AUDIO DIGITIZATION

Abstract

A method of digitizing an audio track carried on an elongate recording medium, such as a movie film, includes transporting the recording medium containing the audio track past a reader to enable sequential reading of the audio track. The reading of the audio track generates an analog output signal. The method also includes sensing a rate of transportation of the recording medium, and sampling the analog output signal at a sampling rate determined on the basis of the sensed rate of transportation to digitize the analog output signal. A system for digitizing audio is also disclosed.

Claims

1. A method of digitizing an audio track carried on an elongate recording medium, the method including: transporting the recording medium containing the audio track past a reader to enable sequential reading of the audio track; reading the audio track with the reader to generate an analog output signal; sensing a rate of transportation of the recording medium; and sampling the analog output signal at a sampling rate determined on the basis of the sensed rate of transportation to digitize the analog output signal.

2. The method according to claim 1, wherein sampling the analog output signal includes: generating a modulator clock signal having a modulator clock frequency based on the sensed rate of transportation; and applying the modulator clock signal to an analog-to-digital converter to digitize the analog output signal.

3. The method according to claim 2, wherein the method further includes adjusting the modulator clock frequency based on an output signal received from an encoder configured to represent the rate of transportation of the recording medium.

4. The method according to claim 2, wherein the modulator clock frequency is determined based on an adjustment frequency and a base clock frequency.

5. The method according to claim 1, wherein sensing the rate of transportation of the recording medium includes: determining a rotational frequency or rotational speed of at least one rotational component of a transport mechanism that carries the recording medium.

6. The method according to claim 1, wherein sensing the rate of transportation of the recording medium includes any one or more of the following: monitoring a rotation of at least one rotational component of a transport mechanism that carries the recording medium; monitoring a rotating element rotationally coupled to at least one rotational component of the transport mechanism, monitoring a linear motion of an element moved by at least one rotational component; and receiving one or more output signals from an encoder rotationally coupled to a rotational component of the transport mechanism.

7. The method according to claim 6, wherein the encoder rotates in synchronization with the rotational component of the transport mechanism.

8. The method according to claim 6, wherein the rotational component of the transport mechanism includes any one or more of a roller, a gear, a sprocket, a capstan, a spindle, an axle, or a spool.

9. The method according to claim 1, wherein reading the audio track includes one or more of: receiving an optical signal representing an optically encoded audio track; or sensing a magnetically encoded audio track.

10. The method according to claim 1, further including: forwarding the digitized analog output signal to a buffer memory, monitoring a status of the buffer memory, and adjusting a modulator clock frequency based on the status of the buffer memory.

11. The method according to claim 1, further including: determining a reference frequency for the digitized analog output signal, based on one or more of the following: a measured rotational frequency or speed of at least one rotational component of a transport mechanism that carries the recording medium; and an input signal that drives a rotational motion of at least one rotational component of the transport mechanism.

12. An audio digitization system, including: a reader configured to read audio information recorded on a recording medium and generate an analog output signal; a transport mechanism configured to move the recording medium carrying the audio information to be digitized, past the reader; and an analog-to-digital converter configured to sample the analog output signal of the reader at a sample rate, wherein said sample rate is dependent on a rate of transportation of the recording medium.

13. The audio digitization system according to claim 12, further including: a clock signal generator configured to generate a modulator clock signal, wherein the analog-to-digital converter is configured to sample the analog output signal of the reader on the basis of modulator clock signal.

14. The audio digitization system according to claim 12, further including a sensing system arranged to sense the rate of transportation of the recording medium.

15. The audio digitization system according to claim 14, wherein the sensing system senses the rate of transportation of the recording medium at a location adjacent to the reader.

16. The audio digitization system according to claim 12, wherein the sensing system is configured to output a reference clock signal that is as one or more of: directly as the clock signal, and by a clock signal generator to generate the clock signal for use by the analog to digital converter.

17. The audio digitization system according to claim 12, wherein the sensing system can include any one or more of the following: an encoder rotationally coupled to a component of the transport mechanism which rotates in synchronization with the transport of the recording medium past the reader, wherein said encoder is optionally coupled to any one or more of a roller, gear, sprocket, capstan, spindle, axle, spool, or the like; a rotating element rotationally coupled to any one of the above.

18. The audio digitization system according to claim 12, wherein the audio digitization system is configured to: transport the recording medium containing the audio track past a reader to enable sequential reading of the audio track; read the audio track with the reader to generate the analog output signal; sense a rate of transportation of the recording medium; and sample the analog output signal at a sampling rate determined on the basis of the sensed rate of transportation to digitize the analog output signal.

