GB2079113A - Networks for suppressing mid-frequency modulation effects in compressors, expanders and noise reduction system - Google Patents

Abstract

Signals in the high and/or low frequency extremes of a signal transmission system, in which those extremes are subject to errors in the transmission channel, are attenuated so as not to control the action of a compressor feeding the transmission channel and so as to be attenuated in the compressor output to the channel. Consequently, a complementary expander fed by the channel is not subject to control by transmission channel errors. A filter network or networks is or are used having a characteristic, referred as a spectral skewing characteristic, with an abrupt shelf or roll-off and a corner frequency selected to attenuate signals in the frequency extreme or extremes and well within the usable frequency range of signals normally carried in the system. A-10 db shelf at around 10 kHz is illustrated for audio applications. To restore an overall flat frequency response a complementary network (or networks) may be employed with the expander, the network having a "de-skewing" characteristic. <IMAGE>

Description

(12)UK Patent Application ig)GB (11) 2 079 113A (21) Application No

8119973 (22) Date of filing 29 June 1981 (30) Priority data (31) 163950 180771 (32) 30 Jun 1980 22 Aug 1980 (33) United States of America (US) (43) Application published 13 Jan 1982 (51) INT CLI H04B 1/64 (52) Domestic classification H4R 22V PCX (56) Documents cited GB 2068197A (58) Field of search H4R (71) Applicant Ray Milton Dolby 50 Walnut Street San Francisco California 94118 United States of America (72) Inventor Ray Milton Dolby (74) Agents Reddie Et Grose 16 Theobalds Road London WC1X 8PL (54) Networks for suppressing mid-frequency modulation effects in compressors, expanders and noise reduction system (57) Signals in the high and/or low frequency extremes of a signal transmission system, in which those extremes are subject to errors in the transmission channel, are attenuated so as not to control the action of a compressor feeding the transmission channel and so as to be attenuated in the compressor output to the channel. Consequently, a complementary expander fed by the channel is not subject to control by ERRATA

SPECIFICATION NO. 2079113A

Page 3, line 64for fequency read frequency -ine 79 for furter read further Page 4, line 7 for with read within line 129for ahd read and Page 5, line 41 for feedword read feedforward line 71 after hearing delete this insert - that Page 6, line 6 for and read an line 87for pararaters read parameters line 95, for abut read about Page 7, line 16for rectifiction read rectification THE PATENT OFFICE 18 May 1984 signals at frequencies subject to transmission channel errors. A filter network or networks is or are used having a characteristic, referred as a spectral skewing characteristic, with an abrupt shelf or roll-off and a corner frequency selected to attenuate signals in the frequency extreme or extremes and well within the usable frequency range of signals normally carried in the system. A- 10 db shelf at around 10 kHz is illustrated for audio applications. To restore an overall flat frequency response a complementary network (or networks) may be employed with the expander, the network having a "de-skewing "characteristic.

1 GB2079113A 1 SPECIFICATION

4. 5 1 50 Networks for suppressing mid-frequency modulation effects in compressors, expanders and noise reduction systems The present invention is concerned in general with circuit arrangements which alter the dynamic range of signals, namely compressors which compress the dynamic range and expanders which expand the dynamic range. The invention is particularly useful for treating audio signals but is also applicable to other signals.

Compressors and expanders are normally used together (a compander system) to effect noise reduction; the signal is compressed before transmission or recording and expanded after reception or playback from the transmis- sion channel. However compressors may be used alone to reduce the dynamic range, e.g. to suit the capacity of a transmission channel, without subsequent expansion when the compressed signal is adequate for the end pur- pose. In addition, compressors alone are used in certain products, especially audio products which are intended only to transmit or record compressed broadcasts or pre-recorded signals. Expanders alone are used in certain products, especially audio products which are intended only to receive or play back already compressed broadcast or pre-recorded signals. In certain products, particularly audio recording and play back products, a single device is often configured for switchable mode operation as a compressor to record signals and as an expander to play back compressed broadcast or pre-recorded signals.

Transmission channels are known to have frequency dependent characteristics. Consequently the frequency spectrum of received or played 'back signals is altered and when compressed expansion of the signals is degraded by the transmission channel frequency characteristics.

In compander type noise reduction systems, complementarity requires not only that the expander have essentially the inverse characteristics of the compressor, but also that the transmission channel between the compressor and expander preserve relative signal amplitudes at all frequencies within the bandwidth of signals compressed. As received at the expander, changes in the relative levels caused by the transmission channel are indistinguishable from signal processing by the compressor. Hence, errors in the transmission channel cause the expanded signals to differ from the input signals to the compressor.

