NL192652C - Circuit member for reducing medium overload effects in signal recording and signal transmission systems. - Google Patents

Description

1 192652

Circuit member for reducing medium overload effects in signal recording and signal transmission systems

The present invention relates to a circuit member for changing the dynamics range of signals applied to or taken from a recording or transmission medium, in particular a compressor for decreasing the dynamics range or an expander for increasing the dynamics range, in which in each case a compressor and expander in common application before or after the recording or transmission medium respectively form a noise suppression system, the recording or transmission medium being exposed to a saturation or overload effect within the frequency range, because the circuit member changes the dynamics range , each consisting of at least one circuit with a main signal path, which is linear with respect to the dynamics range, a combination circuit in the main road, and a further path whose input is connected to the input or output of the main road and whose output is connected to the co combination circuit is connected, the further road providing a signal which increases the main road signal, at least in the upper part of the frequency band, by means of the combination circuit in the compressor and counteracts the main road signal in the expander, which is however limited, that the signal from the further road in the upper range of the dynamic input range is limited to a value smaller than the main road signal.

The invention is particularly useful for handling audio signals, but can also be applied to other signals, for example video signals.

The upper part of the frequency band typically extends upwards from a value of several hundred Hertz, for example 300 to 400 Hz, for audio applications, although a higher value can also be taken. In the upper part of the input dynamics range, for example from -10 dB to +10 dB with respect to a reference level, the signal from further away is small with respect to the main road signal.

Circuits of the type described in the opening paragraph are well known and widely used, and are disclosed, for example, in U.S. Pat. Nos. 3,846,719 and 3,903,485. They are referred to as two-way compressors and two-way expanders (dual path). The signal from the further road increases the main road signal in the compressor, but counteracts the main road signal in the expander. A type I configuration (e.g., according to U.S. Patent 3,846,719) is commonly used for audio applications, and a type II configuration (e.g., according to U.S. Patent 3,903,485) is commonly used for video applications.

In a recording or transmission system, compressors and expanders are normally used together (a compander system) to achieve noise reduction; the signal is compressed for transmission or recording and expanded upon reception or playback from the transmission channel. However, compressors can also only be used to reduce the dynamic range, for example to take into account the capacity of a transmission channel, without a subsequent expansion, when the compressed signal is sufficient for the end purpose. Additionally, compressors are used on their own with certain products, especially audio products, the intent of which is to transmit or record 40 uncompressed broadcast or pre-recorded signals.

Expanders by themselves are used with certain products, especially audio products, the purpose of which is only to receive or play back already compressed broadcast or recorded signals. In certain products, particularly audio recording and audio playback products, a single device is often constructed to switch to operate as a compressor to record 45 signal, or as an expander to play compressed broadcast or recorded signals.

Compressors and expanders, respectively compander systems, which operate independently of frequency are known, for example, from the German Offenlegungsschrift 2,803,751.

In contrast, the invention relates to compressors and expanders which, in addition to providing compression or expansion, also effect level-dependent compensation. With magnetic recording the need for such compensation arises due to the tendency of a magnetic tape to be output or to saturate, especially at high frequencies. Several proposals have been made for obtaining the compensation, several of which have been discussed in Rundfunkttechn. Mitteilungen, Volume 22 (1978), H.2, pages 63 to 74. A need 55 for compensation may also arise with other recording or transmission media sensitive to high level saturation or overload effects at certain frequencies. The portion of the frequency band affected by the occurrence of saturation is usually located at the very top of the band - that is, 192652 2 that its upper limit is that frequency, which is considered the upper end of the audio or video frequency band in each given practical case. As an upper limit, in the case of audio, for example, 15 kHz or 20 kHz can be taken and in the case of video, for example, 4 MHz or 6 MHz.

It is possible to provide a high frequency break point circuit behind the compressor and a compensating boost circuit for the expander, however this does not provide level dependent compensation. An alternative is to place the breakpoint circuit in front of the compressor and the boost circuit after the expander. A drawback of each of these techniques is that signals at all levels are subjected to the same kinking effect (and subsequent increase) so that there is a significant decrease in the noise reduction achieved.

10 In the article in Rundfunktechn. Mitteilungen is suggested that the loss of noise reduction can be allowed if the noise reduction without such techniques is up to 20 dB. This is only partially true. In practice, particularly with regard to some compact cassette tapes, the tape saturation effects extend down to frequencies of the order of 2 kHz and the change in compensation required to account for this will lead to a significant increase in the audible noise.

Attempts have been made to overcome this problem by making the response of the kink and increment circuits level dependent (see U.S. Pat. No. 4,072,914). In the case of the break point switching, the break point is steeper at higher levels than at lower levels. For the boost circuit, the boost characteristic is also steeper at higher levels than at lower levels. A drawback of this approach is the necessary increase in circuit complexity.

