JPH0245385B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates generally to recording and transmitting systems;
More particularly, it relates to a circuit arrangement for changing the dynamic range of a signal processed by such a system, ie a compressor for compressing the dynamic range. Although the invention has particular application to processing audio signals, it can also be applied to other signals, including video signals.

Compressors and expanders are usually used together to reduce noise (compander systems). The signal is compressed before transmission or recording and decompressed after reception or playback from the transmission channel. However, the compressor can be used alone to narrow the dynamic range, for example to match the capacity of the transmission channel, without subsequent expansion if the compressed signal is suitable for the final purpose. Additionally, compressors are used exclusively in certain products, especially audio products intended only for transmitting or recording compressed broadcast or prerecorded signals. Expanders are also used exclusively in certain products, especially audio products intended only to receive or play back already compressed broadcast signals or prerecorded signals. Additionally, in some products, particularly audio recording and playback devices, a single device can function not only as a compressor for recording a signal, but also as an expander for playing back a compressed signal (or a previously recorded signal). They are often configured so that they can be used by switching.

More specifically, the present invention relates to a compressor that performs level-dependent equalization in addition to compression. There is an increasing demand for such an equalization effect in magnetic recording. This is because magnetic tape tends to become saturated, especially at high frequencies. Various proposals have already been made to ensure equalization, some of which are listed in the annual report of the Broadcasting Technology Report (Rundfunktechn. Mitteilungen), Vol. 22, No. 2 (1978), p. 63-74. published on the page. There is also an increasing demand for equalization for other recording or transmission media that are susceptible to high level saturation or overload effects at certain frequencies.

One possibility (but not capable of providing level-dependent equalization) is to provide a high frequency roll-off circuit after the compressor. An alternative is to provide a roll-off circuit before the compressor. The disadvantage of such a technique is that the signal at all levels is subject to the same roll-off effect, which greatly reduces the amount of noise reduction that can be obtained. It has been suggested that if the amount of noise reduction is about 20 decibels without using such technology, the loss in noise reduction can be tolerated (paper in ``Broadcast Technology Report''). However, this is only partially correct. In fact, when we investigated some compact cassette tapes, we found that the tape saturation effect was down to about 2KHz, and if we changed the equalization to account for this saturation effect, the audible noise would increase significantly. do.

Attempts have been made to overcome this problem by configuring roll-off circuits to operate in a level-dependent manner (see US Pat. No. 4,072,914). The roll-off is more rapid at high levels than at low levels. One of the drawbacks of this method
First, the configuration described above results in significant circuit complexity.

A proposal to install a high-frequency amplification circuit in front of the compressor control circuit was also published in the ``Broadcasting Technology Report'', but the resulting roll-off is preferably effective mainly at a high level, but the effect is It has been pointed out that this can only be recognized in the range of intermediate level signals. On the other hand, the pre-emphasis used in video recording systems can lead to high frequency saturation problems. A similar problem can be seen with FM broadcasting.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a compressor which adequately solves the above-mentioned problem, ie a compressor for effective and simple problem solving.

A related issue, although not discussed in the prior art, is that equalization (roll-off) at low frequencies is desirable if compression is also effective in the low frequency range. Compression in the low frequency range is effective in reducing hum, and this can be accomplished using a wideband compressor or a compressor with circuitry operating separately at low and high frequencies. The main purpose of the equalization action in the low frequency range is, for example, the 3180usec (at 50Hz) built into many magnetic tape recording devices which causes tape saturation problems in the low frequency range as in the case of organ music. +3dB) to cancel the effect of the recording boost.

Another object of the invention is to provide a device which, if required, can additionally carry out the required equalization even in the low frequency range.

Yet another object of the invention is to increase the headroom, and therefore the dynamic range, of a recording medium that is subject to saturation.

The starting point of the present invention is that a linear main signal path with respect to the dynamic range, a coupling circuit on the main signal path,
A compressor having a separate path whose input is connected to the input or output of the main signal path and whose output is connected to a coupling circuit, the separate path amplifying the main path signal passing through the coupling circuit at least in the upper frequency band. but,
At the top of the input dynamic range, the special signal is to emit a signal that is limited to be smaller than the main path signal.

The upper part of the frequency band typically lies above values in the hundreds of hertz, eg, 300-400Hz, although higher values may be used. For example, at the top of the input's dynamic range ranging from -10 dB to +10 dB with respect to the reference level, the off-path signal is smaller than the main path signal.

