DE102017123068A1 - System for synchronizing audio or video recordings - Google Patents

Abstract

The invention relates to a system (1) for synchronizing audio or video recordings, comprising: a tone generator (2) for generating an audio reference signal (10) comprising an ID (14) and a temporal mark (13) and by means of a loudspeaker to the environment, a plurality of sound recording devices (3a-3d) which receive the sound generator (2) emitted audio reference signal (10) and each a sound recording (8a-8d) and optionally also a video recording (7b, 7c) and a computer unit (4), which is in communication with the sound recording devices (3a-3d) and which in each of the sound recordings (8a-8d) produced by the sound recording devices (3a-3d) in the audio reference signal (10 ) and the individual sound recordings (8a-8d) or optionally also video recordings (7b, 7c) synchronized therewith in accordance with the temporal marking (13).

Description

The invention relates to a system for synchronizing audio or video recordings recorded by different recording devices at the same time.

BACKGROUND OF THE INVENTION

For movie or sound recordings, multiple recording devices are usually used simultaneously to record a scene from different perspectives or the sound at different locations. The various recordings are usually stored in a data processing device and can be reworked or cut there with the aid of audio or video editing software.

For example, if a movie scene or song is recorded simultaneously by multiple recorders, the individual audio or video data is often processed independently. There is thus no relative temporal relationship (no synchronization relationship) between each audio or video track. When cutting the shots, the cutter would have at each point where a cut is to take place within a scene, the corresponding, i. simultaneously recording, video frame, or the same positions within the audio sequences of two audio or video recordings. Historically, the flap was used here. This procedure is very time-consuming because all recordings have to be manually searched and edited.

In order to facilitate this work, so-called "locks" are used in professional film productions, such as the company Ambient Recording GmbH, with which synchronizes the recordings made by the individual recording devices, d. H. can be put into an exact temporal relationship. The mentioned locks each have a timer, which provides a very precise time code, which is then recorded along with the sound or video recordings. The individual audio or video recordings can be synchronized automatically. However, each recorder requires its own lockit, which connects directly to the recorder via a cable. In addition, the individual locks must be synchronized prior to recording so that they deliver the same time code at the same time. This method is therefore relatively complicated and expensive.

From the US Pat. No. 8,205,148 B1 In addition, a method is known with which different media sources can be synchronized. In these methods, similarities between two audio tracks are searched for and, depending on the correlation of the audio signals, the temporal relation to each other is determined.

In both methods, there is no possibility to transmit additional meta-information during the recording, so that the editor has to manually review / listen to all the recordings in order to assign them to a specific recording situation (eg scene 1 , Shot 2 , Take 4 ).

OBJECT OF THE INVENTION

It is therefore an object of the present invention to provide an alternative system for synchronizing sound or video recordings as well as a similar method which operates simply and reliably, is also inexpensive and saves additional time in post-production.

This object is achieved according to the invention by the features specified in the independent claims. Further embodiments of the invention will become apparent from the dependent claims.

• - einen Tonerzeuger zur Erzeugung eines Audio-Referenz-Signals, das eine ID und eine zeitliche Markierung umfasst und mittels Lautsprecher an die Umgebung ausgesendet wird,
• - mehrere Tonaufzeichnungsgeräte, die das vom Tonerzeuger ausgesendete Audio-Referenzsignal aufnehmen und jeweils eine Tonaufnahme machen und gegebenenfalls auch weitere Informationen ( z.B. Video) aufzeichnen, und
• - eine Rechnereinheit, welche die Ton- und ggf. weitere Aufzeichnungen lesen kann und in den einzelnen, von den Tonaufzeichnungsgeräten erzeugten Tonaufnahmen jeweils die im Audio-Referenzsignal enthaltene ID und die zeitliche Markierung ermittelt und die einzelnen Tonaufnahmen oder gegebenenfalls auch damit gekoppelte Videoaufnahmen anhand der zeitlichen Markierung synchronisiert.

According to the invention, a system for synchronizing sound or video recordings is proposed, which comprises at least the following components:

• a tone generator for generating an audio reference signal comprising an ID and a time mark and sent out to the environment by means of a loudspeaker,
• a plurality of sound recording devices which record the audio reference signal sent by the sound generator and record sound in each case and optionally also record further information (eg video), and
• a computer unit which can read the audio and possibly further recordings and in each of the sound recordings produced by the sound recording devices each determines the ID contained in the audio reference signal and the temporal mark and the individual sound recordings or possibly also coupled video recordings based on synchronized time mark.

