ATSC A/85 Broadcast Loudness Validator

ATSC A/85 is the recommended practice behind US loudness law: the CALM Act made its target binding for US broadcast television as of December 13, 2012, and the FCC enforces it. The anchor number is -24 LKFS (LUFS) with a ± 2 LU tolerance, so the pass window runs from -26 to -22 LKFS, measured over the full program with the ITU-R BS.1770-4 K-weighted, gated method — A/85 measures the whole program, not a dialogue-gated subset. Commercials, promos and programs are held to the same target, which is exactly what the CALM Act polices: ad breaks stop jumping out. The maximum true peak is -2.0 dBTP, oversampled; broadcast chains add transmitter limiting and re-encodes after delivery, so many engineers limit masters at -2.3 dBTP to stay safely inside. ATSC DTV audio is 48 kHz (AC-3/E-AC-3), so non-48 kHz files are flagged. Dialnorm is the metadata field that tells consumer decoders how loud the program is; this tool cross-checks the dialnorm you plan to encode against the measured loudness (recommended value: the measurement rounded to a whole dB) — when they disagree, playback normalisation is wrong even though the audio measured clean. A short-term (3 s) timeline chart shows whether loud moments stay proportionate. Measurement uses FFmpeg's ebur128 filter; files are only read, never modified.