METHOD FOR PROCESSING A DIGITAL SOUND SIGNAL

Abstract

The present invention relates to the field of digital processing to restore the auditory impression of a "vinyl sound". The vinyl microgroove record was born in 1948; the sound information was restored by the movement of a needle transmitting the deviations of the groove to an electromagnetic or piezoelectric transducer which transforms these vibrations into an electrical signal. During the manufacture of the disc matrix, the high frequencies are reinforced and the low frequencies are attenuated according to an RIAA curve. This allows for a more regular engraving dimension and prevents the high frequencies from getting lost in the background noise and the low frequencies from sending the chisel into the neighboring turn. Playback is carried out with a correction according to the inverse RIAA curve. Figure 1

Description

Title of the invention: METHOD FOR PROCESSING A DIGITAL SOUND SIGNAL Field of invention

[0001] The present invention relates to the field of digital processing for reproducing the auditory impression of a “vinyl sound” from a reproduction on digital audio equipment.

[0002] The vinyl microgroove record was born in 1948; the sound information was restored by the movement of a needle transmitting the deviations of the groove to an electromagnetic or piezoelectric transducer which transforms these vibrations into an electrical signal. During the manufacture of the disc matrix, the high frequencies are reinforced and the low frequencies are attenuated according to an RIAA curve. This allows for a more regular engraving dimension and prevents the high frequencies from getting lost in the background noise and the low frequencies from sending the chisel into the neighboring turn. Reading is carried out with a correction according to the inverse RIAA curve.

[0003] The imperfections resulting from this “mechanical” reading (wear, hiss, surface noise, speed fluctuations, crackles, etc.) led in the 2000s to the preference for digital recordings on physical media (CD, DVD-audio) or dematerialized media (digital radio, DAB, “streaming”).

[0004] Recently, there has been a return of interest in vinyl records, not only out of nostalgia, but also because of a more musical listening perception. This difference in perception is explained by the specificities of vinyl record recording, which in the absence of sampling and because of the "physical" constraints imposed by the formation of the grooves, and resulting in particular in typical parameters (RIAA curve), which provide a particular "coloring" of a signal that is easier to listen to, in which more details are brought to audible thresholds and by a sound perceived as more musical, warmer and more "punchy".

[0005] In order to be able to reconcile the advantages of a digital signal, a priori unalterable, and the particularity of the “vinyl sound”, to use this term designating a somewhat subliminal perception which is difficult to characterize technically, it has been proposed in the prior art to apply filters to modify the spectral curve of the sound signal and simulate the “vinyl sound” from a digital signal. State of the art

[0006] Several patents from the company DOLBY (trade name) are known in the state of the art, describing acoustic filter systems for modifying an audio signal to introduce sound effects and spectral corrections. Patents US9825598B2 or US10466957B2 or US10834493B2 describe various variations of active acoustic filter systems for applying audio processing functions including adding echo or reverb to ambient sound and / or detecting and canceling an echo in the ambient sound, noise reduction processing. These patents also mention that the audio processing function may include "vinyl emulation" effects consisting of adding scratch sounds and pops.

[0007] We also know the article "Benefiting from the pleasure of vinyl sound ... from digital sources" published on the site http: / / www.carbonl3.fr / vsound.html reporting on processing aimed at reproducing vinyl sound from a digital source, mainly by increasing the level of low frequencies (e.g.: +6 dB for Fr below 150 Hz) and a "home-made dynamic correction", which provides a more "crunchy" sound color, and allows a gain of 3 dB of margin.

[0008] We also know of “plugin” type applications that allow you to simulate the dust and scratches on a vinyl record, for example “iZotope Vinyl” (trade name).

[0009] Disadvantages of the prior art

[0010] The solutions of the prior art result in a sound that is closer to a caricature of vinyl sound, adding to the defects of digital sound the certainly characteristic but nevertheless detrimental defects of vinyl records. Less experienced listeners will of course find the particular ambiance of microgroove records.

