Lecture 7-Mixing And Stereo Recording

Question 1

What is a mixer?

Answer 1

An electronic device for combining “mixing”, routing and changing levels, frequency content and/or dynamics of audio signals.
Digital mixing consoles convert the incoming audio signal to digital and use software DSP to process the signals. Analog mixing consoles use physical electric circuits to perform the processing (all hardware). Music studios typically use analog consoles. Film and TV studios typically use digital consoles.

Question 2

What are the parts of a mixer?

Answer 2

A mixer is divided into channels. Each channel corresponds to a single audio signal. No matter what size of console, the same logic applies to each channel.

Question 3

Input stage:mic and line levels:

Answer 3

Line and mic levels refer to the voltage of an audio signal. Microphones are typically connected to mic level inputs. Most other audio devices use line level.

Question 4

What does mic level describe?

Answer 4

The voltage generated by a microphone when it picks up sound. Mic outputs are normally quite low, and one millivolt (1mV), Around 60dB lower than line level.
Pre-amps have a gain control to provide amplification between 0 to 60dB approx.

Question 5

What does mic level require?

Answer 5

Amplification

Question 6

What is phantom power?

Answer 6

Usually 48V, necessary for powering condenser microphones.

Question 7

Mic and line levels:

Answer 7

A line level signal is approximately 1V or about 1,000 times as strong as a mic-level signal. Nominal value of around 1.23 volts=+4sBu (0.775V reference).
0dBu=0.775 volts.
Decibel units for measuring voltage. A logarithmic voltage ratio with a reference voltage of V0=0.775volt-0dBu

Question 8

What is dBv?

Answer 8

A logarithmic voltage ratio with a reference voltage of V0=1.0000volt-0.dBv
The home audio level of -10dBv means 0.3162volts, that is 7.78dBU.
The studio recording level of +4dBu means 1.23 volts.

Question 9

Good sounding mic preamps are one of the most important considerations in recording

Answer 9

Often engineers will not choose to use the preamps on the console may use ‘outboard’ preamps but these can be costly.

Question 10

DI boxes:

Answer 10

Direct Injection (DI) of a signal means that a microphone is bypassed and the instrument (always electric) is plugged directly into the console or recording device. Converts an unbalanced 1/4inch output into a balanced XLR mic output. 
Active boxes, can provide gain to the audio signal and therefore need electronics requires battery or AC power. Sometimes it has enough gain to be able to replace the mic amp and connect directly to the DAW. Passive boxes, provide no gain and don't require power.

Question 11

Unbalanced connectors:

Answer 11

Not good at suppressing noise from outside interference.

Question 12

Balanced connectors:

Answer 12

Helps reduce noise and good for long cable runs.

Question 13

Signal processing section:

Answer 13

Complexity of signal processing stage depends on mixer type. Normally contains an EQ section and may also contain a dynamics processing section. Auxiliary sends: we can send a portion of the signal on the channel to an effects processor such as a reverb unit.

Question 14

Panning section (L-R):

Answer 14

Can position sound in the stereo field.

Question 15

Fader control:

Answer 15

Can control the overall mix level of the audio signal.

Question 16

PFL (pre-fader listening):

Answer 16

Can listen to the audio without fader control.

Question 17

Mute:

Answer 17

Mutes the audio signal

Question 18

Coincident mics:

Answer 18

Supports level difference derived stereo reproduction. Blumlein and XY.

Question 19

Spaced mics:

Answer 19

Spaced pairs record timing differences. Disadvantages: unstable imaging, not good for source localisation, mono incompatibility. Advantages: you can use omi mics, less coloration at low frequencies, good for capturing diffuse soundfields, good sense of spaciousness. Distance between mics: 1/2 and 1/3 of the width of the sound stage.