compressor 22 and (if desired) the expander 24. Figure 4 shows a corresponding implementation in connection with the sliding band compressor 26 and the expander 28. In this equivalent implementation, the spectrum bias network (s) 28 is located in a side path control circuit 30 that controls a frequency controlled step circuit or a slip filter circuit 32. A second spectrum beam network 34 is located at the side path output. If desired, similar nets can be placed in the side path of the expander. The implementations of Figures 1 and 2 are considered to be more advantageous than the embodiments of Figures 3 and 4, as the former act at all signal levels and thus also reduce channel congestion or band saturation phenomena at the extreme end (s) of the band.

Although only one bypass is shown in Figures 3 and 4, multiple bypasses may also be used, such as in U.S. Patent No. 3,846,719. In addition, the bypasses may be implemented such that the compressor bypass is feedback and the expander bypass is feedforward as in U.S. Patent 3,903. 485. When a compressor and expander are used, for example, in Audio, May, 1981, p. 20-26, two-way devices of the type described in series, it is sufficient to use the spectrum skew network (s) in the first serial device of the compressor and (if desired) in the last serial device of the expander, although the implementation of Figure 1 is preferred.

Most preferably, the spectral skew characteristic has: a) The cut-off cutoff frequency in the range where the transmission channel or recorder response, including the expander bandpass filter, is reasonably reliable, i.e. somewhat below (above) the frequency where the transmission channel or recorder response is uncertain and begins to decrease.

b) A steep drop in level to obtain a clearly defined limit for the frequencies controlling the circuit.

c) A clearly defined shape after level reduction to allow easy implementation of the inverse characteristic in the repetition as a whole to maintain a uniform frequency response (if desired).

is 79428 (d) A shape that makes optimal use of the sensitivity characteristics of hearing to low-level noise, which means a decrease in frequency response that is as sharp and deep as possible without appreciably increasing the noise level when using the inverse characteristic.

Although the spectral skew characteristic effectively reduces the noise attenuation effect above (below) the sharp cutoff frequency if the frequency is greater than about 8 kHz (or less than about 50 Hz), the increased noise is not audible given the ear sensitivity to low levels of high and low frequency noise, especially if the noise level is very low as when using the invention in a recorder associated with a recorder. The somewhat surprising feature of this invention has been experimentally confirmed.

The justification for this treatment can also be found in the form of the CCIR noise-weighing curve shown in Figure 6. The curve tracks the sensitivity of the human ear to low-level noise. It is observed that the sensitivity is low at low frequencies and also drops very sharply after a peak at about 6-7 kHz. The need to maintain psychoacoustic noise attenuation at frequencies above 8-10 kHz is thus small. This provides a high frequency response to the perceived ability of noise attenuation systems to subjectively provide relatively high noise attenuation, even if low frequencies are treated little or not at all. With good design, hum can be eliminated, which is the only low-frequency noise in cassette-tape recording that is subjectively annoying.

In professional-grade noise reduction systems, where low-frequency noise reduction is used to back up unexpected noise problems, noise reduction below the lowest probable noise component (i.e., 50 Hz) is generally not required.

1 «79428

The use of a spectrum skew network, especially at the high frequency end of the spectrum, does not make it unnecessary or replace the total band limiting filter, sometimes referred to as a "multiplex filter" ("MPX"). The reasons for this have been set out earlier. As noted in this case, the band-limiting filters commonly used for both recording and playback have a number of functions that have little to do with the tasks discussed here. Thus, even in the case of an ideal signal channel, it is desirable to use in coding and decoding both 1) band-limiting filters and 2) spectrum skew and correction networks.

The spectral skew nets (2, 10, 12, 18, 28, 34) cause a sharp step, drop or level drop as shown in Figure 5. The purpose of the dashed lines is to show that the final response of the high (low) frequencies does not have to be exactly as shown by the solid lines.

One suitable form of spectral skew network (s) (2, 10, 12, 18, 28, 34) is a sharp low-pass (high-pass) filter with an 18 dB / octave ramp and a cut-off frequency for the rapidly decreasing CCIR noise weighing curve (Figure 6). inside the part and below (above) the upper cut-off frequency of the transmission channel. A cutoff frequency of about 8-10 kHz (50 Hz at the low frequency end) is suitable for a high quality deck with a usable though uncertain response up to about 15 kHz (or 30-60 Hz at the low frequency end).

The network may also consist of a step network with a step level of about 10 dB as shown in Figure 5.

