ISP, Inter Sample Peak or TP, True Peak, refers to peaks in the analogue 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 is depends on the converter used. In some cases the actual peak can be as much as 3dB 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 don’t react as intended.
    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.
  2. 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 so it will be meaningless.
  3. 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 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.

You can sort the result by clicking the column headings.

DeveloperPluginTrue PeakdBTP comments
Slate DigitalFG-XNo+1.04
KuassaKratos Maximizer 2Yes-0.45This is without Oversampling – search the list for the result with Oversampling turned on
KuassaKratos Maximizer 2Yes-1.07Result with 4x Oversampling – search the list for the result without Oversampling
TBProAudioLAxLimit2Yes-0.94Remember to change detection from "Peak" to "ISP"
A.O.MInvisible Limiter 1.7.5Yes-0.58
LVC-AudioLimited-Z 1.0.1No+1.29This is measurements of the free version. Their paid Limited-MAX 1.0.2 are ISP aware but we haven't tested it. 
AudacityLimiterNo-0.18Soft Limit
MagixSeqouia sMax11 v13.0Yes-1.11Balanced setting. The Full Scale peaks ended up at -1.16dBFS which means that Seqouia compensate for TP by lowering the overall gain. 
ToneBoosterBarricade v3.1.3yes-0.97
VladgsoundLimiter No6 v1.0.2b WinYes-0.92Free
VladgsoundLimiter No6 OSXYes-0.97Free
DMG AudioLimitless v1.03Yes-0.51
HofaIQ-Limiter v1.07Yes-0.64
112dBBig Blue Limiter v1.1.5No-0.23
CockosEvent Horizon v5.1No+1.09Comes with Reaper
CockosMaster Limiter v5.1No-0.10Comes with Reaper
CockosNP1136 v5.1No+1.69Comes with Reaper
Final Mix SoftwareM1 Limiter Lite v1.0.1No+0.59
AbletonLimiterNo+0.03
FabFilterPro-LNo v1.13-0.87Pro-L does not handle ISP automatically – you need to use Lookahead and keep an eye on the meter.
IzotopeMaximizer v7.0Yes-1.0As stated above – This is the result from IzotopeRX – Measurements with other tools varied between -1.0 to -0.8
IzotopeVintage Limiter v7.0Yes-1.0As stated above – This is the result from IzotopeRX – Measurements with other tools varied between -1.0 to -0.8
KazrogKClipYes-0.87
PresonusLimiter (Studio One 3.2)Yes-0.40
PSPXenon v1.52Yes-0.92We used version 1.3 at first which did not give as good results (-0.61). Upgrade if you're using an older version.
Sonnox LimiterNo+0.37Says in the manual that there's a TP mode but does not seem to be present in the version we used
UADPrecision LImiterYes+0.08
WavesWLMYes-0.83
Nugen ISL2 v2.0Yes-0.85
Lively Audio Maxwell Smart v2.0Yes-0.90Free – Windows only
Thomas MundtLoudMax v1.15No+0.08Free
Cockos v5.1Zero Crossing MaximizerNo+0.68Comes with Reaper
Brainworxbx_limiter v1.3No+0.21
AvidDyn 3 v12.5No+0.36
FabFilterPro-MBNo-0.98Not a limiter per se but handle TP very well if you use max Lookahead. This result is with Single Band – max ratio – both Att/Rel at 20%
Massey L2007 Plugin Revison 5604No+0.11
MeldaProductionMDynamicsLimiter v9.21No-0.85
MeldaProductionMLimiter v9.21No+1.09
MeldaProductionMMultibandLimiter v9.21No+0.54
VoxengoElephant v4.4No-1.0Oversampling at 8x
VoxengoElephant v4.4No+0.05Oversampling at Auto – We added this option to make everyone aware of how important oversampling while limiting is. Elephants scored a perfect -1.0dBTP in all measures with x8 oversampling.
PSPOld Timer v2.0No-0.56Not a limiter per se but Sigurdór made it work with a fast attack/release and a high ratio
KjaerhusClassic Master Limiter v1.0.6No-0.08The ceiling is set to -0.2 and you can't change it so this result is with a volume attenuation of -0.8 after the limiter. Without the adjustment it would have been about +0.9dBTP
T-RacksStealth LimiterYes-0.44Mode set to Tight – ISPL at 16X

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.