The mentioned article in Rundfunktechn. Mitteilungen also proposes to place a high-frequency boost circuit for the control circuit of both the compressor and the expander, but again it is noted that the resulting drop is only observable in the mid-level signal range, while it is desirable to effect mainly at higher levels to obtain. In video recording systems, high frequency saturation problems can be caused by the recording enhancement used. A similar problem exists with FM broadcasting.

A similar problem, which has not yet been discussed in the prior art, is that low-frequency compensation (drop in the compressor and increase in the expander) is also desirable when the compression and expansion are also effective at low frequencies. Low frequency compression and expansion are useful for hum noise reduction and can be obtained using broadband compressors and wideband expanders or circuit compressors and expanders operating independently at low and high frequencies. At low frequencies, the main purpose of compensation is to counteract the effect of the 3180 microseconds (+3 dB at 50 Hz) built-in recording boost built into many magnetic tape recorders, which causes low-frequency tape output problems or tape saturation problems, for example with organ music. For example, the lower audio frequency range affected thereby extends downward from about 100 Hz to the lower end of the audio frequency band, which is typically close to 20 Hz.

The object of the invention is therefore to provide a circuit member of the type described in the preamble, with which a level-dependent compensation in the frequency range of interest, 40 or the relevant frequency ranges can be obtained, and which moreover is of simple construction. .

To this end, the circuit member is characterized by a frequency-dependent circuit which is connected only in the main road and effects a reduction in the frequency path in the compressor in that part of the frequency band in which the recording or transmission medium is exposed to the saturation 45 or overload effect, and causes an increase in the frequency path in this part of the frequency band in the expander, the parts of the frequency band and the compressor and the expander being identical in common in a noise suppression system.

The circuit member according to the invention takes advantage of the fact that a circuit member of the type described in the preamble has a frequency-dependent signal path which dominates at low levels 50, so that the influence of the frequency path desired in the region with high levels can be obtained. carried out in the linear main road with respect to dynamics and thereby becomes ineffective at low levels.

The invention is further elucidated hereinafter on the basis of exemplary embodiments with reference to 55 the accompanying drawings. Herein: figure 1 shows a representative response curve of a cassette tape recorder display device, figure 2 an explanatory characteristic curve, 3 192652 figure 3 a block diagram of an embodiment of the invention in the context or type I compression members and expansion members, figure 4 is a block diagram of the invention in the context of type II compression members and expansion members; FIG. 5 is a block diagram of an embodiment of the invention in the context of a two-stage compression expansion system for magnetic tape recording.

The problem of high-frequency saturation is common to all magnetic tape and optical film recorders. It can also occur in highlighting registration and transmission systems of many 10 types including FM broadcasting systems. Although most serious with low speed tape recorders, especially those using low cost tape formulas, it affects the ability to record at high levels even with professional magnetic tape and high quality optical film recorders. That is, the recording medium does not accurately record the applied high level high frequency signals. The main audible effect is intermodulation distortion and a reduction in the high-frequency content of the registration. However, in the context of the invention, such saturation is particularly undesirable because certain combinations of signal levels and frequencies disrupt the complementarity of the expander display. Thus, the expansion member will in some way inaccurately decode the displayed signal 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 parasitic modulation of mid-frequency signals.

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

Figure 1 shows the response of a compact cassette recording display which is well above the average. At a recording level of -20 dB, the response is practically flat to 20 kHz. At higher recording levels, the effects of high frequency band saturation become apparent, causing a significant high frequency drop at a recording level of 0 dB. Typical cassette units exhibit significantly greater saturation effects.

The most direct way to reduce the above-mentioned saturation effects is to change the recording-compensation in such a way that the tape is not driven so hard in the frequency range in which the saturation causes problems. The display compensation is then changed in a complementary manner. Unfortunately, with cassette recording, saturation effects can extend down to frequencies of 2 kHz or so as Figure 1 suggests. Thus, the required change in compensation would result in a significant increase in the audible noise.

The circuit described below effectively enables treatment to reduce high-frequency saturation without sacrificing any significant noise reduction in the treated frequency range. The same technique can be used to reduce low frequency band distortion using a full range noise reduction system, in particular it is noted that in a two way compression or expander the output of the chain is usually supplied at very low 40 signal levels by the further or noise reduction path. in the case of one such device providing a dynamic action of 10 dB, the contributions of the main and noise reduction paths are in a ratio of 1 and 2, 16, respectively. At high signal levels, the roles of the two roads are reversed; the main road provides the predominant signal component and the further road contribution is negligible.

