Multi-Channel Audio


Key Terms:
• Audio Interface
• Outputs
• Line-in Inputs
• Mixer Sub-Out Vs Mixer Main Out
• Mixer Output assignment


Relevant Notions in Logic:

• Preferences -> Audio -> Devices -> Output Device (for 618, choose "MOTU 818mk3 Hybrid")
File -> Project Settings -> Audio -> Surround Format (for 618, choose Quadraphonic)



Notes

Multi-channel Audio

Multi-channel audio simply means having more than one audio stream (i.e., channel) at a time. So strictly speaking, stereo audio (which has two channels, and is what all CDs and MP3s are) is multi-channel audio. However, multi-channel audio generally refers to using MORE than two channels at a time. The important idea to remember is that each channel goes to its own speaker (so two channel = two speakers, four channels = four speakers, etc.).

Any number of channels is theoretically possible, but in practice is limited to the hardware at hand. Some standard formats exist: Quadraphonic (4 channels going to four speakers, setup in a square or rectangle formation), 5.1 (five channels going to five speakers, setup in a quasi pentagon), 7.1 (seven channels going to seven speakers, setup like 5.1 but with added rear speakers), etc.

In
618, we'll be using a Quadraphonic setup.

To make multi-channel audio possible, we need:


1) A computer with software capable of multi-channel output. For us, this is the iMac (or your laptop) and Logic Pro.

2) A multi-channel audio interface. For us this is the MOTU 828mrk3 Hybrid (the black box on the left of the iMac). An interface is a device that converts digital signals from the computer, and converts them to electrical signals to be used by mixer and speakers (see "DAC" from your "digitizing audio" class notes).

3) A mixer with "main outs" and "sub outs" (sometimes called "group outs", or even "bus outs"). For us this is the Mackie VLZ3 (to the left of the MOTU)

4) Speakers connected to the outputs of the mixer. For us, these are the two Genelec in front of the desk, and the two other Genelecs sitting on the cabinets at the back of the room.



Procedure to Obtain Multi-Channel Outputs in 618

First, Send Sounds to the front Speakers



1- Make sure the Mixer, MOTU 828, Speakers, and Computer are all turned on.

2- Make sure the USB cable going from the black box (MOTU 828) is connected to the computer.



3- In logic, go to the preferences and choose "MOTU 828 Hybrid" as the Input and Output device


4- Make sure the Output on your tracks is set to "Stereo Output":


5- Then, make sure the following faders are up on the Mixer:

"MOTU 828 Main L-R"


6- Make sure the "L/R" Output Assignments buttons are
pressed down on the Mixer. The "L/R" button sends audio to the "Main Mix" fader (labelled "Front Speakers", on the bottom right of the mixer), which in turn sends audio to the "Main Left and Main Right" outputs on the back of the mixer, to which the front two Genelec speakers are connected.



7- Play something from Logic and slowly raise the "Front Speakers" fader:

"Front Speakers"



You should now hear something from the front speakers. If you don't, repeat steps 1-7, and also make sure the following buttons (Mutes) are
NOT pressed down on the Mixer:






Then, Send Sounds to the Rear Speakers:

9- Each track can be assigned a specific pair of outputs on the interface. To use the rear speakers in 618, select "Output 3-4" on one of your tracks:



10- Then, set the "MOTU 828 Analog 1-2" faders to about -10dB on on the Mixer:


11- Make sure the "3-4" Output Assignments buttons are
pressed down on the Mixer. The "3-4" button sends audio to the "Sub 3 and 4" faders (labelled "Back Speakers", on the bottom right of the mixer), which in turn send audio to the "Sub 3 and Sub 4" outputs on the back of the mixer, to which the Back two Mackie speakers are connected.



12- Play something from Logic and slowly raise the "Back Speakers" faders:

"Back Speakers"



You should now hear something from the Back speakers. If you don't, repeat steps 1-7, and also make sure the Mutes on strips 3 and 4 of the Mixer are NOT pressed down.


This is the simplest way to send sounds to the rear speakers. If you want to have one sound go from the front speakers to the rear speakers, then you must duplicate that sound's track (by pressing the "Create New Track with Duplicate Settings" button)….


…..change the outputs of that new track to "Output 3-4", and edit the sound between the two tracks:





To Use the Surround Panner (Best Way to Send Sound to All Four Speakers):

1-Go to File->Project Settings->Audio, and select "Quadraphonic" in the Surround Format:


2-On one of your tracks, choose "Surround" as the output:


This will automatically change the panner to reflect the surround format:



3- Use automation to control the Angle and Diversity (i.e., the amount of spread amongst the speakers):