Such differences can be significant and audible depending on the spectral content of the signals. With high compression /expansion ratios, errors in the transmission channel become more significant. Typically, the most audible effect is not the direct effect on very high or very low frequency signals themselves, but rather the modulation effect on signals between the bandwidth extremes that is caused by failure of the extreme high and low frequency signals to reach the expander. For discussion purposes this effect will be referred to as the mid-band modulation effect.

In wideband companders an amplitude error at the controlling frequency will manifest itself to the same degree in all other portions of the spectrum. This may or may not be acceptable. In sliding band companders, as described hereafter, i the controlling frequencies are at the bandwidth extremes, errors at such frequencies are substantially multiplied at mid-frequencies. (Conversely, if the controlling frequencies are at mid-frequencies, as they usually are, then any errors at the frequency extremes are reduced; this is an ad- vantage of sliding band companders.) "Transmission channel" as used herein is defined to include all portions of the system between a compressor and an expander.

In the case of noise reduction systems that record compressed signals on relatively narrow bandwidth media such as slow speed magnetic tape, e.g. Compact Cassettes, it is particularly difficult to provide accurate complementary expansion of the compressed sig- nals. This is due to the inability of such systems to provide a flat signal amplitude response particularly at low and high frequencies. However, such problems exist even in professional co rn pressor/ expander systems.

The inadequacy occurs after the compressor and can be caused by the recorder, the tape, the playback unit or any combination thereof, including bandwidth limitation filters. Similarly in other systems, such as FM or satellite broadcasting, the errors occur after compression in the transmitter, the broadcast signal carrying media, the receiver or combinations thereof.

In practical compressor-expander systems it has been found necessary to incorporate sharp cutoff filters before the compressor and before the expander. The filters are at least low pass, but preferably also include high-pass sections. Such band-limitation filters have corner fre- quencies at the edges of or outside the useful bandpass of the system in order not to limit the system bandwidth. Such filters have several functions:

a) Attenuation of subcarrier components and the 19 kHz pilot tone used in FM broadcasting, in order to avoid bias "birdie" beats (whistles) and mistracking of encode and decode noise reduction processor circuits.

b) Attenuation of the tape recorder bias which may leak in the signal circuits, in order to avoid encoder/decoder mistracking. - c) Attenuation of rf or supersonic components in the encoder input signal which may otherwise result in audible intermodulation products and/or bias "birdies."

2 POOR QUALITY G132079 113A 2 d) Attenuation ofsupersonic tape noise or other transmission channel -noise at the deco der input, in order to avoid encoder/ decoder mistrackin,g.

e).A signal bandwidth definition to prom ote 70 complementarity in response of the encoder/ decoder.

in professional applications it is &sirable -to employ ahigh frequency bandwidth limitation filter e.q. at 20-25 kliz),and preferably also 75 a iow frequency bandwidth limitation filter (e.g. at 20 Hz).

Strictly speaking, if an -ideal channel exists between the encoder. and decoder, then the input filter to the decoder should be discon nected, as its inclusion may result in some signal situafions in a slight noncom p lementar ity (the encoder signal is subjected toone stage of filtering; the decoder issubjected to two stagesof filtering). However, removal of the decoder input filter is not practical be cause of considerations a) to e) above. There fore, even when there is a good channel between the encoder output and decoder in put, the presenceof the very necessary -deco -der input filter (for protection purposes) results inan inherently non-wcomplementary system under certain signal conditions.!Because of this it is helpful toconsider the expander input filter to be an integral part of the transmission channel.

It is an object of the present invention to provide for the suppression of theadverse effects on complementarity in compander type noise reduction systern caused by -errors in transmission channel amplitude -response.

Stated Jn other words, it is an object of the present invention to provide for the suppres sion of the adverse effects on complementar ity in compander type noise reduction system, s caused by errors in amplitude response that occur between the -compressor and expander It is a particu far object of -the invention -to suppress non-complementarhyeffects at very high (low) frequencies that produce audible effects at mid-frequencies,,(to reduce the mid band modulation effect).

A further object of -the invention is to sup press such adverse effects in systems in which the bandwidth of compressed gignalsexceeds the bandwidth in which the transmission channel amplitude,response isrelatively flat.

It is yet a -.further object of the invention to suppress such adverse effectsin audio tape systems employing Compact -Cassette or other bandwidth restricted media,!including the au., - dio portion of video cassettes and video discs.

