This process of summing or adding various signals into a stereo signal is called summing. The whole idea of summing came about when Audio Engineers and Audio Enthusiasts moved to form an Analog system to the modern digital systems. A lot of engineers believe that the way Analog Sums the channels into stereo is far different from how Audio is Summed Digitally. This gave way for Pro Audio Equipment Manufacturers to find a way to market this way of summing “Outside the Box”, a way to get better quality recordings or mixes. There may be little truth to the statement made in the previous sentence but there’s a lot of confusion amongst engineers.
Here I will list a few Arguments and break them down so we can understand what they mean:
Analog Summing gives more headroom!
There is nothing that could be more wrong than this statement.
Look at this image of how a Summing Mixer is used in a modern digital setup.
Here as you can see the Audio Tracks from your DAW are converted from Digital to Analog and sent to the Summing Mixer, which in case of the above image is a Dangerous Music Box.
Here these Analog Signals get Summed from the DBOX into two channels i.e L and R and go back into your A/D converter and then to your Monitors.
It’s pretty obvious what makes this argument a fail, here irrespective of where the audio is being summed Analog we are still converting it back into a digital signal that is, it is going through the A/D conversion once again.
We know that the headroom of a system (Digital System) is defined by its bit depth. What it means is that the headroom of your recordings/mixes/masters will be defined by the bit-depth of your session. A session in 24bit will have 16,777,216 values and hence has a headroom of dB= 20 log(16777216/1) = 144dB as opposed to the 96dB of the 16bit system.
So irrespective of your signal going from Digital into analog Summing it eventually comes back into your A/D convertor and the limit on your headroom is set again by the bit depth of your convertor.
So simply put, summing does not increase the headroom of your mixes.
Analog Summing changes the tone of the Mix/ Digital Summing Sounds Fake
I started by doing this test where we took a track that was summed in the Analog domain and a track that was summed digitally and did a Null Test.
A null test is when you take two tracks that are identical and put one out of phase with the other. If both are essentially the same signal then the signals should cancel each other out completely and you would hear nothing. But, in this case, the two files weren’t the same and understandably so when the signal is going through the circuitry of the analog summing mixer it does lose some top end and some of the bottom end as well.
This means two things, that by summing in an analog domain certain frequencies are added and some that are removed on your signal.
This addition of Analog sound is not always pleasing, and this is where my argument comes in. Sometimes, some tracks and mixes could use that summing but it isn’t something without which you can’t mix.