GB2385420A - Measuring the perceived loudness of an audio signal - Google Patents

Abstract

The perceived loudness of an audio signal is measured by filtering the audio signal with a filter 92 designed to select a parameter representative of the perceive loudness level as a function of frequency. The resultant audio signal can then be output 98 to a to a metering device such a display to give an indication of the perceived noise levels at a location. The device is designed for giving a better loudness indication of isolated dialogue phases such as those occurring in commentary channels or descriptive audio services.

Description

Apparatus and Method for Measuring Loudness This invention relates to an

apparatus and method for measuring loudness for a programme audio signal.

It is well known for audio signals with varying programme contents to vary widely in 5 reproduced perceived loudness, despite the actual signal excursions of all the signals being of comparable amplitudes. In an attempt to indicate loudness, known programme level meters and metering methods, such as those covered by IEC Standard 60268 parts 7 and 10, are designed with some electrical and/or mechanical compensations for perceptual loudness factors. However, these meters primarily serve the engineering function of signal level excursion indication, and 10 the compromise loudness indication is often found to be inadequate or inaccurate.

Many types of specific audio loudness meter have been specified in the literature, for example those covered by US-A-3,031,528 and US-A-4,52S, 501. Typically the specifications of

such meters include means to filter the input audio signal frequency response according to an 15 estimate of the perceived loudness against frequency. For example, the filtering characteristics may follow those characteristics specified in the ISO 226 standard at a given estimate of the nominal listening level. Compensation means may also be incorporated in the metering method to compensate for the increase in perceived loudness as the audio bandwidth of the programme signal increases. In order to display the resultant measurement a form of visual display with 20 controlled rise and fall times, or with a controlled integration time, is typically included.

Such meters have not enjoyed widespread use in the past, possibly because of the overriding importance of signal level control in order to prevent circuit overloading which might result in audible distortion. Moreover, the introduction of additional and possibly unfamiliar

25 visual displays of loudness would be disadvantageous to an operator.

It is an object of the invention at least partially to overcome the foregoing disadvantages.

According to a first aspect of the present invention there is provided a loudness measuring 30 apparatus for a programme audio signal, the apparatus comprising an audio input for inputting the programme audio signal to be measured, frequency filter means having parameters representative of perceived loudness levels as a function of frequency connected to the audio input to filter the programme audio signal and to output an output audio signal representative of perceived loudness of the programme audio signal for input to a programme level meter.

Advantageously, the parameters are selectable dependent on a sound pressure level of the programme audio signal.

Conveniently, there is also provided bandwidth compensator means for dividing the programme audio signal into sub-bandwidths of the signal and outputting sub-bandwidth signals 5 representative of a signal strength in each sub-bandwidth respectively, and summation means for summing the subbandwidth signals to compensate for perceived loudness dependent upon a width of bandwidth occupied by the programme audio signal.

Advantageously, the summation means includes means for comparing the signal strength in the sub-bandwidths and compensating for mutual loudness masking effects of the respective sub 10 bandwidth signals.

Conveniently, the summation means is arranged to perform a non-arithmetic summation.

Preferably, the summation means includes a non-linear summation look-up table for compensating for the loudness masking effects.

Conveniently, there is provided sample and hold means for maintaining the output audio 15 signal at an immediately preceding output level during a predetermined period of time during which a signal level of the output audio signal falls below a predetermined threshold level.

Advantageously, the sample and hold means comprises comparator means for determining when the signal level falls below the predetermined threshold level.

Conveniently, the apparatus comprises a visual display unit for displaying a 20 representation of the output audio signal.

Advantageously, the visual display unit comprises a audio oscillator modulated by a modulator dependent on the output audio signal.

In a feature of this invention, the loudness measuring apparatus is provided in combination with a programme level meter for receiving the output audio signal and displaying the perceived 25 loudness level.

Advantageously, bypass means are provided for selectively inputting the programme audio signal to the apparatus for generating an output signal for input to the programme level meter or for inputting the programme audio signal directly to the programme level meter.

