DK156356B - CIRCUIT TO REDUCE OVERLOAD EFFECTS IN SIGNAL RECORDING AND TRANSMISSION SYSTEMS - Google Patents

Description

DK 156356 B

The invention relates to a coupling device for changing the dynamic range of signals supplied to a recording or transmission medium or sensed by one, namely a compressor for narrowing the dynamic range or an expander 5 for expanding the dynamic range, wherein the compressor the expander, respectively, upon placement in front of or after the recording or transmission medium, respectively, forms a noise reduction system and the recording or transmission medium is subjected to a saturation or overload effect outside the frequency range, in which the switching device changes the dynamic range, consisting of at least One coupling with a main signal path which is linear with respect to the dynamic range, a combination circuit in the main signal path and an additional signal path whose input is connected to the input or output of the main signal path and whose output is connected to the combination circuit , wherein the additional signal path produces a signal which in each decreases in the upper part of the frequency range and also the highway signal1 by means of the combination circuit in the compressor and in the expander counteracts the highway signal, however 20 is so limited that the signal of the additional signal path in the upper range of the dynamic range in the input is limited to a value which is less than the highway signal,

The upper part of the frequency band typically extends from a value of a few hundred Hz, for example 300 to 400 Hz in an upward direction, when a higher frequency can also be used. In the upper part of the input dynamics range (for example -10 dB to + 10 dB relative to a reference level), the signal of the additional signal path is small relative to the signal from the main signal path.

Such coupling devices are known and widely used - cf. U.S. Patent No. 3,846,719 and U.S. Patent No. 3,903,485. They are referred to as two-way compressors and 35-expanders. The signal of the additional signal path raises the main path signal in the compressor and counteracts the main path signal in the expander. A Type I configuration (U.S. Patent No. 3,846,719) is generally used in connection with audio and video

DK 156356 B

a Type II configuration (U.S. Patent No. 3,903,485) is generally used in connection with video.

Compressors and expanders are usually used together (com- ponent system) to achieve noise reduction. The signal is compressed before transmission or recording and expanded after receiving or playing from the transmission channel. However, compressors can only be used to reduce the dynamic range, for example, to adjust the capacity of a transmission channel without subsequent expansion, when the compressed signal is appropriate for the respective use. In addition, compressors are used only in certain products, especially audio products, intended to transmit or record only compressed signals or pre-recorded signals. Expand alone is used in some products, especially audio products, which are only used for receiving or reproducing already compressed broadcasts or pre-recorded signals. In certain products, especially audio recording or reproduction products, a single device is often arranged for optional operation as a compressor for recording signals or as an expander for reproducing compressed broadcasts or pre-recorded signals.

Compressors and expanders, respectively, are operating systems that operate frequency dependent, cf. German Publication No. 2,803,751.

The invention also relates to the compression of expanders which, in addition to providing compression or expansion, also affects the level in dependence on a counter distortion.

In magnetic recording, the need for such a counter-distortion (attenuation in the compressor and gain in the expander) arises from the tendency of the magnetic tape to saturation, especially at higher frequencies. Several proposals have been made to achieve nodal irrigation. Several of these proposals are discussed in - Rundfunktechnik Mitteilungen, year 22 (1978) H. 2, pages 63-75. Need for counter-distortion may also arise in connection with other recording or transmission media exposed 3

DK 156356 B

for high-level saturation or overload effects at certain frequencies. Den de! of the frequency band affected by saturation effects is generally extremely high, i.e. substantially above the range where the upper frequency is considered to be the upper end of the audio frequency band in all practically occurring cases, for example 15 kHz or 20 kHz or another value in the case of audio or 4-6 MHz in the case of video.

