by DonHopkins | 2025-10-30 21:14:30+00:00
Even more complex and reflectively imaginative than the Fourier Transform is the mighty Cepstrum!

https://en.wikipedia.org/wiki/Cepstrum

It’s literally a "backwards spectrum", and the authors in 1963 were having such jolly fun they reversed the words too: quefrency => frequency, saphe => phase, alanysis => analysis, liftering => filtering

The cepstrum is the "spectrum of a log spectrum," where taking the complex logarithm turns multiplicative spectral features into additive ones, laying the foundation of cepstral alanysis, and later, the physiologically tuned Mel-frequency cepstrum used in audio compression and speech recognition.

https://en.wikipedia.org/wiki/Mel_scale

>The mel scale (after the word melody)[1] is a perceptual scale of pitches judged by listeners to be equal in distance from one another. [...] Use of the mel scale is believed to weigh the data in a way appropriate to human perception.

As Tukey might say: once you start doing cepstral alanysis, there’s no turning back, except inversely.

Skeptics said he was just going through a backwards phase, but it turned out to work! ;)

https://news.ycombinator.com/item?id=24386845

DonHopkins on Sept 5, 2020 | parent | context | favorite | on: Mathematicians should stop naming things after eac...

I love how they named the inverse spectrum the cepstrum, which uses quefrency, saphe, alanysis, and liftering, instead of frequency, phase, analysis and filtering. It should not be confused with the earlier concept of the kepstrum, of course! ;)

https://en.wikipedia.org/wiki/Cepstrum

>References to the Bogert paper, in a bibliography, are often edited incorrectly. The terms "quefrency", "alanysis", "cepstrum" and "saphe" were invented by the authors by rearranging some letters in frequency, analysis, spectrum and phase. The new invented terms are defined by analogies to the older terms.

>Thus: The name cepstrum was derived by reversing the first four letters of "spectrum". Operations on cepstra are labelled quefrency analysis (aka quefrency alanysis[1]), liftering, or cepstral analysis. It may be pronounced in the two ways given, the second having the advantage of avoiding confusion with "kepstrum", which also exists (see below). [...]

>The kepstrum, which stands for "Kolmogorov-equation power-series time response", is similar to the cepstrum and has the same relation to it as expected value has to statistical average, i.e. cepstrum is the empirically measured quantity, while kepstrum is the theoretical quantity. It was in use before the cepstrum.[12][13]

https://news.ycombinator.com/item?id=43341806

DonHopkins 7 months ago | parent | context | favorite | on: What makes code hard to read: Visual patterns of c...

Speaking of filters and clear ergonomic abstractions, if you like programming languages with keyword pairs like if/fi, for/rof, while/elihw, goto/otog, you will LOVE the cabkwards covabulary of cepstral quefrency alanysis, invented in 1963 by B. P. Bogert, M. J. Healy, and J. W. Tukey:

cepstrum: inverse spectrum

lifter: inverse filter

saphe: inverse phase

quefrency alanysis: inverse frequency analysis

gisnal orpcessing: inverse signal processing

https://en.wikipedia.org/wiki/Cepstrum

https://news.ycombinator.com/item?id=44062022

DonHopkins 5 months ago | parent | context | favorite | on: The scientific “unit” we call the decibel

At least the Mel-frequency cepstrum is honest about being a perceptual scale anchored to human hearing, rather than posing as a universally-applicable physical unit.

https://en.wikipedia.org/wiki/Mel-frequency_cepstrum

>Mel-frequency cepstral coefficients (MFCCs) are coefficients that collectively make up an MFC. They are derived from a type of cepstral representation of the audio clip (a nonlinear "spectrum-of-a-spectrum"). The difference between the cepstrum and the mel-frequency cepstrum is that in the MFC, the frequency bands are equally spaced on the mel scale, which approximates the human auditory system's response more closely than the linearly-spaced frequency bands used in the normal spectrum. This frequency warping can allow for better representation of sound, for example, in audio compression that might potentially reduce the transmission bandwidth and the storage requirements of audio signals.

https://en.wikipedia.org/wiki/Psychoacoustics

>Psychoacoustics is the branch of psychophysics involving the scientific study of the perception of sound by the human auditory system. It is the branch of science studying the psychological responses associated with sound including noise, speech, and music. Psychoacoustics is an interdisciplinary field including psychology, acoustics, electronic engineering, physics, biology, physiology, and computer science.