SE447525B - COMPRESSOR, EXPANDER AND NOISE REDUCTIONS INCLUDING A COMPRESSOR AND EXPANDER - Google Patents

Description

10 15 20 30 35 40 f447 525 2 expandern. Alternatively, the damping circuit can be placed before the compression. and the raising circuit after the expander. A disadvantage of all method of this kind is that signals of all levels are exposed for the same attenuation (and subsequent increase), which results in a significant reduction in the noise reduction achieved. In the aforementioned article in "Rundfunktechn. Mitteilungen" states that this impaired noise reduction can be tolerated if the magnitude of the noise reduction play without methods of this kind is as high as 20 dB. This is however only partially correct. In practice, saturation effects extend magnetic tapes, in particular for certain compact cassette tapes, as low as 2 kHz and the correction change required 'race for consideration of this entails a significant increase in audible noise.

Attempts to eliminate this problem have been made by make the frequency response of the attenuation and increase circuits level dependent (U.S. Pat. No. 4,072,914). For the attenuation circuit, the case is faster at high levels than at lower. For the raise circuit, the raise curve is also steeper at high levels than at low. A disadvantage of this solution is the concomitant increase in circuit complexity.

In the above mentioned article there is also a suggestion that place a high frequency boost circuit in front of the control circuit for both the compressor and the expander, but it is also mentioned that the resulting the attenuation is only noticeable within the medium signal range, while it is desirable that the effect be obtained mainly at high levels. In video recording systems, high frequency saturation problems are caused by the frequency dependence used in the recording the level increase. A similar problem occurs with FM radio shipments.

An object of the invention is thus to provide one better solution to the problems discussed above, namely one solution that is both simple and effective. Although not discussed in connection with the prior art, presents a related problem in that low frequency correction (attenuation increase in the compressor and increase in the expander) is desirable if pressure and expansion occur even at low frequencies. Such, at low frequencies companies compression and expansion exist benefit in reducing humming disorders and can be achieved during of broadband compressors and expanders or of compressors and expands with circuits operating independently at low and high frequencies. At low frequencies, the main purpose is 10 15 20 25 30 35 40 3 447 525 with the correction to counteract the effect of the 3180 ps (+ 3 dB at 50 Hz) recording boost built into many tape recorders and which give rise to problems in measuring the band at low frequencies, for example in organ music.

Another object of the invention is to enable it required correction can also be performed at low frequencies if required.

A further object is to increase the upper limit and with the dynamic range of a recording medium exposed to saturation.

The invention is based on a compressor or an expander contained in comprising a main signal path which is linear with respect to the dynamics area, a combination circuit located in the main track, and an external lane, which has its entrance connected to the main course output and its output with the combination circuit, which further path via the combination circuit emits a signal which at least within an upper part of the frequency band raises or counteracts the main band nans signal but which is limited so that it, within the upper part of the input dynamic range, is smaller than the main path signal.

The upper part of the frequency band typically extends from a value of a few hundred Herz, say 300-400 Hz, but also a higher one value can be used. In the upper part of the dynamic range of the signal, for example from -10 dB to +10 dB relative to a reference level, is the signal in the additional path is small compared to the main path signal.

Compressors and expanders of this kind are well known and widely used. intended to use; examples of this are described in U.S. Pat. No. 3,846,719. US-PS 3,903,485 and US-PS Re 28 426. They may be referred to as compressors and double track type expanders. The signal of the additional path the main path signal in the compressor but lowers or counteracts the main path signal in the expander. A configuration called type I (see for example US-PS 3,046,719) is commonly used in audio applications. and a configuration called type II (see for example US-PS 3,903,485) commonly used in video applications.

In the compressor case, according to the invention, it is a frequency dependent circuit connected only in the main circuit and causes a lowering of the frequency curve within the part of the frequency band to which it is exposed saturation.

In the expander case, according to the invention, a frequency dependence circuit connected only in the main circuit and causes an increase of the frequency curve within the part of the frequency band to which it is exposed 10 15 20 25. 30 35 40 447 525 saturation. _ This frequency-dependent circuit located in the main path is connected between the starting point of the additional course and the combination circuit, both in the compressors and the expanders and at type I and type II configurations.

The invention also relates to a complete noise reduction system. with the features specified in the following requirements.

Said portion of the frequency band exposed to saturation is usually the very upper part - ie corresponding to that in each practical case given the upper value, whether this is 15 kHz, 20 kHz or any other value at audio fall or for example 4-6 MHz at the video case.

