All mixers modify the signals that pass through them. Even with the controls “zeroed” the changes to bandwidth, phase, and timing can be observed by comparing the output signal to the input signal. There will also be the introduction of noise and possibly electromagnetic interference (EMI).
To compare these attributes for a lineup of contemporary mixers, Doug Woosley of Sound Ideas conceived and organized a digital mixer study, with the results available here at the SynAudCon website. Doug’s roles as an integrator and front-of-house mixer give him the opportunity to mix on various digital consoles.
On multiple occasions he’s noticed significant tonal coloration when following some commonly used routing practices on digital mixers, such as routing a channel to an output bus via multiple paths. His ears told him something was wrong, and this study was designed to find out what. The mixer lineup includes stock items along with samples provided by manufacturers. I scripted and performed the tests, and formatted the results for presentation. Dennis Birkemeier produced the results matrix.
Again, go here if you want to bypass the rest of this article and cut to the chase. Once there, click on the “Summing” plot in the matrix for each mixer. If you see a comb filter (a series of peaks and nulls in the response) then there is a routing configuration for that mixer that should be avoided. The “Timing” tab will show the routing that produced it. The Timing and Summing plots are color-coded the same (e.g., green plot in Summing corresponds with green plot in Timing).
The main objective of the study was to investigate timing issues; however, we performed additional tests to evaluate some other responses. Out of the very large number of tests that could have been performed, we selected the frequency response magnitude, frequency response phase, EMI susceptibility, common-mode rejection ratio (CMRR), and timing, with the timing information presented in both the time and frequency domains.
The study’s matrix presents the test results. Selecting any of the plots reveals a tabbed report of vector graphics plots for each mixer. The tabs are Levels, Phase, Timing, and Summing. Here’s a description of each.
The Levels tab presents a number of measures in the frequency domain. These include the frequency response magnitude (blue), EMI susceptibility (red), noise floor (gray), ideal CMRR (orange), and the CMRR with an imbalanced source (blue and green) (Figure 1).
Each mixer was tested under identical conditions. The same send and receive cable was moved from mixer to mixer. The input signal was -20 dBV and the mixer gain was set to 30 dB. This produced a +10 dBV output from the mixer – a strong line level signal that should fall short of causing any non-linearities such as clipping, limiting, or compression.
The plots on the Levels tab were smoothed at 1/12-octave by the analyzer to make them more readable and relevant to what can be perceived by human hearing. The frequency range limits for the frequency response magnitude were the maximum achievable with the Audio Precision APx515 analyzer (2 Hz – 80 kHz). The band limits were reduced for the CMRR, noise floor, and EMI susceptibility tests, zooming in on the part of the spectrum where power line noise is the typically the culprit. This makes it much easier to compare the results without being distracted by data that is outside of the scope of the study.
Frequency Response Magnitude. All mixers are band-pass filters. This means that there is necessarily a high-pass response at the low frequency end of the spectrum and a low pass response at the high frequency end. This is a good thing, since a “DC to daylight” response could allow all sorts of parasitic junk to enter the mixer and be amplified.
Some of the mixers have a gentle frequency roll-off at the frequency extremes. Others have a brick wall response. These reflect both the philosophy of the mixer’s designer and the limitations on bandwidth imposed by the mixer’s sample rate.
Noise Floor. It’s important to realize that no perfect electronic components exist. All will have residual noise. Note that the level plot shows a 100 dB range (+20 dBV at top, -120 dBV at bottom) with the signal level near the top and the residuals near the bottom.
This underscores the importance of establishing a good component and system gain structure. If an artifact is at -80 dB relative to the signal, it’s not likely to be audible unless your gain structure makes it audible. The objective is not the absence of noise, but the inaudibility of noise. The noise floor plot (grey) shows the output of the mixer with no input signal.
Pin 1. When multiple “ground” paths exist between audio components, parasitic currents can flow through them and enter an audio device via the cable shield connections on the analog input and output jacks. These “ground loops” have been the bane of the audio practitioner for decades.
The “Pin 1” test is based on AES48 and the research of Ralph Morrison, Neil Muncy, Bill Whitlock, Ray Rayburn and others. It’s designed to reveal a susceptibility to these shield currents at power line frequencies. To perform the test we deliberately injected a parasitic current into the input shield connection (pin 1 on the XLR jack). A measurement was made of the output with no input signal, with pins 2 and 3 connected through a 100-ohm resistor, simulating the presence of an ideal source.
The parasitic signal is a half-wave rectified 60 Hz sine wave at 100 mA (Figure 2). This produces a 60 Hz fundamental and a string of harmonics, emulating the parasitic currents that circulate around the ground paths of a sound system.
In the ideal case, the spectrum of output noise floor would be unchanged. In the real world the parasitic signal may show up, indicating a susceptibility to Shield-Current-Induced-Noise (SCIN). This susceptibility has been dubbed the “Pin 1 Problem” and in severe cases steps may need to be taken to mitigate the resultant hum and buzz.
The presence of the parasitic signal in the noise floor suggests that special precautions may be necessary for channels that may have a ground loop condition, such as when driven from another device that is powered by a different electrical outlet than the mixer. Common solutions include the “lifting” of cable shields at balanced inputs, and the use of EMC XLR connectors.
Common Mode Rejection Ratio. The CMRR describes the ability of a balanced input (or output) to reject a signal that is identical on the plus and minus terminals. Such a signal is termed common mode and it should be rejected by the input. Signals that are different between the input terminals are differential mode and should be accepted by the input (Figure 3). A balanced output driving shielded twisted-pair cable is designed to render parasitic interference signals as common mode and the desired audio signal as differential mode.
The orange trace on the Levels plot shows the ideal CMRR through the power-line frequency range. Ideal in this case means that the driving source (the analyzer) has an instrumentation-grade output impedance balance, meaning that the output impedances looking back into the plus and minus terminals are exquisitely matched. Unfortunately, this is not typically the case with audio gear due to design philosophy and component tolerances, so an additional CMRR test was warranted.
CMRR Imbalance. Bill Whitlock has pointed out that in the real-world few outputs are perfectly balanced, and any imbalance can impair the CMRR of an electronically-balanced input. The CMRR Imbalance test was formulated by Whitlock to reveal the CMRR of an input under real-world conditions. The ideal CMRR test is repeated but with a 10-ohm resistor placed in series with pin 2 and pin 3 respectively. Any degradation of the ideal CMRR can be observed on the plot. Solutions to CMRR reduction due to source impedance imbalance include high quality input transformers and special analog IC chips.