FI76456C - KRETSANORDNING FOER MINSKNING AV OEVERBELASTNINGSEFFEKTER HOS INSPELNINGS- ELLER TRANSMISSIONSMEDIUM I INSPELNINGS- OCH TRANSMISSIONSSYSTEM. - Google Patents

Description

Circuit arrangement for reducing the congestion phenomena of a storage or transmission medium in storage and transmission systems 7645 6

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

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

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More particularly, the present invention is directed to compressors and expanders that, in addition to compression or expansion, provide level-dependent correction. This type of correction must be used for magnetic recording, as the magnetic tape tends to saturate at particularly high frequencies. Many suggestions for the implementation of the repair have been made in the past, several of which have been discussed in Rundfunktechn. Mitteilungen, Jahrg. 22 (1978) H. 2, pp. 63-74. Correction may also be required for other storage or transmission media that may experience high levels of saturation or overload at certain frequencies.

One possibility (which, however, does not allow for level-dependent correction) is to use a high frequency attenuation circuit after the compressor and a compensating boost circuit before the expander. Alternatively, the damping circuit may be located before the compressor and the boost circuit after the expander. One disadvantage of this method is that signals of all levels are subjected to the same attenuation effect (and consequent emphasis) so that the amount of noise attenuation achievable is greatly reduced. In the above-mentioned publication Rundfunktechn. A paper by Mitteilungen states that a reduction in the amount of noise attenuation can be tolerated if the noise attenuation without this method is as high as 20 dB. This is only partially true. In practice, especially with some C-cassette tapes, the tape saturation phenomena extend downwards up to 2 kHz, and the correction change required to account for this results in a significant increase in audible noise.

Attempts have been made to solve the problem by making the responses of the attenuation and enhancement circuits level-dependent (U.S. Patent 4,072,914). In the case of an attenuation circuit, the level drop is steeper at high levels than at low levels. In the highlight circuit, the highlight characteristic is likewise steeper at high levels than at low levels. The disadvantage of this approach tl 3 7645 6 is the complexity of the circuits it causes.

The publication Rundfunktechn. Mitteilunge's article has also proposed placing a high-frequency boost circuit in front of both compressor and expander control circuits, but the publication also emphasizes that the resulting level drop is only noticeable in the medium signal range when the effect is mainly to be obtained at high levels. Recording pre-emphasis used in video recording systems can cause high frequency saturation problems. A similar problem occurs with FM broadcasts.

The object of the invention is thus to provide a better solution to the problems discussed above, i.e. a solution which is at the same time effectively effective and simple.

A problem with the above is that the use of correction at low frequencies (attenuation in compression and emphasis in expansion) is desirable if compression and expansion also affect at low frequencies, although this has not been addressed in previous publications. Low frequency compression and expansion are useful in attenuating hum-like noise and can be achieved by using broadband compressors and expander or compressors and expanders with high and low frequency independent circuits. At low frequencies, the main purpose of the correction is to reduce the pre-emphasis of the 3180 yUS (+3 dB at 50 Hz) recording included in the recorders, which causes tape saturation problems at low frequencies, e.g., in organ music.

It is an object of the invention to provide a solution by which the required correction can, if desired, also be carried out at low frequencies.

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Yet another object of the invention is to increase the risk of overshoot and thus the dynamic range of the saturable storage medium.

The invention is based on a compressor or expander comprising a main signal path linear with respect to the dynamic range, a circuit connecting the main path and an additional path connected to the input or output of the main path and an output connecting circuit, the additional path providing a signal emphasizing or weakening the main path at least at the top of the frequency band. by means of a signal connecting circuit but which is limited so that at the top of the dynamic range of the input the signal of the additional path is smaller than the signal of the main path.

The upper part of the frequency band typically starts at a few hundred hertz, e.g. from 300-400 Hz, although a higher value may be used. At the top of the dynamic range of the input, e.g., between -10 dB and +10 dB with respect to the reference level, the auxiliary path signal is small compared to the main path signal.

Compressors and Expanders of this type are previously known and commonly used, examples of which are disclosed in U.S. Patent Nos. 3,846,719, 3,903,485 and Re 28,426. Equipment of this type is referred to as two-way compressors and expanders. The uplink signal increases the main path signal in the compressor and weakens in the expander. In audio signal applications, the Type I form (e.g., U.S. Patent 3,046,719) is commonly used, and the Type II form (e.g., U.S. Patent 3,903,485) is commonly used in video applications.