As a tone generator, for example, any conventional mobile device, such as a smartphone, can be used on which runs a corresponding software application.

The sound or video recordings made by the sound recording devices are, as mentioned above, synchronized depending on the temporal mark contained in the audio reference signal. The term "synchronizing" should be understood to mean that based on the at least one common reference time is determined in the individual sound recordings contained temporal marking. The sound sequences or video frames contained in the recordings can then be z. B. along a common (time) bar are positioned by the sound sequences or video frames contained therein, ie the corresponding audio or video data, supplemented with certain information, ie be referenced. This information can be z. B. be a temporal information. Additionally or alternatively, another information or reference size, such. For example, numbering, metadata or a relational reference can be used to reference the audio or video data.

The ID contained in the audio reference signal serves to uniquely identify the respective audio or video recording. When editing or editing the sound or video recordings thus related, that is, simultaneously made, recordings can be easily identified and z. B. be grouped.

An ID can basically include any characters, such as. As numbers, letters, special characters or a code. In the simplest case, for example, the recordings made by a recording device can be made with numbers " 1 . 2 . 3 . 4 . 5 , etc .... ".

According to a preferred embodiment of the invention, it is provided that the audio reference signal contains further information (metadata), in particular information relating to the sound or video recording made. This additional information or metadata, for example, scene information such. B. "Scene 3 , Take 2 ", Camera settings, a title, a project name, a time, geographical position data, etc. include. The information can be entered, for example, by the user of the system on his mobile device running the software application according to the invention. Alternatively, the software application could also generate such metadata automatically.

The temporal marking contained in the audio reference signal can be, for example, one or more pulses, a chirp, pink noise or a pseudo-random sequence. The temporal marking should be an audio signal that stands out well from other environmental sounds and can be detected easily and timely. The person skilled in various realization options from the prior art are well known.

The present invention is particularly relevant to those sound recorders that include a clock generating a clock in which the digital sound sequences or video frames are then generated. The clocks of different audio recording devices operate at about the same frequency, but the clock frequencies are not exactly the same in the rule. For example, a first clock may clock slightly faster than 48 kHz and a second clock slightly slower than 48 kHz. In the subsequent sound or video editing, this means that sound sequences or video frames with the same reference (for example, the sample or the video frame No. 5000 in two different video recordings) do not correspond to each other, because they were recorded at different times. In order to correct this systematic timing error (this is also referred to as clock error), it is proposed to generate an acoustic clock correction signal, send it via the sound generator to the environment via sound and record it with the sound recording devices located in range.

As a clock correction signal, for example, two or more pulses in a defined time interval, or a periodic signal whose period is determined accurately enough, can be used. In the sound or video recordings is then determined how many sound sequences or video frames in a period or period - or a multiple thereof - are included. As a result, the difference of the clock frequencies can be measured with each other and taken into account in the sound or video editing. In the case of a slightly higher clock frequency of a clock, the sound sequences or video frames of the associated audio or video recording can be played, for example, a little slower; in the case of a slightly lower clock frequency of a clock, however, the sound sequences or video frames can be played a little faster. The sound or video recordings of different recording devices can thus be played synchronously in time. In addition, the above-mentioned reference of the individual sound or video recordings can be corrected accordingly, so that simultaneously recorded sound sequences or video frames can be found quickly and clearly.

According to one embodiment of the invention, in addition to the ID and the temporal marking, the audio reference signal may also comprise a start sequence by means of which the audio reference signal can be identified. The start sequence may be, for example, a sinusoidal signal of a defined length.

The clock correction signal may be part of the audio reference signal, but it could also be a separate audible signal.

According to a specific embodiment of the invention, the temporal marker and the clock correction signal may be one and the same signal. The temporal marking can also be part of the clock correction signal.

A sound recording device according to the invention may be a device which exclusively records sound. Under this name, however, are synonymous devices that can make both sound and video or other media recordings. In a sound recording device that can make both sound and video recordings, the sound and video data are preferably timed with each other. In this case, the audio track - if it contains an audio reference signal - can be used to synchronize the video recordings of different recording devices, i. to be in temporal relation to each other.

