Inter-Sample peaks are one confusing topic! People sometimes freak out after reading about them on forums around the web and they call me to make sure the master they just received from us is ISP’s free!
Before we delve into the technical details, we need to realise there are 3 camps:
1. People who claim ISP are the reason why modern music sounds so bad and they should be avoided like the plague
2. People who admittedly keeps an eye on them but as long they do not interfere with the quality of the final result can live happily with them
3. People who don’t give a proverbial…
(I am in the #2 camp BTW)
Sure there are situations where ISP’s are not welcomed at all (e.g. an MFiT delivery) but by and large, their presence deserves little attention (unless audible distortion occurs).
ISP AND HOW ARE THEY CREATED
Inter-Sample peaks are a phenomenon strictly related to the DA conversion (digital to analogue) where all numbers from the digital files are converted back into an analogue waveform.
Nothing can go beyond the 0dBfs, but that applies only to the digital realm. Once you are back to the analogue world, all bets are off and we have to deal with some unpredictable outcomes.
This is why, sometimes, waveforms pushed close to the 0dBfs (digital domain) once converted back to analogue ( because of the filtering involved in the DA conversion) could reproduced peaks who are above the 0db and push the converter into the red (clipping | this is what the red light on your DA is there for btw).
ISP are simply an attempt by the DA converter to recreate,by interpolation, data that was lost during clipping (squashed wave = 1111111111111111 = no resolution = data loss).
Naturally, the better the DA stage, the more headroom, the less noticeable the ISP’s
This is why a master that sounds fine in our studio could end up sounding horribly on a cheap CD player or sound-card.
HOW TO PREVENT THEM
It is fairly obvious to understand why ISP’s has been associated with excessive levels and the infamous “Loudness War”… but not so much how to prevent them.
For sure dropping the level of the final master is a good solution…but by how much?
Luckily for us, there are tools we can use to make sure we do not have too many of them… or none at all!
Simple True-peak metering plugins and specialised plugins by Sonnox, NuGen, Apple and many more, offer different takes on how to solve this issue
Running your meter in oversampling is NOT a definite solutions but it can help to keep an eye on things.
By running your meter in oversampling, you force the meter to “model” a typical interpolation applied by the D/A conversion (by some sort of filtering) and to then feed an ISP clip indicator.
As I said, this is not bullet-proof and cannot be used in critical professional settings (such as the delivery of MFiT masters) but it can be helpful as a rough guide while both mixing or mastering.
The End of ISP’s
As the end of the “Loudness War” might be imminent after big players like Spotify, YouTube and Apple Music force a loudness normalization to their streams (in other words, you want it hot? It will sound crippled online…your choice) more and more masters are held back to a more conventional (and more musical) overall level. This in itself means ISP’s will be automatically reduced drastically.
For more info, the first part of the promo video by Fab Filter for their Pro-L limiter plugin is very well done. Check it out 😉