[0011] Purists, for their part, will only partially find the acoustic specificities of vinyl sound in the solutions of the prior art. Solution provided by the invention

[0012] In order to overcome these drawbacks, the present invention relates, in its most general sense, to a method for processing a pair of stereophonic signals, characterized in that it comprises the following steps: - An initial step of reading a physical vinyl microgroove record on a turntable, sampling the pair of stereophonic signals and recording said digital stereophonic signals constituting the reference stereophonic signals, then processing comprising:

[0013] a) An iterative modification step consisting of applying a change of tone to increase the pitch for said stereophonic signals initially referenced for the first modification, then to the pair of signals obtained during the previous iteration,

[0014] b) an additional step consisting of summing said stereophonic digital signals resulting from the last iteration and said reference stereophonic digital signals to produce a pair of corrected digital stereophonic signals.

[0015] Preferably, the number of iterations is between 3 and 8.

[0016] Advantageously, the method comprises an additional step of reducing the stereophonic image of said pair of corrected digital stereophonic signals.

[0017] According to a variant, said additional step of reducing the stereophonic image consists of adding to each channel of said corrected digital stereophonic signals a central monophonic signal resulting from the processing of the two channels of said corrected digital stereophonic signals.

[0018] Advantageously, said processing of said corrected digital stereophonic signals consists of adding to the signal of each of the channels the opposite signal of the other channel and adding the two resulting signals to provide a central monophonic signal.

[0019] According to a preferred embodiment, said initial step of reading a vinyl record is repeated with a plurality of turntables, and said pair of reference stereophonic signals corresponds to the selection of the pair of signals corresponding to one of said turntables as a function of a selection criterion depending on characteristics specific to said turntables.

[0020] According to another variant, the method comprises an additional step consisting of applying a post-processing of said pair of digital stereophonic signals corrected by an impulse response filter, by an impulse response corresponding to a capture of an impulse signal from an acoustic and electronic assembly for reading a vinyl record.

[0021] According to another variant, the method comprises an additional step consisting of generating a print of a reference listening system [that on the reference turntable] in stereo and binaural at a distance of between 20 and 300 cm and applying a filter making it possible to ensure maximum compatibility with the HRTF profiles.

[0022] According to a particular embodiment, the method comprises processing by a crosstalk filter having an angle greater than 3 degrees and less than 320 degrees.

[0023] Advantageously, the method comprises an additional step consisting of applying an equalization processing of the listening medium in order to restore a signal identical to the original.

[0024] According to another variant, the method comprises an additional step consisting of applying a specific processing to the frequencies between 150hz and 300hz by impulse filtering with binaural impulse responses comprising a delay modification of 25%, by applying to each corrected signal a low-pass filter and weighted addition of said corrected signal and the signal processed by impulse filtering.

[0025] Advantageously, said processing comprises a modification of a value between 10 and 20% of the “left” component of the “right” channel of the impulse response as well as of the “right” component of the “left” channel of the impulse response.

[0026] According to another variant, said method comprises an additional step consisting of applying a specific processing to the frequencies between 12000hz and 22000hz with a specific processing to the harmonics between 2 and 7 without modification of the harmonic 1, by applying to each corrected signal a high-pass filter and weighted addition of said corrected signal and the processed signal.

[0027] According to a particular embodiment, the method comprises an additional step consisting of applying a processing of frequencies lower than 150 Hz in mono via a reduction of the stereo tracks with application of an acoustic filter.

[0028] According to another variant, the method comprises an additional step consisting of applying a multi-band 3D acoustic correction filter defined by the capture distance of the reference system and the volume of the harmonics.

[0029] According to another variant, it comprises an additional step consisting of applying a 3D acoustic correction filter applied to the fundamental.

[0030] Advantageously, the method comprises an additional step of modifying the stereophonic image applied to step b) and to the fundamental of the signals corrected on the 3D signal.

