A lot of times I have seen limiters on the master channel that have a threshold set to -0.2 etc. On asking why, I got the answer “Well, it doesn’t clip and its loud enough”. Well, of that half of it is true. It is loud enough, but it doesn’t clip, I dont think so.
There are 2 kinds of peaks that we should be aware of. Digital peaks and Intersample peaks. Digital peaks are the peaks that you se on your DAW. They represent the signal level on the digital domain. This is what is usually looked at to see if there is a clip. Well, isn’t that all that is needed? Unfortunately no. Intersample peaks area what create the brittleness. What are intersample peaks?
Intersample peaks, or True Peaks, are the peaks that occur between samples. How is that possible? Well, the digital domain is fine, but our D-A conversions have a role to play there. This is where we ultimately listen from. The Signals are converted back to analog and in analog, there is no straight line between dots. The Converter will reconstruct the waveform, and if you look at the picture, you can see the peak. How is this peak so important? This tiny peak that occurs is usually noticeable in the high frequency spectrum. This frequency spectrum is what imparts fatigue which we usually associate with brittle sound. Prolonged listening to this will cause us to be irritable and not emotionally responsive to the sound. In films, especially, if dialogues or effects constantly hit this, it is easy to get uncomfortable and find it sharp. This need not necessarily be from the EQ. Ofcourse, there are modern DA converters that have these headroom, but not all.
In the cinema, you have AC3, DTS or the digital cinema plays the wav files. AC3 and DTS (apt-X) are lossy codecs, and introduce this distortion during the encoding. The wav files in a digital player can also have these depending on the DA converter used.
How can we work around this?
Well, one is to mix to proper levels. And the second way is to use intersample limiters if at all needed. The Avid Pro Limiter and Izotope’s Ozone are some examples of Intersample Limiter or True Peak limiters. They can look at peaks that can occur between samples. Now, it is impossible to accurately determine what the True peak may be in a real world scenario as the reproduction may not be accurate. But these limiters do a very good job in that. This is also the reason why the LUFS loudness metering also has an inclusion for True Peak measure against the normal Digital Peak measure to determine clipping. The R128 and ITU-R BS-1770-3, use these to determine peaks.
One very important thing to note is that all of these relate to one simple fact. Headroom. The better you leave in terms of headroom, the better sound you get in reproduction. In the end, mixing is not just about trying to do it visually and looking at numbers. They are there as feedback. And what do we do with feedback? Act on it, not depend on it.