Method for processing a digital sound signal for vinyl disc emulation

Abstract

The present invention relates to the field of digital processing in order to reproduce the auditory impression of a "vinyl sound". Vinyl records were created in 1948; the sound information was reproduced by the movement of a needle transmitting the deviations in the groove to an electromagnetic or piezoelectric transducer that converts said vibrations into an electrical signal. When manufacturing the disc matrix, high frequencies are boosted and low frequencies are reduced according to an RIAA curve. This enables a more regular engraving dimension to be achieved and prevents high frequencies from being lost in background noise and low frequencies from sending the engraving point into the neighbouring turn. Playback is corrected according to the reverse RIAA curve.

Description

METHOD FOR PROCESSING A DIGITAL SOUND SIGNAL FOR VINYL RECORD EMULATION 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. Playback 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 due to 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 due to 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 that 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] Known in the state of the art is patent application US2011060436 describing a method of stereo biaural audio recording, in which the notes stored in a first channel of the recording are stored in the same sequence in a second channel of the recording but are shifted in frequency by the same value, and a method for processing an audio signal, consisting of:

[0007] - acquire a set of data capable of defining an audio signal;

[0008] - select a frequency offset;

[0009] - generate a first digital audio recording by synthesizing it according to the data set;

[0010] - generate a second digital audio recording by synthesizing it according to the data set and the frequency offset;

[0011] - select a channel from the first audio recording, select a channel from the second audio recording, mix the two channels into a third audio recording that includes at least two channels.

[0012] Also known is the article Jot, Jean-Marc & Walsh, Martin. (2010). Center-channel processing in virtual 3-D audio reproduction over headphones or loudspeakers. 128th Audio Engineering Society Convention 2010. 3. 1935-1944 concerning virtual 3D audio processing systems for spatial enhancement of recordings played over headphones or loudspeakers generally produce a less convincing effect on sound components located in the center. The virtual 3D audio processing algorithm described makes it possible to improve the spatial enhancement effect on headphones or loudspeakers, even for sound components located in the center of the stereo image, to preserve the timbre and balance of the original recording and to produce a more stable "phantom center" image in the loudspeakers.The proposed improvements are particularly beneficial in computer and television audio systems or in home theater "soundbar" speakers.

[0013] Patent application US2019379995 describes a method of providing a head-related transfer function (HRTF) for application to an input audio signal for audio localization in headphones, comprising:accessing a pair of binaural acoustic impulse responses (BAIRs) individualized for a listener in a reference position;accessing a plurality of binaural acoustic impulse responses (BAIRs) for the listener corresponding to additional positions relative to the listener;deriving a plurality of transfer functions for converting binaural acoustic impulse responses (BAIRs) for the reference position relative to the listener at each of the additional positions by dividing each of the plurality of pairs of binaural acoustic impulse responses (BAIRs) for the additional positions by the pair of binaural acoustic impulse responses (BAIRs) for the reference position;receiving a signal indicating a change in head orientation, and selecting a pair of the plurality of transfer functions in response to the signal and correspondingly to the signal; andapplying the pair of binaural acoustic impulse responses (BAIRs) of the reference position and the selected pair of the plurality of transfer functions to the input audio signal to localize the audio in the headphones.;

[0014] The article “Pitch extraction and separation of overlapping speech” PN Denbigh, J. Zhao, Speech Communication, Volume 11, Issues 2–3, 1992, Pages 119-125, ISSN 0167-6393 describes algorithms that separate two overlapping signals. These algorithms depend on accurate measurements of the pitch of the target voice. The first algorithm uses a single microphone and the important feature is to exploit the onset of voicing to extract its pitch in the presence of interference. The second algorithm uses two microphones and its major feature is also to exploit the direction of the target voice.

[0015] The article "Audio content analysis" by Alexander Lerch et al. doi = {10.48550 / ARXIV.2101.00132} presents the general process of audio content analysis and examines basic approaches to audio content analysis. The various tasks of audio content analysis are classified into three categories: music transcription, musical performance analysis, and music identification and categorization.

