Recently while helping a studio calibrate their set up and map their cutterhead we ran into a problem where we were getting quite severe phase problems. After many tests across the entire system it came down to being something so simple that it was overlooked initially, their Fab Filter Pro Q plugin which was being used for their head mapping was set to ‘Zero Latency’ and then with two instances of the plug in, with multiple bands of EQ, this caused a right mess. After a few clicks of a button and setting to ‘Linear Phase’ mode and remapping, order was restored. So, we thought it is worth mentioning.
For anyone who is interested in mapping their heads for a flat response, after reading this blog post, head here for full information on how to do it.
When cutting to disc, accurate frequency compensation for your cutterhead is essential. But not all EQ is created equal and if you’re mapping your cutterhead using non-linear phase EQ, you might be introducing phase distortion that can actually harm your cuts.
Most EQs (like those running in zero latency or natural phase modes) are minimum-phase EQs. These introduce phase shift around each EQ band, meaning certain frequencies are delayed more than others. In stereo, this causes the left and right channels to drift out of alignment at different frequencies. This behaviour is actually common in mastering EQ software because it can, at times, behave more like an analogue unit. Analogue EQs naturally introduce some phase shift due to the way their circuits work, and many engineers find this imparts a subtle warmth or depth to the sound. Digital minimum-phase EQs are often designed to mimic this behaviour, which is why they’re preferred in musical contexts, but it’s not ideal for precision tasks like cutterhead calibration.
The problem with using this type of EQ when mapping and compensating for your cutterhead’s natural response is that we are trying to work with two drivers identically, so there is no phase smearing at all. Any phase shift can cause problems when cutting, sometimes totally ruining a cut and resulting in severe groove instability. Phase issues can lead to a dull, empty sound, loss of stereo imaging and clarity, or in the worst case, cause the groove to break as the cutterhead lifts out of contact.
Linear phase EQ keeps all frequencies time-aligned, preserving the integrity of your test signals. It’s crucial when measuring your cutterhead’s natural frequency response and applying compensation curves to flatten it. Linear phase mode ensures that what you’re measuring and what your cutterhead is reacting to, is pure and phase-accurate.
Your cutterhead only responds to the signal you feed it, so if that signal has hidden phase distortion or imbalance, your cuts will too. Linear phase EQ is non-negotiable when mapping a cutterhead. It’s not about ‘sounding good’; it’s about preserving accuracy during critical calibration.