Itis a further object of the invention to reduce the non-complementary modulation of low levelmediurn frequency.(e.-g. 500 Hz-1 kHz, or so) signals when present With ex tremely high frequency signals e.g. above 10 kHz) in suchaudio tape -systems.

It is a further object to reducenon-comple 6.5 mentary effects introduced by the use of ex- pander input filters.

lt is yet a further object of the invention to reduce the above effects in sliding band systems of the ype described hereinafter.

An bandwi - dthrestricted systems the fre- quency bandwidth ofcompressed signals appr.oaches,or,--exceeds the usable bandwidth of the record/playback transmission channel and thus such systems areespecially susceptible to errors in record /playback frequency response, particularly in the region of the higlY andlow frequency band edges.

There are two aspects to the problem: 1) the compressoroutput bandwidth may exceed 80 thebandwidth in which the record/playback res ponseis relatively flat in that seWcontained audio device, and 2) the compressor output bandwidth of thedevice used in preparing. prerecorded tapes or discs may exceed the B5 bandwidth inwhich the audiodevice playback response is flat.

WhileerrprS may theoretically occur anywhere in the spectrum, as explained further below, the invention is directed to suppressing the effect of errors at the bandwidth extremes of the system,. particularly to suppressing the.mid-band modulation effects caused by such errors.

The invention which is the solution to the mid-band Modulation problem is rather surprising in its simplicity. More specifically, the signals processed by the compressor are subjected to an. abrupt high,(and /or low) frequency.dro-,of,f with a corner frequency well within the useful bandpass of the system, somewhat below (above) the frequency at which the transmission channel or record/ playback response has substantial errors. Sign als processed by the expander are preferably subjected to acomplementary boost so that an overall flat frequency is maintained. If lack of overall flat frequency response is accepta-. ble, the invention may beincorporated only in the compressor portions of the compander system.

Thus, in accordance with the present invenlion, a portion (or portions) of the signal spectrum in which transmission channel or record /playback response has errors is (are) attenuated to a level such that the attenuated portionis substantiallyexcluded from controllingthe compression and expansion.

In, accordance with the invention, the spec,tm'.1 content of the sianals Processed by the 2 1.20 compressor is alterei or s6wed (---spectral skevifing") such that the compressor action is s.ignifjcantiyJess susceptible to the influence of signals beyond the abrupt roWoff frequency.

One preferredembodiment of the invention is to locate a suitable filter (or network) in the isi,gn;Efl path of Ibecompressor input (and preferably a complementary filter in the signal path of thee.xpander output). This approach is preferred because itrequires theleast amount 1 Z 3 1 2 1 50 GB2079113A 3 of circuit components and it operates at all signal levels. However, an equivalent arrangement is to employ two filters: one in the control circuit path of the compressor and the other in the signal path of the compressor output. Similarly, in the expander, one filter is located in the control circuit path and the other in the signal path of the expander input. Further alternatives are possible in the case of a dual path compressor or expander (for example as disclosed in US-PS 3,846,719 and US-PS Re 28,426): a suitable filter may be located only in the input of the processor side path so as to effect both the signals passing therethrough and also the control circuit of the side path or the equivalent two filter arrangement may be employed where one filter is placed in the side path control circuit path and the other in the side path signal output path.

It is known in compander systems to provide a roll off at high frequencies in the compressor side of the system and to provide a complementary boost in the expander side of the system in order to reduce tape saturation, for example (US-PS 3, 846,719, US-PS 4,072,914, Rundfunktechn. Mitteilungen, Jahrg. 22 (1978) H. 2, pp. 63-74.) However, the roll off is gentle and is not sufficient to keep high level signals at high (low) frequencies in the uncertain response region of the system from affecting the compressor as is accomplished with the present invention.

It is known from the above disclosures to place anti-saturation filters at various places in the signal path; before and after the compressor/expander and within the compressor/expander including at the same time in its control circuit. Thus there are many possible locations of antisaturation filters in the compressor and expander signal paths that will serve the purpose of reducing the high frequency signals applied to the recording tape. In contrast, the circuitry according to the present invention must be more precisely located because it is intended to affect the compressor and consequently the expander by altering the frequency spectrum of signals that the compressor processes.

Although the present invention is not directed to alleviating saturation effects it nevertheless will assist in avoiding tape saturation to some extent. However, because the corner frequency of the abrupt drop-off -is ordinarily located at a frequency above (below) the region in which saturation begins to odcur, tape saturation is preferably treated by other means as for example as described in Audio, May, 1981,pp.20-26.

