I think it’s a misconception to say we should always use pass filters (High Pass Filter [HPF]/Low Pass Filter [LPF]) at the start of the mastering process to remove unwanted frequency. This should have already been done in the mixing process by the correct application of pass filters to each individual track. I’ll come back to how we do that later.
If pass filters have been used affectively during the mixing process and the mix is well balanced in general, any pass filters we would use during mastering would be in conjunction with shelving/parametric equalisation to shape the overall tone of a track. Not because we want to remove unwanted frequency from the session mix or that isn’t available in principle on the end output format i.e.; CD audio 16bit 44.1kHz or any other unreasoned rational along those lines.
There is good reason to say we should use a HPF before compressing to remove unwanted low energy. If present it will affect the overall threshold and reaction of the gain reduction. Though again when mastering we would only apply a HPF because they have not been correctly applied in the mixing process and not generically because we are using compression.
It is also possible a DC offset could have been introduced by a poor digital conversion during creation of a session mix from an analogue source. This type of offset can easily be removed by a HPF well below the usable audio threshold. Most DAW’s have excellent DC offset removal tools.
If we’re not using pass filters initially to correct session mixing errors discussed above what we do use them for is the key to good tonal shaping in mastering. The use of pass filters in conjunction with board shelving shifts will correct most tonal issues in a good session mix, but without the application of pass filtering we will be boosting frequencies at the extreme ranges of the shelving.
An example of this would be; if we applied a high shelf boost to brighten/excite a dull mix this shift will move all the frequency above the set filter point. At the uppermost frequency range, 16khz and above, the harmonic detail will be enhanced which is not a bad thing if we want to enhance a dull mix. But, if this boost is not tamed by a LPF we are most likely, in making an equal loudness comparison (see previous posts), to have made the mix sound better/meet more of our aim to brighten the track but it would sound more brittle because of this harmonic excitement.
If we apply the LPF to tame the extreme top end boost (say 17/18kHz 6db/Oct) this will roll off and shape the top end, removing this brittle edge leaving our ‘good’ sounding EQ shift without unwanted artefacts. This shelving/pass filter combination for tonal shaping has been the basis of why many sort after EQ units (Pultecs/EMI/Manley) for the mastering scenario sound effective and musical to our ears.
You can in principle create a similar shaped equalisation [EQ] curve using a parametric boost. Though this sounds tonally quite different because of the phase distortion inherent in the EQ (assuming it is not linear phase) and the nature of most mastering EQ’s. These tend to have Gerzon elements to the active frequency point. This is a peak/dip focus which gives the shelf a smoother more focused sound that cannot be achieved with a single parametric alone.
The main point I’m making above is tonally shaping in a mix is a combination of parts of the EQ unit to achieve the desired sound. The pass filter is a key element of this, meaning we are not in mastering generically adding pass filters to ‘remove unwanted frequency’ from the outset. They are being applied for a good reason to tame and control the use of EQ. In a future post I will discuss in detail the use of EQ in the mastering scenario and its different types and application.
Coming to the mixing process, this is where the pass filters can be applied with most positive effect on the quality of the mix, hence its master. A good rule of thumb is you should have as many HPF/LPF filters as you have channels in the mix. The rational for this is if we have pass filters at the start of the chain on every channel of the mix we’ll achieve the same positive effect we’ve discussed earlier when applying EQ or compression, effects and so on.
Equally in removing unwanted information from the mix at source it reduces the over average RMS, hence we have achieved more usable dynamic range within our system which can only be a good thing.
In application we should be listening to each track individually and applying the pass filters at the extremes of the required frequency range of the instrument. It should not sound when bypass/engaged as if we are removing part of the instrument we want.
Again remember it’s the effect this has on the processing after rather than the pass filter itself. It’s easy to be too aggressive and remove the weight (low end) especially. Take your time and listen – don’t assume you don’t need a certain frequency range. Always trust your ears and make an equal loudness comparison. In general, the effect of the pass filters will be slight so there is no need for equal loudness to hear if we have correctly set the filters.
Another point to note here from a mix cohesiveness point of view is if we have used the same pass filters on all our channels and they all have the same roll off to them it will inherently tie-in our mix. In my opinion this is one of the many reasons classic consoles sound cohesive. Bearing this in mind classic Neve channel strips use 12db/Oct as do SSL though some are also 18db/Oct or switched. If you are going to achieve the required tie-in effect of pass filters in the mix it’s important to use the same type and roll off throughout, unless corrective filters are require on some elements. If you’re also trying to achieve a ‘sound’ you should research what was used originally to make it and replicate as best you can with the resources you have available. i.e. use an appropriate roll off.
In practise the judicious application of pass filters in a mix without question will make your mix sound clearer, more focused, tighten and round the bass end, smooth and create clarity in the high end. It is incorrect to think this can be achieved by pass filters at the mastering stage. Because it’s not about the pass filter itself but the effect the pass filter has on all other processing after the filters are applied.
If you take this good practise onboard and start to apply pass filters appropriately in your mixes you’ll hear the difference, your mastering engineer will hear the difference and be able to create an improved final product which, at the end of the day, is what we are all interested in. It’s the same old audio engineering principle. If you want to get it right, get it right in the first place.