19. The audio digitization system according to claim 12, wherein the reader includes any one or more of: an optical reader configured to read an optically encoded audio track; and a magnetic reader configured to read a magnetically encoded audio track.

20. A system including an audio digitization system as claimed in claim 18, wherein the system comprises one or more of: a telecine, a motion picture film scanner, and a video digitizer.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0069] FIG. 1 is a schematic diagram of an audio digitization system according to one embodiment.

[0070] FIG. 2 is a diagram of a motion picture film illustrating the location of an analog audio track in one embodiment.

[0071] FIG. 3 is a schematic diagram showing additional details of an audio digitization system of the type illustrated in FIG. 1.

[0072] FIG. 4 is a schematic diagram showing additional details of another embodiment of an audio digitization system of the type illustrated in FIG. 1.

DETAILED DESCRIPTION

[0073] An exemplary audio digitization system embodying aspects of the invention is shown in FIG. 1. The digitization system 10 is operative to scan a continuously moving elongate recording medium 11 and to digitize the analog sound information recorded on the recording medium 11. FIG. 2 shows an example of a recording medium 11 that may be used in conjunction with the system of FIG. 1. In this example, it is a film 11a. While a preferred embodiment will be described in the context of motion picture film digitization and thus the recording medium 11 is motion picture film having sound recorded as one or more tracks on the film 11a, one would understand that other embodiments of the present invention may be adapted for use with other forms of analog encoded recording medium that requires a transport mechanism, such as reel-to-reel magnetic tape, phonograph cylinders, gramophone records, or the like.

[0074] The film 11a includes one or more analog sound tracks 14 along one or both edges to provide monaural or binaural audio information for the sequence of film frames 13a, 13b. These sound tracks 14 can be of magnetic or photographic form, depending on the recording technology that was used to record the sound. A synchronization track 12, or also known as reference track may also be provided which generally includes a plurality of apertures. Each aperture or a group of apertures may be associated with a respective frame 13a, 13b. The reference track 12 is generally provided for the purposes of transporting and stabilizing the film 11a during film movement. It may also be used as a standard measuring reference with certain camera systems to refer to the size of the frame.

[0075] Now with reference to FIG. 1 again, the recording medium 11 is carried by a transport system 15 to move the recording medium 11 at a substantially constant speed past a reader head 16 to read the analog sound information stored. In the embodiment shown, the reader head 16 has an associated scanning position, where the recording medium 11 is scanned as it is transported past the reader head 16. In this example, the reader head 16 is an optical reader. On one side of the recording medium 11, there is positioned a light source 17 illuminating the recording medium's 11 optical sound track. On the opposite side of the recording medium 11, a reader head 16 is disposed to receive the light transmitted through the recording medium 11 and to produce analog output signals representative of the light transmitted through the recording medium 11. The analog output signals are then transmitted to an analog to digital converter (ADC) 18 to be converted into a digital form. Although FIG. 1 shows a system where sound tracks are optically stored and read, one should understand that aspects of the invention can be readily adapted to be used in other systems where sound tracks may be recorded magnetically and the reader head may be of a magnetic type. In some embodiments, the recording medium 11 can include audio tracks encoded in multiple forms, e.g., optical and magnetic, and the system can include reader heads corresponding to each.

[0076] In accordance with the present disclosure, the analog to digital converter 18 is configured such that it samples the analog output of the reader head 16 at a sampling rate determined on the basis of the sensed rate of transport of the recording medium 11. This sampling rate differs from the target sampling rate of the digitization by a small fraction and increases or decreases to compensate for variations in the transportation speed of the recording medium 11 relative to the reader head 16.

[0077] The rate of transport of the recording medium 11 may be determined in various ways. In one embodiment, the rate of transportation of the recording medium 11 may be determined based on the rotational frequency or speed of at least one rotational component 19 of the transport mechanism 15 that carries the recording medium 11. More specifically, the system 10 may monitor the rotation of a rotational component 19 of the transport mechanism 15 that carries the recording medium 11, or monitor a rotating element (not shown) that is rotationally coupled to the rotational component 19 of the transport mechanism 15. In some cases, the system can monitor the linear motion of an element (not shown) that is moved by a rotational component of the transport system. In a particularly preferred embodiment, rotational component 19 is a precision roller over which the recording medium 11 passes. Such a roller can be machined and mounted on bearings which ensure that the roller is precisely circular (or as precise as machining tolerances practically permit) and is mounted on bearings in such a manner as to run as precisely concentrically as possible. Furthermore, multiple reader heads mounted on diametrically opposing sides of the roller 19 and can be provided to remove the effect of any lack of rotational concentricity in the roller 19.