One possibility (which does not, however, provide a level-dependent counter-distortion) is to provide a high-frequency damping circuit after the compressor and a compensating gain circuit in front of the expander. Alternatively, the damping circuit may be placed in front of the compressor and the gain circuit after the expander. One disadvantage of this is that the signals at all levels are subjected to the same attenuation (and subsequent amplification), so that the noise reduction does not become so great. It is stated in Rundfunktechnik Mitteilungen that loss in noise reduction can be tolerated if the noise reduction is only 20 20 dB. This is only partially the case. In the case of compact cassette tapes, there is in practice saturation all the way down to 1 2 kHz, and the necessary change in counter distortion to take into account this would lead to a significant increase in the audible noise.

25

This problem has been solved by making the reproduction of the attenuation and amplification circuits level dependent - cf.

U.S. Patent No. 4,072,914. In the case of the damping circuit, the damping is steeper at high levels than at 30 low levels. For reinforcement circuits, the gain characteristics are also steeper at high levels than at low levels. One disadvantage of this approach is that the circuit becomes more complex.

35 ï Rundfunktechnik Mitteilungen it has also been proposed to place high frequency amplifier circuits in front of the control circuit of both the compressor and the expander, but it is also pointed out that the resulting attenuation is only evident in the mid-level range,

DK 156356B

where you first and foremost want to achieve the effect at high levels. In video recording systems, shark frequency saturations may be caused by the preamplification used during recording. A similar problem exists with FM broadcasts.

5

A similar problem is that a low-frequency counter-distortion (attenuation in the compressor and gain in the expander) is solved when the compressor and the expander are also operating at low frequencies. A 1-frequency compression and expansion ion only serve to reduce hum, and it can be obtained by means of broadband compressors and expanders or compressors and expanders with couplings operating independently at low and high frequencies. At these frequencies, the problem consists in being able to counteract the effect of a 3180 psec. recording amplification (+ 3 dB at 50 Hz) built into multiple magnetic tape devices and causing tape equipment and saturation problems at low frequencies, such as organ music.

The relevant lower frequency range extends, for example, from about 100 Hz to the lower end of the audio frequency band, typically within the range of 20 Hz.

The object of the invention is therefore to provide a coupling device of the kind mentioned above, which enables a level dependent counter distortion in the frequency range (s) and which is easy to incorporate.

A coupling device of the kind mentioned above is characterized by the characteristics of claim 1. naughty.

30 This utilizes the existence of a frequency-dependent signaling pathway that dominates at low levels such that the influence of the frequency path in the hi-level region can be made in view of the dynamics of the linear highway, and thus cannot be effective at low levels.

35

The invention will be explained in more detail below with reference to the drawing in which: -

DK 156356 B

FIG. 1 shows a representative reproduction curve of a cassette tape recorder; FIG. 2 examples of transfer characteristics, 5 fig. 3 is a block diagram of an embodiment of the system according to the invention in connection with type I compressors and expanders; FIG. 4 is a block diagram of the system according to the invention in connection with type II compressors and expanders; and FIG. 5 is a block diagram of one embodiment of the system of the invention in conjunction with a two-stage compander system 15 for recording on magnetic tape.

Common to all magnetic tape recorders and optical film apparatus are the problems associated with frequency saturation. However, frequency saturation can also occur in preamplified 20 recording and transmission systems of several types including FM transmitter systems. Although frequency saturation is particularly severe in slow tape recorders, especially tape recorders who use cheap tape recorders, it also affects registration at higher levels; even with professional 25. magnetic tapes and optical f in high quality Imps devices. The recording medium does not accurately record the high level input high frequency signals. The important horrible effect is intermodulation distortion and a reduction in the high frequency content of the recording. Such a saturation is undesirable since, at certain combinations of signal levels and frequencies, it destroys the play complementarity of the expander. The expander will thus, depending on the saturation graph, make an inaccurate decoding of the recorded signal. The main audible effect is usually an exaggeration of the high-frequency loss of the expander, but may also include a false modulation of intermediate frequency signals.

The embodiment shown is primarily intended for use in ----- 6

DK 156356 B

may also be used in conjunction with professional magnetic tapes, optical film recording and reproduction, and transmission systems in general.

