About

This utility manipulates the temporal resolution of audio signals while maintaining the original frequency domain characteristics. Commonly known as time-stretching, this process allows users to slow down rapid speech for transcription or speed up slow content for efficiency. Precision is maintained through the browser's native signal processing pipeline, ensuring that the fundamental frequency (pitch) remains constant regardless of the playback rate r.

For transcriptionists and language learners, acoustic clarity is paramount. Lowering playback speed often introduces spectral artifacts or "muddiness" in the low-end frequencies. This tool incorporates a dedicated Digital Signal Processing (DSP) chain - specifically a parametric equalizer - to attenuate low-frequency noise and boost the standard vocal presence range (1kHz - 4kHz). The A-B loop functionality provides granular control for repetitive listening, essential for decoding complex phonemes or rapid dictation.

Formulas

The relationship between the new duration t_new and the original duration t₀ is inversely proportional to the playback rate r.

t_new = t₀r

Where r is a dimensionless scalar such that 0.5 ≤ r ≤ 2.0. The pitch preservation algorithm uses Granular Synthesis or Phase Vocoding (implementation dependent) to maintain frequency f:

f_out ≈ f_in for all r

Reference Data

Target Speed (r)	Time Dilation (Δt)	Frequency Response	Primary Application	Cognitive Impact
0.25x	400%	High artifact risk (Transient smearing)	Forensic Phonetics, Micro-detail analysis	Maximum Focus
0.50x	200%	Pitch-locked, Minor phasing	Verbatim Transcription, Music Transcription	High Precision
0.75x	133%	Transparent (Near-native)	Language Acquisition, Accent Reduction	Learning State
1.00x	0%	Native Source	Reference / Baseline	Neutral
1.25x	-20%	Accelerated Intelligibility	Lecture Review, Slow Speaker Compensation	Efficiency
1.50x	-33%	Rapid Comprehension	Podcast Skimming, News Digest	Information Density
2.00x	-50%	Syllabic Compression	Content Locating, "Search by Ear"	Scanning
3.00x	-66%	Tone/Pitch Only	Structure Analysis, rapid review	Abstract

Frequently Asked Questions

No. This tool utilizes the `preservesPitch` attribute of the HTML5 Audio Engine. This decouples the playback rate from the sampling rate, allowing the duration to change while the pitch (frequency) remains locked. You will not hear the "chipmunk" effect at high speeds or the "monster" effect at low speeds.

The Vocal Boost engages a BiquadFilterNode configured as a "peaking" filter. It applies a +6dB gain centered at 2.5kHz (the "presence" range of human speech) and simultaneously applies a high-pass filter at 150Hz to cut low-frequency rumble. This makes consonants crisper and easier to distinguish when audio is slowed down.

The loop precision is tied to the browser's `timeupdate` event frequency (typically 250ms) and the audio sampling rate. For standard mp3/wav files, the loop point is accurate enough for sentence or phrase repetition but may not be sample-perfect for seamless musical looping.

No. All processing occurs locally within your browser using the `File API` and `Web Audio API`. Your audio files never leave your device, ensuring complete privacy and security.