One thing we see all the time when people EQ their cutterheads using software like FabFilter Pro-Q or Toneboosters Equalizer Pro is that they don’t gain stage properly before they send audio through the EQ. Often people load their audio in at it’s applied level, for example and audio master hitting 0dBfs, run it through their EQ curve, push the top end by 10–20dB to compensate for their heads EQ and assume everything is fine because nothing is showing red on the DAW meters. But what actually happens is the EQ itself clips internally long before the output meter ever sees it, and that completely destroys the sound and flat response you’re trying to make.
Digital EQ isn’t unlimited headroom, in fact it adheres to 0dBfs, so when you boost a frequency the signal gets louder at that part of the spectrum. So if your audio is already sitting near 0dBfs and you boost the top end, the boosted frequencies hit the 0dBfs ceiling inside the plugin. At that point the waveform gets clipped and you get digital distortion which is never good. This will translate as distortion, harsh sibilants and loss of highs. Often with head mapping the high end needs 10-20dB boosts, so if your audio is already at 0dBfs, that’s 10-20dB of audio going into the digital red and clipping.
A reason that a few people have mentioned to us this that they didn’t notice due to their DAW meters still hitting 0dBfs without clipping, but this is only because the clipping happened inside the EQ, not after it. So from the DAWs meters it may look fine, but the audio is already damaged.
To sort this there are two ways, one is to adjust the audios volume or to normalise it (we use Reaper as all edits are non destructive) or to turn down the output of the plug in itself. We normally tell people to normalise audio to around -27lufs, this usually gives plenty of head room. If cutting a mastered LP, we normalise the first song to -27lufs and then adjust all other songs by the same amount to keep the overall intended volume relative from song to song. Normalising audio the a specific volume also means your cutting chain is calibrated as all audio going to your cutter amp will be relatively close to what you expect. As long as the headroom exists before the EQ, the plugin won’t clip internally and the high end will stay clean and open. You must always check the plugin’s internal meter to confirm.
These steps are crucial to getting clean cuts when using EQ software and can save a lot of headaches!