GB2342023A - Audio signal processing - Google Patents

Abstract

An audio signal processor, particularly a dynamics processor such as for signal compression, noise gating or peek limiting, routes an input audio signal in parallel to a main signal path 6 and a side signal path 7. The side signal path has means 30, 40, 50, 70 for deriving a processing parameter to be applied to the signal in the main path by means of a linear multiplier 10. A user adjustable first delay device 80 provides a look-ahead delay which delays the signal in the main path 6 relative to the signal in the side path 7 so that the processing parameter is applied to the main path signal at an appropriate time, having regard to inherent delays in the side path signal processing. A second variable delay device 90 provides a common delay to the main and side path signals. The delay device 90 may be located after the multiplier 10 or before the input signal is divided into paths 6 and 7. A delay controller 100' controls the delay devices 80, 90 so that the sum of the two delays provided by these devices 80, 90 is substantially constant. A user control 65 may set control functions 60 such as an attack time and a decay time. The look ahead delay provided by delay device 80 may be altered in response to the attack time set by user control 65 so that as the attack time decreases so does the delay of device 80, and the delay of device 90 increases such that the sum of the delays remains the same.

Description

2342023 AUDIO PROCESSIN This invention relates to the processing of audio

signals.

In many modem audio signal processing devices, such as audio mixing consoles, "dynamics" processing may be applied to signals which are input to mixing desks, the aim being to produce a non-linear change in the dynamic profile of a sound with the least possible audio disturbance. Examples of dynamics processing techniques include compression to reduce the dynamic range of a signal, or noise gating to remove low-level components from a signal.

In a typical dynamics section, a control parameter is derived from the signal and applied (e.g. via a signal multiplier) to the signal to effect a non- linear change to the signal. To derive the control parameter, the input signal is split so that the processing needed to derive the control parameter is conducted by a "side chain" with the control parameter being applied to a main signal, i.e. separate from the side chain signal.

The side chain processing is generally not instantaneous, and so the control parameter derived by the side chain processing is delayed with respect to the temporal position in the main signal from which that control parameter was derived. Also, some dynamics processes impose their own delay - for example, in a compressor the degree of gain applied to a signal might reduce at a particular "attack" rate when the signal C C) level increases. This can mean that the full effect of a gain reduction is not reached until a certain time after the signal level increase which prompted the gain reduction. During that intervening period, the input signal may have been processed at too high a gain. So, there is a tradeoff between compressors which have to fast a gain variation to give a quick response to sudden level changes but possibly leading to unnatural distortion of the signal due to envelope modulation, fast rates being needed for signal control, and those having slow attack times to give lower sonic intrusion but possibly not responding quickly enough to sudden gain changes.

To overcome these difficulties, therefore, it is known to subject the main signal path to a similar delay so that when the control parameter is applied to the signal it is 2 applied at the correct temporal position in the main signal path. This technique is known as 'look ahead' dynamics. This invention provides audio processing apparatus comprising: means for routing an input audio signal in parallel to a main audio signal path and a side audio signal path; the side audio signal path having means for deriving a processing parameter to be applied to the signal of the main audio signal path; an adjustable first variable delay device arranged to provide a first, differential, delay between the main and side audio signal paths; a second variable delay device arranged to provide a second, common, delay to the main and side audio signal paths; and a delay controller for controlling the delay of the second delay device so that the sum of the first and second delays is substantially maintained at a desired delay period.

The invention recognises that in dynamics processing it may be advantageous I tD C to change the look ahead time and thus the delay in the main signal path, to investigate the effect of such variation on the dynamics processing applied to a particular signal.

However, when the look-ahead delay is changed the overall deiay of the dynarnics'processor is likewise changed. Other signal processing operations may be occurring in parallel with the dynamics processing, however, so it is advantageous if the dynamics processing stage has a reliably constant delay. The invention enables this by providing a delay in the common signal path (before or after the side chain splits off) together with a delay controller. The overall delay and the attack time is set by the user or by signal processing systems, and the delay controller adjusts the delay in the common signal path in order to keep the overall delay of the dynamics processor substantially constant.

In preferred embodiments, where for example the dynamics processing provides a gain compression or peak limiting function, the gain variation has a finite I C) t> attack time, but it can be advantageous to vary that attack time - in effect, varying the 0 )0 rate at which the system gain varies in response to a sudden change in input level.

3 With a longer attack time, the dynamics processor will take longer to reach a target gain in response to a particular impulse, so the lookahead delay (the delay applied to the main signal path) can be correspondingly increased, with the other delay reduced to maintain a substantially constant overall delay as described above. Similarly, a reduction in attack time leads to a reduction in the look-ahead delay but an increase in the common path delay to compensate. In this way, an appropriate amount of lookahead delay is used for the current value of the attack time.