The tone generator can be connected to the computer unit via an additional communication connection in order to transmit to the computer unit the data transmitted by the tone generator via sound and possibly also other inputs via a different transmission path, ie not via sound. The additional communication connection may be, for example, a wired or wireless connection, such as a wireless connection. B. a Bluetooth or WIFI connection, over which the said data are transmitted in particular digitally.

The computer unit is preferably set up to carry out a correlation analysis between the time signal originally sent by the tone generator and each one of the sound recording devices acoustically received. With the help of the correlation analysis, the temporal marking can be clearly and accurately recognized.

• - Erzeugen eines Audio-Referenzsignals, das eine ID und eine zeitliche Markierung umfasst, mittels eines Tonerzeugers, und Übertragen des Audio-Referenzsignals via Schall an die Umgebung;
• - Aufnehmen des vom Tonerzeuger ausgesendeten Audio-Referenzsignals mit mehreren Tonaufzeichnungsgeräten, die jeweils eine Tonaufnahme und gegebenenfalls auch eine Videoaufnahme erzeugen; und
• - Für jede der von den Tonaufzeichnungsgeräten gemachten Tonaufnahmen: Ermitteln der im Audio-Referenzsignal enthaltenen IDs der zeitlichen Markierung.
• - Für alle Tonaufnahmen mit gleicher ID: Synchronisieren der einzelnen Tonaufnahmen oder damit gekoppelter Videoaufnahmen in Abhängigkeit von der zeitlichen Markierung mit ggf. Korrektur des Uhrenfehlers.

The present invention also relates to a method for synchronizing sound or video recordings recorded by different recording devices, the method comprising at least the following steps:

• - generating an audio reference signal comprising an ID and a temporal mark by means of a tone generator, and transmitting the audio reference signal via sound to the environment;
• Recording the audio reference signal transmitted by the tone generator with a plurality of sound recording devices, each of which generates a sound recording and possibly also a video recording; and
• - For each of the sound recordings made by the sound recorders: Determine the IDs contained in the audio reference signal of the time mark.
• - For all sound recordings with the same ID: Synchronization of the individual sound recordings or video recordings coupled with it as a function of the time marking with correction of the clock error if necessary.

list of figures

• 1 eine schematische Ansicht eines Systems zum Synchronisieren von Ton- oder Videoaufnahmen, die von verschiedenen Aufzeichnungsgeräten aufgenommen wurden;
• 2 eine schematische Darstellung der in einer Aufnahme enthaltenen Daten;
• 3 eine schematische Darstellung eines beispielhaften Audio-Referenzsignals;
• 4 ein Verfahren zum Generieren eines Audio-Referenzsignals;
• 5 ein Verfahren zum Synchronisieren bzw. Referenzieren von Ton- oder Videoaufnahmen, die von verschiedenen Tonaufzeichnungsgeräten gemacht wurden; und
• 6 eine Anordnung mehrerer Aufnahmen, die von verschiedenen Tonaufzeichnungsgeräten gemacht wurden, wobei die einzelnen Aufnahmen entlang einer gemeinsamen Zeitleiste zeitlich positioniert sind, so dass gleichzeitig aufgenommene Audiodaten oder Videoframes unmittelbar übereinander angeordnet sind.

The invention will now be described by way of example with reference to the accompanying drawings. Show it:

• 1 a schematic view of a system for synchronizing sound or video recordings that have been recorded by different recording devices;
• 2 a schematic representation of the data contained in a recording;
• 3 a schematic representation of an exemplary audio reference signal;
• 4 a method of generating an audio reference signal;
• 5 a method of synchronizing sound or video recordings made by different sound recorders; and
• 6 an arrangement of several recordings, which were made by different sound recording devices, wherein the individual recordings are positioned along a common timeline, so that simultaneously recorded audio data or video frames are arranged directly above one another.

Embodiments of the invention

1 shows a schematic block diagram of a system 1 to synchronize sound 8th or video data 7 , simultaneously from different sound recording devices 3a - 3d were recorded. As can be seen, the system includes multiple sound recorders 3a - 3d that have a transmission path 19 with a computer unit 4 keep in touch. The individual sound recording devices 3a - 3d are set up to record at least one audio signal. One or more of the sound recording devices 3a -3d can also include, for example, a camera to record videos.