[0031] Detailed description of a non-limiting example of embodiment of the invention,

[0032] Other characteristics and advantages will emerge from the following description of the invention, a description given by way of example only, referring to the appended drawings in which: - [Fig.l] [Fig.l] represents a graphical representation of an algorithm implemented in the method which is the subject of the invention. General principles

[0033] The aim of the invention is to post-process a digital audio file to enable listening with headphones, with “modern” equipment, preserving the impression of listening to a microgroove record, characterized by the characteristic signature of their defects: • Stereophonic image limited by the maximum physical extension of the grooves • Deformation: Adjustment of the amount of deformation and the shape of disc deformation, from no deformation to completely melted and deformed edges. • Dust: simulation of the amount of dust that has settled on the surface of the disc. • Year: Record player models from different decades using filter responses. • Wear: simulation of the effect of a record that has been played too often, from brand new to a few thousand revolutions. • Mechanical noise: added turntable rumble and motor noise. • Spin Down: Simulation of the sound of slowly stopping the playback of a recording, by modulating both the speed and the playback frequency.

[0034] The objective is to obtain, through headphones, a listening experience consistent with vinyl mastering on a reference system while retaining all the advantages of the medium, as well as the restitution of the space of the two speakers (left and right) and of the listener, essential for an identical restitution of an existing system on a hi-fi system. Digital processing

[0035] The digital sound signal is subject to a set of processing operations described below and illustrated by [Fig.l].

[0036] The first step (1) consists of reading a vinyl record (30) on a turntable (40) and sampling this pair of stereophonic signals to record a reference digital file (35) taking into account all the audio characteristics of reading a vinyl record on a turntable. These reference files (35) are recorded in a memory, during an initial step of the method. Listening to a digital file

[0037] When listening to a digital file (45), a series of digital stereo signal processing operations are carried out. We begin by creating a harmonic of the fundamental frequency of this signal. To create an artificial harmonic, we duplicate the signal (step 2) and we perform a frequency shift on the duplicated component in a ratio of x2 to create a harmonic 1 (step 3). Thus, the frequencies of 100 Hz are transformed into frequencies of 200 Hz, without the duration of the musical sequence changing. This processing can be carried out by applications of the “voicechanger” type (trade name).

[0038] Several iterations of this harmonic creation step are carried out, then an addition of the reference signal (35) is carried out with the signal resulting from the last iteration (step 4).

[0039] The number of iterations is determined empirically, based on the evolution of the signals of one iteration compared to the signals of the previous iteration. If the variation is imperceptible, the processing is stopped. Typically, the optimum number of iterations is between 3 and 8, and generally five iterations lead to a good result.

[0040] This first processing of the digital signal (45) integrates the characteristics specific to vinyl records by this step of combination between the digital signal (45) and the reference signal (35) recorded during the initial step.

[0041] A combination of one or more additional treatments is then carried out: - Step (5): modification of the stereophonic image resulting from step (4) and the fundamental of the corrected signals on the stereophonic signal via dynamic processing defined by the content of the source - Step (6): Generation of the imprint of a reference Hifi system in stereo and binaural at a distance between 20 and 300 cm.

[0042] This step aims to produce a binaural audio signal from a sound capture method adapted to the morphology of the human head. It consists of recording from an artificial head (36) a reference signal reproduced by loudspeakers arranged at a distance of between 20 and 300 centimeters to record an impulse response (37). This impulse response (37) will then be used for binaural reproduction with headphones. - Step (7): This step consists of adding a filter ensuring maximum compatibility with the transfer functions relating to different HRTF head profiles (head-related transfer function) - Step (8); Using a crosstalk filter with an angle greater than 3 degrees and less than 320 degrees - Step (9): Equalization of the listening medium in order to restore a signal identical to the original - Step (10): Specific processing at frequencies between 150hz and 300hz with binaural impulse responses including a delay modification of 25% as well as a modification of the left signal of the right impulse response as well as the right signal of the left impulse response by a value between 10 and 20%. - Step (11): Specific treatment for frequencies between 12000hz and 22000hz with specific treatment for harmonics between 2 and 7 without modification of harmonic 1 - Step (12): Processing of frequencies below 150Hz in mono via a reduction of the stereo tracks with application of an acoustic filter. - Step (13): Multi-band 3D acoustic correction filter defined by the capture distance of the reference system and the volume of the harmonics - Step (14): 3D acoustic correction filter applied to the fundamental - Step (15): modification of the stereophonic image applied to step b) and to the fundamental of the corrected signals on the 3D signal - Step (16): Use of reference signals to generate the characteristic noise of vinyl - Step (17): Processing of the signal from step (16) with a set of specific spatialized impulse responses.