According to a second aspect of the invention there is provided a method for measuring 30 loudness of a programme audio signal, the method comprising the steps of: a) filtering an applied programme audio signal with frequency filter means having parameters representative of perceived loudness levels as a function of frequency; and b) outputting an output audio signal

representative of perceived loudness of the programme audio signal for input to a programme level meter.

Conveniently, step a) includes the step of selecting the parameters dependent on a sound pressure level of the programme audio signal.

5 Advantageously, the method comprises the further steps of dividing the programme audio signal into sub-bandwidths of the signal and outputting sub-bandwidth signals representative of a signal strength in each subbandwidth respectively and summing the sub-bandwidth signals to compensate for perceived loudness dependent upon a width of bandwidth occupied by the programme audio signal.

10 Conveniently, the step of summing the sub-bandwidth signals includes the step of comparing the signal strength in the sub-bandwidths and compensating for mutual loudness masking effects of the respective subbandwidth signals.

Preferably, the step of summing the sub-bandwidth signals comprises performing a non arithmetic summation.

IS Conveniently, the step of summing the sub-bandwidths includes using a non-linear summation look-up table for compensating for the loudness masking effects.

Advantageously, the method comprises the further step of sampling and holding the output audio signal at an immediately preceding output level during a predetermined period of time during which a signal level of the output audio signal falls below a predetermined threshold 20 level. Conveniently, the step of sampling and holding the output audio signal comprises using comparator means to determine when the programme signal level falls below the predetermined threshold level.

Preferably, the method further comprises the step of displaying a representation of the 25 output audio signal on a visual display unit.

Advantageously, the step of displaying a representation of the output audio signal comprises modulating an audio oscillator using a modulator dependent on the output audio signal. Digital audio signal environments are now common, and within them a nominal sound 30 reproduction level can be referred to the digital signal excursion level at any point in a signal chain from source to listener. Thus, a margin of signal excursion before overloading occurs may be measured and managed at any point in this type of signal chain. In many cases such signal

level management can be undertaken without operator intervention. For this reason, a metering system based on loudness may prove more effective for balancing the audio content of a programme or recording, now that most audio signal chains utilise digital coding. The multi-

channel stereo content of many digital audio programmes also favours metering based on 5 loudness criteria, because a single loudness reading is easier to interpret than multiple signal level displays which are otherwise necessary.

If a loudness indication is used to control the loudness of the dialogue content of an audio programme an additional benefit should accrue, namely that the intelligibility of the reproduced 10 dialogue to the listener should improve. This is because any particular listener will more easily be able to optimise the reproduced audio level to suit the listening environment, without having constantly to adjust the reproduced level as the programme loudness varies.

Given these advantages accruing from an audio metering method based on an estimation of 15 the reproduced loudness, the present system offers improvements in the use of a level adaptive filter algorithm, and in addition enables the re-use of currently installed and familiar programme level meters by converting the audio signal loudness measurement into a single loudness based continuous electrical output tone or bit-stream. This continuous electrical output may then be used to replace the normal programme signal input into known installed programme level 20 meters.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: 25 Figure I is a schematic diagram of an apparatus in accordance with this invention; Figure 2 is a more detailed schematic diagram of a signal frequency filter of the apparatus of Figure 1; Figure 3 is a more detailed schematic diagram of a bandwidth compensator of the apparatus of Figure 1; 30 Figure 4 illustrates a look-up table for use in a integrator of the apparatus of Figure 1; Figure 5 is a circuit diagram for a sample and hold circuit used in the apparatus of Figure 1; Figure 6 is a schematic diagram of a visual display unit used in the apparatus of Figure 1; Figure 7 is a flowchart of a method used in this invention.

35 In the Figures, like reference numerals denote like parts.

The apparatus shown in Figures 1 and 2 has audio inputs 1 connected by lines 10 to an input 110 of a signal frequency filter 2. Although three input channels are illustrated in Figure 1, two by broken lines, it will be appreciated that the invention has equal applicability to a system

5 having fewer or more input channels. The frequency filter 2 has filter characteristics corresponding to a known dependency of perceived loudness on frequency and outputs an audio signal representative of the perceived loudness from an output terminal 120 of output line 20.