[0016] Examples of music transcription systems include musical key detection, fundamental frequency detection, and musical structure detection. Music performance analysis systems provide an overview of approaches to rhythm and tempo detection and musical performance evaluation. Music classification systems covered are audio fingerprinting, musical genre classification, and musical emotion recognition. The chapter concludes with a discussion of current challenges in the field and speculation about prospects.

[0017] Disadvantages of the prior art

[0018] Prior art solutions result in a sound that is closer to a caricature of vinyl sound, adding to the defects of digital sound the characteristic but nonetheless detrimental defects of vinyl records. Less experienced listeners will of course find the particular atmosphere of microgroove records.

[0019] Purists, for their part, will only partially find the acoustic specificities of vinyl sound in the solutions of the prior art.

[0020] Solution provided by the invention

[0021] 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 having the characteristics set out in claim 1.

[0022] It involves 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:

[0023] (a) An iterative modification step consisting of applying a change in 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,

[0024] b) an additional step 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.

[0025] Preferably, the number of iterations is between 3 and 8.

[0026] Advantageously, the method comprises an additional step of reducing the stereophonic image of said pair of corrected digital stereophonic signals.

[0027] 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.

[0028] 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.

[0029] According to a preferred embodiment, said initial step of playing 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 according to a selection criterion depending on characteristics specific to said turntables.

[0030] 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.

[0031] According to another variant, the method comprises an additional step consisting of generating a footprint 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 to ensure maximum compatibility with the HRTF profiles.

[0032] 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.

[0033] Advantageously, the method comprises an additional step consisting of applying an equalization processing to the listening medium in order to restore a signal identical to the original.

[0034] 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.

[0035] Advantageously, said processing comprises a modification by a value of 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.

[0036] 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.

[0037] According to a particular embodiment, the method comprises an additional step consisting of applying processing of frequencies below 150 Hz in mono via a reduction of the stereo tracks with application of an acoustic filter.

[0038] 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.

[0039] According to another variant, it includes an additional step consisting of applying a 3D acoustic correction filter applied to the fundamental.

[0040] Advantageously, the method comprises an additional step of modifying the stereophonic image applied in step b) and to the fundamental of the signals corrected on the 3D signal.

[0041] Detailed description of a non-limiting example of embodiment of the invention.

[0042] 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: The represents a graphic representation of an algorithm implemented in the method which is the subject of the invention.

[0043] General principles

[0044] The purpose of the invention is to post-process a digital audio file to allow 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 Warp: adjustment of the amount of warp and the shape of the disc's warp, from no warp to completely melted and warped edges. Dust: simulation of the amount of dust that has settled on the disc's surface. Year: Models of record players 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: addition of turntable rumble and motor noise.Spin Down: Simulates the sound of a record slowly stopping playback, by modulating both the speed and frequency of playback.

[0045] The objective is to obtain on headphones a listening experience consistent with vinyl mastering on a reference system while retaining all the advantages of the medium, as well as the reproduction of the space of the two speakers (left and right) and of the listener, essential for an identical reproduction of an existing system on a hi-fi system.

[0046] Digital processing

[0047] The digital sound signal is subject to a set of processing operations described below and illustrated by the.

[0048] The first step (1) consists of playing 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 playing a vinyl record on a turntable. This reference file (35) is recorded in a memory, during an initial step of the method.

[0049] Listening to a digital file

[0050] 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 perform a frequency shift on the duplicated component in a ratio of x2 to create a harmonic 1 (step 3). Thus, 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 such as "voicechanger" (trade name).

[0051] 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) of processing said digital file (45).

[0052] 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 optimal number of iterations is between 3 and 8, and generally five iterations lead to a good result.

[0053] 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.

[0054] We then proceed to a combination of one or more additional treatments: 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 audio source Step (6): Generation of the imprint of a reference Hifi system in stereo and binaural at a distance between 20 and 300 cm.

