This intriguing but ultimately frustrating technique is something that promises much and on most material usually disappoints but it offers an interesting perspective on commercial mixes on anything and on some material can offer genuinely useful results. I refer to it as “centre channel removal” this isn’t referring to the centre channel in a surround format but to auditioning the sides or difference channel from stereo material with the S component summed into mono.
Sum And Difference
This technique relies on the convention that vocals are almost always panned in the middle of mixes. Because of this it is possible to use the fact that centrally panned elements are represented nearly identically in the left and right channels.
If one of the two stereo channels is inverted using a polarity invert, or “phase” button and the results are summer into mono, the common elements in the left and right channels (the sum of the two channels) cancels out, thereby removing the information in the centre of the mix.
What Gets Cancelled?
The vocals usually cancel well but stereo reverb and stereo delays don’t, often resulting in disembodied vocal effects. This can be very interesting, offering a glimpse of the vocal treatments fully exposed. Snare drums often get removed as does anything else panned centrally, luckily guitars are frequently panned left and right and backing vocals are very commonly panned wide and escape cancellation. The bass and kick drum are almost always panned centrally, however some of this bass cancellation can be avoided using filters and routing.
Feeding Bass Around The Centre Channel Removal
By setting two audio paths featuring a pair of filters, one set to high pass just below the vocal and another set to low pass as high as possible without including too much vocal, and by inverting the polarity of one channel of only the high pass filtered is is possible to apply the centre channel removal to only the midrange and top end of the track. This can restore the missing bass though invariably bass and kick drum sounds extend up into the midrange and this is lost.
This technique can help but isn’t a magic bullet, it puts low frequency energy back into the mix but the missing harmonics from bass instruments gives a rather subby sound to the results.
Using centre channel removal - auditioning the difference channel, the ‘sides’ component from MS, it is possible to listen in to details that might otherwise be obscured by the elements common to both channels. With the bass, kick, snare vocals and other centrally panned elements out of the way reverbs, backing vocals, guitars and the like are revealed.
If you try this on a track which has been compressed using lossy compression such as mp3, AAC or compressed music streaming platforms you’ll notice how prominent the detrimental artefacts introduced by the compression become. This is extremely useful. It can help refine your awareness of compression artefacts, by making them more obvious is is easier to spot them when they are back in context and being masked by the centre channel information.
In the example in the video above, an uncompressed WAV mixdown of a track is auditioned with the centre channel removed and compared with the same track aggressively compressed using 64kbps compression for really significant compression artefacts.
Previewing Compression Artefacts In Real Time Using Plug-ins
Although it is possible to preview compression artefacts to some extent using centre channel removal, this is very much an offline process - you have to render a file before you can hear the artefacts it will create. Sonnox currently offer two excellent tools for previewing lossy compression without the need to render a file first. They also include dedicated tools allowing you to fine tune settings to find the best balance between compression and sonic quality. They are:
Sonnox Codec Toolbox
Codec Toolbox is an affordable Plug-In to encode your music for the internet. It includes metering to help avoid potential overs, different lossy compression settings require different amounts of headroom to avoid overdriving the algorithm. You can use Codec Toolbox to audition codecs in real time before committing to encode, ensuring your music sounds as good online as it does in the studio.
Codec Toolbox consists of two applications: the Toolbox Plug-In for auditioning your mix, and the Toolbox Manager for encoding it and adding metadata.
Sonnox Fraunhofer Pro Codec
For a more comprehensive solution the Fraunhofer Pro-Codec Plug-In can be compare up to five audio codecs in real time. Its display helps you focus on key issues such as codec effect on frequencies, audible artefacts and codec-induced overloads.
Using this tool you can optimise your mix for specific formats, such as AAC or mp3, before encoding it using the Plug-In or the separate Manager application for background batch processing.
What Is The Difference Between Fraunhofer Pro Codec and Codec Toolbox?
With two tools which perform the same task but a different price points (Pro Codec costs significantly more) what are the key differences? To see a comprehensive list visit the Sonnox Website but the most significant differences are that the Codec Toolbox is limited to a . maximum of 16 bits and 48KHz and the facilities you have available to guide your choice of settings. The Pro Codec offers comprehensive metering and the ability to audition multiple compression settings against uncompressed versions in real time.