Such compressors are well known;
Widely used. Some examples of compressors are described in US Pat. No. 3,846,719, US Pat. No. 3,903,485, and US Pat. Such a compressor is called a dual path compressor. The off-path signal amplifies the main path signal passing through the compressor. configurations of the type (e.g., disclosed in U.S. Pat. No. 3,046,719) are commonly used in audio equipment, while
The type (eg, as disclosed in US Pat. No. 3,903,485) is commonly used in video equipment.

The compressor of the present invention is characterized by a frequency-dependent circuit that is connected only to the main path and serves to reduce the frequency response in the frequency band portions affected by saturation.

The frequency-dependent circuit provided in the main path is connected between the point where the input of the off-path is connected and the coupling circuit in both types of compressors.

Said part of the frequency band affected by saturation is usually the highest frequency band part, which in practical use is considered to be the upper end of the audio frequency band. in particular,
For audio, it is 15KHz, 20KHz, etc., and for video, it is in the range of 4 to 6KHz.

In an audio device according to the invention in which the compressor is adapted to operate at a low frequency, the frequency-dependent circuit typically operates from about 100 Hz onwards.
Reduces the frequency response in the lower region of the audio frequency band at the lower end of the 20Hz range.

The present invention will now be described in detail with reference to the accompanying drawings, which illustrate the invention.

The problem of high frequency saturation is common to all magnetic tape recording devices and optical film recording devices. This problem is also present in many types of pre-emphasized recording and transmitting systems, including FM broadcast systems. Although this problem is very severe for low-speed tape recording devices, especially those that use low-cost tape formats,
Even high quality professional magnetic tape and optical film recorders have an undesirable effect on their ability to record at relatively high levels. That is, the recording medium cannot accurately record the high level, high frequency signals applied to the recording medium. The main audible effects are intermodulation distortion and a reduction in the proportion of high frequencies included in the recording.
However, such high frequency saturation problems are not at all desirable in connection with the present invention. This is because, for certain combinations of signal levels and frequencies, when such high frequency saturation occurs, reproduction complementarity is significantly impaired. Therefore, depending on the degree of high frequency saturation, decoding of the reproduced signal becomes inaccurate to some extent. The main audible effect is usually an exaggeration of high frequency losses, but the audible effects can also include false modulation of intermediate frequency signals.

Although the illustrated embodiment is primarily related to a cassette tape recording and reproducing apparatus, the present invention may also be applied to high-end magnetic tape recording and transmitting systems and optical tape recording and transmitting systems for professional use.

FIG. 1 shows the response of a commonly used cassette type recording/playback device. At a recording level of -20dB, the response is almost flat up to 20KHz. At recording levels higher than this, the effects of tape's high frequency saturation become apparent, and at recording levels of 0 dB there is a very high frequency roll-off. Conventional cassette devices exhibit saturation effects that are significantly greater than this.

The most accurate way to reduce the saturation effects described above is to vary the recording equalization in such a way that it does not overexcite the tape in the frequency range where high frequency saturation is a serious problem. Then, the reproduction equalization is modified in a complementary manner. Unfortunately, in the case of cassette recording, the 2K
The saturation effect may extend to frequencies around Hz. Changing the equalization state as required results in a significant increase in audible noise.

Using the circuits described below, it is possible to perform level-dependent equalization that reduces high frequency effects without significantly sacrificing noise reduction in the processing frequency domain. The same technique can be used to reduce low frequency tape distortion if a full area noise reduction system is employed. In particular, in double-path compressor circuits, in most cases a separate noise reduction path is provided to ensure a very low signal level circuit output. For such equipment with 10dB dynamic operation,
The contribution ratio of the main path and the noise reduction path is 1:2.16. Furthermore, if NdB compression effect is desired,
If the contribution of the main route is 1, the contribution of the alternate route is expressed by the following equation.

Antilog of log(N/20) - 1 At high signal levels, the roles of both paths are reversed, with the main path providing the main signal and the contribution of the secondary path being negligible. That is, the frequency response of the dual path compressor is substantially determined by the frequency response of any frequency dependent circuitry in the main path at high signal levels.

