GB2079114A - Circuit arrangements for reducing media overload effects in signal recording and transmission systems - Google Patents

Abstract

A frequency dependent circuit is located in the main signal path of a dual path compressor or expander in which the main signal path is linear with respect to dynamic range and a further path provides a signal which boosts or bucks the main path by way of a combining circuit, but which is so limited that in the upper part of the input dynamic range the further path signal is smaller than the main path signal. The frequency dependent circuit in the compressor is called an antisaturation network with a characteristic FAS such as to effect a reduction in frequency response in the frequency band affected by overload in a transmission channel fed by the compressor, typically the upper end of the overall frequency band. The expander which is fed by the channel has a complementary antisaturation circuit with a characteristic F'AS providing a complementary increase in frequency response. The arrangement provides level dependent equalization without significantly affecting the noise reduction effect of a compressor/expander system because in dual path compressors and expanders the output of the circuits at very low signal levels is provided mostly by the further path. Thus, at high levels, where channel overload is a problem, the frequency dependent circuit is effective, while at low levels, where it is not desired or needed, the equalization effect is reduced. <IMAGE>

Description

(12)U K Patent Appi ication 9)G B (i 1) 2 0 7 9 114 A (21) Application No

8119974 (22) Date of filing 29 Jun 1981 (30) Priority data (31) 163950 180771 (32) 30 June 1980 22 Aug 1981 (33) United States of America (US) (43) Application published 13 Jan 1982 (51) INT CLI H04B 1/64 (52) Domestic classification H4R 22V DCX (56) Documents cited (54) Circuit arrangements for reducing media overload effects in signal recording and transmission systems (57) A frequency dependent circuit is located in the main signal path of a dual path compressor or expander in which the main signal path is linear with respect to dynamic range and a further path provides a signal which boosts or bucks the main path by way of a combining circuit, but which is so limited that in the upper part of the input dy- ERRATA

SPECIFICATION NO. 2079114A

Pagr-.2, line 5 for part read path line 6for combinating read comb i g Page 4, line 1 to 4 delete whole lines insert thus, F'A5 FAS + F, FiAS Z = - - - X 1 + F2F'M line 6 to 9.,et-lete whole lines insert F'AS - 1 FAs line 16 v 20 delete whole lines insert thus (F1AS - F2) z= - X 1 - -Fl FAs like 93 for gently read gentle YHE PATENT OFFICE 18 May 1984 fed by the compressor, typically the upper end of the overall frequency band. The expander which is fed by the channel has a complementary antisaturation circuit with a characteristic F'As providing a complementary increase in frequency response. The arrangement provides level dependent equalization without significantly affecting the noise reduction effect of a compressor/expander system because in dual path compressors and expanders the output of the circuits at very low signal levels is provided mostly by the 1 1 GB2079114A 1 SPECIFICATION

Circuit arrangements for reducing media overload effects in signal recording and transmission systems The present invention is concerned in general with recording and transmission systems and with circuit arrangements which alter the dy namic range of signals handled by such sys tems, namely compressors which compress the dynamic range and expanders which ex pand the dynamic range. The invention is particularly useful for treating audio signals but is also applicable to other signals, includ ing video signals.

Compressors and expanders are normally used together (a compander system) to effect noise reduction; the signal is compressed be fore 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 com pressed 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 broadcast or pre-recorded signals.

Expanders alone are used in certain products, especially audio products which are intended only to receive or playback already com pressed broadcast or pre-recorded signals. In certain products, particularly audio recording and play back products, a single device is often configured for switchable mode opera tion as a compressor to record signals and as an expander to play back compressed broad cast or pre-recorded signals.

More specifically, the present invention re lates to compressors and expanders which, in addition to providing compression or expan sion, also effect level dependent equalization.

In magnetic recording, the need for such equalization arises because of the tendency of magnetics tape to saturate, particularly at high frequencies. Various proposals have been made for achieving the equalization, several being discussed in Rundfunktechn. Mitteilun gen, Jahrg. 22 (1978) H. 2, pp 63-74. A need for equalization can also arise with other recording or transmission media which are susceptible to high level saturation or overload effect at certain frequencies.