In operation of the system 1 are all sound recording devices 3a - 3d switched on and take depending on the version simultaneously either audio or audio and video signals from the Environment up. The associated, from the sound recording devices 3a - 3d generated sound 8th and video recordings 7 be sent to the computer unit 4 where they can be reworked or cut using software.

The system 1 further comprises a tone generator 2 for generating an audio reference signal 10 by means of which of the various sound recorders 3a - 3d taken pictures 5 respectively. 5a - 5d (please refer 6 ) can be synchronized. A subsequent processing or cutting process can be significantly simplified. The cutter can in particular easily and accurately to a specific point within the individual shots 5 . 5a - 5d jump and simultaneously edit recorded sound sequences or video frames.

At the tone generator 2 For example, it may be a mobile terminal, such. As a smartphone, a tablet computer or the like, which includes an integrated speaker.

That from the tone generator 2 emitted audio reference signal 10 is used by all sound recorders 3a - 3d received via microphone. The corresponding sound waves are denoted by the reference numeral 20 designated. The audio reference signal 10 as exemplified in 3 serves to provide a temporal reference between those of different sound recorders 3a - 3d taken pictures 5a - 5d so that simultaneously recorded audio sequences or video frames can be easily and quickly identified as such and edited. In addition, several audio or video tracks can be played synchronously or displayed in the correct time position.

2 schematically shows the structure of a typical recording 5 (or the recording data), as used by the sound recording devices 3a - 3d is created. In the illustrated embodiment, the recording includes 5 both a sound recording 8th as well as a video recording 7 , In addition, it contains a time signal 9 and metadata 6 ,

The sound recorder in question 3a - 3d in this case has a clock generator which generates a clock signal via which the sound 8th and video recording 7 are coupled with each other in time, whereby 8th and video recording 7 there is a synchronization relationship.

The above metadata 6 may include, but is not limited to, a file name, a recording time, codecs used, camera or microphone settings, location data, or any other information.

3 shows a schematic representation of a typical audio reference signal 10 as it is from the tone generator 2 is sent via sound. The audio reference signal 10 in this embodiment comprises a start sequence 11 , Metadata 12 as well as a time marking 13 , For a startup sequence 11 For example, it can be a sinusoidal signal of a specific frequency and length. It essentially serves to better detect the audio reference signal 10 , The in the audio reference signal 10 contained metadata 12 are preferably modulated by a method known in the art, e.g. B. demodulated by means of phase-shift keying or frequency-shift keying and on the receiver side again.

The metadata 12 include at least one ID 14 for identification of the sound recording devices 3a - 3d currently made recordings 5 . 5a - 5d , At the ID 14 For example, it can be a sequence of numbers, any string, or any other code that contains the audio reference signal 10 and thus the current recording 5 clearly identifies. In addition, the metadata can 12 also a time information, scene information, such as "scene 2 , Take 5 , Information about camera settings, geodata or other parameters 15 include.

The time marking 13 For example, a chirp, that is, a sinusoid rising in frequency, may be pink noise, a pseudorandom sequence, or at least one pulse. It is to be chosen so that the exact time position can be determined from the recording.

Furthermore, the Audi reference signal 10 Also include a clock correction signal by means of which the clock error of the sound recording devices can be determined. As a clock correction signal, for example, two or more pulses in a defined time interval, or a periodic signal whose period is determined accurately enough, can be used. In the sound or video recordings is then determined how many sound sequences or video frames in a period or period - or a multiple thereof - are included. Thereby, the difference of the clock frequencies from the clocks of the different sound recording devices can be measured and taken into account in the sound or video editing. In the case of a slightly higher clock frequency of a clock, the sound sequences or video frames of the associated audio or video recording can be played, for example, a little slower; in the case of a slightly lower clock frequency of a clock, the sound sequences or video frames, however, something be played faster. The sound or video recordings of different recording devices can thus be played laterally synchronously. In addition, the above-mentioned reference of the individual sound or video recordings can be corrected accordingly, so that simultaneously recorded sound sequences or video frames can be found quickly and clearly.

The on the tone generator 2 Running software application preferably offers the user the option of certain metadata 12 such as entering scene information. Other metadata 12 , such as Eg the ID 14 , are preferred for each audio reference signal 10 automatically generated.

