Loudness Meter

Standard-compliant loudness measurements

• Library:

• Audio Toolbox / Measurements

Description

The Loudness Meter block measures the loudness and true-peak of an audio signal based on EBU R 128 and ITU-R BS.1770-4 standards.

Ports

Input

Port_1 — Input signal

matrix | 1-D vector

• Matrix input –– Each column of the input is treated as an independent channel. If you use the default Channel weights, specify the input channels in order: [Left, Right, Center, Left surround, Right surround].

• 1-D vector input –– The input is treated as a single channel.

Data Types: single | double

Output

M — Momentary loudness measurement

column vector

The block outputs a column vector with the same data type and number of rows as the input signal.

Data Types: single | double

S — Short-term loudness measurement

column vector

The block outputs a column vector with the same data type and number of rows as the input signal.

Data Types: single | double

TP — True-peak value

real scalar

The block outputs a real scalar with the same data type as the input signal.

Dependencies

To enable this port, select the Output true-peak value parameter.

Data Types: single | double

Parameters

If a parameter is listed as tunable, then you can change its value during simulation.

Channel weights — Linear weighting applied to each input channel

[1, 1, 1, 1.41, 1.41] (default) | nonnegative row vector

The number of elements of the row vector must be equal to or greater than the number of input channels. Excess values in the vector are ignored.

The default channel weights follow the ITU-R BS.1170-4 standard. To use the default channel weights, specify the input to the Loudness Meter block as a matrix whose columns correspond to channels in this order: [Left, Right, Center, Left surround, Right surround].

It is a best practice to specify the channel weights in order: [Left, Right, Center, Left surround, Right surround].

Tunable: Yes

Use relative scale for loudness measurements — Specify block to output loudness measurements relative to target level

off (default) | on

• On — The loudness measurements are relative to the value specified by Target loudness level (LUFS). The output of the block is returned in loudness units (LU).

• Off — The loudness measurements are absolute, and returned in loudness units full scale (LUFS).

Tunable: No

Target loudness level (LUFS) — Reference level for relative loudness measurements

–23 (default) | real scalar

For example, if the Target loudness level (LUFS) is –23, then a loudness value of –24 LUFS is reported as –1 LU.

Tunable: Yes

Dependencies

To enable this parameter, select the Use relative scale for loudness measurements parameter.

Output true-peak value — Add output port for true-peak value

off (default) | on

When you select this parameter, an additional output port, TP, is added to the block. The TP port outputs the true-peak value of the input frame.

Tunable: No

Inherit sample rate from input — Specify source of input sample rate

on (default) | off

When you select this parameter, the block inherits its sample rate from the input signal. When you clear this parameter, you specify the sample rate in Input sample rate (Hz).

Tunable: No

Input sample rate (Hz) — Sample rate of input

44100 (default) | scalar

Tunable: Yes

Dependencies

To enable this parameter, clear the Inherit sample rate from input parameter.

Simulate using — Specify type of simulation to run

Code generation (default) | Interpreted execution

• Code generation –– Simulate model using generated C code. The first time you run a simulation, Simulink^® generates C code for the block. The C code is reused for subsequent simulations, as long as the model does not change. This option requires additional startup time but the speed of the subsequent simulations is comparable to Interpreted execution.

• Interpreted execution –– Simulate model using the MATLAB^® interpreter. This option shortens startup time but has a slower simulation speed than Code generation. In this mode, you can debug the source code of the block.

Tunable: No

Model Examples

Compare Loudness Before and After Audio Processing

Measure momentary and short-term loudness before and after compression of a streaming audio signal in Simulink®.

Open Model

Trigger Gain Control Based on Loudness Measurement

This model enables you to apply dynamic range compression to an audio signal while staying inside a preset loudness range. In this model, a Compressor block increases the loudness and decreases the dynamic range of an audio signal. A Loudness Meter block calculates the momentary loudness of the compressed audio signal. If momentary loudness crosses a -23 LUFS threshold, an enabled subsystem applies gain to lower the corresponding level of the audio signal.