Embodiments of the invention will now be described with reference to the accompanying drawings, throughout which parts are referred to by like references and in which:

Figure I schematically illustrates a dynamics processor according to a first embodiment of the invention; and Figure 2 schematically illustrates a dynamics processor according to a second embodiment of the invention.

Referring now to Figure 1, an input signal 5 is split into a main signal 6 and a C, C Cn side chain signal 7. The side chain signal 7 is used to derive a control parameter which is to be applied to the main signal.

The side chain signal 7 is supplied to a peak detector 30 which determines the envelope of the signal 7 for further processing. The detected envelope is then converted into a decibel representation by a log converter 40. This is so that the time constants, control values and other constants used by the dynamics processing element 50 can relate directly to a decibel law, which in turn makes the implementation of the dynamics processing element more simple and intuitive.

Appropriate processing then takes place in a parameter processor 50 to derive a control parameter, such as a gain control parameter in the case of a compressor, as determined by the user who sets control functions 60 (e.g. an attack time, a decay time and so on).

Once the parameter processing has been performed, the resulting parameter is transformed back into linear form by a log decoder 70. It is then multiplied with the 0 3 main siernal 6 at a linear multiplier 10.

4 Whilst the side chain signal proceeds through these different processing stages ZD it may undergo considerable delay, and the nature of the processing itself - e.g. an attack time dictating how the system reacts to sudden signal level changes - can introduce a delay between a change in the input signal occurring and the control parameter fully reacting to that change. In order to apply the derived control parameter to the main signal at an appropriate time, a variable delay stage 80 applying a delay -cl is incorporated into the main signal path.

In order to keep the overall delay of the dynamics processing stage constant, a further variable delay stage 90 applying a delay '12 is incorporated into the common 10 signal path 8. In this embodiment this occurs after the linear multiplier 10 but it could 0 equally occur before the input signal 5 is split. The delay times of both of the variable delay stages 80, 90 are set by a delay controller 100 which acts to keep the total delay Cr 1 + 'C2) at a substantially constant value set by the user at the controls 110.

Figure 2 illustrates a second embodiment substantially similar to the first embodiment, but in which a controller 60' controls both the parameter processor 50 and a delay controller 100', in response to user control 65 of a system attack time.

It is usual with gain compression and peak limiting devices employing side chains as shown in Figure 2, to apply a gain variation (e.g. via the multiplier 10) having a finite attack time. The choice of attack time is open to variation by the mixing engineer, to achieve a desirable processing effect. However, in previous systems having a fixed look-ahead delay, too fast an attack time will cause the signal to "suck out" as the gain changes quicker than the delay period. Too slow an attack time may allow overshoot.

Accordingly, in this embodiment the look-ahead delay period rl and the common path delay periodr2 are controlled by the delay controller in response to the controller 60. As before, the delay controller ensures that the sum of the two delays, T1 + T, remains substantially constant, but the look-ahead delay T1 is altered in response to the attack time as set by the user with the control 65. So, as the attack time decreases, the delay T1 is also decreased, for example in a linear relationship to attack )o time. (As an alternative, a law relating attack time to delay period can be established empirically and stored in a simple read-only memory or applied using a microprocessor). In this way, an appropriate look-ahead delay period is used for different values of attack time, but the overall delay of the apparatus remains at least substantially constant.

6

Claims (1)

1. Audio processing apparatus comprising:

means for routing an input audio signal in parallel to a main audio signal path and a side audio signal path; the side audio signal path having means for deriving a processing parameter to be applied to the signal of the main audio signal path; an adjustable first variable delay device arranged to provide a first, differential, delay between the main and side audio signal paths; a second variable delay device arranged to provide a second, common, delay to the main and side audio signal paths; and a delay controller for controlling the delay of the second delay device so that the sum of the first and second delays is substantially maintained at a desired delay period.

is 2. Apparatus according to claim 1, in which the desired delay period is a predetermined constant delay period.

J. Apparatus according to claim 1, in which the desired delay period is user-adjustable.

4. Apparatus according to any one of claims 1 to 3, in which the first delay device is user-adjustable.

5. Apparatus according to any one of claims 1 to 3, in which:

the side signal path comprises means for generating a gain control parameter for controlling a gain applied to the signal of the main signal path so that gain variation occurs at a rate no greater than a user-adjustable attack rate; and the first variable delay device is operable to apply a delay dependent upon the attack rate.

7 6. Audio mixing apparatus comprising apparatus according to any one of claims 1 to 5.

7. Audio processing apparatus substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.