4 shows a diagram of a general method for creating an audio reference signal 10 , The procedure begins in step S1 with that a user on the tone generator 2 , such as a smartphone, metadata 12 enters. The on the tone generator 2 running software application generated in step S2 an ID 14 , From the metadata entered by the user 12 and the automatically generated ID 14 it will be in step S3 a modulated audio reference signal 10 generated, then in step S4 played by loudspeaker. The audio reference signal 10 spreads via sound and is from all sound recording devices 3a - 3d received about the same time. The audio reference signal 10 is part of every shot 5 that of the individual sound generating devices 3a - 3d is done.

5 shows schematically a method with which of the individual sound recorders 3a - 3d made sound 8a - 8d or video recordings 7b . 7c can be synchronized. After the sound recording devices 3a - 3d each made recordings 5a - 5d to the computer unit 4 have been transferred, search in step S10 one on the computer unit 4 running editing software in the audio tracks of each recording 5a - 5d First, the start sequence 11 the individual audio reference signals 10 , Once the individual start sequences 11 are identified, the processing software then determines in step S11 each in the audio reference signals 10 included time marker 13 , In this step, watch error and impulse response of the room may also be analyzed. Assuming that from the tone generator 2 played audio reference signal 10 from all sound recording devices 3a - 3d is received about simultaneously, thereby a common reference time is known at which a particular video frame or a particular audio sequence was recorded. In step S12 becomes the position of the time marker 13 relative to the time signal 9a . 9b , etc. stored. All other sound 8a - 8d and video recordings 7b . 7c can then be clearly referenced, so that each video frame or each sound sequence within a first recording (eg recording 5b) in relation to a video frame or a sound sequence within a second recording (eg the recording 5c) is clearly marked and thus can be edited automatically.

Unless the different sound recorders 3a - 3d spatially distributed so that significant differences in transit time in the sound transmission of the audio reference signal 10 If necessary, these can be deducted later. For this purpose, z. B. the geodata of the individual sound recording devices 3a - 3d and the tone generator 2 be used. Alternatively or additionally, there would be other mathematical approaches to calculate such differences in transit time, which are well known to those skilled in the art. So z. B. by means of the tone generator 2 a further correction signal can be emitted via sound - or another of the aforementioned acoustic signals can be used for this purpose - with which the characteristic of the room (Hall) can be measured. The resulting information (such as the impulse response of the room) can eventually be used to correct the digital audio or video recordings. This preprocessing can also be used to improve demodulation in step S13 serve, if the process is impaired, for example because of a long reverberation. As a correction signal can z. As an impulse or pseudorandom noise serve. Methods known to those skilled in the art are known in the art.

In step S13 Finally, those in the audio reference signals 10 contained metadata 12 demodulated and all collected data (possibly together with the over 17 transferred data) in step S14 saved.

A particularly accurate determination of the temporal marking 13 can be done, for example, that between the temporal markings contained in the individual sound recordings 13 and originally from the tone generator 2 generated temporal mark 13 a correlation analysis is performed. This is the original time marker 13 eg constant, ie in S3 and S13 known according to condition, or it may be about a separate communication connection 17 to the computer unit 4 be transmitted. In particular, the transmission does not take place via sound, but preferably in digital form, for example via a radio link. After carrying out the method described above, the individual shots 5a - 5d and further, subsequently made recordings in any way z. B. grouped by the metadata, arranged in time and edited.

6 Finally, shows a schematic arrangement of various, from the sound recording devices 3a - 3d taken pictures 5a - 5d , The in the individual audio data 8a - 8d included time marker 13 is in each case marked by a triangle symbol. The individual shots 5a - 5d are temporally positioned so that the temporal markings 13 on a global timeline 18 be at the same time position. Instead of referencing using a global timeline 18 can of course also another reference size, such. As a sequence of numbers, or other reference values are used. Contains the audio reference signal 10 Also, a proportion with which the clock error of the various clocks can be corrected, so the systematic speed difference of the clock in S11 determined. Based on this information will be the recordings 5a - 5d according to the maturities 9a - 9d corrected, if supported by the further processing.