FIG. 1 shows a representation of a compact cassette recorder that is above average. At a recording level of -20 dB, the reproduction is essentially flat up to 20 kHz. At higher recording levels, the effects of high frequency saturation become prominent and give rise to a significant high frequency attenuation at a recording level of 0 dB. Typical cassette units exhibit significantly greater saturation effects.

The most immediate method for reducing the above saturation effect is to change the counter distortion during registration 15 in such a way that the tape is not operated so hard in the frequency range where saturation causes difficulties. The counter distortion during playback is then changed in a complementary way. While recording in a cassette, saturation effects can extend down to 2 kHz or soin shown in FIG. 1. The necessary counter distortion will thus result in a significant increase in the audible noise.

The circuit described below enables signal processing to reduce high frequency saturation without thereby losing any significant noise reduction in the treated frequency range.

The same technique can be used to reduce low-frequency band distortion in case a "full-range" noise reduction system is used. In a bidirectional compressor or expander circuit, the output of the circuit at very low signal levels is usually provided by additional signal paths or noise reduction paths. In the case of such a device providing a dynamic range of 10 dB, the contributions of the main and noise reduction paths are in ratios 1 and 2.16. At high signal levels, the roles of the two signal paths are switched; the high signal path provides the dominant signal component, while the contribution of the additional signal path may be neglected.

The saturation or distortion-reduction effect is based on the nuunction of the pIanH approach. On modf nourann. rif »r ti 1 ve ïehr ï naep 7

DK 156356 B

the desired reduction in the high or low frequency drive is conveniently located in the main signal path of the compressor. As shown in FIG. 2, which illustrates the operation of a high frequency distortion reduction circuit, is essentially the full effect of the counter distortion at high signal levels obtained by a corresponding reduction in the high frequency saturation. However, at low levels, the counter-distortion effect is reduced since the bi-drag of the noise reduction pathway is significant. For example, the anti-saturation network provides attenuation of 12 dB at a particular 10 frequency, so that, considering phase conditions, the low level of power will be

0.25 κ 1 + 2.16 = 2.41 = 7.6 dB

15 A reduction of 12 dB in the high level of registration has been obtained at the expense of a loss of 2.4 dB in noise reduction effect. Such a distortion reduction is only required at the highest frequencies, for example at 15 hKz. At lower frequencies, the necessary saturation reduction is less by a correspondingly smaller loss in noise reduction effect. At lower frequencies, the primary purpose will be to counteract the power of 3180 psec.

(^ 3 dB at 50 Hz) the regeneration gain built into several tape recorders, causing low-frequency tape saturation problems, for example, with organ music 25 as mentioned above.

For audio applications, the requirements for a suitable anti-saturation network can be determined as follows. The maximum usable output level of the tape, optical film, FM channel, or 1g channel is determined at low frequencies for medium frequencies and at higher frequencies up to the highest frequencies of interest. The resulting maximum output level curve is compared to a representation of the energy distribution as a function of the frequency of normally occurring music and sounds. A 35 such image is shown in the Journal of the Audio Engineering Society, Vol. 21, No. 5, June 1973, pages 357-362. The difference between the two curves is an indication of the necessary high-level anti-saturation characteristics. When such characteristic-

DK 156356 B

Once determined and implemented, the resulting compression characteristics must be checked to determine if the anti-saturation network has caused an increase in the compression ratio at certain frequencies or levels. If this is the case, changes can be made to the limiting characteristics of the noise reduction path and / or in the case that several compressors (or expanders) are connected in series, the anti-saturation characteristic can be distributed along the devices.

10

Similar considerations come to mind in the case of video devices. VCRs often use high-frequency gain, which can cause FM over-modulation problems. The over-modulation tendency of a particular recording system can be measured and an appropriate degree of anti-saturation can be provided. If type I or type II compressors or expanders are put into operation, there will be a further tendency for over-modulation as a result of small residual surges (in some few percent) from the compressors. These surges can be compensated by the compressor according to the invention. (In audio systems, there is also a tendency for residual overshoot compensation).