In a further development of audio of the present invention, the compressor or expander operating even at low frequencies sees, the frequency-dependent circuit causes a fall (compressor) or an increase (expander) in the frequency curve also within the lower one part of the audio frequency band (for example, from about 100 Hz and down to the lowest value of the band, which is typically close to 20 Ilz).

The invention is described in more detail below in the form of some embodiments. and with reference to the accompanying drawing.

Fig. 1 shows a representative frequency response of a cassette tape recorder. lare. Pig. 2 is a characteristic shown for explanatory purposes Fig. 3 is a block diagram of an embodiment of in connection with type I compressors and expanders

Fig. 4 is a block diagram of an embodiment of the invention in in connection with type II compressors and expanders. Pig. 5 is one block diagram of an embodiment of the invention in connection with a double-stage compander for magnetic tape recordings.

The problem of high frequency saturation is common to all recording devices for magnetic tape and for optical recording on Film. The problem can also occur with treble recording and transmission systems of many kinds, including FM radio systems tem. Although the problem is most pronounced with tape recorders working at low speeds, especially when inexpensive tape types are used, it allows for high-level recording even in high-quality lithative, professional magnetic tape recorders and optical recording. Thus, the recording medium does not register correctly applied high-level high-frequency signals. The essential The audible effect of this is intermodulation distortion and a curve scale. the finding 10 20 25 50 35 40 5,447,525 reducing the high frequency content of the recording. In connection with According to the present invention, however, such saturation is particularly undesirable. because at certain combinations of signal levels and frequencies it overrides the expander's display complementarity. Expand will thus decode the playback signal incorrectly in an output stretch depending on the degree of saturation. The most essential audible the effect usually consists of an exaggeration of the high frequency desire in the expander but may also involve false modulation of the frequency signals.

The embodiment shown is considered primarily in the tape with cassette recorder (for recording / playback) even if the invention can also be applied in professional recording to quality tapes, for optical recording on film and for recording and transmission systems in general.

Pig. 1 shows the frequency response of a cassette recorder (for "Compact Cassettes") of a better-than-average type. During a recording level of -20 dB, the frequency response is essentially flat to 20 kHz.

At higher recording levels, the effects of high frequency saturation of the band is obvious and causes a high frequency drop at a recording level of 0 dB. Typical cassette units exhibit significantly greater saturation effects.

The easiest way to reduce the above saturation effect is to change the recording correction in such a way that the tape not driven so hard within the frequency range where saturation results problem. The playback correction is then changed to complementary way. Unfortunately, with cassette recordings, the saturation effects can extend down to frequencies as low as 2 kHz or as is indicated in Fig. 1. The required change of the correction would thus resulting in a noticeable increase in audible noise.

The circuit described below allows effective reduction of high-frequency saturation without the need for significant sacrifice with regard to noise reduction within the treated frequency the area. The same method can be used to reduce low frequency band distortion when an entire frequency range covering hrus reduction systems are used. Note especially that at one with double lanes provided compressor or expander circuit, so the circuit originates output signal at very low signal levels for the most part from it extra or additional path, i.e. the noise reduction path. By a such a device with a dynamic effect of 10 dB amounts to the relative the contributions from the main path and the noise reduction path to 1 resp. 10 15 20 25 30 35 447 525 6 2.16. At high signal levels, the roles of the two paths are reversed; the main path provides the dominant signal component, and the contribution from the second lane is negligible.

The saturation or distortion reduction effect is based on the above observations. A correction circuit that provides the necessary reduction of the high or low frequency drive placed race in the main path of the compressor. As can be seen from Fig. 2, the function of a circuit for reducing the high frequency at high signal levels is obtained with substantially full correction. effect with concomitant reduction of the high frequency saturation.

However, at low signal levels, the correction effect is reduced by as the contribution from the noise reduction path becomes significant. If for example, the anti-saturation network provides as much as 12 dB of attenuation at a certain frequency, is obtained - apart from phase considerations - that the effect at low levels will be 0.25 X 1 + 2.16 = 2.41 = 7.6 dB Thus, a reduction of 12 dB is obtained in the high level recording. drive for a noise reduction loss of 2.4 dB. So powerful distortion reduction is required only at the highest frequencies for example, at 15 kHz. At lower frequencies, the required lower the saturation reduction, with correspondingly lower desire for noise reduction effect. At low frequencies the main thing is the purpose of counteracting the impact of it built into many tape recorders the recording increase of 3180 Ps (+3 dB at S0 Hz) which gives rise to problems of band saturation at low frequencies, e.g. wise in organ music, as already mentioned.