One suitable form of network is a narrowband band-stop filter with a center frequency of about 16 kHz (20 Hz) and a Q-value such that the cut-off frequency is about 8-10 kHz (40 Hz) and the stop attenuation depth is about 10 dB. A double-tuned (scattered) narrowband filter can also be used, especially in professional applications, to obtain a wider overall band, with the second band being located a fraction of an octave (e.g. 1/3 octave) above the first band.

An attenuation depth of about 10-15 dB has been shown experimentally to be sufficient to eliminate the midband modulation phenomenon even in more severe cases. However, even a depth of only 6 dB has been found to substantially improve midband modulation phenomena, especially when the level drop is very sharp, e.g., 18 dB / octave.

If a uniform overall response is desired, a similar network and / or complementary networks 8, 10, 14, 18, 28 and 34 are used in the playback section of the system.

To more fully understand the advantages of the invention, the characteristic curves of consumer level audio signals for C-cassette recorders and players are considered. Figures 7-9 show representative measured high frequency response curves at input levels low enough to avoid tape saturation for several typical C-cassette recorders. Figure 10 shows representative high frequency response curves at multiple levels for another type of C-cassette recorder. It is observed that in Fig. 7 the response of the recorder drops rapidly above 10 kHz. In Figure 8, the response begins to rise at about 10 kHz and has a clear peak at about 17 kHz. The response in Figure 9 has a high frequency peak at 15 kHz and a rapid decrease above this frequency. The response in Figure 10 at the level of -20 dB at which saturation is avoided is almost ideal, being substantially uniform up to 20 kHz. However, such a good response is rare.

Thus, about 10 kHz is a good choice for the cut-off frequency of a high frequency spectral skew network because the response curves in Figures 7-10 show that at levels below ib 79428 saturation, the response of a typical properly tuned recorder has few errors below 10 kHz. As a result, the spectrum skew network ensures at most levels that the playback control signal is virtually free of level errors caused by very high frequency response uncertainties. In addition, a cut-off frequency of about 10 kHz is a good choice for such devices, since that frequency is in the sharply decreasing part of the CCIR noise weighing curve (Fig. 6) and thus the ear can accept some reduction in noise attenuation.

When selecting an appropriate cut-off frequency for a spectrum skew network, the designer can select near frequencies other than 10 kHz based on system parameters. For example, in the case of a good quality transmission channel, a higher cutoff frequency may be acceptable. For C-cassette devices of the type whose characteristic curves are shown in Figures 7-10, the acceptable high frequency cutoff frequency may range from about 8 kHz to perhaps 11 or 12 kHz.

Although the use of a filter of about 18 dB / octave is desirable to ensure that high level high frequency (low frequency) signals do not control the compressor, even a ramp of only 12 dB / octave is substantially sufficient to achieve the objectives of the invention for most signals. Attenuations steeper than 18 dB / octave make it difficult to achieve complementary correction and are more expensive.

The required slope of the filter level drop depends in part on the sensitivity of the compressor to signals above the filter cutoff frequency. As an example, a sliding-band two-way compressor such as that disclosed in U.S. Patent No. 28,426 is considered. In this apparatus, high frequency pre-emphasis is used in the compressor control circuit, so that when a signal of the type shown in Fig. 11 is applied to the compressor (such a signal may be generated by broadband shock sound), the control circuit 19 79428 changes the energy spectrum of the signal to that shown in Fig. 12. There is one peak in the pre-highlighted signal spectrum. After filtering, this peak generates a DC control signal which controls the smooth operation of the compressor.

Figure 13 shows the uncertain responses of the recording channel shown for four exemplary C-cassette recorders a, b, c and d. The effect of the spectrum of Figure 12 provides four different spectra to the control circuit of the expander (decoder), followed by four different DC control signals. The result is clearly decoding errors.

In this case, a suitable spectrum bias network characteristic causes the expander to decode in each case a similar DC signal as shown in Fig. 15. A network characteristic with a cutoff frequency at about 10 kHz, such as the characteristic curve in Fig. 5, is suitable. It should be noted that the network does not eliminate the slip of the frequency band, in fact the slip can be reduced at most slightly. However, slip (or compression in the band splitting system of U.S. Patent No. 3,846,719) can then be corrected during playback.

It should be noted that Figure 15 shows the primary object of the present invention, which is to ensure the presence of the same peak in the spectrum of the AC control signal at the rectifier point of both the compressor and the expander.