[0078] FIG. 3 shows more details of the embodiment of FIG. 1. In this embodiment, the transport mechanism 15 includes an encoder 20 rotationally coupled to the roller 19 of the transport mechanism 15. The encoder 20 is configured to generate output signals indicating the rotational position of the encoder 20. For example, the encoder 20 may be configured to generate any suitable number of pulses for each complete revolution of the roller 19. In one form, 512 pulses can be generated, but in other embodiments a higher number, say 1000, may be generated. The output signals of the encoder 20 are received and analyzed by a velocity estimator 21. The velocity estimator 21 has a high frequency clock (e.g., on the order of 150 MHz) and records the number of counts between pulses received from the encoder 20. From this, it calculates fluctuations in the rotational speed of the roller 19 during transport. If it is determined that there is speed variation (from the expected speed), the velocity estimator 21 generates a frequency adjustment signal which is then used in conjunction with a base clock frequency to generate a modulator clock signal for the ADC converter 18.

[0079] As will be appreciated by one of ordinary skill in the art, the sample rate used for digitizing an analog audio signal will typically be around 50 kHz in order to capture the sounds in frequency band having a bandwidth approximately equal to that of human hearing. In the present example, the ADC 18 has a target sample rate of 48 kHz. In this example, the ADC 18 employs a Delta Sigma modulator. As will be appreciated by those skilled in the art, the Delta Sigma modulator has a single bit ADC and this oversamples the analog signal using at a frequency that is much higher than the target sampling rate, e.g., 48 kHz. The oversampling is used by the Delta Sigma modulator to provide the bit depth required for the digitizing the signal. The output signals from the delta sigma ADC may, in some embodiments, be averaged (using the sample rate converter, as described below) and combined to generate a signal equivalent to a conventional ADC sampling at the target frequency, e.g., 48 kHz.

[0080] The ADC 18 is supplied with a modulator clock signal that governs when the ADC takes each successive sample. In this example, the modulator clock frequency is centered at 12.288 MHz, but may vary depending on the required sampling frequency of the output sound signal and in order to maintain the desired sampling frequency at various transport speeds/disturbances. In the present example, the Delta Sigma ADC operates in a manner equivalent to a conventional 48 kHz ADC with a bit depth of 8-bit. In other embodiments, a higher or lower bit depth or sample rate can be used, e.g., having a 24 bit 96 kHz output. According to the preferred embodiment of the disclosure, the modulator clock frequency is adjusted (up or down relative to the central frequency) based on the output of the velocity estimator 21 to compensate for sensed speed variations that occurred during transport of the recording medium 11. The variation is preferably in the range of 2.048 MHz to 15.36 MHz, but higher or lower rates may be used. In the case that the instantaneous transportation speed of the recording medium 11 is slower than desired, the rotational frequency of the roller 19 will be reduced. This is then sensed by the encoder 20, and the velocity estimator 21 will adjust down the frequency of the modulator clock signal. This then increases the time between samples taken by the ADC 18. Conversely, if the transport speed fluctuates above its predetermined speed, the modulator clock frequency will be increased and the actual sample rate attained by the ADC 18 increased accordingly, e.g., between 8 kHz and 60 kHz according to the preferred modulator clock frequency variation noted above.

[0081] Conceptually, consider the simplistic case of a sinusoidal audio wave which should be played back as a constant frequency tone. To sample this tone, it is sufficient to take a sample at each peak and trough of the analog signal, as indicated by ovals marked on waveform 30. Due to transportation speed fluctuations during playback, the analog audio signal read output by the audio reader 16 will not be a perfect sinusoid, but will have portions in which the waveform is compressed in time and portions where it is stretched in time. When a constant sample rate is used (in an attempt to sample the idealized position of the peak and trough of the waveform), the peaks and troughs are missed. However, using an embodiment of the present disclosure, the sampling rate used for digitization of the analog signal is adjusted, e.g., by adjusting the clock signal governing the sampling rate, to match the time-domain disturbances in the waveform. By doing this, the sample that is intended to be taken at the waveform peak more accurately coincides with the peak, and the sample intended to be taken at the waveform trough more accurately coincides with the trough, as indicated by ovals marked on waveform 32. Although the bit stream of the ADC is derived at a variable sample rate, it is output from the ADC at a fixed target bit ratewhich in the above example is 48 kHz.