Preferably, the filter characteristics are adjustable dependent on an intended reproduction sound level of the audio input. For this purpose, as illustrated in Figure 2, the input 110 of the signal 10 frequency filter 2 is connected in parallel by lines 101, 102 and 103 to three filters 21, 22 and 23 respectively corresponding to known dependencies of perceived loudness on frequency at different sound levels. Although three filters are illustrated it will be understood that the number of filters may be varied, dependent on the specificity required. The outputs of the respective filters are connected by lines 211, 212 and 213 respectively to respective inputs of a selector 15 switch 26, the output of which is connected by line 261 to the output terminal 120 of the signal filter 2. However, the output of the selector switch is also connected by line 262 to an input of an overall signal level detector 24 which is used to control the selector switch 26 to select an output from one of the three filters 21, 22 and 23 dependent on the strength of the signal detected by the signal level detector 24. Preferably a known hysteresis circuit 25 is included in series between an 20 output line 241 for the comparator 24 and a control line 251 for the selector switch, to prevent the selector switch hunting between filters during short-lived variations in sound level.

It will be appreciated that the audio output from the signal filter 2 may be fed directly to a known programme level meter, for example as a continuous monotone audio signal of varying sound level, to provide an indication of the perceived loudness level of the audio signal.

25 Preferably, however, as illustrated in Figures 1 and 3, the audio output from the frequency filter 2 is passed in series through a bandwidth compensator 3 and a summation or integrator module 4 before presentation to a programme level meter 100 at an audio output 8. For this purpose, as illustrated in Figure 3, the output line 20 from the signal frequency filter 2 is connected to an input terminal 310 of the bandwidth comparator 3. The input 310 is connected in 30 parallel to an array of band-pass filters 31, 32, 33 and 34 to separate the signal into sub bandwidths of, for example, one octave. Referring to a first filter 31, which is illustrative of the operation of each of the filters 31, 32, 33, 34, the output of the band-pass filter 31 is connected by line 311 to a sub-bandwidth signal level detector 312 to determine the signal level within the

sub-bandwidth and an output is provided by line 313 from the signal level detector 312 to an output terminal 314 of output 315 dependent on the strength of signal detected.

Inputs from each of the channels input at 10 are separately frequency analysed in this way.

Therefore the output 30 from the comparator 3 comprises separate outputs along four lines 315, 5 325, 335, 345 from each of the four filters illustrated respectively, for each of the respective input channels.

As best illustrated in Figure 4, the lines 30 receiving outputs from the bandwidth compensator 3 are separately input to a non-arithmetic integrator 4 to perform a non-linear summation of the inputs using lookup tables 41 in a manner to be described, to take account of 10 the dependence of the perceived loudness on the bandwidth of the input programme audio signal at input 10 and to take account of the effect of masking of one band by another on the perceived loudness. An audio signal representative of the perceived loudness is output from the summation process along line 411 to an output terminal 420 of the integrator 4. It will be understood that 15 this audio signal can be presented to a sound level meter to provide an indication of the perceived sound level of the signal input at terminal 10.

However, preferably, as illustrated in Figure 1, output from the integrator 4 is connected by line 40 to a sample and hold circuit 5, a suitable embodiment of which is illustrated in Figure 5.

The line 40 is connected to an input 510 of the sample and hold circuit 5, and the input 510 is 20 connected by line 501 to a switch 51 connected in series with a resistor 52 and over line 521 to an output 520 of the sample and hold circuit 5. An end of the resistor 52 remote form the switch is connected to a capacitor 53 by a line 522, the capacitor being connected to earth potential by an opposing line 531. The input 510 is also connected by a line 502 branching from line 501 to a signal detector 54, the output of which is connected by line 541 to a first input of a comparator 25 55, a second input of the comparator 55 being connected to earth potential and an output line 552 of the comparator 55 acts as a control line of the switch 51.

