- algorithm based on ITU-R BS.1770-4
- implemented in plain javascript using Web Audio API
- clone repository and open index.html file in Browser (tested with FF 60.1.0esr)
- audio files sample including test files from ITU-R-BS.2217 and a couple of others located under /files subdirectory
- open audio file via Select File Button
- depending on the files size and format this may take some seconds
- adjust parameters if needed an Play
- or visit github pages: https://goepfert.github.io/loudness_adaption_demo/
Interval Size The file (stream) will be segmented in time intervals of given size. This is the smallest segment a loudness can be calculated. Defaults to 400ms.
Overlap between Intervals The Intervals can overlap each other. Defaults to 75%.
Window Size How many seconds the algorithm shall look back if enough data is available.
AudioBufferSource -> Live Loudness Calculation (Web Audio Script Processor) -> Gain Control -> Post Loudness Calculation (Web Audio Script Processor) -> Volume Meter (Web Audio Script Processor) -> Audio Out
Live Loudness The current calculated loudness from the start of the file (if not in loop) or since last reset (Button: Reset Loudness Calculation). The total time is limited to the Window Size parameter (default: 30 seconds).
Target Loudness The target loudness in LKFS before any audio processing.
Target Gain The gain to compensate the difference between Live Loudness and Target Loudness. The target gain is directly applied with an exponential ramp and a decay constants that can be parametrised in dB/s. (The timeConstant is set to get 95%, for details see here). The gain correction can be bypassed by unchecking the checkbox apply gain orrection. Remark: There is no clipping control implemented.
Loudness after gain control The Loudness is (re)calculted after setting the target gain with a fixed Window Size of 1 second.
A simple attempt to check if there is clipping. It checks if there are samples with values near the maximum (abs(value) > 0.98) within the last 250 ms. If so, it meter graph turns red.