As with the known anti-saturation arrangements, the present invention does result in a loss of noise reduction effect in the region in which the signals are abruptly attenuated as a function of fequency. However, the corner frequency of the abrupt drop off for the upper frequency end Of the system is at a relatively high frequency in the region in which the human ear is much less sensitive to noise. With respect to use of the invention at the low frequency end of the system, the ear is also less sensitive and also there be essentially no audible noise at such frequencies in properly engineered systems even with reduced noise reduction. The invention contemplates spectral skewing only at the high and/or low frequency extremes because in practical systems those frequency regions are the only ones in which the transmission channel amplitude response has significant errors. A furter reason is that an impairment of the noise reduction can be tolerated at these extremes due to the response of the human ear.

In another known system, a 12dB/octave bandpass filter having corner frequencies of about 20 Hz and 10 kHz is employed in the control circuit of a wide band consumer audio tape compander type noise reduction system (sold under the trademark "dbx 11"). However, this arrangement does not achieve the results of the present invention because no filter is provided in the signal path and a high level high (low) frequency signal beyond 10 kHz (or below 20 Hz) is amplified in accordance with the level of whatever signals are present with the bandpass of the control circuit. Hence, such an arrangement provides excessive amplification of high level, high (low) frequency signals (outside the 20 Hz- 10 kHz band) when high amplitude signals within the 20 Hz- 1 OkHz band are not present, resulting in overdriving the transmission channel.

It is also known in a video noise reduction system (US-PS 3,846,719) to provide a vari- able-Q notch filter responsive to chroma level, centred on the color television subcarrier frequency, in order to prevent the chroma signals from choking the compressor and expander and thus eliminating noise reduction at frequencies below the color subcarrier frequency. Thus, the filter center is within the bandwidth extremes of the signals in the system and within the predictable response of the system transmission channel and, moreo- ver, is not for the purpose of overcoming record/playback response errors, which purpose underlies the present invention.

It is further known to provide 12 dB/octave low pass, band pass and high pass filters in the side paths of dual path compressors and expanders (US-PS 3,846,719; Journal of the Audio Engineering Society, Vol. 15, No. 4, October, 1967, pp. 383-388.) However, these filters are for an entirely different pur- pose, namely band splitting of the noise reduction action in separate side paths, and do not affect the upper or lower frequency extremes of the compressor input signals.

Likewise, the sharp cut off band limitation filters described earlier have corner frequen- POOR OUALITY 4 GB 2 079 113A 4 cies purposely located outside the useful sys tem bandwidth because they are not intended to affect the upper or lower frequency bandwidth extremes.

Heretofore, it has been considered undesir able -to substantially alter the frequency spec trum with the usable system bandwidth. For example in professional recording it is consid ered unthinkable to locate the corner fre- quency of any upper band-limitation filter below 20 kHz. Similarly, in FM broadcasting, a 15 kHz upper bandpass limit is rigorously maintained throughout all the audio signal stages.

Although the present invention is not limited to use with any particular type of:noise reduction companding system and it will improve the operation of all types of companders, including wide band companders, it is particularly useful for use with sliding band noise reduction systems. Examples of such systems are to be found in US-PS Re 28,426 and US-PS 3, 757,254. In such known sliding band circuits, high frequency audio comprBs- sion or expansion is achieved by applying high frequency boost ifor compression) or cut (for expansion) by way of a high pass filter with a variable lower corner frequency. As the signal level in the high frequency band in- 30_ creases, the filter corner frequency slides upwardly so as to narrow the boosted or cut band and exclude the useful signal from the boost or cut.

In sliding band devices one effect of uncer- tainties in record/playback response is the resulting modulation of low level -medium frequency signals when high frequency signal in the region of record/playback uncertainties are present at the compressor input. In dual path sliding band devices of the --type such as described in US-PS Re 28, 426, this effect can be suppressed by providing the abrupt high frequency drop-off in the side path of the device. While such a configuration provides the abrupt drop-off only at medium and low signal levels, it is adequate to substantially satisfy the objects of the invention in dual path devices of both the sliding band type (US-PS Re 28,426) and the fixed band type (US-PS 3,846,719).

The invention will be described in more detail, by way of example, with reference to the accompanying drawings, in which:

Figures 1-4 are block diagrams showing alternative placements of the spectral skewing network(s) according to the invention.

