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CCRMA Classroom

The CCRMA Classroom (Knoll 217) is our main teaching space.

2006 photo of CCRMA Classroom by Ge Wang

In the front (not shown in the photo) is a projection screen, white board, pair of loudspeakers (Mackie HR824), and presenters’ desk.

The audience/students sit in rows of desks with convenient power outlets.


An LG GRU510N 4K laser projector is mounted to the ceiling.

Audiovisual Selection

Use the Kramer system to plug in and choose one video source.

Use the mixer to mix the audio (including stereo from the selected HDMI source).

Audio System

Actually it’s now an Allen & Heath CQ-12T; maybe you want to download their CQ MixPad app (under “Software Downloads”) at the manufacturer’s resources page.

XXX update all references to Mackie 1202

The heart of the Classroom’s audio system is this Mackie 1202-VLZ3 mixer:

Photograph of the Mackie 1202 audio mixer with labeled inputs and ouputs.

The Mackie’s Alt 3/4 output feeds Zoom; therefore pressing “mute” on a channel actually means “Zoom only”. To completely eliminate a sound source, turn the gain knob fully counterclockwise.

Audio Inputs

lav mic
Wireless lavalier microphone for presenter
room mic
PZM mic affixed to the ceiling of the room that does a decent job of carrying the sound of people in the room
From the dangling stereo audio cable with a 1/8 inch (aka 3.5mm) TRS unbalanced analog stereo connector, meant to plug into the headphone jack of a laptop, phone, etc.).
Stereo audio decoded from the Kramer’s one selected video source.
Stereo sound, in analog, from the Linux machine’s connected MOTU 1248 audio interface
Zoom (Mac mini)
sound from the Mac mini dedicated to running Zoom, straight from its headphone jack in 1/8” stereo unbalanced analog.

Audio Outputs

We use almost all (!) of the Mackie’s audio outputs (not including inserts), namely (in stereo pairs):

“MAIN” (XLR on back)
Classroom overhead stereo loudspeakers; controlled by “overall” / MAIN MIX knob.
“MAIN OUT” (1/4” on top)
[not currently used]
“AUX SEND 1/2” (1/4” on top)
To Mic 2 of the “Drum Machine”; “left” channel (aka #1) unfortunately controlled by “SEND” knob of “AUX 1 MASTER”, which should be left at “U” (for “unity gain”)
“CONTROL ROOM” (1/4” on back)
To the Ballroom Adam stereo speakers, via an “Alto Professional Stealth Pro 2-Channel Wireless Audio System for Active Loudspeakers” (manufacturer’s page) (so that no speaker cables will be in the way of shutting the door between Classroom and Ballroom); controlled by both “overall” / MAIN MIX and “ballroom” / CTL ROOM/SUBMIX.
“ALT OUTPUT” aka “L/3” and “R/4” (1/4” on back)
To Mic 1 of the “Drum Machine”; controlled by “overall” / MAIN MIX, consisting only of the channels for which the MUTE aka ALT 3-4 button is pressed. This is specifically so we can send the overhead room mic to Zoom without amplifying it in the Classroom, but it works for lav mics (to make a perfectly-audible-in-person presenter more clear over Zoom) or any other “muted” sound sources intended for streaming, recording, live processing, etc.

Expanded A/V System

Labeled panoramic photo of the presenter’s desk, January 2022, just days before the demise of the “good old Extron” (replaced with the Kramer).

What’s going on with all this gadgetry?!?

Video Switcher

Photograph of ATEM video switcher with labeled buttons. Note that the power is off because the power input (upper right corner of photo) is physically unplugged (making it easier to read the labels in this photo).
Photograph of ATEM video switcher with certain buttons lit up to show the current selections, e.g., video source #1 “Extron” (now Kramer). The labels are easier to read in the other photo.

The Blackmagic ATEM video switcher (“Drum Machine”) is the heart of the video production system. It has vast capabilities; we use it primarily to select which (combination of) camera view(s) will be recorded and/or sent to Zoom. From Zoom’s point of view, this device is the connected webcam (and microphone); whatever the ATEM is outputting (e.g., your laptop video via the Kramer, with your face superimposed in a small picture-in-picture rectangle) is “your camera” as far as Zoom (or any software on the Mac mini) is concerned.

