Is your EQ good enough?
EQ is by far the most important tool when mastering, so yours should be up to scratch. Analogue hardware EQs marketed for mastering typically have tons of headroom, very little distortion, low noise floors… and a high price!
The good news? These boxes are easy to tick in the digital domain. Any ‘clean’ EQ plugin will have practically infinite headroom, and negligible noise and distortion, so your DAW’s bundled EQ will do very nicely.
Ringing the bell
Digital EQs can be susceptible to a different issue, however, which manifests when you sweep a bell-shaped band up near the Nyquist frequency (ie, half the audio sample rate). Some digital EQs will start to ‘cramp’ out of shape: the band becomes much narrower and sharper than it should be, and the bell shape becomes asymmetric with a steeper upper slope. You can see this behaviour very clearly with DDMF’s IIEQ Pro. Set a bell-shaped boost at 20kHz, with your DAW’s sample rate set to 44.1 or 48kHz, and switch between the Digital and Analogue bell shapes. The Analogue type works fine, with the lower half of the bell creating a lovely smooth ‘air’ boost, while the Digital type results in a pretty useless narrow spike.
Just a phase
But there’s another issue, which we can clearly demonstrate with FabFilter’s Pro-Q 2. This EQ doesn’t cramp in shape at all, but if we set it to Zero Latency mode and analyse the phase response, we see some asymmetry again, which wouldn’t happen with an analogue EQ. Switching to Natural Phase mode fixes this issue, with just a few samples of latency as the penalty.
Cramping of phase response is a much less significant issue than cramping of frequency response, and might be inaudible in many cases. However, switching between Zero Latency and Natural Phase modes in Pro-Q 2, can reveal surprising differences, with the latter mode often sounding more open and airy than the former at the same settings.
If you don’t have an EQ with a de-cramped phase response, download Tokyo Dawn Labs’ free SlickEQ. With only three bands, it may not be flexible enough to be your only mastering EQ, but it will provide wonderfully airy high frequency boosts with analogue-style frequency and phase response.
Alternatively, their commercial SlickEQ M plugin provides everything you’re likely to need in a mastering EQ (minus linear phase modes, see below), and includes sophisticated auto EQ and match EQ functions. You can also enable subtle saturation, that was unavoidable in even the most expensive analogue EQs.
Speaking of this analogue ‘colour’, although saturation can be sonically beneficial sometimes, your default starting point when mastering should probably be clean and transparent. FabFilter Pro-Q 2 is arguably the king of ‘clean’ digital EQs: bristling with features, yet incredibly intuitive and ergonomic, and beautiful to look at as a bonus.
DMG’s Equilibrium also has many fans. This one’s a tweaker’s delight, with an incredible degree of control. Equilibrium can be overwhelming, but the learning curve’s worth it.
A bit on the side
All of the EQ plugins that we’ve mentioned so far offer mid/side processing. While this can doubtless be very useful, it isn’t essential to have it built into your EQ, as you can manually convert to M/S and back again, and route the separate mid and side signals through separate EQ instances.
Alternatively, you can run two instances of the free SlickEQ in series, with one set to process Sum and the other set to process Difference. The interface is compact, so you can therefore have both visible at the same time.