The saturation reduction effect or distortion reduction effect is based on the above-mentioned observation. An equalizer is arranged in the main path of the compressor which provides the desired saturation reduction or distortion reduction during high-frequency excitation or low-frequency excitation. As shown in Figure 2, which illustrates the effect of using a high frequency distortion reduction circuit at high signal levels, it is possible to obtain an almost perfect equalization effect, thereby reducing high frequency saturation. . However, at low signal levels, the equalization effect is reduced. This is because the contribution of the noise reduction path increases. For example, if the anti-saturation network provides attenuation on the order of 12 dB at a particular frequency, ignoring phase considerations, the low signal level effect can be expressed as:

0.25 x 1 + 2.16 = 2.41 = 7.6 dB, that is, a 12 dB reduction in high-level recording excitation was obtained for a 2.4 dB loss in noise reduction effect. Such a high degree of distortion reduction can be achieved by e.g.
It is only needed for very high frequencies such as 15KHz. At lower frequencies, the required saturation reduction is less than that described above, and the loss of noise reduction effect is correspondingly reduced. At low frequencies, the main problem is that the 3180usec (+30dB at 50Hz) recording boost built into many tape recorders causes low frequency saturation problems, for example in the case of organ music as mentioned above. This will cancel out the effect.

In the case of audio equipment, the requirements for a suitable anti-saturation network can be determined as follows. The maximum usable output level of tapes, optical films, FM channels, etc. is determined in the range from low frequencies to intermediate frequencies, and the maximum usable output is also determined in a relatively high frequency range up to the highest frequency. The resulting maximum output level curve can be compared to a trajectory plotting the energy distribution as a function of frequency that matches commonly heard music or speech sounds. An example of such a plot is from Journal of the Audio Engineering Society, Volume 21, No. 5, 357, June 1973.
This is published in the inventor's paper published on page 362. The difference between the two curves is representative of the required high level anti-saturation characteristics. Once such characteristics are determined, the resulting compression characteristic curve must be checked to determine whether the anti-saturation network results in an increase in compression ratio at any frequency or level. If so, the limiting characteristics of the noise reduction path should be changed accordingly, or if several sets of compressors connected in series are used, the anti-saturation characteristics should be distributed among the devices.

A similar idea can be applied in the case of video equipment. Video recording devices often perform high frequency pre-emphasis, which often results in FM overmodulation problems.
If a compressor of the type disclosed in US Pat. However, such overshoots can be compensated for according to the invention (in the case of audio systems, too, there is a tendency to try to compensate for residual overshoots). However, it is preferable to perform complementary correction on the reproduction side. This allows a flat frequency response to be maintained at all levels. The following analysis illustrates the type of correction required.

The left halves of FIGS. 3 and 4 respectively show a mold and a mold compressor according to the invention in contrast to the expander in the right half. In the figure, F1 indicates the transfer characteristic of a known level-dependent compressor circuit provided in a separate path, FAS indicates a transfer characteristic of a frequency-dependent saturation prevention circuit according to the present invention, that is, a roll-off circuit provided in the main path, and + indicates a coupling circuit that couples both paths.

With the above configuration, in this device, the low level signal x
However, since the main path signal becomes dominant only for high-level signals, the saturation frequency band is attenuated according to the frequency setting conditions, and the actual response of the compressor is Characteristics deteriorate.

Referring to Figure 3, which illustrates the configuration of a type of dual-path compressor and expander, the input signal to the compressor is denoted by x, the signal passing through the information channel is denoted by y, and the output signal of the expander is denoted by z. represent F ₁ and F ₂ represent the separate transfer characteristics of the compressor and expander, respectively;
Let F _AS represent the transfer characteristic of the anti-saturation network. Furthermore, let F′ _AS represent the required compensation characteristic of the decoder.

_{y = ( F AS + F 1 ) _ _ _ _} And if F′ _AS =1/F _AS , then z=x.

Similar derivations can be applied to configurations of the type illustrated in FIG.

y=F _AS ×F ₁ F _AS y and z=F′ _AS y−F ₂ y Therefore, z=(F′ _AS −F ₂ )/1/F _AS −F ₁ × If F ₁ =F ₂ And if F′ _AS =1/F′ _AS , then z=x.

The above equations not only indicate that the two noise reduction networks must be identical, as is clear from the applicant's prior invention, but also indicate that the decoder's anti-saturation compensation network is used in the encoder. This indicates that the circuit must have the opposite characteristics to the characteristics of the existing circuit network. Simple corrections can be made using a combination of resistors and capacitors, but for more complex corrections, feedback techniques can be employed, especially to ensure the inverse characteristics required of the decoder.