The output 520 of the sample and hold circuit may be directly connected to the input of the programme level meter, or, as illustrated in Figure 1, may be connected to an input 610 of a visual display unit 6, as best seen in Figure 6. The visual display unit comprises a audio 30 oscillator 62 the level of which is modulated over line 612 by a modulator 61 driven over line 601 by the audio output signal received at input 610. The modulator 61 provides an output at output 620 and which may be used to drive the input of a programme level meter.

However, as further shown in Figure 1, preferably the output terminal 620 is connected by line 60 to one input branch of a manually operable bypass switch 7, the output of which is connected to the audio output 8 for connection to the programme level meter 100. A second input branch of the bypass switch 7 is directly connected by line 11 to the audio input 10.

5 The operation of the apparatus of Figure 1 is best illustrated by the flowchart shown in Figure 7 in conjunction with Figure 1. An audio signal, input, step 90, at the audio input 1, is subjected to an operator decision, step 91, whether the audio input is to be compensated for perceived loudness or presented directly to a progranme level meter connected to output terminal 8. If no compensation is to be carried out the switch 7 is manually switched to pass the 10 input signal directly to the output terminal 8.

If loudness compensation is to be used, the bypass switch is set to receive an input from the visual display unit 6 and the incoming audio signal is frequency filtered, step 92, using a filter with a known characteristic of perceived loudness as a function of frequency. Preferably one of the filters 21,22,23 is selected, step 93, having a characteristic dependent on the signal strength 15 of the incoming signal as measured by the overall signal level detector 24 shown in Figure 2, which controls the selection switch 26. Hunting between such filters is prevented by the hysteresis circuit 25 located between the signal level detector 24 and the control line 251 of the selector switch 26.

Output from the frequency filter is divided into sub-bands, step 94, by the bandwidth 20 compensator 3. The separate outputs from each of the subbands are then integrated, step 96, by the integrator 4. In performing the integration a look-up table 41 is preferably used to compensate for the effect on perceived loudness of the bandwidth of the input audio signal and to compensate for masking of one sub-bandwidth by another. The existence of such masking is disclosed in chapter 3 of BCJ Moore: An introduction to the psychology of hearing; Academic

25 Press, 1989. The requirement for such compensation can be understood by considering two related signal channels, in a form such as a tsvo- channel stereo, which are presented to a loudness meter in anti-phase. The electrical summation would represent zero loudness, whilst the audible loudness from two spatially separated reproduction sources might be only slightly reduced from the audible loudness where both signals were in-phase.

As illustrated, the compensated output from the integrator 4 may be sampled and held, step 97, by the sample and hold circuit 5, one analogue embodiment of which is shown in Figure S as described hereinabove. The circuit is capable of maintaining an integrated summation value of the near instantaneous signal loudness value obtained from the integrator 4, during periods of

relative silence. Relative silence is defined as occurring when the input loudness summation value drops below a value determined by the threshold of the comparator 55. Whilst the loudness value remains below this threshold, switch 51 remains open, and the loudness value held in the capacitor 53, is that effectively integrated by the combination of resistor 52 and capacitor 53 5 during those periods when the switch 51 is closed. This renders a better loudness indication of isolated dialogue phrases, such as those that can occur in commentary channels, or in descriptive audio services. It also enables the loudness reading to be more easily assessed by an operator at a casual glance, rather than having to study the meter readings for a period of time.

A visual display of the perceived loudness may be realised by the arrangement in Figure 6, 10 comprising a simple audio oscillator 62 whose level is linearly modulated in modulator 61 by the integrated loudness value output from the sample and hold circuit 5.

An audio signal representative of perceived loudness is thus output from the visual display unit 6 to the audio output 8 for input to the programme level meter 100 known per se.

Claims (1)

1. A loudness measuring apparatus for a programme audio signal, the apparatus comprising an audio input for inputting the programrne audio signal to be measured, frequency filter means 5 having parameters representative of perceived loudness levels as a function of frequency connected to the audio input to filter the programme audio signal and to output an output audio signal representative of perceived loudness of the programme audio signal for input to a prograrnme level meter.