One possibility (but which fails to provide level dependent equalization) is to provide a high-frequency roll-off circuit after the compre sor and a compensating boost circuit before the expander. An alternative is to put the roll off circuit before the compressor and the boost circuit after the expander. A disadvan tage of any such technique is that signals at all levels are subject to the same roll-off action (and subsequent boost) so that there is a 130 significant decrease in the amount of noise reduction which is obtained. It is suggested in the Rundfunktechn. Mitteflungen article that the loss of noise reduction can be tolerated if the amount of noise reduction without such techniques is as high as 20 dB. This is only partially true. In practice, particularly with respect to some Compact Cassette tapes, tape saturation effects extend down as low as 2 kHz and the change in equalization required to take account of this will lead to a significant increase in audible noise.

Attempts have been made to overcome this problem by making the response of the roll-off and boost circuits level-dependent (US-PS 4,072,914). In the case of the roll-off circuit the roll-off is steeper at high levels than at low levels. For the boost circuit, the boost characteristic is likewise steeper at high levels than at low levels. A disadvantage of this approach is the consequential increase in circuit complexity.

In the Rundfunktechn. Mitteilungen article there is also a proposal to put a high-fre- quency boost circuit in front of the control circuit of both the compressor and the expander but it is also there pointed out that the resulting roll-off is only noticeable in the medium level signal range, whereas it is desired to obtain the effect principally at high levels. In video recording systems high frequency saturation problems may be caused by the recording pre-emphasis used. A similar problem exists in FM broadcasting.

It is therefore one object of this invention to provide a better solution to the problems discussed above, namely a solution which is both effective and simple.

Although not discussed in the prior art there is a related problem in that low frequency equalization (roll-off in the compressor and boost in the expander) is desirable if the compression and expansion are effective at low frequencies also. Low frequency compres- sion and expansion are useful for hum noise reduction and may be achieved using wide band compressors and expanders or compressors and expanders with circuits operating independently at low and high frequencies. At low frequencies, the primary objective of equalization is to counteract the effect of the 3180 usec ( + 3 dB at 50 Hz) recording boost built into many magnetic tape recorders, which causes low frequency tape saturation problems, e.g. with organ music.

Another object of the invention is to provide a means whereby the required equalization can be effected additionally at low frequencies, if required.

A further object of the invention is to increase the headroom and accordingly, the dynamic range of recording media subject to saturation.

The starting point of the invention is a compressor or expander comprising a main 2 GB 2 079 114A 2 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 part and its output connected to the combinating circuit, the further path providing a signal which, at least in an upper part of the frequency band, boosts or bucks the main path signal by way of the combining circuit, but which is so limited that, in the upper part of the input dynamic range, the further path signa' is smaller than the main path signal.

The upper part of the frequency band typically extends up from a value of some hun- dreds of Hertz, say a value of 300-400 Hz, although a higher value may be used. In the upper part of the input dynamic range, say d B to + 10 d B, relative to a reference level, the further path signal is small relative to the main path signal.

Such compressors and expanders are well known and widely used, examples being described in US-PS 3,846,719, US-PS 3,903,485 and US-PS Re 28,246. They are designated dual path compressors and expanders. The further path signal boosts the main path signal in the compressor but bucks the main path signal in the expander. A Type I configuration (for example as in US-PS 3,046,719) is generally used for audio applications and a Type II configuration (for example as in US-PS 3,903,495) is generally used for video applications.

In the case of the compressor the invention is characterized by a frequency dependent circuit connected only in the main path and providing a reduction in frequency response in the part of the frequency band affected by saturation.

In the case of the expander the invention is characterized by a frequency dependent circuit connected only -in the main path and providing an increase in frequency response in the part of the frequency band affected by satura- tion.

The frequency dependent circuit in the main path is connected between the further path take off point and the combining circuit, in both compressers and expanders, both Type 1 and Type II.

The invention. also provides a complete noise reduction system as characterized in the claims.

The said part of the frequency band af- fected by saturation is usually the upper extreme-that is, said upper extreme is whatever upper frequency is regarded as the top end of the audio frequency band in any given practical case, whether this is 15 kHz or 20 KHz or some other value, in the case of audio, or such as 4-6 MHz in the case of video.

In an audio development of the invention, where the compressor or expander also acts at low frequencies, the frequency dependent cir- cuit also provides a reduction (compressor) or increase (expander) in frequency response in the lower part of the audio frequency band (e.g., extending downward from about one hundred Hertz. towards its lower extreme, which extreme is typically in the region of 20 Hz.