[0082] The ADC's output bit stream 32 is passed to the audio buffer 34 at its predetermined bit rate. However, it may be necessary to adjust the output bit stream to coincide with that required downstream. This is achieved by using a sample rate converter 36. The sample rate converter receives a reference frequency, e.g., from a component of the transport system, such as roller 38. The reference frequency represents the transport rate of the recording medium 11 relative to the requirement of the output signal. For example, if the digitized output signal represents audio for a 24 frames per second (fps) film, but the actual transport rate for film being digitized is only 12 fps, then twice as many samples will have been accumulated by the buffer memory 34 than are needed to generate the output audio signal. The sample rate converter 36 corrects the output signal for this mismatch.

[0083] In another example, if the digitized output signal represents audio for a 24 frames per second (fps) film, but the actual transport rate for film being digitized was 30 fps, then too few samples will have been accumulated by the buffer memory 34 than are needed to generate the output audio signal. This case is illustrated in wave form 31, in which the sample points marked by ovals are directly produced by the ADC 18. However, this represents an undersampling of the analog signal (essentially because too many frames were digitized each second). In this case, the sample rate converter 36 outputs a data stream comprising 6 data point marked by squares instead, of the 5 sample values marked by ovals. The mechanism for performing sample rate conversion can be any suitable technique known to those skilled in the art.

[0084] A second feedback mechanism can also be applied to control the accumulation of digitized audio in the buffer memory 34. As will be appreciated, the buffer memory 34 must contain sufficient data to feed the sample rate converter, but not overflow. This is controlled by monitoring the buffer usage and generating, using a PID controller 37, a control signal that adjusts the output of the velocity estimator 21 up and down accordingly. The level of adjustment performed by the PID controller 37 will typically be very low frequency (e.g., below audible frequency) and hence will not affect output audio quality.

[0085] FIG. 4 shows a second embodiment of a system of the type shown in FIG. 1. Common features have been given matching reference numerals and will not be described in detail again. The system of the present embodiment differs from that of FIG. 3 in that the present embodiment does not include a sample rate converter 36. This is achieved because the modulator clock signal for the ADC converter 18 is allowed to vary over a wider frequency range than in the previous example.

[0086] By permitting a wider frequency variation by the ADC 18, it is possible for the ADC 18 to handle greater frequency variations, both in terms of the extent of wow and flutter being corrected for, and also in terms of the range of target sampling rate, e.g., between 8 kHz and 60 kHz, although higher or lower sampling rates may be used.

[0087] Again the transport mechanism 15 includes an encoder 20 rotationally coupled to the roller 19 of the transport mechanism 15. The encoder 20 is configured to generate output signals indicating the rotational position of the encoder 20. The output signals of the encoder 20 are received and analyzed by a velocity estimator 21. The velocity estimator 21 records the number of counts between pulses received from the encoder 20. As in the previous embodiments, any suitable number of encoder pulses can be used, but about 1000 per rotation is believed to be suitable. From this, it calculates fluctuations in the rotational speed of the roller 19 during transport. If it is determined that there is speed variation (from the expected speed), the velocity estimator 21 generates a frequency adjustment signal which is then used in conjunction with a base clock frequency to generate a modulator clock signal for the ADC converter 18. The modulator clock signal governs when the ADC takes each successive sample. The modulator clock frequency is adjusted (up or down relative to the central frequency) based on the output of the velocity estimator 21 to compensate for sensed speed variations that occurred during transport of the recording medium 11. The ADC's output bit stream 32 is passed to the audio buffer 34 at its predetermined bit rate. In this example, digitized output signal may be read directly from the buffer memory 34 without resampling. As should be appreciated, the present embodiment can work over the same target sampling rates (e.g., 8 kHz to 60 kHz, or up to 96 kHz or higher) and bit depths (e.g., 8 bit to 24 bit or other bit depth as needed) as the embodiment of FIG. 3.

[0088] A second feedback mechanism can also be applied to control the accumulation of digitized audio in the buffer memory 34. As will be appreciated, the buffer memory 34 must contain sufficient data to output data at the correct sample rate but not overflow. This is controlled by monitoring the buffer usage and generating, using a PID controller 37, a control signal that adjusts the output of the velocity estimator 21 up and down accordingly. The level of adjustment performed by the PID controller 37 will typically be very low frequency (e.g., below audible frequency) and hence will not affect output audio quality.

[0089] It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.