In the case of a compressor, the invention is characterized by a frequency-dependent circuit which is connected only to the main path and which causes a reduction in the frequency response in the part of the frequency band in which saturation acts.

In the case of Ercspander, the invention is characterized by a frequency-dependent circuit connected only to the main road and

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5 76456 which causes an increase in the frequency response in the part of the frequency band affected by the saturation.

The frequency-dependent circuit of the main path is connected between the branch point of the auxiliary path and the connecting circuit in both compressors and expanders, both in type I and type II.

The invention furthermore relates to a complete noise reduction system which is characterized by what is stated in the claims.

Said part of the frequency band in which the saturation acts is usually the upper end of the band, i.e. the upper end is the highest frequency considered in this practical case as the upper limit of the audio frequency range, either 15 kHz, 20 kHz or other value for audio signals or e.g. 4-6 MHz.

In an application of the audio signals of the invention, where the compressor or expander also operates at low frequencies, the frequency dependent circuit further reduces (in the compressor) or increases (in the expander) the frequency response at the bottom of the audio frequency band (e.g. from about 100 Hz below to typically 20 Hz).

The invention will now be described in detail by way of example with reference to the accompanying drawings, in which Figure 1 shows a representative cassette recorder / player response curve, Figure 2 shows an illustrative characteristic curve, Figure 3 is a block diagram of a type I compressor and expander embodiment, Figure 4 is a type II and a block diagram related to expander, Fig. 5 is a block diagram of an embodiment of the invention 6 76456 related to a two-stage compander system for magnetic recording.

High frequency saturation is a common problem in all recorders using magnetic tape and optical film. It can also occur in a variety of pre-highlighted recording and transmission systems, including FM broadcast systems. Although the problem is worst with low-speed recorders, it affects the recording capacity of even high-quality professional-level recordings using magnetic tape and optical film. In other words, the recording medium does not accurately record the high-level, high-hair signals input to it. The main audible effects are central modulation distortion and a decrease in the proportion of high frequencies in the recording. However, in the context of the present invention, such saturation is particularly undesirable because it interferes with the complementarity of the expander during reproduction with certain combinations of signal levels and frequencies. As a result, the expander erodes the reproduced signal to some extent erroneously depending on the degree of saturation. The main audible effect is usually that the expander exaggerates the loss of high frequencies, but the effects may also include bias modulation of mid-frequency signals.

The disclosed embodiment is primarily intended for use with cassette recorders / players, although the invention is equally applicable to professional-grade magnetic tape or optical film recording and recording and transmission systems in general.

Figure 1 shows the response of a clearly better than average C-cassette recorder / player. At the recording level of -20 dB, the response is substantially uniform up to 20 kHz. At higher recording levels, the effects of high frequency saturation of the tape become apparent, causing considerable

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frequencies at the recording level O dB. With typical cassette units, the impregnation effects are substantially greater.

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The most straightforward way to reduce the saturation phenomenon described above is to change the pre-emphasis of the recording so that the tape is not so strongly guided in the frequency range where saturation is a problem. The repeat correction is then changed complementary. Unfortunately, in cassette recording, saturation phenomena can extend down to frequencies of up to about 2 kHz, as shown in Figure 1. The required change in correction would thus lead to a substantial increase in audible noise.

The circuit shown below effectively enables processing that reduces high frequency saturation without sacrificing significant noise attenuation in the processed frequency range. The same method can be used to reduce tape distortion at low frequencies when using a full-range noise reduction system. In particular, it should be noted that in the two-way compressor or expander circuit, the output is for the most part provided by an additional path, i.e. a noise attenuation path. In the case where one such device produces a dynamic effect of 10 dB, the ratios of the proportions of the main and noise attenuations are 1 and 2.6. At high signal levels, parts of the roads change, the main road forms the predominant signal component, and the proportion of the auxiliary road is insignificant.