In sliding band systems of the above type, the spectral skew network is particularly useful in attenuating the center frequency modulation caused by high level high frequency signals acting on the compressor that cannot be reproduced and fed to the expander. This modulation phenomenon, which is very rare in the case of real music sound sources, is related to the basic operation of a sliding device in the case of incomplete signal channels. The sliding frequency characteristic is effectively controlled by the dominant signal and can cause audible effects when that signal is present at high frequencies that cannot be recovered during reproduction. When the dominant signal occurs at high frequencies significantly above the frequency of the low-level mid-frequency signal, the mid-frequency modulation phenomenon is audible if the high-frequency signal is intermittent such as percussion and cymbal sounds and the recorder does not reproduce it at the same level. In this case, the medium frequency signals are modulated with respect to the amplitude even after decoding, because the high frequency signal causes a change in the sliding frequency response without complementary expansion in the repeater decoder. It should be noted that the phenomenon is not essentially a tape-saturation phenomenon, it may be due to inaccurate pre-magnetization and correction or loss of sound gap, incorrect azimuth setting or the like. However, it is clear that the phenomenon is worse if saturation also occurs in the control frequency range.

This problem of low-level medium-frequency modulation phenomena is better understood with reference to Fig. 16, which shows the results obtained when a series of test frequency curves using a 15 kHz signal ranging from 0 to -60 dB and below, and low-level -65 dB sweep test frequency, generated by a sliding system by recording and reproducing.

It is observed that as the amplitude of the dominant 15 kHz signal increases from -60 dB, e.g. to -50 dB, the output of the compressor changes by about 2 dB. This 2 dB change must be accurately maintained in the recording process because the center frequency dynamics depends on the dominant signal (a change of about 10 dB occurs in the 1 kHz range). Thus, each error occurring in the playback frequency 2i 79428 increases substantially in the center frequency range.

If the noise reduction system handles low frequencies, a similar effect occurs at low frequencies. A very low rumble in the compressor input signal can control the compressor circuits. If the recorder does not play rumble components, signal modulation phenomena occur at the output of the expander.

Claims (13)