Although it is possible to operate with only one anit saturation network in the encoder, there is usually also a complementary correction on the play side, so that flat reproduction curves are maintained at all levels. Subsequent analysis shows the necessary correction type.

FIG. 3 shows some type I bidirectional compressor and expander configurations. The input to the compressor is called x, the signal in the information channel is called y, and the output of the expander is called z. F1 and F2 are the transmission characteristics of the additional signal path of the compressor and the expander, and F2 s is the transmission characteristic of the anti-saturation network. Fg is the necessary compensation character connector in a complementary anti-saturation network.

9

DK 156356 B

y = (phase + fi) x

and Z ~ yF'AS * zF2f'AS

5 It is thus said that

f'asFas + F ^ F1AS

z = -—--

1 + F2F'AS

10

It is seen that z = x »if Fj = F2 and F'as =

FAS

A similar derivative is used in connection with the type II configuration in fi9-4: y = FAsx + FiFAsy and z = F'ASy “p2y 20

One has such that (F's - F2)

Z --- X

1 p 25 F!

Fas

It is seen that z = y if Fj = F2 and F'as s ~ - “phase 1 0

In addition to the two noise reduction networks being identical, as was also known from the applicant's previous publications, the anti-saturation compensation network in the decoder should also have a characteristic inverse to the characteristic of the network used in the encoder. Simple corrections 35 can be obtained by passive circuits, for example RC combinations, while more complex corrections can be provided by backlinking techniques, in particular to obtain the inverse characteristic required in the decoder.

10

DK 156356 B

The FIG. 5 is used in a two-stage Type I configuration, which is primarily intended for magnetic tape recording and reproduction.

The embodiment shown utilizes cascade-coupled compressors 52 and 54 to obtain an increased degree of compression and corresponding cascade-coupled expanders 56 and 58. The invention may be used in conjunction with one-stage bidirectional compressors and expanders. Each of the compressors has a main signal path 10 containing a combination circuit 12, so as to the main signal path, the output of the additional signal path N1, N2, whose input is connected to the input of the corresponding main signal path. The expanders have main signal paths 14 and combination circuit 16 which subtract from the highway signal1 the output of the additional signal path N2, N1, whose input is connected to the output of the corresponding main signal path.

Such compressor and expander configurations are well known in themselves and will therefore not be described in detail. However, there are two main forms of the additional path Nj or 1 N2. In an alternative (Figs. 7 and 8 of U.S. Pat.

3,846,719) is a filter followed by a controlled limiter which progressively limits as the signal level rises by a unidirectional and smoothed control signal. Another alternative 25 (United States Patent RE 28,426) is a sliding tape pass, whose pass band is progressively narrowed by the control signal, thus eliminating large signal components from the output of the filter, which is preferably arranged in series with a fixed lift pass. The corner frequency of the sliding bandpass 30 is at a standstill of about 375 Hz, providing a progressively narrowed high pass range depending on the control signal.

Compressors 52 and 54 have high-frequency and / or low-frequency anti-saturation networks 74, 76 which affect the main signal components. Getting the reproduction side of the tape recorder T has the first and second expander 56 and 58 complementary networks acting on the main signal components. Alternatively, such a compensation verse may be provided in only one of the compressors and in

DK 156356 B

the corresponding expander. The compensation can, for example. take the form of a weak high frequency attenuation in the 'encoder (compressor) which grows at very high frequencies, for example above 10 kHz and complementary amplifications in the decoder (expander).

5

Due to the fact that the high-frequency overload reduction is applied only to the main signal components, the low-level components in the side channels are unaffected and the overload reduction does not affect the noise reduction very much. Consequently, the channel-10 overload reduction can be extended to the intermediate frequency range.

