How Pitch-Preserving Speed Change Works
Convertio uses the WSOLA (Waveform Similarity Overlap-Add) algorithm — the same time-stretching method used by professional DAWs and media players. Unlike simple fast-forward that makes voices sound like chipmunks, WSOLA separates tempo from pitch.
The algorithm divides audio into overlapping segments, then repositions and crossfades them to create natural-sounding speed changes. The result: your audio plays faster or slower while voices and instruments maintain their original pitch and character.
Quality is best within the 0.5x–2.0x range. Beyond these limits, audible artifacts may appear as the algorithm stretches or compresses samples past their natural boundaries. For most use cases — transcription, interview review, voice acting — the standard range delivers transparent results.
Speed Settings Guide
| Speed | Duration Change | Best For |
|---|---|---|
| 0.5x | 2× longer | Detailed transcription of fast speakers |
| 0.75x | 33% longer | Standard transcription speed, interview review |
| 1.0x | Original | Format conversion only |
| 1.25x | 20% shorter | Quick review of long Voice Memos |
| 1.5x | 33% shorter | Condensed playback of recordings |
| 2.0x | 50% shorter | Rapid scan of lengthy interviews |
M4A to WAV Speed Change: Voice and Production
M4A Voice Memos from iPhone are the most common source for this workflow. Journalists slow interview recordings to 0.75x for accurate transcription. Voice actors adjust demo pacing — speeding up a 35-second take to fit a 30-second slot, or slowing a rushed read for more deliberate delivery.
The WAV output integrates directly into professional workflows. Podcasters can drop the speed-adjusted WAV into their DAW session without format conversion. Audio editors get uncompressed files ready for further processing — normalization, noise reduction, or multitrack mixing.
For students and researchers, slowing lecture recordings (often saved as M4A on iPhone) to 0.75x makes dense academic content easier to process and note-take.
Journalist workflow: Record on iPhone → slow to 0.75x → convert to WAV → transcribe in your preferred tool. No quality loss from re-encoding.