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

Figure 1 shows a representative response curve of a cassette tape recorder/ reproducer:

Figure 2 shows an explanatory characteris-_ tic curve.

Figure 3 is a block diagram of an embodi-, ment of the invention in the context of Type I compressors and expanders.

Figure 4 is a block diagram of the invention in the context of Type 11 compressors and expanders.

Figure 5 is a block diagram of an embodiment of the invention in the context of a dual stage compander system for magnetic tape recording.

The problem of high frequency saturation is common to all magnetic tape and optical film recorders. It may also occur in pre-emphasized recording and transmission systems of many kinds, including FM broadcasting systems. Although it is most severe in low speed tape recorders, particularly those using low cost tape formulations, it affects the ability to record at higher levels even in high quality professional magnetic tape and optical film recorders. That is, the recording medium does not accurately record the high level, high frequency signals applied to it. The main audible effect is intermodulation distortion and a reduction in high frequency content of the recording. However, in the context of the present invention, such saturation is especially undesirable because with certain combinations of signal levels and frequencies it upsets the playback complementarity of the expander. Hence, the expander will accurately decode the played back signal to some extent depending on the degree of saturation. The main audible effect is usually an exaggeration of the high frequency loss by the expander but it may also include a spurious modulation of mid-frequency signals.

The illustrated embodiment is considered primarily in the context of a cassette tape recorder recorder/ reproducer although the invention is applicable as well to professional quality magnetic tape, optical film recording and recording and transmission systems in general.

Fig. 1 shows the response of a well above average Compact Cassette recorder/reprodu- cer. At a recording level of - 20 dB the response is substantially flat out to 20 kHz. At higher recording levels the effects of high frequency tape saturation become apparent, causing a substantial high frequency roll off at a record level of 0 dB. Typical cassette units 3 p GB2079114A 3 exhibit substantially greater saturation effects.

The most straightforward way of reducing the above saturation effect is to change the recording equalization in such a way that the tape is not driven so hard in the frequency range in which saturation is troublesome. The playback equalization is then altered in a complementary way. Unfortunately, in cassette recording, saturation effects may extend down to frequencies as low as 2 kHz or so as Fig. 1 suggests. The required change in equalization would thus result in a signicant increase in audible noise.

The circuit described below effectively per- mits a treatment to reduce high frequency saturation without sacrificing any significant noise reducation in the treated frequency range. The same technique may be used to reduce low frequency tape distortion where a full-range noise reduction system is employed. In particular, note that in a dual path compressor or expander circuit the output of the circuit at very low signal levels is provided mostly by the further or noise reduction path.

In the case of one such device providing 10 dB of dynamic action, the contributions of the main and noise reduction paths are in the ratios of 1 and 2.16, respectively. At high signal levels the roles of the two paths are reversed; the main path provides the predominant signal component, and the further path contribution is negligible.

The saturation or distortion reduction effect is based on the above observations. Namely, an equalizer providing the required reduction in high or low frequency drive is placed in the main path of the compressor. As shown in Figyre 2, which shows the operation of a high frequency distortion reduction circuit, at high signal levels essentially the full effect of the equalization is obtained, with a consequent reduction in high frequency saturation. However, at low levels, the equalization effect is reduced, since the contribution of the noise reduction path becomes significant. If, for example, the antisaturation network provides for as much as a 12 dB attenuation at a particular frequency, then, ignoring phase considerations, the low level effect will be 0.25 X 1 + 2.16 = 2.41 = 7.6 dB That is, a 12 dB reduction in high level recording drive is obtained for a 2.4 dB loss in noise reduction effect. Such a high degree of distortion reduction would be required only at the highest frequencies, at 15 kHz, for example. At lower frequencies the required reduction in saturation would be less, with a correspondingly reduced loss of noise reduction effect. At low frequencies, the primary objective would be to counteract the effect of the 3180 usec ( + 3 dB at 50 Hz) recording boost built into many tape recorders, which causes low frequency tape saturation problems, e.g. with organ music, as already men- tioned.