Figure 5 is a response curve showing generally the characteristics of a. high frequency spectral skewing network and complementary de-skewing network for use in a cassette tape recorder/ reproducer.

Figure 6 is the standard CCIR noise weighting characteristic curve.

Figures 7- 10 are representative response curves of typical cassette tape recorder/repro- 130 ducers.

Figures 11- 16 are representative curves useful in understanding the invention.

Referring now to Figs. 1-4, generalized block diagrams are shown to illustrate the various locations where spectral skewingnetworks according to this invention can be located.

In Fig. 1, which shows the simplest and preferred embodiment, the spectral skewing network (having a low or high frequency section) or networks (having both low and high S frequency sections) 2 is (are) located in the input signal path to a compressor 4, the output of which is applied to a transmission- channel N. On the reproduce side of the channel 'N, a complementary expander 6 acts on the reproduced signal and applies it to an optional spectral de-skewing network(s) 8'that has (have) a characteristic(s) complementary to the network(s) in the compressor input path. This location of the network is particularly advantageous where the compressor and expander each include two or more series devices such as described in Audio, May, 1981,pp.20-26.

Fig. 2 illustrates an equivalent configuration, whrich in practice is not preferred because it is more complex, requires additional circuitry and is more expensive. In this equiva- lent configuration a spectral skewing network(s) 10 is placed in the compressor control circuit and a further spectral skewing network(s) 12 in the compressor signal output path. If theoptional de-skewing is employed 1,00 in the reproduce side, a complementary deskewing network(s) 14 is employed in the expander signal input path and a spectral skewing network(s) 10, that has the same characteristics as the network(s) 10 in the 1,05 compressor control circuit, is placed in the expander 6 control circuit. The characteristics of networks 4 and 12 may differ somewhat from that of network(s) 2 and from each other to obtain the same overall results as net- work(s) 2. This observation applies also to networks 14 and 16. Where compressor 4 and expander 6 each include series devices, such as described in the Audio article, a -spectral skewing network(s) 10 is required 11.5 only in the control circuit of the first compressor device and a network(s) 12 is located only in the output path of the first compressor series device, and (optionally) a network(s) 14 onlyin -the input path of the last expander series device with a network(s) 16 only in its control circuit.

Figg. 3 and 4 illustrate placement of the -spectral skewing networks in the side paths of dual path compressors anclexpanders. Such compressor and expander configurations are well known in themselves and will therefore not be described in great detail. However, there are two main forms for the further path N (20). One alternative (Figs. 7 ahd 8 of US-PS 3,846,719) is a filter followed by a 1 1Q 1 50 GB2079113A 5 controlled limiter which is caused to limit progressively, as the signal level rises, by a rectified and smoothed control signal. Another alternative (US-PS RE 28,426) is a sliding band high pass filter whose pass band is progressively narrowed by the control signal so as to exclude large signal components from the output of the filter. Advantageous corner frequency values for the variable filters are about 375 Hz in the quiescent state but become progressively narrower high pass in response to the control signal. Dual path compressors and expanders (singly or in series) can also be employed in in the configuration of Figs. 1 and 2.

In Fig. 3 the spectral skewing network(s) 18 is (are) placed in the input signal path of the noise reduction side path circuitry 20 of the compressor 22 and (optionally) the expander 24. In Fig. 4, an equivalent configuration is shown in connection with a sliding band type compressor 26 and expander 28. In this equivalent configuration, a spectral skewing network(s) 28 is placed in the side path control circuit 30 which controls a frequency variable shelf or sliding band filter circuit 32. A further spectral skewing network 34 is placed in the side path output. Optionally, the same networks are located in the expander side path. The configurations of Figs. 1 and 2 are preferred to those of Fig. 3 and 4 because the former act at all signal levels and hence also reduce channel overload or tape saturation effects at the band extreme(s).

reciprocal characteristic during reproduction or playback to retain an overall flat frequency response (if desired).

d) A shape which takes optimal advantage of the low level noise sensitivity characteristic of human hearing this is, frequency re sponse drop-off as steep and as deep as possible without introducing a noticeable in crease in noise level when employing the reciprocal characteristic during playback.

While the spectral skewing characteristic effectively reduces the noise reduction action above (below) the sharp corner frequency, if the frequency is above about 8 kHz (or below about 50 Hz), the increased noise will not be audible in view of the human ear's response to low-level high and low frequency noise, especially if the noise level is extremely low as it is when the present invention is used in a tape recording compander. This somewhat surprising aspect of the invention has been verified experimentally.