The action to reduce saturation or distortion is based on the above findings. That is, a equalizer that reduces the required control at high or low frequencies is placed on the main road of the compressor. As can be seen from Figure 2 showing the operation of the high frequency distortion reducing circuit, the high signal levels provide a substantially full correction effect and the consequent reduction in the saturation of the high 8 76456 frequencies. At low levels, however, the correction effect decreases as the proportion of noise attenuation paths becomes significant. If the anti-saturation network provides, for example, 12 dB attenuation at a certain frequency, then the effect at low levels becomes when the phase is ignored

0.25 · 1 + 2.16 = 2.41 = 7.6 dB

That is, a 12 dB reduction in control in high-level recording is obtained with a 2.4 dB reduction in noise reduction effect. Such a large reduction in distortion is only needed at the highest frequencies, for example 15 kHz. At lower frequencies, the required reduction in saturation is smaller and the loss of noise attenuation is correspondingly smaller. At low frequencies, the primary goal is to attenuate the effect of the 3180 μs (+3 dB at 50 Hz) recording enhancement built into many recorders, which causes tape saturation problems at low frequencies, e.g., in organ music as mentioned above.

In audible signal applications, the requirements of a suitable anti-saturation network can be determined as follows. The maximum usable output level of a tape, optical film, FM channel, or other is determined between low and mid frequencies and at high frequencies up to the highest frequency of interest. The resulting maximum output level curve is compared to the energy distribution graph plotted as a function of the frequency of normally occurring music and speech sounds. Such a graph is presented in the applicant's publication "Journal of the Audio Engineering Society", Vol. 21,

Well. 5, June, 1973, p. 357-362. The difference between the curves indicates the required anti-saturation characteristic curve for the high levels. Once such a characteristic has been determined and implemented, the resulting compression characteristics must be checked to determine whether the anti-saturation network has caused an increase in the compression ratio at any frequency or

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9 76456 levels. If so, the limiting characteristics of the noise reduction paths can be changed in a suitable manner and / or in the case of several compressors (or expanders) connected in series, the anti-saturation characteristic can be shared between the devices.

A similar consideration applies to video devices. Video recorders often use high-frequency pre-emphasis, which can result in FM over-modulation problems. The tendency of any recording system to over-modulation can be measured, and anti-saturation measures of the type and amount can be used. When using compressors or expanders of the type disclosed in U.S. Patent Nos. 3,046,719 (Type I) and 3,903,485 (Type II), there is an additional tendency for over-modulation caused by small residual overruns of the compressors (a few percent). These excesses can be compensated for by the present invention. (In audible signal systems, a compensating effect of residual overshoot is also obtained).

Although operation is possible in the form that a saturation network is placed in the encoder unit alone, it is preferable to provide the repeater side with a complementary correction so that a uniform frequency response is maintained at all levels. The following analysis shows the nature of the correction required.

Figure 3 shows a type I two-way compressor and expander combination. Let the compressor input signal be x, the information channel signal y and the expander output signal z. and F2 represent the transmission characteristics of the compressor and expander auxiliary paths and the transmission characteristics of the anti-saturation network FAG. F'Äg is the required compensation characteristic of the ecocoder.

10 7645 6 Υ = (Fas + Fx) x and 2 = yF'AS "ZF2F'AS 'j ° ten t? * TP +" P TP 1 AS AS 1 AS v z = ----- x

1 + F F Χ 2 * AS

Consideration shows that z = x if F ^ = F2 and F'Ag =.

The corresponding line applies to the type II form shown in Figure 4: ^ = FASX + FlFASy and z = F'ASy - F2y, so (F1 - F) 1 AS 2;

Z - '' · '- V

F ~ F1 AS 1

Consideration shows that z = x if F ^ = F £ and F'Ag = -.

It is clear from the above that both the noise attenuation networks must be identical, as is known from the applicant's previous publications, and that the decoder anti-saturation circuit must have an inverse characteristic compared to that used in the encoder. Simple corrections can be achieved passively, e.g. with RC combinations and more complex ones, feedback technique can be used, especially in the decoder to achieve the required inverse characteristic.

Figure 5 is a block diagram of a two-stage type I configuration of the present invention intended primarily for recording and reproducing using magnetic tape.

In the illustrated embodiment, series-connected compressors 52 and 54 are used to achieve a higher compression ratio, and series-connected expanders 56, 58, respectively. The invention is equally applicable to single-stage compressors.

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n 76456 in two-way compressors and expanders. Each compressor has a main path 10 which includes a connecting circuit 12 which sums the output of the main path additional path N1, N2, the input of which additional path is connected to the input of the corresponding main path. The expanders have main roads 14 and connecting circuits 16, which subtract from the main road signal the additional path N2, the output of the additional path being connected to the output of the corresponding main road.

