EE2F2 - Music Technology 3. Mixing

Mixing Basics In the simplest terms, mixing is just adding two or more sounds together. Of course, things are rarely that simple! Mic 1 Mic 2 Mixer +

Simple Mixer Schematic Pre- amps EqualisationFadersPanningStereo Bus L OUT R OUT Inputs Three input channels, stereo output

Pre-Amplifiers Input signals can be very different amplitudes: Line level: Approx 1 V RMS from electronic instruments. Mic level: Approx 1 mV RMS from microphones. The pre-amplifier is just a voltage amplifier with an adjustable gain that can be varied over around a 60 dB range (i.e. from around 1 to 1000)

Terminology – Decibels A way of expressing a ratio between two powers. For example, the gain of an amplifier is: NB. If the output power equals the input power:

Equalisation (EQ) Equalisation is a form of filtering similar to the bass/treble controls on a Hi-Fi. + Low-Pass Filter High-Pass Filter Gain Frequency LPF HPF

Fixed Band EQ + Low-Pass Filter High-Pass Filter Gain Frequency LPFHPF Band-Pass Filter BPF

Graphic Equalisers A graphic equaliser is just fixed band EQ but with lots of bands. E.g. 30-band stereo graphic equaliser

Parametric EQ With fixed band EQ, only the gain of each band can be varied. With parametric EQ, the gain and the centre frequency (and sometimes the bandwidth) of the bands can be varied. In advanced parametric equalisers, the required frequency response can be drawn on a screen (e.g. using a mouse). This gives much more flexible control over the frequency response curve.

EQ Examples Untreated Mid-band cut Low frequency boost High frequency boost Mid-band boost, low and high cut Gain Frequency

EQ Applications Equalisation is applied for many reasons. Common examples are: Correcting a non-uniform microphone response Suppressing resonant modes Enhancing vocal clarity Suppressing high-frequency noise (hiss) Suppressing low-frequency rumble (e.g. traffic) Modifying wide-band sounds (e.g. cymbals) to avoid masking other parts Changing sounds for creative effect

Faders HI MID LO Pre-Amp MIC LINE EQ PAN FADER 1 HI MID LO Pre-Amp MIC LINE EQ PAN FADER 2 HI MID LO Pre-Amp MIC LINE EQ PAN FADER 3 HI MID LO Pre-Amp MIC LINE EQ PAN FADER 4MAIN MIX LEFT RIGHT HI MID LO Pre-Amp MIC LINE EQ PAN FADER 5 HI MID LO Pre-Amp MIC LINE EQ PAN FADER 6 POWER METER

Fader Example Faders are used to balance the mix, setting all parts at the right level. With a small number of parts this is pretty easy. Example: Mixing three tracks, Drums, Bass and Organ in real time…

Panning Most mixers have more than just one output. Typically, they have at least two – the left and right stereo channels. Each input channel has a ‘pan’ control that ‘places’ the sound within the stereo image. Amplitude-only (Blumlein) stereo is used. Examples: Pan control fully anti-clockwise, signal just comes out of the left speaker. Pan control fully clockwise, signal just comes out of the right speaker. Pan control centred, signal just comes out of both speakers equally; it appears in the centre of the stereo image.

Effects In addition to the left and right output channels, additional outputs are often available for routing sounds to effects units. Each input channel has a separate control to determine how much of it gets mixed into each ‘effects send’ output channel. FX Unit L OUT R OUT Effects Send

Other Facilities Metering Meters are usually provided to monitor the output levels and, sometimes, the inputs as well. Mute/Solo Buttons are often provided on each channel to: Mute – Temporarily turn off that channel Solo – Temporarily turn off all channels except that one Insert Points Insert points allow the circuit to be broken and an effect unit directly inserted. Can be useful as some effects are better applied before equalisation rather than after.

Automation To help operators manage large mixing desks, some form of automation is often used. The fader positions can be logged by computer and repeated automatically by: Setting the gain of each channel using voltage controlled amplifiers (VCAs) Physically moving the faders using small electric motors. The VCA method is cheaper but motorisation is often preferred for reasons of: Visual feedback Lower noise It looks cool

Summary Mixers are used to add together multiple sound sources either: From different microphones/instruments From different tracks of a multi-track recorder As well as just adding signals together, mixers also: Provide equalisation facilities Fade and pan signals Route signals to effects units Allow automatic control