Justification for this treatment can also be found in the shape of the CUR noise weight ing curve, shown in Fig. 6. The curve follows the low-level noise sensitivity of the human ear. Note that the sensitivity is low at low frequencies, and also falls off very rapidly above a peak at about 6-7 kHz. Thus, there is a reduced psycho-acoustic need for main taining substantial noise reduction at frequen cies above 8-10 kHz. This is the high fre quency counterpart of the observed ability of noise reduction systems to provide a subjec Although in Figs. 3 and 4 only a single side 100 tively large amount of noise reduction even path is shown, several side paths may be employed such as in, for example, US-PS 3,846,719. Further, the side paths may be configured such that the compressor side path has a feedback configuration and the expander side path a feedword configuration, as, for example in US-PS 3,903,485. Where series dual path devices are employed in a compressor and expander, such as the type described in Audio, May, 198 1, pp. 20-26, it is sufficient to employ a spectral skewing network(s) in the first series device in the compressor and (optionally) in the last series device in the expander, although the configuration of Fig. 1 herein is preferred.

The spectral skewing characteristic has preferably:

a) A drop-off corner frequency within the region in which the transmission channel or tap recorder response, including that of the expander band-passfilter, is reasonably dependable - that is, somewhat below (above) the frequency at which the transmission channel or tape recorder response is uncertain or begins to roll off.

b) An abrupt drop-off, to provide a well defined limit for the frequencies controlling the circuit.

c) A well defined shape following the drop- off, so as to permit the easy creation of the though the low frequencies may be treated little if at all. Good engineering can eliminate hum, which in cassette tape recording is the only low frequency noise which is subjectively troublesome.

In professional noise reduction systems, in which low frequency noise reduction is provided as an insurance against unexpected hum problems, there is usually little need for any noise reduction below the lowest hum component likely to be encountered (i.e. 50 Hz).

Especially at the high frequency end of the spectrum, the use of a spectral skewing net- work does not obviate or replace an overall band limitation filter, sometimes popularly known as a "multiplex filter" ("MPX"). The reasons have been given previously. As discussed, a band limitation filter, usually used in both recording and playback, has several functions only peripherally related to those in these discussions. Therefore, even in the case of an ideal signal channel, it is desirable to have in encoding and decoding both:

1) bandwith limitation filters; and 2) spectral skewing and de-skewing networks.

The spectral skewing network(s) (2, 10, 12, 18, 28, 34) provide an abrupt shelf, dip or drop-off as shown in Fig. 5. The dashed lines 6 GB2079113A 6 are intended to show that the ultimate high frequency (low frequency response) need not be precisely as shown in the solid lines.

One- suitable form of the spectral skewing network(s) (2, 10, 12, 18, 28, 34), is a sharp low-pass (high-pass) filter, with and 18 dB/octave slope, and having a corner frequency within the rapidly declining portion of the CCIR noise weighting curve (Fig. 6) and below (above) the upper cut off frequency of the transmission channel. A corner or cutoff frequency of about 8-10 kHz (50 Hz at the low frequency end) would be suitable for a high quality tape deck having a useful but uncertain response out to some 15 kHz (or 30-60 Hz at the low frequency end).

The network could also take the form of a shelf network with about a 10 dB floor such as shown in Fig. 5.

Another suitable form of the network is a notch filter with a center frequency of about 16 kHz (20 Hz), a Q such that a corner frequency of about 8-10 kHz (40 Hz) is obtained, and a depth of some 10 dB. A double tuned (stagger tuned) notch filter can also be used, especially in professional applications, in order to provide a wider overall notch, the second notch being placed some fraction of an octave above the first notch (say 1/3 octave).

A depth of some 10-15 dB has been proven experimentally to eliminate the midband modulation effect in the most difficult cases. However, a depth of as little as 6 dB has been found to make a substantial improvement in the mid-band modulation effect, particularly when the drop-off is very abrupt such as 18 dB per octave.

If a flat overall response is desired, the same network and/or complementary networks 8, 10, 14, 18, 28, and 34 are employed in the reproduce or playback portion of the system.

Referring particularly to the characteristics of consumer Compact Cassette audio tape recorder and playback devices, in order to appreciate the invention more fully, reference is made to Figs. 7-9 which show representative measured high frequency response curves at input levels low enough to avoid tape saturation for several typical Compact Cassette recorders. Fig. 10 shows representative high frequency response curves at several input levels for another typical Compact Cassette recorder. Note that in Fig. 7, the recorder response falls off rapidly beyond 10 kHz. In Fig. 8, there is a rise in response starting about 10 kHz with a pronounced peak at about 17 kHz. The response in Fig. 9 has a high frequency peak at 15 kHz with a rapid fall off of response beyond that frequency. The response in Fig. 10 for a level of - 20 dB, which avoids saturation, is nearly ideal, being substantially flat out to 20 kHz. Such a good response is, however, rare.

