null test

null A (PCM source) against the encoded version. residue concentrated above ~16 kHz and at quiet passages = the codec dropping pre-echo and high frequencies. residue at clean tones = quantisation distortion.

null an undithered truncation against the same source dithered. the residue is the dither — typically a ~−93 dBFS noise floor for TPDF dither at 16-bit, with the noise-shaped variants showing characteristic high-frequency tilt.

null pre- and post-plugin renders. the residue is exactly what the plugin added or removed. EQ shifts show as bands in the spectrogram, dynamics processing shows time-varying envelopes, reverbs show late tail energy.

null a 24-bit master against its 16-bit truncation. clean truncations leave just the LSBs as residue (~−96 dBFS); dithered conversions leave a smooth dither noise floor instead.

null a 96 kHz master against its 48 kHz down-converted version (after the 48 kHz file is up-sampled back to 96). residue concentrates above 24 kHz — the content the down-conversion shed — and shows the resampling filter's transition slope at the cutoff.

null two bounces of the same DAW session. a perfect null is the only proof that two exports are bit-identical. anything else means a routing, automation, or summing change between the two takes — find it.