22 79428 Patent Requirements A signal compressor for use in a signal transmission system, wherein the compressed signal is supplied to a transmission channel in turn connected to an expander, the compressor compression being controlled according to the frequency and amplitude level characteristics of the input signal and the compressor control input being provided with a filter element, the control signal frequency range substantially bandwidth, with the exception of high and / or lower frequency extremes, characterized in that a network (2) is arranged at the input of the compressor (4), the characteristic curve (Fig. 5) of which has a sudden attenuation in the frequency range of the transmission system that is as steep as possible and large without the inverse of the expander (6, 8) required to achieve a uniform frequency response between the compressor input and the expander output (Fig. 5) causing a significant increase in the noise level so that the effect of the transmission channel (N) curve (Fig. 13) Between the slave (4) and the expander (6) at extremely high and / or low frequencies of the usable frequency band of the signal coming to the compressor (4) at the end of the transmission channel (N) or at the output of the expander (6) is blocked by simultaneously attenuating the compressed signal. or lower extremities and keeping these substantially away from the control input of the compressor (4) (Fig. 1). 2. A signal compressor for use in a signal transmission system, wherein the compressed signal is fed to a transmission channel, which in turn is connected to an expander, the compressor is controlled according to the frequency and amplitude characteristics of the input signal and a filter element is provided at the compressor control input. usable bandwidth of the input signal, with the exception of high and / or low frequency extremes, characterized in that both the compressor control signal control circuit of the compressor (4) and the signal output circuit 23 79428 are provided with a network (10, 12) with its own transmission curve (Fig. 5) attenuation that is as steep and large as possible without the inverse of the expander (6, 14) required to provide a uniform frequency response between the compressor input and the expander output (Figure 5) causing a significant increase in noise level ni that the effect of the characteristic curve of the transmission channel (N) (Fig. 13) between the compressor (4) and the expander (6) at extremely high and / or low frequencies of the usable frequency band of the signal entering the compressor (4) at the end of the loose channel (N) or expander (6) ) to the output signal is blocked by simultaneously attenuating the upper and / or lower extremes of the frequency of the compressed signal and keeping these substantially out of the control input of the compressor (4) (Fig. 2). 3. A signal compressor for use in a signal transmission system, wherein the compressed signal is fed to a transmission channel, which in turn is connected to an expander, the compressor compression is controlled according to the frequency and amplitude characteristics of the input signal. usable bandwidth except for high and / or low frequency extremes, said signal compressor including a main signal 11a of a circuit element having constant dynamics and a side signal 11a of a circuit element having an input connected to either the input or output of the main signal circuit element and the output of the main signal circuit is limited, characterized in that a network (18) is arranged at the input of the side signal circuit element (20), the characteristic curve of which (Fig. 5) has a sudden attenuation in the frequency range of the transmission system, which is n as steep and large as possible without the inverse of the expander (24) required to provide a uniform frequency response between the compressor input and the expander output (Fig. 5) 24 79428 cause a significant increase in noise level so that the transmission channel (N) characteristic curve (Fig. 13) ) and the expander (24) at extremely high and / or low frequencies of the usable frequency band of the signal entering the compressor (22) at the end of the transmission channel (N) or at the output of the expander (24) is prevented by simultaneously attenuating the side signal frequency upper and / or these are substantially out of the control input of the compressor (22) (Figure 3). A signal compressor for use in a signal distribution system, wherein the compressed signal is fed to a transmission channel, which in turn is connected to an expander, the compression of the compressor being controlled according to the frequency and amplitude level of the input signal. the frequency range substantially covers the usable bandwidth of the compressor input signal except high and / or low frequency extremes, said signal compressor including a main signal 11a of a circuit element having constant dynamics and a side signal 11a comprising an output of the circuit element having an input connected to either the main signal input and is connected to a circuit element of the main signal and whose output amplitude is limited, characterized in that both the control signal control circuit (30) and the output of the signal circuit circuit are provided with a network (2S, 34) having a characteristic curve (Fig. 5) in the frequency of the transmission system: a sudden attenuation in the range that is as steep and large as possible without the inverse of the expander (27) required to provide a uniform frequency response between the compressor input and the expander output (Fig. 5) causing a significant increase in noise level so that the transmission channel (N) Fig. 13) the effect between the compressor (26) and the expander (27) at extremely high and / or low frequencies of the usable frequency band of the signal entering the compressor (26) at the end of the transmission channel (N) or at the output of the expander 2542428 (27) is prevented, while attenuating the side signal upper and / or lower frequency ranges of the frequency and keeping these substantially out of the control input of the compressor (26) (Figure 4). Signal compressor according to one of Claims 1 to 4, characterized in that the abrupt attenuation of the female curve is from about 12 dB / octave to 18 dB / octave. A signal compressor according to any one of claims 1 to 5, characterized in that said circuit elements (30) comprise at least one band-stop network in which the upper and / or lower frequency regions are attenuated. Signal compressor according to Claim 6, characterized in that the band-stop network (28) comprises a narrow-band band-stop filter in the high and / or low frequency extremes. Signal compressor according to Claim 6, characterized in that the band-stop network (28) comprises a step network in the high and / or low frequency extremes. Signal compressor according to one of Claims 1 to 8, characterized in that the magnitude of the sudden attenuation is 6 to 15 dB. An expander for use in a transmission system, wherein the compressed signal is fed to a transmission channel to the input of which a compressor signals a compressor according to any one of claims 1 to 9, characterized by a network (8) having a frequency response mirror image of the compressor network or networks. Figure 1). 26 79428 Π. An expander for use in a transmission system to which a compressed signal is fed via a transmission channel to the input of which a compressor signals a signal compressor according to one of claims 1 to 9, characterized in that both the control signal control circuit of the expander (6) and its signal input circuit are provided with a network (10, 14). the frequency response is a mirror image of the frequency response of the network or networks of the compressor (4) (Figure 2). An expander for use in a transmission system to which a compressed signal is passed through a transmission channel to the input of a signal compressor according to any one of claims 1 to 9, said signal compressor including a main signal 11a circuitry having constant dynamics and a side signal 11a further connected to either the input or output of the main signal circuit element and the output of which is connected to the main signal circuit element and the output amplitude of which is limited, characterized in that a network (18) with a frequency response mirror image of the compressor network or networks is arranged at the input of the side signal circuit element (20). An expander for use in a transmission system to which a compressed signal is passed through a transmission channel "to the input of which a compressed signal is supplied by a signal compressor according to any one of claims 1-9, said signal compressor comprising a main signal with constant dynamics and further a side signal 11a a circuit element, the input of which is connected to either the input or output of the main signal circuit element and the output of which is connected to the main signal circuit element and the output amplitude is limited, characterized in that both the expander (27) control signal control circuit (30) and the side signal circuit element (20) are networked (28, 34), the frequency response of which is a mirror image of the frequency response of the compressor network or networks (Fig. 4).