2. An apparatus as claimed in claim 1, wherein the parameters are selectable dependent on a 10 sound pressure level of the programme audio signal.

3. An apparatus as claimed in claims 1 or 2, wherein there is also provided bandwidth compensator means for dividing the programme audio signal into sub-bandwidths of the signal and outputting sub-bandwidth signals representative of a signal strength in each sub-bandwidth respectively, and summation means for summing the sub-bandwidth signals to compensate for 15 perceived loudness dependent upon a width of bandwidth occupied by the programme audio signal. 4. An apparatus as claimed in claim 3, wherein the summation means includes means for comparing the signal strength in the sub-bandwidths and compensating for mutual loudness masking effects of the respective sub-bandwidth signals.

20 5. An apparatus as claimed in claims 3 or 4, wherein the summation means is arranged to perform a non-arithmetic summation.

6. An apparatus as claimed in claims 3 to 5, wherein the summation means includes a non-

linear summation look-up table for compensating for the loudness masking effects.

7. An apparatus as claimed in any of the preceding claims, wherein there is provided sample 25 and hold means for maintaining the output audio signal at an immediately preceding output level during a predetermined period of time during which a signal level of the output audio signal falls below a predetermined threshold level.

8. An apparatus as claimed in claim 7, wherein the sample and hold means comprises comparator means for determining when the signal level falls below the predetermined threshold 30 level.

9. An apparatus as claimed in any of the preceding claims, further comprising a visual display unit for displaying a representation of the output audio signal.

10. An apparatus as claimed in claim 9, wherein the visual display unit comprises a audio oscillator modulated by a modulator dependent on the output audio signal.

5 11. A loudness measuring apparatus as claimed in any of the preceding claims in combination with a programme level meter for receiving the output audio signal and displaying the perceived loudness level.

12. A combination as claimed in claim 11, wherein bypass means are provided for selectively inputting the programme audio signal to the apparatus for generating an output signal for input to 10 the programrne level meter or for inputting the programme audio signal directly to the programme level meter.

13. A method for measuring loudness of a programme audio signal' the method comprising the steps of: a) filtering an applied programme audio signal with frequency filter means having parameters representative of perceived loudness levels as a function of frequency; and b) 15 outputting an output audio signal representative of perceived loudness of the programme audio signal for input to a programme level meter.

14. A method as claimed in claim 13, wherein step a) includes the step of selecting the parameters dependent on a sound pressure level of the programme audio signal.

15. A method as claimed in claims 13 or 14, comprising the further steps of dividing the 20 programme audio signal into sub-bandwidths of the signal and outputting sub-bandwidth signals representative of a signal strength in each sub-bandwidth respectively and summing the sub-

bandwidth signals to compensate for perceived loudness dependent upon a width of bandwidth occupied by the programme audio signal.

16. A method as claimed in claim IS, wherein the step of sumTning the subbandwidth 25 signals includes the step of comparing the signal strength in the sub-bandwidths and compensating for mutual loudness masking effects of the respective sub-bandwidth signals.

17. A method as claimed in claims 15 or 16, wherein the step of summing the sub-bandwidth signals comprises performing a non-arithmetic summation.

18. A method as claimed in any of claims IS to 17, wherein the step of summing the sub 30 bandwidths includes using a non-linear summation lookup table for compensating for the loudness masking effects.

19. A method as claimed in any of claims 13 to 18, comprising the further step of sampling and holding the output audio signal at an immediately preceding output level during a predetermined period of time during which a signal level of the output audio signal falls below a predetermined threshold level.

S 20. A method as claimed in claim 19, wherein the step of sampling and holding the output audio signal comprises using comparator means to determine when the programme signal level falls below the predetermined threshold level.

21. A method as claimed in any of claims 13 to 20, further comprising the step of displaying a representation of the output audio signal on a visual display unit.

10 22. A method as claimed in claim 21, wherein the step of displaying a representation of the output audio signal comprises modulating an audio oscillator using a modulator dependent on the output audio signal.

23. An apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

15 24. A combination substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

25. A method substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.