Compressor and expander combinations of this type are known per se and will therefore not be explained in detail. However, there are two main forms for the supplementary road or N2. Another alternative (Figures 7 and 8 of U.S. Patent 3,846,719) consists of a filter followed by a controlled limiter adapted to provide a progressive limitation as the signal level increases by means of a rectified and filtered control signal. Another alternative (U.S. Patent No. Re 28,426) is a sliding band high pass filter, the pass band of which is progressively limited by a control signal to exclude large signal components from the filter output, which filter is preferably in series with a fixed high pass filter. The cut-off filters for the slip-band filters are preferably at about 375 Hz at rest, but the high-pass band narrows progressively under the influence of the control signal.

The first and second compressors 52 and 54 each have anti-saturation networks 74, 76 for high frequencies and / or Diene frequencies (and / or any other specified frequency or frequency range) that affect the main signal components. On the playback side of the recorder T, the first and second expanders 56 and 58 have complementary networks that affect the main signal components. Alternatively, the compensation can only be implemented in the second compressor and only in the corresponding expander. The compensation may, for example, consist of a slight decrease in the high frequencies in the encoder (compressor), which increases fc '76456 12 at very high frequencies, e.g. above 10 kHz, and a corresponding emphasis in the decoder (expander).

Since the reduction of the high frequency overload of the channel is focused only on the main signal components, the low level components of the side channels are not affected, and thus the reduction of the overload does not significantly affect the noise attenuation. Reducing channel overload can thus extend down to the center frequency range.

The anti-saturation networks of encoders 74, 76 and the complementary networks of decoders 78, 80 are located on the main signal paths. This arrangement reduces the noise attenuation effect very little because at low levels most of the signal is provided by the noise attenuation side chain. Therefore, it is possible to use anti-saturation networks which together provide, for example, a gradual decrease in high frequencies of 3-6 dB / octave (and a corresponding emphasis in the decoder) extending quite low in frequency, e.g. 2-3 kHz, to correct saturation phenomena without significant noise reduction. . As an example, the frequency response at the level of 0 dB in Fig. 1 shows that the decrease in high frequency response due to saturation starts at about 2 kHz. Because anti-saturation networks are in the main channels, only the signals that cause saturation are affected. As shown in Figure 1, the low-level signals do not cause significant tape saturation.

If desired, half of the 3-6 dB / octave decrease and half of the complementary gain in each of the networks 78 and 80 can be provided in each of the networks 74 and 76. If the entire response decrease and emphasis is placed in one network 76 and 78, the compression or expansion ratio of the compressor or expander increases in the level and frequency range associated with the compressor or expander. However, in a consumer-level system, the use of a single network is sufficient.

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Claims (3)

A circuit device for changing the dynamic range of signals measured to or from a recording or transmission medium, i.e. compressor for limiting the dynamic range or expander for expanding the dynamic range, whereby a compressor and an expander in common use before and after the recording or transmission medium, respectively, form a noise reduction system, whereby the recording or transmission medium can be subjected to a saturation or overload condition within the frequency range at which the circuit device changes the dynamic range, which circuit comprises at least one circuit with a main signal path which is linear with respect to the dynamic range, and an additional path having its input connected to the main path input or output and its output with a combination circuit which further path through the combination circuit outputs a signal which at least within an upper part of the frequency band through the combination circuit in the compressor raises the main path signal and in the expander counteracts the main path signal, but which is limited so that, within the upper part of the input dynamics range, it is less than the main railway gna, characterized in that it comprises frequency dependent circuit means (F? 74, 76 or F '; 78, 80), which is connected AS AS only to the main path (xy; 10 or yz; 14), and which causes in the compressor (52, 54) within a portion of the frequency band a lowering of the frequency curve, whereby saturation or overloading of recording - or the transmission medium is avoided, which in the expander (56, 58) causes an increase in the frequency curve within this part of the frequency band, the parts of the frequency band being identical in the compressor (52, 54) and in the expander (56, 58). they were used together in a noise reduction system. 2. A circuit device according to claim 1, wherein the signals are within the audio frequency band and the recording or transmission medium is subject to possible saturation or overload effect from and at a fixed frequency in the upper