[0055] 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 placed 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 on headphones.Step (7): This step consists of adding a filter ensuring maximum compatibility with the transfer functions relating to different head profiles HRTF (head-related transfer function)Step (8): Use of a crosstalk filter with an angle greater than 3 degrees and less than 320 degreesStep (9): Equalization of the listening medium in order to restore a signal identical to the originalStep (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 processing of frequencies between 12000hz and 22000hz with specific processing of harmonics between 2 and 7 without modification of harmonic 1Step (12): Processing of frequencies below 150hz in mono via a reduction of the stereo tracks with application of an acoustic filter.Step (13): Multiband 3D acoustic correction filter defined by the capture distance of the reference system and the volume of the harmonicsStep (14): 3D acoustic correction filter applied to the fundamentalStep (15): Modification of the stereophonic image applied to step b) and to the fundamental of the corrected signals on the 3D signalStep (16): Use of the reference signals to generate the characteristic noise of vinylStep (17): Processing of the signal from step (16) with a set of specific spatialized impulse responses.

[0056] The objective is to obtain, through 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 reproduction of the space of the two speakers (left and right) and of the listener, essential for an identical reproduction of an existing system on a hi-fi system.

Claims

A method for processing a pair of stereophonic signals characterized in that it comprises: a) An initial reading step consisting of playing a vinyl record (30) on a record player (40) and sampling this pair of stereophonic signals to record a digital reference file (35) taking into account all the audio characteristics of playing a vinyl record on a record player, this reference file (35) being recorded in memory, during an initial step of the method, b) Then, during playback of a digital file (45), a processing step comprising the following steps: i. An iterative modification step consisting of applying a pitch change to increase the pitch for said stereophonic signals initially of reference for the first modification, then to the pair of signals obtained during the previous iteration, ii.an additional step consisting of summing said digital stereophonic signals resulting from the last iteration and said reference digital stereophonic signals to produce a pair of corrected digital stereophonic signals. A method for processing a digital sound signal according to claim 1 characterized in that the number of iterations is between 3 and 8. A method for processing a digital sound signal according to claim 1 characterized in that it comprises an additional step of reducing the stereophonic image of said pair of corrected digital stereophonic signals. A method for processing a digital sound signal according to the preceding claim, characterized in that said additional stereophonic image reduction step 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. A method for processing a digital sound signal according to the preceding claim, characterized in that 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. A method for processing a digital audio signal according to claim 1, characterized in that said initial step of reading a vinyl record is repeated with a plurality of turntables, and in that said pair of stereo reference signals corresponds to the selection of the pair of signals corresponding to one of said turntables according to a selection criterion based on characteristics specific to said turntables. A method for processing a digital audio signal according to claim 1 or 2, characterized in that it comprises an additional step consisting 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) then being used for binaural playback on headphones, by a processing comprising the following steps: Step (7): addition of a filter ensuring amaximum compatibility with transfer functions related to different HRTF (head-related transfer function) profiles 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 to reproduce a signal identical to the original. Step (10): Specific processing of frequencies between 150 Hz and 300 Hz with binaural impulse responses, including a 25% delay modification and 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 processing of frequencies between 12,000 Hz and 22,000 Hz with specific processing of harmonics between 2 and 7 without modification of the 1st harmonic. Step (12): Processing of frequencies below 150 Hz in mono via reduction of the stereo tracks with application of aAcoustic filter. Step (13): 3D multiband 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 in step b) and to the fundamental of the corrected signals on the 3D signal. Step (16): Use of the 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. A 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 between 20 and 300 cm and applying a filter to ensure maximum compatibility with HRTF profiles. A 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. A 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 treatment to the listening medium in order to reproduce a signal identical to the original. A 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 specific processing to 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. A method for processing a digital sound signal according to the preceding claim, characterized in that said processing involves a modification of a value between 10 and 20% of the "left" component of the "right" channel of the signal processed by the impulse response, as well as of the "right" component of the "left" channel of the signal processed by the impulse response. A 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 specific processing to frequencies between 12000hz and 22000hz with specific processing to harmonics between 2 and 7 without modification of the 1st harmonic, by applying to each corrected signal a high-pass filter and weighted addition of said corrected signal and the processed signal. A 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 processing of frequencies below 150Hz in mono via a reduction of the stereo tracks with application of an acoustic filter. A method for processing a digital sound signal according to claim 1 characterized in that it comprises an additional step consisting of applying a 3D multiband acoustic correction filter defined by the capture distance of the reference system and the volume of the harmonics. A 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 to the fundamental. A method for processing a digital sound signal according to at least one of the preceding claims, characterized in that it 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.

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