WO1988003695A1

WO1988003695A1 - Analogical system for noise reduction in magnetic recordings

Info

Publication number: WO1988003695A1
Application number: PCT/IT1986/000079
Authority: WO
Inventors: Mario Zagni
Original assignee: H.F.C. Sas Di Menetti & Zagni
Priority date: 1986-11-04
Filing date: 1986-11-04
Publication date: 1988-05-19

Abstract

The working principle consists of the circumstance that along with the musical signal, a codified signal is recorded which steadily indicates the percentage of compression on the inlet signal. In the subsequent expansion phase, the codified signal is read in order to quantify the value to give to the expansion and to exactly recreate the initial conditions of the audio signal. This codified signal consists of a tension covering a frequency extent outside of the audio frequency band (ultrasounds), in one point of the characteristic tension frequency still to use, even if dampened, for the purposes of the magnetic recording; a kind of frequency modulated signal which turns out to be proportional to the piloting signal of the VCA.

Description

"Analogical syston for noise reduction in magnetic recordings" The invention refers to the improvement in the analogical system of the noise reduction in the magnetic recordings which mainly allows the keep unchanged the imprecisions of the magnetic recorders to which in the recordings it is combined, giving back, during the out let from the expander, the same true sound as it had when entering the compressor.

At present, in the musical recording systems, both professional and high fidelity ones, the cαmpression-expansdlon system of the audio signal is used with a view to getting the best recording and, conse¬ quently, to reduce the ground noise. The proceeding can be carried out according two subssequent phases: in the first one, the compres¬ sion of the signal is performed by me4ns of a codification form; in the second phase its expansion is obtained which brings the signal again to the initial conditions through a decodification. In a system of audio recording, the most critic^ equimpent, from the poin of view of the signal-noise rate, is represented by the magnetic recorder or magnet-phone, which in the case of a profes- sional device, allows a signal-noise rate of apporximately 60-65dB against the 80-1OOdB of the microphones and amplifiers. In order to obviate this inconvenience, the signal is compressed which is sent to the recorder: i.e. with reference level of CdStι(0,775 V) , for instance, the level signals are raised which were under this reference, and those .above it are lowered, in order to get the recording phase as good as possible. In -the reproduction phase of the recorded ribbon, to obtain again the dynamic features of the inlet signal, the exactly inverse operation is carried out in com parison with the one made in the recording phase; this operation is called signal expansion. The system acts as a noise reducer in the abovementioned phase, because in order to bring the recorded signal again to the initial conditions, the signal must be reduc¬ ed to the lowest level by contemporarily damping - as a consequen¬ ce - the noise introduced into the recorder. In the present proceed ings, the cαπpressed signal which constitutes^'the basic element to reach the decertification, cna be altered in the sebsequent expan- ^* sion phase. This in connection with level changes, if any, introduc ed by the recorder or other means that has been interposed, as for instance the presence of a telephone line.

The invention systan avoids ttiat the decodification data become untrue by sending to the compressor outlet not only the elaborated signal coming from the compressor inlet, but by further accompany¬ ing another check signal which turns out to be the only useful pa- rameter for the decodification purposes.

The system foresees two phases: the first one is the cαnpression phase and consists in raising the signals that are below a thres¬ hold reference, and by lowering the signals that are above the said reference; the second phase carries out the expansion of the recorded signal to get again the initial conditions before the si gnal compression. The signal passes from the inlet stage 1 through two filters 2 and 3 one of which passes-high and the other passes- low, and through two resistances 4 and 5 it reaches an amplifier 6 checked in tension. This tension checking is derived from .an ini- tial signal taken after the high-pass filter 2 and is sento to a _f rectifier 7 which transforms the inlet alternate tension into a continue one the value of which is proportional to the amplitude of the initial signal. From the rectifier 7 outlet, the signal is sento to a logarithmic amplifier 8 which makes the check signal apt to pilot the amplifier 6 checked in tension. This check signal is sent, at the same time, on line 9, to a converter 10 of tension- frequency as well as to the above mentioned amplifier 6 which is tension-checked. The tension-frequency converter 10 transforms the check tension of line 9 into a changing frequency one out of the primary signal range. The outlet from the tension controlled aιrpli_ fier 6_%is sent to a band rejection filter 11 which eliminates the components of the inlet signal, if any, the frequency of which is identical to the frequencies generated by the tension-frequency con verter 10. The filter 11 exit and the converter 10 one are sent to a sunming circuit 12 which carries out the algebriσ .sum of the two signals and, by means of an outlet stadium 13, sends both signals to the outlet of the ccmpressor ready to pass to the recording pha¬ se. For the expansion in the phase of signal reproduction, at the recorder outlet, through an inlet stadium 14 the signal is contem porarily sent to a band rejection filter 15 and to a band passing filter 16 apt for selecting the che Lng signal between the two si_^ gnals which are present at the compressor exit. Such check signal is sent to a squaring up circuit 17 which makes this signal apt to pilot a frequency-tension converter 18. At this stage outlet 18, the signal has been created again which is present on the ccmpres¬ sor line 9, i.e. the signal which controlled the .amplifier 6 check ed in tension by the compressor. To obtain the ccmplementary effect of the cαπpressione, i.e. the expansion^*effect, the signal is sent on line 9 to an inverting stage 20, which will invert the phase of the same signal and send it to an amplifier 21 with checked tension. The primary signal on line 22, at the filter 15 exit, reaches VCA 21. From outlet 21, through two resistances 23 and 24, the signal goes on high-passing filter 25 and another low-passing filter 26, which r have both a complementary aim as to filter 2 and 3, present in the compressor section. At the outlet of these two filters 25 and 26, the signal reaches an automatic deviator 27 which, if activated, sends the signal to an exit stage 28 which makes it apt for utili¬ zation.

