Common methods

The rules on this page bind every stage of the pipeline. Units and confidence have their own page, Units, which this page does not restate. The tags are those of the pipeline page.

Samples in f64

Samples, spectra and every intermediate value are f64 from the decoder on, at the stem’s native rate [fact]. f32 appears only where a file format stores 32-bit floats [fact]. Analysis never resamples; only a consumer with a fixed input rate does [fact].

The window

The analysis window is the periodic Hann, $w[n]=0.5-0.5\cos (2\pi n/N)$ for $n=0\dots N-1$: the first $N$ samples of a symmetric window of length $N+1$ [fact]. Its main lobe is 4 bins wide null to null, and its -3 dB width is 1.44 bins [fact]. The symmetric form would shift every bin slightly [fact]. A four-term Blackman-Harris window, with lower side lobes and a wider main lobe, and the rectangular window exist for comparison only [fact].

The frame rule

• Frames are centred and zero-padded, with hop $N/4$ [fact].
• At 44.1 kHz the frame length is 8192 samples when the note list has a median f0 below 100 Hz, else 4096, and 4096 without a note list [fact]. At another rate the length is the power of two nearest, in log, to that length times $f_s/44100$: the same at 32 and 48 kHz, twice it at 88.2 and 96 kHz, four times at 176.4 and 192 kHz, half at 22.05 kHz [fact]. Harmonics $f_0$ apart need $N\ge 4f_s/f_0$ for the 4-bin Hann main lobe; at 48 kHz, 4096 points resolve harmonics down to 47 Hz, and 8192 down to 23 Hz, as do 16384 points at 96 kHz [fact].
• The stereo screen uses 8192 points for every f0, and the spectrogram’s zoom panel 16384 [fact].
• layers.json records the length used as conventions.frame_size [fact].

Active frames and statistics

• A frame is active when its RMS lies within 40 dB of the loudest frame’s [fact, proposal for 40 dB]. The stereo screen takes the RMS over both channels [fact].
• A statistic over active frames is the median and the interquartile range [fact]. Both resist outliers, such as the frames of a note onset [fact].
• Quantiles interpolate linearly between the sorted values at position $q(n-1)$, the convention of the common numeric libraries [fact]. The IQR is the 75th minus the 25th percentile [fact].
• A statistic without an active frame is no estimate with the reason silent_stem, never a number [fact]. One with fewer than 16 active frames is no estimate with the reason too_few_frames: the first and last two frames of a clip reach into the zero padding, and a median over few frames follows them [fact, proposal for 16].

Conventions next to every number

Published tools disagree on weights, floors, windows and statistics, so a number means something only with its convention [fact]. Every number in the stem group of layers.json has a convention under the same key in conventions, and a test fails when a field lacks one [fact]. The convention names the weighting, the window, the floor and the statistic [fact].

Golden vectors

Expected values in tests come from an independent reference, never from our own code [fact]. The testing page gives the commands.

• A pinned Python generator computes each reference value once, from a recipe that the JSON vector stores [fact]. The interpreter and every package are pinned with hashes, so a second run writes byte-identical files [fact].
• Each vector holds the recipe, the generator, the tool versions, the values and the tolerance [fact]. The Rust test rebuilds the signal from the recipe and compares with $|\text{actual}-\text{expected}|\le \text{abs}+\text{rel}|\text{expected}|$ [fact].
• Noise comes from a SplitMix64 generator that both languages implement bit for bit [fact]. A statistic over noise is still compared as a statistic, with a tolerance that covers its spread [fact].
• CI never runs Python. It checks that every vector has its fields and names the pinned versions [fact].
• The figures that task issues quoted live as prototype tables with their recipe. A figure the recipe does not reproduce is recorded with a reproduced: false flag, and the committed table replaces it [fact].