45 The saturation or distortion reduction effect is based on the above observations. Namely, a compensation device providing the desired reduction in high or low frequency drive is placed in the main road of the compression member. As shown in Figure 2, which indicates the operation of a high-frequency distortion reduction circuit, at high signal levels, essentially the full effect of the compensation is obtained with a consequent reduction in the high-frequency saturation. At low levels, however, the compensation effect is reduced as the contribution of the noise reduction path becomes important. For example, if the anti-saturation network delivers as much as a 12 dB attenuation at a certain frequency, without considering 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 a high level recording drive is obtained 55 for a loss in noise reduction effect of 2.4 dB. Such a high degree of distortion reduction would only be necessary at the highest frequencies, for example at 15 kHz. At lower frequencies, the desired reduction in saturation would be less with a correspondingly reduced loss of noise reduction effect. At 192652 4 low frequencies, the main goal would be to counteract the effect of the 3180 psec (+3 dB at 50 Hz) recording boost built into many tape recorders that cause low frequency tape saturation problems, for example with organ music, as already mentioned.

In audio applications, the requirements of a suitable anti-saturation network can be determined as follows 5. The maximum usable output level of the band, optical film, FM channel, or otherwise, is determined at a low to mid frequency and at high 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 normal music and speech sounds. Such a plot is reported in Journal of the Audio Engineering Society, vol. 21, no. 5, June 1973, p. 357-362.

The difference between the two curves is an indication of the required high level anti-saturation characteristics. Once such characteristics have been determined and performed, the resulting compression characteristic curves must be checked to determine whether the anti-saturation network has caused an increase in the compression ratio at each frequency or level. If so, appropriate changes can be made to the limiting characteristics of the noise reduction path and / or, where several arrays of connected compression members (or expansions) are used, the anti-saturation characteristic may be distributed among the devices.

Similar considerations apply in the case of video devices. Video recorders frequently use high-frequency highlighting, which can result in FM overmodulation problems. The overmodulation tendency of any given recording system can be measured and an appropriate type and degree of anti-saturation applied. When compression members and expansion members of the above-mentioned Type I and Type II embodiments are used, there will be a further overmodulation tendency provided by small residual overshoots (a few percent) of the compression members. These excesses can be compensated for with the invention.

25 (Audio systems also tend to compensate for residue overshoot).

While it would be possible to operate with only the anti-saturation network in the encoder, it is preferred to make a complementary correction on the display side so that a flat frequency response is maintained at all levels. The following analysis shows the type of correction needed.

In Figure 3, which shows the type I compressors and two-way expansion device construction, it is assumed that the input signal to the compression device x is the signal in the information channel y the output signal of the expansion device z. That F , and F2 are the transfer characteristics of the further away from the compression member and the expansion member, respectively, and FAS are the transfer characteristic of the anti-saturation network. Let the F'as be the required compensation characteristic in the decoder. y = (Fas + pi) x

and z = yF ^ - zF2F'AS

F'aSFaS + F, F'AS dusz = A1 * SFF, 1 ASX 1 + * ~ 2Γ AS

40 Inspection shows that z = x when F, = F2 and F'as = έγ ~ · Γ

A similar derivation applies to the type II construction shown in Figure 4: y = Fasx + F ^ Asy and z = F '^ y - F2y 45 so z = —— x ΈΓ— F1

FaS 1 Inspection shows that z = x when F, = F2 and F'as = έ ~ · i axis

The above shows not only that the two noise reduction networks must be identical as known from applicant's previous submissions, but that the anti-saturation compensation network in the decoder 50 must have an inverse characteristic to that of the network used in the encoder. Simple corrections can be obtained passively, such as by RC combinations, while more complex corrections can apply feedback techniques, in particular to obtain the inverse characteristics required in the decoder.

In Figure 5, a block diagram of the invention is shown in a two-stage type I construction 55 primarily for use in magnetic tape recording and reproduction.

The indicated embodiment uses cascaded compression members 52 and 54 to provide one

Claims (3)

