How to measure loudness according to EBU R 128¶

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

1. Initialization¶

Create a loudness analyzer with sample rate and channel layout:

ebu_r128<float> loudness(48000, arrangement_speakers(arrangement_for_channels(channel_number)), /* 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: * 1 = 100ms (default), 4800 samples at 48kHz * 2 = 50ms, 2400 samples at 48kHz * 3 = 33ms, 1600 samples at 48kHz * 4 = 25ms, 1200 samples at 48kHz

2. Feeding Data¶

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

loudness.process_packet({ left, right }); // stereo example

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

3. Reading Results¶

After each packet, retrieve metrics:

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 — Lower and upper loudness range boundaries
LRA = RH − RL

At least 1.5 s of silence must follow the signal before final measurement to ensure correct gating.

4. Example (Raw File Measurement)¶

ebu_test.cpp provides a full working example:

ebu_test input.raw 2

Reads 32-bit float interleaved audio, deinterleaves by channel, processes through ebu_r128, and prints:

M = -23.0
S = -23.1
I = -23.0
LRA = 2.1

5. Verification¶

ebu.cpp includes reference tests validating compliance with EBU R128 for stereo and multichannel inputs at 44.1 kHz and 48 kHz. These confirm numerical alignment within ±0.1 LU across configurations.