The presence of the checking signal at the outlet of the squaring circuit 17 enables the realization of a self-activating circuit ob¬ tained by means of a rectifying filter 29 and a comparator 30 which controls the automatic deviator 27.

The expanser is capable of distinguishing, .among the different si¬ gnals which have been recorded, the ones corresponding to its own decodification.

A form of execution is illustrated in an indicative way by the dra wings of Table 1, where Fig. 1 is the block system of the compres¬ sion device, while Fig. 2 is the block scheme of the expander.

The components, the cαπplaxientary and ccmbining means may be fore- seen in different ways.

Claims

Claims.

1) Analogical systan for noise reduction in magnetic recordings, characterized by the fact that it foresees two phases: the first one is the compression phase and consists in raising the signals

-5 that are below a threshold reference, and by lowering the signals that are above the .said reference; the second phase carries out the expansion of the recorded segnal to get again the initial con c ditions before the signal compression. The signal passes from a inlet stage (1) through two filters (2 and 3) one of which pass- 0 es-high .and the other passes-low, and through two resistances (4 and 5) it reaches an amplifier (6) checked in tension. This tension checking is derived from an initial signal taken after the high-pass filter (2) and is sent to a rectifier (7) which transforms the inlet alternate tension into a continue one the 5 value of which is proportional to the amplitude of the initial si_ gnal. From a rectifier (7) outlet, the signal is sent to a loga¬ rithmic amplifier (8) which makes the check signal apt to pilot a amplifier (6) checked in tension. The check signal is sent, at the same time, on a line (9) , to a converter (10) of tension-frequen- 0 cy as well as to the above mentioned amplifier (6) which is tension checked. The tension-frequency converter (10) transforms the check tension of line (9) into a changing frequency one out of the pri¬ mary signal range. The outlet from the tension controlled ampli¬ fier (6) is sent to a band rejection filter (11) which eliminates the components of the inlet signal, if any, the frequency of which is identical to the frequencies generated by the tension-frequency converter (10). The filter (11) exit and the converter (10) one are sent to a suππύng circuit (12) which carries out the algebric sum of the two signals and, by means of an outlet stadium(13) ,sends both signals to the outlet of the compressor ready to pass to the recording phase. For the expansion in the phase of signal reprcduc_ tion, at the recorder outlet, through an inlet stadium(14)the si¬ gnal is contanporarily sent to a band rejection filter (15) and to a band passing filter (16) apt for selecting the checking signal „ between the two signals which are present at the compressor exit. Such check signal is sent to a squaring up circuit (17) which makes this signal apt to pilot a frequency-tension converter (18) . At this stage outlet (18) , the signal has been created again which is present on the compressor line (9) , i.e. the signal which con- 0 trolled the amplifier (6) checked in tension by the compressor. To r obtain the complementary effect of the compression, i.e. the ex¬ pansion effect, the signal is sent on line (19) to an inverting stage (20) , which will invert the phase of the same signal and send it to an amplifier (21) which checked tension. The primary 5 signal on line (22) , at the filter (15) exit, reaches VCA 21.

From outlet (21) , through two resistances (23 and 24) the signal goes on high-passing filter (25) and another low-passing filter (26) ,which have both a complementary aim as to filter (2 and 3) , present in the ccmpressor sectJLon. At the outlet of these two fi 0 ters (25 and 26) , the signal reaches an automatic deviator (27) which, if activated, sends the signal to an exit stage (28) which makes it apt for utilization.

2) Analogical system for noise reduction in magnetic recordings, 5 as per claim 1, characterized by the fact that the presence of the checking signal at the outlet of the squar.ing circuit (17) enables the realization of a self-activating circuit obtained by means of a rectifying filter (29) and a comparator (30) which controls the autαnatic deviator (27). 0

3) Analogicalsystait for noise reduction in magnetic recordings, as per claim 2, characterized by the fact that the expanser is capable of distinguishing among the different signals which have been recorded, the ones corresponding to its own decodification; 5