Max Time-Stretch Factor in Audacity Paulstretch
This article explains the maximum time-stretch capabilities of the Paulstretch effect within Audacity. It covers the specific numerical limits imposed by the software in a single pass, how users can achieve extreme stretch ratios beyond those interface limits, and key practical considerations regarding project file sizes and rendering performance.
In Audacity, the maximum time-stretch factor achievable in a single application of the Paulstretch effect is 100x (a stretch factor of 100.0). The software restricts the "Stretch Factor" input field and slider to a range between 1.0 and 100.0. Applying this maximum setting extends one second of source audio into 100 seconds, or a three-minute song into a five-hour ambient wash.
Exceeding the 100x Limit Through Chaining
While the dialog restricts a single pass to 100x, there is no technical limit preventing users from applying the effect multiple times to the same audio. By chaining consecutive passes, the stretch factors multiply:
- Two passes at 100x: Yields a 10,000x stretch factor (1 minute becomes ~6.9 days).
- Three passes at 100x: Yields a 1,000,000x stretch factor.
To achieve factors higher than 100, apply the Paulstretch effect at 100x, select the resulting stretched waveform, and apply the effect again at your desired multiplier.
Audacity vs. Standalone PaulXStretch
The 100x limit is unique to Audacity’s native implementation of the algorithm. The original standalone software developed by Nasca Octavian Paul (Paul’s Extreme Sound Stretch) and the modern PaulXStretch plugin allow stretch factors exceeding \(10^{18}\) in a single session. If extreme stretch values are needed without manual chaining, using the standalone version or VST plugin is often more efficient.
Practical Limitations
When pushing Audacity's Paulstretch tool to its upper limits, several practical constraints arise:
- Processing Time: Extreme stretch factors require the spectral phase randomization algorithm to calculate thousands of overlapping fast Fourier transform (FFT) windows, resulting in significant render times.
- Track Length Limits: Audacity supports long tracks, but extreme stretching can create file sizes that exceed format standards (such as WAV's standard 4 GB limit, requiring RF64 or export format adjustments).
- Time Resolution Loss: At maximum stretch values, all original attack and rhythmic transients are completely dissolved, leaving only continuous spectral pads and drone textures.