This guest post comes from Barry Gardner, mastering engineer at SafeandSound online mastering
The 24 bit advantage
These days almost all digital audio workstations have the option to record and operate at 24 bit resolution. In fact it is highly likely that the vast majority of musicians and engineers are already setting their DAW’s up to work at 24 bit. 24 has a number of advantages over operating at 16 bit. I am going to explain a very important advantage which may not be as obvious. It relates to mixing reference levels on your stereo master output bus. Firstly we have to look at record levels because this is where the initial advantages occur. When you record at 24 bit resolution you have a much greater theoretical dynamic range. This means that the noise floor of the recording in the digital realm is much lower than recording at 16 bit. It is theoretically 48 dB lower than at 16 bit. So as a consequence there is no need to record anywhere as hot as when using 16 bit. In fact, a suggested recording level for 24 bit would be to have an average signal level of -18dBFS. You could even have peak signals at this level without detriment to audio fidelity. With peaks at -18dBFS at 24 bit your noise floor is still theoretically 30dB better than that at 16bit. An additional bonus effect is that to obtain peak levels of -18dBFS your mic preamps, compressors and mixer channels will be operating at a lower electrical level which means a cleaner and clearer recording.
Ideal operating levels
I always find this analogy of great value when speaking about digital signal levels and their relationship to analogue metering and electrical values (voltage). A large format console such as a NEVE, DDA or SSL has rows of VU meters present. A nominal channel operating level for many signal types is peaks at 0VU. An exception to this rule would have been instrument with lots of high frequency transient information such as hi hats and cymbals, these would have been recorded at -7VU. This is because the VU meter ballistics under read the level present on these types of signal and they used to more easily distort tape. On these mixing consoles (and in fact most 8 bus mixers of semi pro origins) 0VU would equate to -18dBFS on a digital meter. That might come as a bit of a surprise in part due to the logarithmic scaling on digital peak metering systems and -18dBFS looking very low as a reading relative to the size of the meter itself. However 0VU and -18dBFS referenced to a +4dBU operating level both equate to 1.23 Volts as an electrical potential. So this suggests that at 24 bit an optimal working level would be -18dBFS on your channel signals. It seemed to work for those working with a large format recording console so why not you in your DAW?
Headroom in mixing
All of this follows on nicely to mixing gain structure, the L/R bus output metering on an SSL console would often see something between 0Vu and +6Vu. This is perfect as the console’s electronics would have headroom to spare and so the monitoring electronics would be nice and clear. When you mix in your DAW I suggest an optimal working method is to peak an audio signal with lots of transient information such as a snare or kick drum to -18dBFS on the stereo master output. Then balance all other instrumentation against this reference instrument. Try not to be tempted to adjust the reference signals level too much or you will lose your reference. The beauty of doing this is you have now created ample headroom for the mix so you will not find yourself clipping or having to pull the master fader back at a later stage as you run out of headroom. Another substantial bonus is the analogue electronics in your digital to analogue convertor (audio interface outputs) will be operating at a less high signal level meaning clearer and cleaner monitoring. This means your mix decisions should be enhanced as a positive side effect. You may find you will need to turn your monitoring up to compensate for the lower operating levels. This is a small price to pay for the benefits from the point of recording all the way through to mixing. Don’t worry about maximized loudness at this point, whether you’re self mastering, or sending it to a local or online mastering engineer, you will be happy to have this extra headroom.