In the case of audio applications, the requirements of an appropriate saturation networks are determined as follows. The maximum useful the output level from the tape, the movie's optical audio recording, the FM channel e.dyl. determined at low and medium frequencies and at higher frequencies up to the highest frequency of interest. The obtained The curve above the maximum output level is compared with a curve of the distribution as a function of the frequency of normal occurrence music and speech. Such an energy distribution curve is found in "Journal of Audio Engineering Society", Vol. 21, no. June 5, 1973, sid. 357-362. The difference between the two curves is an indi- cation on the required anti-saturation properties at high spring. Once these characteristics have been determined, the the compression curves are checked for the purpose of determining 10 15 20 25 7 447 525 whether the anti-attenuation network has given rise to an increase in the pressure ratio at any frequency or level. If this is In this case, appropriate changes can be made to the noise reduction path characteristic boundary data and / or can, if several cascading coupled compressors (or expanders) are used, anti-saturation the curve is distributed over the devices.

Similar considerations apply to video devices. In video tape recorders are often used high frequency increase, which can give rise to to problems with FM overmodulation. The overmodulation tendency of a particular recording system can be measured and appropriate degree of anti-saturation is introduced. If compressors and ex- panders of the above-mentioned types I and II, an additional more overmodulation tendency generated by small overshoots (a few percent) from the compressors. These overshoots can compensated by the present invention. (Also in audio systems available a tendency to compensate for overshoots). Although it is possible to work only with anti-saturation network in the coding unit, it is preferable that a complementary correction is introduced at the reproduction side so that flat frequency response maintained at all levels. The following analysis shows the type of fatal correction.

Consider Fig. 3, which shows double-track compressor and external pander constructions of type I, and let x be the input of the compressor signal, the y signal in the information channel and the output of the z expander signal. Let F1 and FZ be the transfer function for the compressor resp. further path of the expander, and allow FAS to be anti-saturation transmission function of the network. Let F'AS be the decoder's experience poor compensation function.

V = (FAS * F13 * and Z = VIN / as' ZFzFAs thus z = ï: A§fA§ -: - ÉlÉLê§- 1 * FZWAS X 1 ïšš ' A corresponding derivation can be performed for that in Fig. 4 Investigation shows that z = x if P1 = FZ and F'AS = showed the structure according to type IT: 10 15 20 25 30 35 447 525 3 y = FASX + F1FASy and z = F'ASy - Fzy (F '-Fl thus z = -ï-A§ - 2- X AS J ..

PHASE' The above not only shows that the two noise reduction networks shall be identical, as is known from the applicant's more constructions, but also to the decoder's anti-saturation network shall have a characteristic which is inverse to that of the encoder turn the net. Simple corrections can be achieved with passive networks, such as RC combinations, while feedback methods can be used jas at more complicated corrections, especially for achieving the inverse properties required in the decoder.

Fig. 5 shows a block diagram illustrating the present invention used in a type I two-stage device, primarily for use Investigation shows that z = x if F1 = FZ and F'As = during magnetic recording and playback.

In the embodiment shown, cascade-coupled compressors are used. sores 52 and 54 in order to achieve stronger compression as well corresponding expanders 56, 58. The invention is also applicable to single-stage, two-lane compressors and expanders. Where- their compressor has a main path 10 with a combination circuit 12, which to the main path adds the output signal from the further path NI, NZ, whose input is connected to the input of the corresponding main course. The expanders are provided with main tracks 14 and combined circuits 16, which from the main path signal subtract the output signal. len from the further path NZ, N1, the input of which is connected with the corresponding main line exit.

Such compressor and expander constructions are in themselves well known and will therefore not be described in detail. However there are two main shapes for the additional path N1 or N2. One alternatives (Figs. 7 and 8 of US-PS 3,846,719) are a filter followed by a controlled limiter, which by a rectified and smoothed nal is caused to perform an increasing constraint when the signal level increases. The second alternative (US-PS Re ɧ'426) consists of a control bare high band pass filter, the pass band of which is gradually made narrower the control signal so that large signal components are excluded from the filter output signal, which is preferably in series with a fixed high-pass 10 15 20 25 30 35 40 9 447 525 filter. Favorable limit frequency values for the controllable band filters is around 375 Hz at rest but gradually becomes narrower high-pass character in response to the control signal.

