WHICH PARTIALS MOVE: A 12-TET PULL ABOVE THE BAND EDGE OF A NEURAL CODEC DECODER

Neural synthesis pulls output tuning toward 12-TET, as recently reported for melto-audio vocoders. We ask where inside a neural audio codec that happens. At low bitrate a decoder transmits a tone’s lower partials and invents the rest above a band edge that rises with bitrate within a codec. The transmitted fundamental stays put through EnCodec, whereas the vocoder we measure moves it by 3.1 cents (a cent is a hundredth of a semitone). The invented partials land most of the way onto the harmonic ladder of the nearest 12-TET pitch, and a pitch estimator fitting across the edge averages the transmitted and invented partials, reporting a displacement no partial underwent: a median of 4.37 cents over twenty-four stimuli. Where the invented partials land follows the decoder’s fine-tuning corpus. When only EnCodec’s decoder is fine-tuned, so that every arm reads bit-identical tokens, retuning its corpus by 33 cents moves the decoder’s grid by 32.7 ± 2.8 cents at seed-level spread, while grid-preserving resamplings of comparable or larger size leave it in place. Classical codecs show no pull. At a 440 Hz reference six of eleven measurable neural conditions show the effect, four of them by more than five cents. Those four hold under every variance treatment and span two codec families; the other two fall below their own detection floors. The practical exposure falls on full-band harmonic fitting, ours more than salience-based tools. After one round trip, 22 of 200 Carnatic and 11 of 60 Turkish makam recordings switch to fitting 12-TET better than their own corpus’s measured degrees, while a period tracker is largely spared. The remedy is simple where it applies: fit no partial above the edge. The band-edge account replaced a pre-registered one that failed, and its registered pass on fifteen fresh checkpoints tests a three-tier prediction, giving p = 0.070 against always predicting the middle tier and 0.375 against always predicting the most common tier.