192652 to obtain enlarged compression and correspondingly cascaded expansion members 56, 58. The invention also applies to single-stage compression members and two-way expansion members, each compression member having a major road 10 with a combination chain 12 which the main road adds the exit of the further road N, N2, the entrance of which is connected to the entrance of the corresponding main road. The expansions have major roads 14 and combination circuits 16 that subtract from the main road signal the output of the further road N2, N1 whose input is connected to the corresponding main road output. The compression and expansion structures are known per se and therefore will not be described in detail. However, there are two main forms of the further away N1 or N2. An alternative (Figures 10, 7 and 8 of U.S. Pat. No. 3,846,719) is a filter followed by a controlled limiter, ensuring that it limits progressively as the signal level rises by a rectified and smoothed control signal. Another alternative (U.S. Patent Re 28,426) is a high-pass sliding band filter whose passband is progressively narrowed by the control signal to exclude large signal components from the filter output and which is preferably in series with a fixed high-pass filter . Advantageously, breakpoint frequency values for the sliding band filters are about 375 Hz in the setting state but progressively narrow in high pass in response to the control signal. The first and second compression members 52 and 54 each have a high-frequency and / or low-frequency (and / or any other particular frequency or frequency range) anti-saturation network 74, 76 that affect the 20 main signal components. On the playback side of the tape recorder T, the first and second expanders 56 and 58 have complementary networks that affect the main signal components. Alternatively, such compensation may be provided in only one of the compression members and in only the corresponding expansion member. For example, the compensation may take the form of a moderate high-frequency drop in the encoder (compression member), increasing at very high frequencies, for example above 10 kHz, and complementary boost in the decoder (expansion member). Since the high-frequency channel overload reduction is only applied to the main signal components, the low-level components in the side channels are not affected, and so the overload reduction does not greatly affect the noise reduction. Accordingly, the channel overload reduction can extend down to the mid-frequency range. The anti-saturation networks in the encoders 74, 76 and the complementary networks in the decoders 78, 80 are located in the main signal paths. This arrangement results in very small losses of the noise reduction effect that most of the signal is supplied at low levels by the noise reduction side chain. Therefore, it is possible to provide anti-saturation networks which, taken together, have a gradual high-frequency drop, for example 3-6 dB / octave (and complementary boost in the decoder) that extends far down in frequency, for example, up to 2-3 kHz. , to deal with saturation effects without a high degree of noise reduction loss. For example, the frequency response at 0 dB in Figure 1 shows a drop in the high frequency response from saturation starting at about 2 kHz. Since the anti-saturation networks are located in the main channels 40 carrying the high-level signals, only those signals that cause the saturation are affected. As shown in Figure 1, scan level signals do not cause significant band saturation. Optionally, half of the 3-6 dB / octave drop in each network 74 and 76 is provided and half of the complementary boost in each network 78 and 80 is provided. When all the drop and chase is arranged in some networks 76 and 78, the associated compression member or expansion member will have an increased compression or expansion ratio at a certain level and a frequency range associated with this compression member or expansion member. In a consumer system, however, the use of a single network is sufficient. 50 Conclusions Switching means for changing the dynamics range of signals applied to or taken from a recording or transmission medium, in particular a compressor for decreasing the dynamics range or an expander for increasing the dynamics range, each comprising a 55 compressor and in common application, before or after the recording or transmission medium respectively, an expander forms a noise suppression system, the recording or transmission medium being exposed to a saturation or overload effect within the frequency range, by the 192652 6 switching means changing the dynamics range, each consisting of at least one circuit with a main signal path, which is linear with respect to the dynamics range, a combination circuit in the main road, and a further road whose input is connected to the input or output of the main road and whose output is connected to the combination circuit, wherein the further road provides a signal 5 which increases the main road signal, at least in the upper part of the frequency band, by means of the combination circuit in the compressor and counteracts the main road signal in the expander, which is however limited such that the signal of the further road in the upper range of the dynamic input range is limited to a value smaller than the main road signal, characterized by a frequency-dependent circuit (FAS; 74, 76 and F'AS, respectively; 78, 80), which is connected only in the main road (xy; 10, respectively yz; 14) and in the compressor (52, 54) effects a decrease of the frequency path in that part of the frequency band, in which the recording or transmission medium is exposed to the saturation or overload effect, and in the expander (56, 58) causes an increase in the frequency path in this part of the frequency band, the parts of the frequency band and the compressor (52, 54) and the expander ( 56, 58) are common in common use in a noise suppression system. The circuit member of claim 1, wherein the signals are in the audio frequency band and the recording or transmission medium from a frequency in the upper part of the frequency band is exposed to the saturation or overload effect, the upper part of the frequency band extending from about 300 Hz extends upwards, characterized in that the frequency-dependent circuit (FAS; 74, 76) in the compressor (52, 54) entails a decrease in the frequency path in the range of the frequency, from which the saturation or overload effect, respectively, in the expander (56, 58) the frequency-dependent circuit (F '^; 78, 80) causes an increase in the frequency response in the same range. Circuit arrangement according to claim 2, which also operates at low frequencies, and wherein the recording or transmission medium is exposed to the saturation or overload effect from a frequency in the lower part of the frequency band, the lower part of the frequency band extending from about Extends downward by 100 Hz, characterized in that the frequency-dependent circuit (FAS; 74, 76) in the compressor also causes a decrease of the frequency downward in the range from the frequency from which the saturation or overload effect begins , respectively, in the expander (56, 58) the frequency dependent circuit (F'AS, 78, 80) also effects an increase in the frequency response in the same range. Hereby 4 sheets drawing