The first and second compressors 52 and 54 are each pre- seen with the main signal components affecting anti-saturation networks 74, 76 of the high and / or low frequency type (and / or someone else certain frequency or frequency range). On the tape playback page player T has the first and second expanders 56, 58 complementary tightly designed networks that affect the main signal components. Alternative such compensation can only be found in one of the compressions. and only in the corresponding expander. The compensation may in the form of a weak high frequency drop in the encoder (compressed sorn), which increases at very high frequencies, for example over 10 kHz, and a complementary increase in the decoder (expander).

Since the reduction of high frequency congestion of the channels performed only on the main signal components, the low level the components of the side channels not to be affected, and thus the overload reduction does not affect to any great extent the noise reduction. Consequently, channel congestion reduction one extend down to the intermediate frequency range.

The encoders' anti-saturation networks 74, 76 and the complementary networks 78, 80 in the decoders are located in the main signal paths. This arrangement mang gives very low noise reduction loss, because the main part of the signal at low levels is supplied by the noise-reducing side chain. It is therefore possible to set up anti-saturation networks such as together have a slight high frequency drop, for example 3-6 dB / octave (and complementary increase in the decoder) which extends relatively far down in frequency, for example to 2-3 kHz, and which allows action of saturation effects without concomitant major noise reduction loss. For example, the frequency curve at 0 dB in Fig. 1 shows a high frequency drop from saturation starting at about 2 kHz.

Since the anti-saturation networks are located in the main channels, which for the high-level signals, only the signals that cause it are affected saturation. As shown in Fig. 1, low level signals do not give rise to saturation in the band.

Half of the mentioned cases of 3-6 dB / octave can be produced by each net 74, 76 and half of the complementary increase in each network 78, 80. If the fall and the increase in their entirety added to only one network 76, 78 each, the associated com- presses and expander to exhibit greater compression and expansion 447 525 10 sion ratio within the_t special level and frequency range as is attached to this compressor and expander. In facilities for private use, however, the use of this single network is adequate.

Claims (9)

11 447 525 ^ Patent claim A compressor comprising a main signal path which is linear with respect to the dynamic range, a combination circuit located in the main path, and a further path having its input connected to the input or output of the main path and its output with the combination circuit, which further path via the combination circuit emits a signal which at least within an upper part of the frequency band raises the signal of the main path but which is limited so that, within the upper part of the input dynamic range, it is smaller than the main path signal, characterized in that a frequency dependent circuit is switched only in the main path and causes a decrease in the frequency curve within a selected part of the frequency band. Compressor according to claim 1 and constituting an audio compressor, characterized in that the frequency-dependent circuit is arranged to give a drop in the frequency curve within the upper part of the audio frequency band. Compressor according to claim 2, and operating even at low frequencies, characterized in that the frequency-dependent circuit causes a drop in the frequency curve also within the lower part of the audio frequency band. An expander comprising a main signal path which is linear with respect to the dynamic range, a combination circuit located in the main path, and a further path, which has its input connected to the input or output of the main path and its output with the combination circuit, which further path via the combination circuit emits a signal which at least within an upper part of the frequency band counteracts the signal of the main path but which is limited so that, within the upper part of the input dynamics range, it is smaller than the main path signal, characterized in that a frequency dependent circuit is switched only in the main path and causes an increase in the frequency curve within a selected part of the frequency band. Expander according to claim 4 and constituting an audio expander, characterized in that the frequency-dependent circuit is arranged to give an increase in the frequency curve within the upper part of the audio frequency band. Expander according to Claim 5 and operating even at low frequencies, characterized in that the frequency-dependent circuit causes an increase in the frequency curve even within the lower part of the audio frequency band. »447 525 Noise reduction plant comprising a compressor and an expander, each comprising a main signal path which is linear with respect to the dynamic range, a combination circuit located in the main path, and a further path having its input connected to the input or output of the main path and its output to the combination circuit. , which further path via the combination circuit emits a signal which at least within an upper part of the frequency band raises the signal of the main path in the compressor and counteracts the signal of the main path in the expander, but which in both cases is limited so that it, within the upper part of the input dynamics range, is smaller than the main path signal, characterized by frequency-dependent circuits connected only in the main paths, which within a selected part of the frequency band cause a lowering of the frequency curve in the compressor and a complementary increase in the frequency curve in the expander. Plant according to claim 7, in which the compressor and the expander operate at audio frequencies, characterized in that the frequency-dependent circuits within the upper part of the audio frequency band cause a drop in the frequency curve of the compressor and a complementary increase in the frequency curve of the expander. System according to claim 8, in which the compressor and the expander also operate at low frequencies, characterized in that the frequency-dependent circuits, even within the lower part of the audio frequency band, cause a drop in the compressor frequency curve and a complementary increase in the expander frequency curve. .