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Measuring Loudness with EBU R128

KFR implements EBU R128 compliant loudness metering through the ebu_r128<T> class. It computes momentary (M), short-term (S), and integrated (I) loudness, along with loudness range (LRA), as defined by EBU Tech 3341 and EBU R128 s3.

1. Initialization

Create a loudness analyzer with sample rate and channel layout:

#include <kfr/dsp.hpp>

using namespace kfr;

constexpr size_t channel_count = 2;
ebu_r128<float> loudness(48000, arrangement_speakers(arrangement_for_channels(channel_count)), /* packet_size_factor */ 1);

The channel arrangement is automatically derived from the number of channels (1–8). Supported configurations include mono, stereo, and multichannel.

packet_size_factor sets the processing block size (valid range 1–6):

  • 1 = 100 ms (default, 10 Hz refresh), 4800 samples at 48 kHz
  • 2 = 50 ms (20 Hz refresh), 2400 samples at 48 kHz
  • 3 = 33 ms (30 Hz refresh), 1600 samples at 48 kHz
  • 4 = 25 ms (40 Hz refresh), 1200 samples at 48 kHz

2. Feeding Data

Process audio in fixed-size packets determined by .packet_size:

const std::array channels{ speaker_type::Left, speaker_type::Right };
ebu_r128<float> loudness(48000, channels);
loudness.process_packet({ left, right }); // stereo example

Each channel is supplied as a univector_ref or similar slice of contiguous samples. For file-based use, ensure data is deinterleaved before passing.

3. Reading Results

After each packet, retrieve metrics:

const std::array channels{ speaker_type::Left, speaker_type::Right };
ebu_r128<float> loudness(48000, channels);
float M, S, I, RL, RH;
loudness.get_values(M, S, I, RL, RH);
  • M — Momentary loudness (400 ms window)
  • S — Short-term loudness (3 s window)
  • I — Integrated loudness (gated average)
  • RL, RH — 10th and 95th percentile loudness, the LRA boundaries
  • LRA = RH − RL

EBU R128 measurements settle only after the signal ends; append at least 1.5 s of silence (roughly 15 packets at the default packet size) before reading the final I and LRA values.

4. Example (Command-Line Tool)

tools/ebu_test.cpp is a full working example: ebu_test INPUT_FILE. It opens the file with create_decoder_for_file (so it accepts WAV, RF64, BW64, W64, FLAC, MP3, AIFF, or CAF), builds an ebu_r128 meter sized for the file's channel count via arrangement_speakers(arrangement_for_channels(channels)), feeds it packet-by-packet as the file is decoded, appends 1.5 s of silence at the end, and prints the final measurements together with the running peak M and S values:

M = -23.0
S = -23.1
I = -23.0
LRA = 2.1
maxM = -20.4
maxS = -21.2

5. Verification

tests/unit/dsp/ebu.cpp checks measured values against reference numbers for stereo and 5-channel inputs at 44.1 kHz and 48 kHz, within ±0.05 LU for stereo and ±0.1 LU for multichannel.