[0043] The objective is to obtain, on headphones (20), a listening experience consistent with vinyl mastering on a reference system while retaining all the advantages of the medium, as well as the restitution of the space of the two speakers (left and right) and of the listener, essential for an identical restitution of an existing system on a hi-fi system.

Claims

Claims

1. Method for processing a pair of stereophonic signals characterized in that it comprises: a. A first step (1) consisting of reading a vinyl record (30) on a turntable (40) and sampling this pair of stereophonic signals to record a reference digital file (35) taking into account all the audio characteristics of the reading of a vinyl record on a turntable, said reference file (35) being recorded in a memory, during an initial step of the method b. When listening to a digital file, a series of processing operations of the digital stereo signal are carried out consisting of: i. creating a harmonic of the fundamental frequency of this signal. ii. carrying out several iterations of this harmonic creation step, then carrying out an addition of the reference signal (35) with the signal resulting from the last iteration.

2. Method for processing a digital sound signal according to claim 1 characterized in that said step of creating a harmonic of the fundamental frequency of this signal, the signal is duplicated and a frequency shift is carried out on the duplicated component in a ratio x2 to create a harmonic 1 so that the frequencies of 100 Hz are transformed into frequencies of 200 Hz, without the duration of the musical sequence changing.

3. Method for processing a digital sound signal according to claim 1 characterized in that the number of iterations is between 3 and 8.

4. Method for processing a digital sound signal according to claim 1 or 2, characterized in that it comprises an additional step consisting of generating an imprint of a reference listening system in stereo and binaural at a distance of between 20 and 300 cm and applying a filter to ensure maximum compatibility with HRTF profiles.

5. Method for processing a digital sound signal according to the preceding claim, characterized in that it comprises processing by a crosstalk filter having an angle greater than 3 degrees and less than 320 degrees.

6. Method for processing a digital sound signal according to at least one of the preceding claims, characterized in that it comprises an additional step consisting of applying an equalization processing of the listening medium in order to restore a signal identical to the original.

7. Method for processing a digital sound signal according to at least one of the preceding claims, characterized in that it comprises an additional step consisting of applying a specific processing to the frequencies between 150 Hz and 300 Hz by impulse filtering with binaural impulse responses comprising a delay modification of 25%, by applying to each corrected signal a low-pass filter and weighted addition of said corrected signal and the signal processed by impulse filtering.

8. Method for processing a digital sound signal according to at least one of the preceding claims, characterized in that it comprises an additional step consisting of applying a specific processing to the frequencies between 12000hz and 22000hz with a specific processing to the harmonics between 2 and 7 without modification of harmonic 1, by applying to each corrected signal a high-pass filter and weighted addition of said corrected signal and the processed signal.

9. Method for processing a digital sound signal according to at least one of the preceding claims, characterized in that it comprises an additional step consisting of applying processing of frequencies lower than 150 Hz in mono via a reduction of the stereo tracks with application of an acoustic filter.

10. Method for processing a digital sound signal according to claim 1 characterized in that it comprises an additional step consisting of applying a multi-band 3D acoustic correction filter defined by the capture distance of the reference system and the volume of the harmonics.

11. Method for processing a digital sound signal according to at least one of the preceding claims, characterized in that it comprises an additional step consisting of applying a 3D acoustic correction filter applied to the fundamental.

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