Thus, the selection of a high frequency spectral skewing network corner frequency of about 10 kHz is a good choice for such devices because the response curves in Figs.

7-10 show that for levels below saturation, the typical recorder in good adjustment has little deficiency in response below 10 kHz. Hence, the spectral skewing network will ensure, at most levels, that there will be essen- lially no level discrepency in the playback control signal caused by uncertainties in re-, sponse at extremely high frequencies. A corner frequency of about 10 kHz is a good choice for such devices because, in addition, that frequency is on the steeply declining portion of the CCIR noise weighting curve (Fig. 6) and thus some loss of noise reduction can be tolerated by the human ear.

In selecting a suitable corner frequency for a spectral skewing network, the designer can select approximate frequencies different from 10 kHz based on the paramters; of his system. For example, in the case of a higher quality transmission channel, a higher corner fre- quency may be acceptable. With respect to Compact Cassette devices of the type whose characteristics are shown in Figs. 7-10, acceptable high frequency corner frequency can range from about 8 kHz to perhaps 11 or 12 kHz. Also, while a filter of abut 18 dB/octave is desirable to assure that high level high (low) frequency signals do not control the compressor, a drop-off as little as 12 dB/octave will substantially meet the objectives of the inven- lion for most signals. Attenuations much sharper than 18 dB/octave present difficulties in providing complementary de-skewing and are more costly.

The required filter drop off rate depends in part on the sensitivity of the compressor to signals beyond the filter cut off frequency. Consider, for example, the dual path sliding band compressor as disclosed in US-PS Re 28,426. In that device, high frequency pre- emphasis is used in the compressor control circuit such that if a signal such as shown in Fig. 11 is applied to the compressor (such a signal might be generated by a wideband percussive sound), the control circuit pre-em- phasis causes the signal to have an energy spectrum as shown in Fig. 12. The preemphasized signal spectrum has a peak. After rectification, this peak provides the DC control signal which controls the compressor's sliding band action.

Fig. 13 illustrates the uncertain frequency responses of the tape recorder channel, shown for four representative Compact Cassette tape recorders a, b, c and d. The effect on the spectrum of Fig. 12 is to cause four different spectra to be present in the control circuit of the expander (decoder), resulting in four different DC control signals. Clearly, errors in decoding will result.

In such a case, a desirable spectral skewing 1 7 GB 2079 113A 7 f. 50 network characteristic causes the expander (decoder) to generate the same DC control signal in each case such, as shown in Fig. 15. A network characteristic having a corner fre5 quency at about 10 kHz, such as shown in Fig. 5 is suitable. Note that the network does not eliminate the sliding of the frequency band; indeed, it may be only slightly reduced. However, the sliding (or compression as in the band splitting system of US-PS 3,846,719) now becomes recoverable during playback, It should be appreciated that the primary object of the invention can be seen in Fig. 1 5-namely to assure the same peak in the spectrum of the AC control signal is present at the rectifiction point in both the compressor and the expander.

With respect to sliding band systems of the type just mentioned, the spectral skewing network is particularly useful in suppressing mid-frequency modulation caused by the presence of high level, high frequency signals in the compressor that are not covered and applied to the expander. This modulation effect, which occurs very rarely in actual music sources, relates to the basis operation of the sliding band device with imperfect signal channels: a predominant signal effectively controls the sliding band frequency character- istic and may cause audible effects when that signal is at a high frequency which is not recovered in playback. When that predominating signal is at a high frequency, significantly above the frequency of a low level mid-fre- quency signal the mid-frequency modulation effect is audible if the high frequency signal is intermittent such as with percussive sounds, brushed cymbals, for example, and is not - reproduced at the same level by the tape recorder. In that case the mid-frequency signals are modulated in amplitude, even after decoding, because the high frequency signal causes the sliding band frequency response to vary without complementary expansion in the playback decoder. It should be noted that this effect is basically not a tape saturation effect; it might be caused by inaccurate biassing and equalization or by gap loss, poor azimuth and the like. However, it is clear that the effect will be worse if there is also saturation in the controlling frequency region.