In audio applications, the requirements of a suitable anti-saturation network may be determined as follows. The maximum usable output level of the tape, optical film, FM channel, or otherwise, is determined at low to medium frequencies and at higher frequencies up to the highest frequency of interest. The resulting maximum output level curve is compared to a plot of energy distribution as a function of frequency for normally encountered music and speech sounds. Such a plot is presented in my paper Journal of the Audio Engineering Societ)1', Vol. 21, No. 5, June, 1973, pp. 357-362. The difference between the two curves is an indication of the required high level anti-saturation characteristics. Once such characteristics are determined and implemented, the resulting compression characteristic curves should be checked to determine if the anti-saturation network has caused an increase in compression ratio at any frequency or level. If so, appropriate changes can be made in the limiting characteristics of the noise reduction path and/or, where several series connected compressors (or expanders) are used, the anti-saturation characteristic can be distributed among the devices.

Similar considerations apply in the case of video devices. In video recorders high fre- quency pre-emphasis is frequently used, which can result in FM overmodulation prob)ems. The overmodulation tendency of any particular recording system can be measured and an appropriate kind and degree of anti- saturation can be provided. If compressors and expanders of the types described in US-PS (Type 1) and US-PS (Type 11) are used, there will be a further overmodulation tendency produced by small residual over- shoots (a few percent) from the compressors. These overshoots can be compensated by the present invention. (in audio systems there is also a tendency to compensate residual overshoots).

While it would be possible to operate with only the antisaturation network in the encoder unit, it is preferable to provide a complementary correction on the playback side, so that flat frequency response is maintained at all levels. The following analysis shows the type of correction required.

Referring to Figure 3, which shows the Type 1 dual path compressor and expander configurations, let the input signal to the compressor be x, the signal in the information channel be y, and the output signal of the expander be z. Let F, and F, be the transfer characteristics of the further path of the cornpressor and expander, respectively, and F,, be the transfer characteristic of the antisaturation network. Let F',, be the required compensating characteristic in the decoder.

y = (FAS + F,)x and z = yF',s - zF2F'AS 4 GB 2 079 114A 4 thus, z= PAsFAS + F, PAS 1 + Fj'M X Inspection shows that z = x if F, = F2 and 70 PAS (PAS F2) thus z = X 1 --Fl FAs Inspection shows that z = x if F, = F2 and 1 PAS = FAs The above shows not only that the two noise reduction networks should be identical, as is known from the applicant's prior art, but that the antisaturation compensation network in the decoder should have an inverse characteristic to that of the network employed in the encoder. Simple corrections can be achieved passively, such as by RC combinations, while more complex ones can employ feedback techniques, especially to achieve the inverse characteristics required in the decoder.

Referring now to Figure 5, a block diagram of the present invention is shown embodied in a dual stage Type 1 configuration primarily for use in magnetic tape recording and reproduction.

The illustrated embodiment employs cascaded compressors 52 and 54 to obtain an enhanced amount of compression and, correspondingly, cascaded expanders 56, 58. The invention is applicable as well to single stage dual path compressors and expanders. Each compressor has a main path 10 including a combining circuit 12 which adds to the main path the output of the further path Nj, N21 whose input is connected to the input of the corresponding main path. The expanders have main paths 14 and combining circuits 16 which subtract from the main path signal the output of the further path N, N, whose input is connected to the corresponding main path output.

1 FAS A similar derivation applies to the Type 11 configuration shown in Fig. 4: y = FAsx + F, Fsy and z = FOASY - F2Y followed by a controlled limiter which is -caused to limit progressively, as the signal level rises, by a rectified and smoothed con trol 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, which is preferably in series with a fixed high pass filter. Advantageous corner frequency values for the sliding band filters are about 375 Hz in the quiescent state but become progressively narrower high pass in response to the control signal.

The first and second compressors 52 and 54 each have high frequency and/or low frequency (and/of any other particular fre quency or frequency range) anti-saturation networks 74, 76 affecting the main signal components. On the reproduce side of the tape recorder T, the first and second expan ders 56 and 58 have complementary net works affecting the main signal components.

Alternately, such compensation may be pro vided in only one of the compressors and in only the corresponding expander. The com pensation, may, for example, take the form of a gently high frequency roll off in the encoder (compressor) increasing at very high frequen cies, say above 10 kHz, and complementary boost in the decoder (expander).

