Mixing Desks - The Channel Strip

Introduction

Alto ZMX164FX USB Mixing Desk

Today I am going to show you how signals move through a mixer, using the Alto ZMX164FX USB analogue mixing board pictured to the right to demonstrate.

Mixing boards look really complicated, but they are modular devices and contain a number of repeated sections.

This board has 12 channel strips, four subgroups, a special effects section, the monitor section, and the main mix fader.

The sections of the board are shown in the diagram to the right.

This lesson is going to focus on the signal flow through the channel strip. I am also going to briefly discuss where the signals go after leaving the channel strips - the sub and main mixes.

The Channel Strip

As a rule, signals flow from the top to the bottom of the channel strip, although there are a couple of exception to this rule, they will be described later.

I am going to describe the mono input channels 1-8, on this mixer channels 9-16 are stereo inputs and are have slightly different operation.

Although the inputs are mono, the output after the fader are a stereo signal.

The Input

Inputs

The signal starts with the microphone or line input at the top. The line inputs are TRS balanced/unbalanced inputs.

The insert is to allow connecting an external effects rack into the mix using a TRS connector.

The line gain allows the level to be adjusted to line level. This is different to a DAW, as the DAW expects the signals to already be at line level.

The peak led illuminates if the signal should rarely, if ever, light otherwise you will have distorted signals from the very start.

The low-cut button switches in a 75Hz low frequency filter to reduce hum from a mains power supply or stage rumbles.

The Equaliser

Signal Routing

The blue knobs are a 3-band EQ with a sweepable MID range.

Aux Sends

The Aux Sends can be thought of as a separate mix

The yellow knobs are the auxiliary sends level controls. These control the signal levels to the two auxiliary bus.

AUX1 and AUX2 can be switched to pre or post fader and can be used for monitor or inputs to an effects and sound processor.

AUX 3 and AUX4 are configured as post faders. AUX4 can be assigned to the onboard effects module.

The PAN control is used to position the signal to the left or right side. In the central position the audio will appear in the centre of the stage. This is also an out of order signal flow.

The Fader Section

Channel Fader
and Options

At the top of the fader section is a peak warning led. Ignore this at your peril, it flashes to warn you when you are reaching signal saturation and possible distortion.

The mute button is equivalent to pulling the fader down, and causes the mute led to illuminate.

The channel can route to sub 1-2, sub 3-4, and the Main LR busses.

The solo button routes the signal from the channel to the control room.

Finally, the fader controls the overall level of the signal sent out to the main and sub busses. On this board, the unity gain is marked with a 0. The fader allows from -∞ to +15 db. The ∞ means that effectively no signal will be heard from the channel.

Ideally you want to keep signals around unity as much as possible, this gives the least risk of distortion occurring in your work.

The DSP

DSP Options

The mixer has a 24 bit digital special effects unit that can add flangers, reverb, chorus, delay and combinations of these. There are 16 preset options for each effect selected.

The effects out control is the signal sent to DFX OUT and can be set between -∞ to +10 db.

The AUX1 and AUX2 controls are used to set the signal level from AUX Return 4 on the channel strip to the AUX Send1 and 2. The range is between -∞ to +15 db.

The Peak LED illuminates if the input signal is too strong. The DSP can be muted, and this causes the peak LED to illuminate.

The board power and phantom power indicators can also be seen on this picture.

The Equaliser

Graphic Equaliser

The board has a nine-band graphic equaliser, and a bypass button.

This allows the fine-tuning of the overall output audio levels. 

Sub groups and Main Mix

Subgroups and Main Mix Faders

This is where the output from all of the channels appears.

The four white sub faders control the levels for each of the sub channels.

The red fader is the main mix, the output that your audience are listening to. This board controls the overall level, unlike some that can control the individual left and right levels.

The channel that each of the sub groups appears on the main mix is controlled by the assign to main mix buttons.

Subgroup assignment

Pressing the LEFT button will assign the subgroup to the left main mix, the RIGHT sends to the right  main mix, and having both selected will send to both channels.

Reflection

I have focussed on the parts of the board that I have used, the only section I haven't discussed are the auxiliary sends and returns. Apart from the built in DSP, I don't have any equipment to connect and experiment with (donations gratefully accepted!)

Creating the lesson has helped me understand the operation of my board in more detail, in particular how the special effects unit operates.

I would like to thank you for your time in reading my lesson on the Channel Strip, and I hope you have gained as much as I have (so much so, that I'm tempted to create lessons on some of the other options for this assignment).

Now I want to go and put theory into practice - apologies to my neighbours!