The anti-saturation networks in the encoder 74, 76 and the complementary networks in the decoder 78 and 80 are located in the main signal paths.

This results in very small losses in the noise reduction effect as the magnitude of the signal at low levels is provided by the noise reduction side chain. It is therefore possible to provide anti-saturation networks which together have a gradual high frequency attenuation of e.g. 3-6 dB / octave (and complementary amplifications in the decoder) extending relatively far in frequency, e.g. to 2-3 kHz to provide saturation effects without particularly large noise reduction losses. The frequency response at 0 dB in FIG. 1 shows e.g. a high frequency attenuation attenuation from the beginning of the saturation at about 2 25 kHz. Since the anti-saturation networks are located in the main channels, only the signals that cause the saturation are affected.

As shown in FIG. 1, low-level signals do not give rise to any significant saturation.

30 Half of the attenuation of 3-6 dB / octave is provided in each network 74 and 76, and half of the complementary gain is provided in each of the networks 78 org 80. If all the attenuations and amplifications are located in individual networks 76 and 78, the associated compressor or expander would generate an increased compression or expansion ratio in the particular level and frequency range of the compressor or expander concerned. However, in a user system, a single network is appropriate.

Claims (4)

1. Switching device for changing the dynamic range of signals supplied to a recording or transmission medium or removed by one, namely a compressor for narrowing the dynamic range or an expander for expanding the dynamic range, the compressor and the expander respectively. when placed before or after the recording or transmission medium, respectively, a noise reduction system is formed and the recording or transmission medium is subjected to a saturation or overload effect outside the frequency range within which the coupling device changes the dynamic range, consisting of at least one coupling with a main signal path which is line -15 s with respect to the dynamic range, a combination circuit in the main signal path and an additional signal path whose input is connected to the input or output of the main signal path, and whose output is connected to the combination circuit, the additional signal path producing a signal which at least for 20 the © Fri. the! of the frequency range ©, the main path signal by means of the combination circuit in the compressor and in the expander counteracts the main path signal, however limited so that the signal of the additional signal path in the upper region of the dynamic range in the input is limited to a value 25 the highway signal, characterized by a frequency dependent circuit (F / \ g; 74, 76 and F'a $; 78, 80. coupled only in the main signal path {XY; 10 and YZ; 14, respectively) and in the compressor (52, 54) a reduction of the frequency response in the de! of the frequency band in which the recording or transmission medium is subject to saturation or overload effect, and in the expander (56, 58) causes an increase in the frequency response in that portion of the frequency band comprising a compressor system consisting of a compressor system ( 52, 54) in front of a recording or transmission medium and an expander (56, 58) after the recording or transmission medium, the parts of the frequency band affected by the frequency-coupling (F ^ §; 74, 76) in the compressor (52, 54) is identical to the parts of the frequency band affected by the frequency dependent circuitry (F1 AS »78» 80) ϊ the expander (56, 58). 2. A coupling device according to claim 1, wherein the signals are in the audio frequency band and the recording or transmitting medium is subject to saturation or overload effect from a frequency in the upper part of the frequency band, the upper one of which! of the frequency band extends upwards from approx. 300 Hz, characterized in that the frequency dependent coupling (F /; 74, 76) of the compressor (52, 54) results in a reduction of the frequency response in the region higher than the frequency at which the saturation or overload effects occur, respectively, in the expander (56, 58) causing an increase in the frequency response in the same area. 15 3. A coupling device according to claim 2, which also operates at low frequencies and wherein the recording or transmission medium is subject to saturation or overload effect from a frequency in the lower part of the frequency band, the lower part 20 of the frequency band extending downwards from approx. 100 Hz, characterized in that the frequency dependent coupling (F / \; 74, 76) in the compressor also gives a reduction of the frequency response in the region below the frequency at which the saturation or overload effect begins the frequency dependent coupling (F * as (78i8 °) in the expander (56, 58) causes a prediction of the frequency response in the same region. 30 35