Because the high frequency channel over load reduction is applied only to the main signal components, the low level components in the side channels are unaffected and thus the overload reduction does not greatly affect the noise reduction. Consequently, the chan nel overload reduction can extend down to the mid-frequency range.

The antisaturation networks in the enco ders, 74, 76, and the complementary net works in the decoders, 78, 80, are located in the main signal paths. This arrangement re sults in very little loss of noise reduction effect, since the bulk of signal at low levels is provided by the noise reduction side chain.

Therefore it is possible to provide antisatura tion networks that taken together have a grad.; ual high frequency roll off, 3-6 dB/octave, for example, (and complementary boost in the decoder) extending quite far down in fre quency, for example to 2-3 kHz, to deal with saturation effects without a high degree of noise reduction loss. The frequency response at 0 dB in Fig. 1, shows, for example, a fall off of high frequency response from saturation beginning about 2 kHz. Since the antisatura tion networks are in the main channels, which carry the high level signals, only those signals Such compressor and expander configura- 125 which cause saturation are affected. As shown tions are well known in themselves and will in Fig. 1, low level signals do not cause therefore not be described in great detail. significant tape saturation.

However there are two main forms for the Optionally, half of the 3-6 dB/octave roll further path N, or N, One alternative (Figs. 7 off is provided in each network 74 and 76 and 8 of US-PS 3,846,719) is a filter 130 and half of the complementary boost is pro- 1 11 11 Y p 4 R GB2079114A 5 vided in each network 78 and 80. If all of the roll off and boost is placed in single networks 76 and 78, then the associated compressor or expander will have an increased compression or expansion ratio in the particular level and frequency region associated with that compressor or expander. However, in a consumer system the use of a single network is ade- quate.

Claims (9)

1. A compressor comprising 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, at least in an upper part of the fre- quency band, boosts the main path signal by way of the combining circuit, but which is so limited that, in the upper part of the input dynamic range, the further path signal is limited to a value smaller than the main path signal, and a frequency dependent circuit means connected only in the main path for providing a reduction in frequency response in a selected part of the frequency band.

2. A compressor according to claim 1 wherein the compressor is an audio compressor, wherein the frequency dependent circuit means produces a reduction in frequency response in the upper part of the audio frequency band.

3. An audio compressor according to claim 2 wherein the compressor also acts at low frequencies, wherein the frequency dependent circuit means also produces a reduction in frequency response in the lower part of the audio frequency band.

4. An expander comprising 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 con- nected to the input or output of the main path and its output connected to the combining circuit, the further path providing a signal which, at least in an upper part of the frequency band, bucks the main path signal by way of the combining circuit, but which is so limited that, in the upper part of the input dynamic range, the further path signal is limited to a value smaller than the main path signal, and frequency dependent circuit means connected only in the main path for providing an increase in frequency response in a selected part of the frequency band.

5. An expander according to claim 4 wherein the expander is an audio expander, wherein the frequency dependent circuit means produces an increase in the frequency response in the upper part of the audio frequency band.

6. An audio expander according to claim 5 wherein the expander also acts at low frequencies, wherein the frequency dependent circuit means also produces an increase in frequency response in the lower part of the audio frequency band.

7. A noise reduction system comprising a compressor and an expander, each having 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, at least in an upper part of the frequency band boosts the main path signal in the compressor, and bucks the main path signal in the expander, by way of the combining circuit, but which is in each case so limited that, in the upper part of the input dynamic range, the further path signal is limited to a value smaller than the main path signal, and frequency dependent circuit means connected only in the main paths for providing a reduction in frequency response in the compressor and a complementary increase in frequency response in the expander in a selected part of the frequency band.

8. A noise reduction system according to claim 7 wherein the compressor and expander act at audio frequencies, wherein the frequency dependent circuit means produce a reduction in frequency response in the compressor and a complementary increase in frequency response in the expander in the upper part of the audio frequency band.

9. An audio noise reduction system according to claim 8 wherein the compressor and expander also act at low frequencies, wherein the frequency dependent circuit means also produce a reduction in frequency response in the compressor and a complementary increase in frequency response in the expander in the lower part of the audio frequency band.

Printed for Her Majesty's Stationery Office by Burgess Ft Son (Abingdon) Ltd.-1982Published at The Patent Office, 25 Southampton Buildings, London, WC2A 1 AY, from which copies may be obtained.