This problem of low-level mid-frequency modulation effects may be better understood by reference to Fig. 16, which shows the results when a series of probe tone curves using a 15 kHz signal at levels from 0 to - 60 dB and below and a sweeping low-level probe tone at - 65 dB are recorded and played back on a sliding band system.

Note that when a dominant 15 kHz signal increases in amplitude from dB to - 50 dB, the output of the compressor changes by about 2 dB. This 2 dB change must be accurately preserved in the recording process because of the dependence of the mid-fre- quency dynamics on this dominant signal (a change of some 10 dB is produced in the region of 1 kHz). Thus any error in playback in the controlling frequency region is multi- plied substantially in the mid-frequency region.

If a noise reduction system treats low frequencies, then there will be a corresponding effect at low frequencies. Extremely low fre- quency rumble in the input signal to the compressor may actuate the compressor circuitry. If the recorder does not reproduce the rumble components, then signal modulation effects at the expander output will be evident.

Claims (17)

1. A signal compressor for use in a signal transmission system in which a compressed signal is transferred to an expander by way of a transmission channel liable to exhibit an uncertain relative signal amplitude response in the region of the high and/or low frequency extremes of the useful bandwidth of signals applied to the compressor, the compressor comprising means whereby its compression is controlled in response to frequency and amplitude level characteristics of the applied signals, and further comprising filter means for substantially excluding said high and/or low frequency extreme regions from controlling the compressor and for attenuating said high and/or low frequency extreme regions in the compressed signal.

2. A signal compressor according to claim 1, wherein the filter means comprises a band rejecting network having at least one steep drop-off with a corner frequency located to attenuate said high and/or low frequency extreme regions.

3. A signal compressor according to claim 2, wherein the band rejecting network has one or more drop-offs if 12 dB/octave to 18 dB/octave.

4. A signal compressor according to claim 2 or 3, wherein the network is located in the input signal path to the compressor.

5. A signal compressor according to claim 2 or 3, wherein the means whereby the compression is controlled comprise a control circuit responsive to the frequency and amplitude of applied signals for controlling compression, and one band rejecting network is located in the control circuit and a further band rejecting network is located in the out- put signal path of said compressor.

6. A signal compressor according to any of claims 2 to 5, wherein the or each band rejecting network comprises a low pass filter in the high frequency extrem region and/or a high pass filter in the low frequency extreme region.

7. A signal compressor according to any of claims 2 to 5, wherein the or each band rejecting network comprises a notch filter in the high and/or low frequency extreme re-

8 GB2079113A 8 gion.

S. A signal compressor according to any of claims 2 to 5, wherein the or each band rejecting network comprises a shelf network in the high and/or low frequency extreme region.

9. A signal compressor according to any of claims 2 to 8, wherein the or each band rejecting network has a depth of at least 6 dB in the high and/or low frequency extremes.

10. A signal compressor according to claim 9, wherein the depth is from 6 to 10 dB.

11. A signal compressor according to any of claims 1 to 3, wherein the compressor has a main signal path which is linear with respect to dynamic range, a combining circuit in the main path, and a further path which has its input connected to the input or output of the main path and its output connected to the combining circuit, the further path providing a signal which boosts the main path signal by way of the combining circuit, but which is limited to a value smaller than the main path signal, the filter means being located only in the further path of the compressor.

12. A signal compressor according to claim 11, wherein the filter means comprises a network located in the input signal path of the further path.

13. A signal compressor according to claim 11, wherein the said means whereby the compression is controlled comprise a control circuit in the further path responsive to the frequency and amplitude of applied signals for controlling compression, and wherein the filter means comprise a band rejecting network located in the control circuit and a further band rejecting network located in the- output signal path of the further path.

14. A signal compressor according to claim 12 or 13, wherein the or each network comprises a low pass filter in the high frequency extreme region and/or a high pass filter in the low frequency extreme regions.

15. A signal compressor according to claim 12 or 13, wherein the or each network comprises'a notch filter in the high and/or low frequency extreme region.

16. A signal compressor according to claim 12 or 13, wherein the or each network comprises a shelf network in the high and/or low frequency extreme region.

17. A signal compressor according to any of claims 1 to 16, in combination with an expander for expanding the dynamic range of signals compressed by the compressor and received via the transmission channel, the expander comprising a circuit arrangement for boosting the attenuated high and/or low frequency extreme regions to provide a substantially flat overall frequency response.

Printed for Her Majesty's Stationery Office by Burgess Et Son (Abingdon) Ltd.-1 982. Published at The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.

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