Open Model

Block Characteristics

Data Types	double | single
Direct Feedthrough	no
Multidimensional Signals	no
Variable-Size Signals	yes
Zero-Crossing Detection	no

Algorithms

The Loudness Meter block calculates the momentary loudness, short-term loudness, and true-peak value of an audio signal. You can specify any number of channels and nondefault channel weights used for loudness measurements. The block algorithm is described for the general case of n channels and default channel weights.

Loudness Measurements

The input channels, x, pass through a K-weighted filter implemented using the algorithm of the Weighting Filter block. The K-weighted filter shapes the frequency spectrum to reflect perceived loudness.

Momentary Loudness

1. The K-weighted channels, y, are divided into 0.4-second segments with 0.3-second overlap. If the required number of samples have not been collected yet, the Loudness Meter block returns the last computed value for momentary loudness. If enough samples have been collected, then the power (mean square) of each segment of the K-weighted channels is calculated:

   $$m{P}_i=\frac{1}{w}{\displaystyle \sum _{k=1}^w{y}_i^2[k]}$$

   • mP_i is the momentary power of the ith segment.

   • w is the segment length in samples.

2. The momentary loudness, mL, is computed for each segment:

   $$m{L}_i=-0.691+10{\log }_{10}\left({\displaystyle \sum _{c=1}^n{G}_c\times m{P}_{\left(i,c\right)}}\right)LUFS$$

   • G_c is the weighting for channel c.

   mL is the momentary loudness returned by your Loudness Meter block.

Short-Term Loudness

1. The K-weighted channels, y, are divided into 3-second segments with 2.9-second overlap. If the required number of samples have not been collected yet, the Loudness Meter block returns the last computed values for short-term loudness and loudness range. If enough samples have been collected, then the power (mean square) of each K-weighted channel is calculated:

   $$s{P}_i=\frac{1}{w}{\displaystyle \sum _{k=1}^w{y}_i^2[k]}$$

   • sP_i is the short-term power of the ith segment of a channel.

   • w is the segment length in samples.

2. The short-term loudness, sL, is computed for each segment:

   $$s{L}_i=-0.691+10{\log }_{10}\left({\displaystyle \sum _{c=1}^n{G}_c\times s{P}_{\left(i,c\right)}}\right)LUFS$$

   • G_c is the weighting for channel c.

   sL is the short-term loudness returned by your Loudness Meter block.

True-Peak

The true-peak measurement considers only the current input frame of a call to your loudness meter.

1. The signal is oversampled to at least 192 kHz. To optimize processing, the input sample rate determines the exact oversampling. An input sample rate below 750 Hz is not considered.

   Input Sample Rate (kHz)	Upsample Factor
   [0.75,1.5)	256
   [1.5,3)	128
   [3,6)	64
   [6,12)	32
   [12,24)	16
   [24,48)	8
   [48,96)	4
   [96,192)	2
   [192,∞)	not required

2. The oversampled signal, a, passes through a lowpass filter with a half-polyphase length of 12 and stopband attenuation of 80 dB. The filter design uses designMultirateFIR.

3. The filtered signal, b, is rectified and converted to the dB TP scale:

   $$c=20\times {\log }_{10}\left(\left|b\right|\right)$$

4. The true-peak is determined as the maximum of the converted signal, c.

References

[1] International Telecommunication Union; Radiocommunication Sector. Algorithms to Measure Audio Programme Loudness and True-Peak Audio Level. ITU-R BS.1770-4. 2015.

[2] European Broadcasting Union. Loudness Normalisation and Permitted Maximum Level of Audio Signals. EBU R 128. 2014.

[3] European Broadcasting Union. Loudness Metering: 'EBU Mode' Metering to Supplement EBU R 128 Loudness Normalization. EBU R 128 Tech 3341. 2014.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.