ISP, Inter Sample Peak or TP, True Peak, refers to peaks in the analog domain. That is, how your peaks will look after your waveform has been converted from a digital stream within your computer, phone, iPod or CD to electrical impulses that can be amplified and played back through your speakers.

Aren’t the waveforms the same as they appear on my screen after the D/A conversion?
No, Your D/A will create a waveform that continuously moves from one sample point to the next. In that process the newly created waveform may peak above the 0dBFS ceiling which individual samples adhere to, thereby causing distortion. Just how bad this distortion depends on the converter used. In some extreme cases, the actual peak can be as much as 3-4dB above the sample peaks detected in the digital domain.

Many of the top mastering business don’t care about ISP so why should I?
No, you don’t have to treat or be aware of ISP but:

1. Digital limiters without oversampling (or other ways of detect ISP) will react to the digital waveform, not the true waveform, and thus won’t react as a limiter should react. This does not mean it will sound bad in any way, just that it doesn’t react as intended.
2. -Tip: If you want to work with limiters that lack oversampling;  up-sample your 44.1 or 48KHz projects to a higher sample rate and by that have the limiter react more accurately.
3. We’ll most probably have a loudness standard for streaming media within a couple of years and that standard will be ISP aware and set at -1dBTP. This means that if your audio peak at +2dBTP it will be turned down by at least -3dB before reaching the end user. All that extra loudness you gained by letting random equipment create a positive peak (that might distort) will be lost.
4. Encoding to lossy formats; ISP will make it harder for encoders to do a good job. If you don’t have the tools or knowledge to check how your audio will perform post-encoding I would recommend to stay away from positive True Peaks.

Since it’s more or less guess-work, or at least really hard to create a real-time limiter that handles ISP perfectly, we thought we’d put as many limiters as possible to the test. So a handful of mastering colleagues and I (Ian Stewart, Sigurdór Guðmundsson, and Johan Eckerblad) went to work.

The Test

We used a mastered track that we boosted by 7.5dB (the peaks were at -1dBTP/dBFS in the mastered file so max Gain Reduction would be 6.5dB), we set the limiter ceiling to -1.0dBFS (or -1dBTP if available in the plugin), rendered it as 44.1KHz, 32bfp wav and measured the True Peaks of the resulting waveforms. Limiters with a result as close to -1.0 dBTP as possible have handled ISPs the best, whereas values over -0.8 dBTP (shown with increasing values in white, through yellow, to red when they exceed 0 dBTP) mean that the limiter has failed to handle ISPs to within the margin of error for measurement we observed during testing (more on that in a bit).

Here’s the track that we used:

https://siggidori.bandcamp.com/track/morbit-orbit

• • “Yes” or “No” in the TRUE PEAK column indicates whether the developer mentions or claims that the plugin handles True Peak or ISP, either in the manual or marketing.  A “No” does not necessarily mean that it doesn’t handle or try to handle ISP, just that it’s not explicitly stated by the developer.

• • The results published here are measurements taken using Izotope RX. Additionally, we did measure with Sequoia, Wavelab, and Nugen, and even though we observed slightly different results, they were negligible. The biggest differences were in iZotope’s products.  Since they probably use the same detection algorithms in all their software, this was expected. The other software measured within a 0.1 to 0.2 dB difference, which still puts iZotope amongst the best performers.

• • Where other settings such as Attack and Release were present we used either the default or adjusted it to a value as close to standard as possible

• • This test did not take into account sonic qualities at all.  That will have to wait for our planned podcast or another post.

• It’s not recommended to use any of the limiters with a positive score in 44.1KHz projects. They should handle ISP better as your sample rate goes up.
• If your favorite Mastering Limiter ain’t here, send us a link so that we can try it out. However, we’re not in the business of buying every limiter out there, so any you’d like us to try must have fully working demo-versions.