In the illustrated embodiment, the audio recording device comprises 3a taken picture 5a metadata 6a , Audio data 8a and a time track 9a , The sound recorder 3b created recording 5b includes metadata 6b , Video data 7b , Audio data 8b and a time track 9b , The sound recorder 3c taken picture 5c includes metadata 6c , Video data 7c , Audio data 8c and a time track 9c , The sound recorder 3d taken picture 5d includes only metadata 6d and audio data 8d ,

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 8205148 B1 [0005]

Claims (12)

A system (1) for synchronizing audio or video recordings comprising: a tone generator (2) for generating an audio reference signal (10) comprising an ID (14) and a temporal mark (13) and emitted by means of loudspeakers to the environment, a plurality of sound recording devices (3a-3d) which record the audio reference signal (10) emitted by the tone generator (2) and respectively generate a sound recording (8a-8d) and possibly also a video recording (7b, 7c), and a computer unit (4) which can read the sound recordings (8a-8d) produced by the sound recording devices (3a-3d) and in each case determines the temporal marking (13) contained in the audio reference signal (10) and records the individual sound recordings (8a-8a) 8d) or possibly also video recordings (7b, 7c) synchronized therewith in accordance with the temporal marking (13). System (1) to Claim 1 , characterized in that at least one of the sound recording devices (3a-3d) also makes video recordings (7) which are temporally coupled to the sound recordings (8). System (1) to Claim 1 or 2 , characterized in that the audio reference signal (10) in addition to the ID (14) and the temporal mark (13) and metadata (12). System (1) according to one of the preceding claims, characterized in that the audio reference signal (10) lifting the ID (14) and the temporal mark (13) also comprises a start sequence (11). System (1) according to one of the preceding claims, characterized in that the sound recording devices (3a-3d) each comprise a clock generating a clock signal on the basis of digital sound sequences or video frames are generated and that the computer unit (4) the differences of the Detected clock frequencies of various clocks and this in the synchronization of sound or video recordings (7,8), which were made by different sound recorders (3a-3d), taken into account. System (1) according to any one of the preceding claims, characterized in that the tone generator (2) via a communication link (17) with the computer unit (4) is in communication via the data taken during or after the recording, such. B. metadata (5a-5d) are transmitted separately to the computer unit (4). System (1) according to one of the preceding claims, characterized in that a cross-correlation between a time mark (13 in 8a-8d) recorded by the sound recording devices (3a-3d) and the original time mark (13) contained in the audio reference signal (10). is carried out. System (1) according to one of the preceding claims, characterized in that the temporal marking (13) comprises one or more pulses, a chirp, pink noise or a pseudorandom sequence. System (1) to Claim 3 , characterized in that the metadata (12) contained in the audio reference signal (10) may include time information, information about the recording, the scene, camera settings or any other information. Method for synchronizing sound (8) or video recordings (7) recorded by different recording devices (3a-3d), the method comprising at least the following steps: - generating an audio reference signal (10) comprising an ID (11) and a temporal mark (13) by means of a tone generator (2), and transmitting the audio reference signal (10) via sound to the environment; Recording the audio reference signal (10) emitted by the tone generator (2) with a plurality of sound recording devices (3a-3d), each of which generates a sound recording (8a-8d) and possibly also a video recording (7b, 7c); and For each of the sound recordings (8a-8d) made by the sound recording devices (3a-3d): determining the temporal mark (13) contained in the audio reference signal (10) and synchronizing the individual sound recordings (8a-8d) or video recordings coupled thereto ( 7b, 7c) as a function of the time mark (13). Method for synchronizing audio (8) or video recordings (7) recorded by different recording devices (3a-3d), characterized in that the tone generator (2) generates a clock correction signal and sends it via sound to the environment, and the clock correction signal is used to adjust the playback speeds of sound (8a-8d) or video recordings (7b, 7c) made by different sound recorders (3a-3d). Method for synchronizing sound (8) or video recordings (7) recorded by different recording devices (3a-3d), characterized in that the tone generator (2) generates a further correction signal by means of whose propagation time differences in the sound transmission of the audio reference signal (10) between the tone generator (2) and different sound recording devices (3a-3d) can be determined, and that the determined transit time differences are used by the computer unit (4) to the temporal position of sound - (8a-8d) or video recordings (7b, 7c) made by different sound recorders (3a-3d).

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