Do I need analogue summing?

Is analogue summing better than digital summing? Do I need analogue summing?

Introduction

The summation of different signals is a relatively simple computing task for one
DAW, a pure addition in which, except possibly due to rounding, no errors should arise.

However, devices are now being sold that are only there to sum signals. Why should this
be useful at all for users of a DAW?

Even the company SPL answers this question in the description of their “Mixdream XP”,
an analog summer, just spongy (SPL, 2014) :

Nowadays, many are asking whether analog summing is better than digital summing. But perhaps the real question is whether digital summing better than analog summing? We at SPL don‘t know of anyone who says so. We do know that summing with the MixDream creates an amazing signal depth, precise local- ization and a wonderful stereo imaging. Moreover, the addition of individual instruments results in soft and pleasant transi- tions. These are analog summing results that we hear – and that you can expect.

Manual of the SPL MixDream Model 2384

Why should an analog device be able to sum an analog-digital-converted signal better than the DAW could do this? Especially since this is (or should be) a simple arithmetic operation?

Let’s figure it out! – In a purely technical way without the guesswork we can read and hear here and there.

Analogue Summing

The analog summation (= addition, mixing) is usually carried out by operational amplifier circuits. Every Analogue Mixer has

To mix two or more Signals, the channels are merged at a junction (node) where they are “summed” according to Kirchhoff’s junction rule.

When combining the different tracks, it is important that a change in one of the incoming signals (level change or switching on and off) does not affect the mix of the other signals.
This is best solved with a 0V summing amplifier (Fig. 1). In contrast to the simple summing amplifiers used earlier, the 0V summing amplifier, as the name suggests, always has a voltage of 0V at the node.

The signals of the individual channels go through the decoupling resistors R1 to the sum bus (node). The red dots in the picture mark the location of the switch for connecting and disconnecting the incoming lanes.
Ideally, all resistors R1 are exactly the same size. Of course these decoupling resistors dampens the signals of the tracks, but the summing amplifier adjusts (amplifies) them again.

With this 0V technology, there should (ideally) be no crosstalk between the individual tracks.

Possible sources of Errors

• Crosstalk between the individual tracks
• Errors could arise in the analog summation due to deviations in the impedances of the various connected channels.

Digital Summing

The Digital Summing should be perfect.

The digital summation works by means of a simple addition¹(Dickreiter, 1997, p. 361), it is one of the easiest operation for a DAW.
According to Bob Katz ²2012, p. 273 there is “absolutely nothing” to complain about digital summing, since adding numbers is one of the easiest exercises for a processor.

If, in a digitally summed end product, the summed product should show errors, it can be assumed that the errors already occurred before the summation, since almost all operations on a digital signal can cause more errors than the actual summation.

In the next chapter we will try to prove these statements by means of measurements.

Measurments

The Stacking Myth

Do I need analogue summing?

One of the most important points in the discussion is:

The difference between analogue and digital summing does not happen at the summing/addition-point itself. It happens before or after it!

There is no question: Good analogue Effects in a Mixer can add wonderful Saturation to your sound and it can make a difference if you add these effects before the summing point or after it.

The discussion with analog summation is, however, whether it is not actually just passing through the various analog components that gives the signal the desired “analog” effect, e.g. in the form of distortion or other influencing of the signal. The fact that the analog summation itself has no use, but rather running through an analog chain, which, according to Bob Katz, is much more sensible to achieve with the digitally ready-mixed stereo summation track, however, in order to save costs and equipment [2].

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