Now, referring to the upper part of FIG. 5, there is shown a block diagram of a two-stage type configuration mainly used for magnetic tape recording and playback.

In the illustration, cascaded compressors 52 and 54 are used to ensure increased compression. The invention can also be applied to single-pass compressors. Each compressa is
Coupling circuit 1 that adds the outputs of separate routes N ₁ and N ₂ to the main route
2, such that the inputs of said separate channels N ₁ and N ₂ are connected to the inputs of the corresponding main channels.

Since the configuration of such a compressor and expander is well known, a detailed explanation thereof will be omitted. However, two main types are adopted for the separate route _N1 or _N2 . One of them is (U.S. Patent No.
No. 3,846,719 (as shown in Figures 7 and 8), the rectified and smoothed control signal provides active limiting as the signal level increases. A controlled limiter is provided after the filter.
The other is a sliding band type high-pass filter (as disclosed in U.S. Reissue Patent No. 28426), whose passband is
The output of the filter, which is preferably connected in series with a fixed high-pass filter, is gradually narrowed by a control signal to remove large signal components from the output of the filter.
The advantageous corner frequency value for a sliding band filter is about 375Hz at rest, but
In response to the control signal, the high corner frequency path becomes progressively narrower.

The first and second compressors 52 and 54 include high frequency and/or low frequency (and/or other specific frequencies or frequency ranges) anti-saturation encoders 74 and 76, with the anti-saturation circuit 74 and 76, the main signal component is affected. Alternatively, such compensation may be performed for only one of the compressors. Compensation can be provided, for example, in the form of a mild high frequency roll-off of the encoder (eg, increasing at frequencies above 10 KHz).

Since overload reduction in high frequency channels is applied only to the main signal components, the low level components in the side channels are not affected, so overload reduction does not significantly affect noise reduction. . As a result, channel overload reduction can be reduced to the intermediate frequency range.

Anti-saturation circuitry provided in encoders 74 and 76 is provided in the main signal path.
As a result of this arrangement, the loss of noise reduction effect can be kept very low. This is because a low level signal is obtained through the chain on the noise reduction side. Therefore, in order to ensure a saturation effect without high noise reduction losses, it has a gentle high frequency roll-off of, for example, 3-6 dB/octave, and a frequency of 2-3 KHz.
It is possible to provide an anti-saturation network with a significantly lower frequency. For example, the frequency response at zero decibels shown in Figure 1 is:
This shows that the high frequency response begins to decrease from about 2KHz, where saturation begins. Since the anti-saturation network is placed in the main channel carrying high level signals, only the signals that cause saturation are affected.

In some cases, a half rolloff of 3 to 6 dB/octave may be applied to each of encoders 74 and 76. If all of the roll-off is taken up by encoder 76 alone, the compression ratio of the associated compressor in the particular level and frequency domain associated with the compressor is increased. However, in consumer devices, it is appropriate to use a single network.

[Brief explanation of the drawing]

Fig. 1 shows a typical response curve of a cassette tape recording/playback device, Fig. 2 shows an example of a characteristic curve, and Fig. 3 shows a compressor of the type according to the present invention compared with an expander. Block
FIG. 4 is a block diagram illustrating a compressor of the type according to the invention in comparison with an expander, and FIG. 5 is a block diagram illustrating a two-system compressor. 10, 14... Main path, 12, 16... Coupling circuit, 52, 54... Compressor, 74, 76...
...Encoder.

Claims (1)

[Claims] 1. Includes a main path and a separate path provided in parallel with the main path, which has a level-dependent equalization effect to prevent saturation or overload due to high-level signals in the operating frequency band of the compressor. A compressor, which is a frequency dependent circuit connected only to the main path in order to reduce the frequency response at a part of high frequencies, and the frequency dependent circuit is connected between the connection point of the separate path and the coupling circuit. a main path that is linear with respect to the dynamic range; a separate path that amplifies the main path signal at least in the high frequency portion but supplies a smaller signal than the main path signal for high-level inputs; and the coupling circuit provided in the compressor. 2. A compressor as claimed in claim 1, wherein the frequency dependent circuit arrangement reduces the frequency response in the upper part of the audio frequency band. 3. A compressor as claimed in claim 2, wherein the compressor also operates at low frequencies and wherein the frequency dependent circuit arrangement reduces the frequency response in the lower part of the audio frequency band.