Q. Is analogue mixing superior to digital summing?

Published in SOS June 2004
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I understand that mixes from DAWs can be improved significantly if, instead of using the digital mix buss within the computer, individual tracks are converted to analogue and then summed/mixed externally. Could you explain the difference between digital and analogue mixing, and whether this analogue approach really does offer significant benefits?

Paul Cooper

Technical Editor Hugh Robjohns: There is nothing necessarily wrong with digital summing (the digital equivalent of 'mixing' in analogue systems), and when performed correctly it is technically equal or superior to analogue mixing. However, many engineers and musicians enjoy the inherent imperfections associated with analogue signal processing, and that may be enough justification to warrant the use of analogue mixing systems in some circumstances.

qa SPL Mix Dream 1.s
SPL's brand new Mix Dream is designed as an external analogue mix buss for digital systems.
SPL's brand new Mix Dream is designed as an external analogue mix buss for digital systems.

In an analogue console the channel signals are mixed together at the group or main stereo busses. This operation is simply performed by adding the instantaneous signal voltages together. The electronic circuit design has to be optimised to provide sufficient headroom to accommodate the signals from a large number of channels, as well as to maintain a sufficiently low noise floor — mix-buss noise is a perennial problem of analogue mixers.

In a digital system the same thing is achieved by adding corresponding sample values together — hence 'digital summing' — but the same requirements remain, namely providing sufficient headroom with a suitably low noise floor. Careful attention must be paid to DSP software to achieve this.

I think it is true to say that there were a number of cases in the early days of DAWs and digital consoles where flawed programming resulted in less than perfect results when mixing large numbers of tracks. The problems concern the proper handling of the large binary numbers that result when adding lots of signal samples together, and the necessary rounding and dithering required to output a fixed-length sample representing the mixed signal.

The notion that analogue mixing is in some way better than digital summing really dates back to a few bad experiences caused by poor programming in those early days, combined with some dubious engineering practices which were not appropriate to the digital domain. Unfortunately, once some 'name' engineer is reported in the press as preferring analogue mixing because it 'sounds better' (in a very particular set of circumstances), the urban myth is established and it is very hard to dislodge!

At the moment, it appears to be fashionable in some quarters to mix in the analogue domain and several analogue manufacturers, spotting a golden opportunity, have produced dedicated outboard analogue mixing systems to cash in on this vogue. However, most of these systems can only handle eight source channels, which is not particularly challenging for either analogue or digital systems. The problems of poor mix-buss engineering (in either domain) only tend to materialise when mixing more than about 30 channels together.

That being the case, even using an outboard analogue mix-buss unit, the chances are that you will have to pre-mix a lot of tracks internally in the DAW before being able to output just eight channels to the analogue box, which really defeats the logic of the argument for using an external analogue mix buss!

From personal experience I've not noticed any kind of mixing-buss issues with any modern, professional console, whether analogue or digital, except in the case of some of the budget analogue mixers which simply don't have the necessary supply rails to provide a lot of headroom. I have, in the past, noticed occasional problems with a couple of early DAWs, and these could be attributed to either poor gain structuring (user error, in other words), poor implementation of plug-in processing (third-party software problems) or, less commonly, to the core DSP of the system itself.

However, there are now more than enough superb-sounding digital consoles and DAWs on the market to give the lie to the argument that analogue summing is inherently better. The most demanding area of the music industry as far as mixing is concerned is probably the film industry. Most big film dubbing theatres use huge digital consoles these days, usually with three engineers working together to mix literally hundreds of source tracks, often with several generations of pre-mixing — and all in the digital domain. The film industry has also been producing release material with a far greater dynamic range than most music-only formats for a very long time. So any inherent problems with digital mix busses would have been revealed a long time ago, and these engineers would be using analogue equipment!

With properly engineered systems running carefully designed software and being operated sensibly, there shouldn't be any problems at all with digital summing. Sure, digital mixing lacks the 'rich sounding' harmonic distortion and benign transient clipping that you can achieve by slightly overdriving an analogue mixer, but if you want to deliberately distort the material in some musical way there are plenty of ways to do that in a more controlled, predictable and less expensive fashion!  

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