Blackmagic ATEM Mini Extreme ISO
The “Drum Machine” (because of the light-up touch pads)
It acts a “webcam” and stereo USB “microphone” to the (mainly dedicated to Zoom) Mac mini
Cameras and the split output of the Kramer. Each has a corresponding labeled and numbered big touch pad for selecting it.
Has its own adjacent small video display monitor.
Best button
Try pressing the M.V button for “multi-view” mode to see all inputs (including audio metering) on the display.
Photograph of ATEM’s video display in its most useful M.V mode (where you can see that the Far Cam in its not-very-useful no-lens mode, and that the Kramer (formerly Extron) is set to the Linux machine)

Mac Mini

There’s nothing too special about this Mac; it’s just for running Zoom (etc.).

Its video input “webcam” (with stereo digital audio) is the “Drum Machine” (so you can video produce your class/lecture/concert/etc. for streaming and/or recording).

Its video output is split (mirrored) to a “Janus-headed” pair of identical video monitors, as well as going to the HDMI 3 input of the Kramer (so you can show Zoom on the projector).

The audio output goes to the audio mixer (so you can hear Zoom in the loudspeakers).

It doesn’t connect to CCRMA accounts, but you can easily just log in as Guest User (with certain restrictions, such as all files being deleted when you log out). Otherwise you need a local account (meaning just for this one computer): ask staff to log in (as admin) and perform Apple’s usual way of making a “User” in the Users & Groups pane of System Preferences.

Recipe for Hosting a Presenter on Zoom

Here’s one way to have somebody who is not in the room teach a class or make a presentation to a live audience in the Classroom.

  1. Turn on both of the Zoom Mac mini’s video displays, so that people on both sides of the presenter’s desk can see the Zoom screen.
  2. Power up the projector
  3. Select 3 on the Kramer. The Zoom Mac mini’s video should now appear on the projector; if not then debug.
  4. Log into the Zoom Mac mini (preferably with your personal local account; otherwise “Guest User”)
  5. Make sound on the Zoom Mac mini. It doesn’t matter how: sound file, website, Zoom, etc. (The Mac mini sound should come out the built-in headphone jack, not via any HDMI or USB audio the machine might recognize.)
  6. Turn up the “Zoom” channel on the mixer. You should hear the Mac mini’s sound; if not then debug.
  7. Decide which local camera to send to Zoom and choose it on the “Drum Machine”. The far camera is nice because it shows backs of audience heads as well as confirming that the people in the Classroom are seeing the right content on the projector. (The camera is good enough that projected content is generally legible even when seen through the camera.) The near camera can be good too, if you turn it to face the audience; then the remote projector can see the faces of the people in the front of the audience.
  8. Make sure that the local room mic is going to Zoom (so the presenter can hear in-person people asking questions and heckling) by turning it up on the mixer with the MUTE/ALT 3-4 button pressed in. It’s wise to confirm the level by looking at the audio meters on the “Drum Machine” “M/V” view while clapping or making a sound.
  9. Ideally somebody in person in the Classroom will sit in front of the Zoom Mac mini to monitor chat, etc.

If/when the current presenter is local, it’s worth going through everything above, and then just make these few additions:

  1. Select the local presenter’s computer video on the Kramer, possibly switching the local projection back and forth between that and the Zoom video on input 3 (like if somebody remote asks an involved question, or when going between local and remote presenters).
  2. Use a microphone on the local presenter, either lav(alier) or handheld. The corresponding mute buttons on the mixer determine whether the mic will go to both zoom and the local loudspeakers (unmuted) or only to zoom (muted), depending on the local preference. (Skipping this step means using only the distant room mic: the zoom sound will be distant and reverberant, so that all but the loudest presenter voices will be inaudible over Zoom without headphones.)
  3. Make sure the local presenter has computer sound through the mixer; if so then it should automatically go to Zoom. When playing sound examples, it’s best to use “Original Sound” type options (because to Zoom these are coming into the “microphone” that is actually our mixer, not via Zoom screen sharing with “share sound”). But if remote people are talking through Zoom through the local loudspeakers, you might want/need to turn off “Original Sound” to avoid feedback/echo.

Linux Machine

A CCRMA Linux Workstation is accessible from the presenters’ desk.

Linux Audio

It currently has a Tascam 2x2 like most other CCRMA Linux Workstations.

Two Different Video Monitors for Linux

Jonathan Abel gets migraine headaches from certain video monitors. To avoid this he has identified an old low-resolution (1280x1024) ViewSonic VP930b “Model VS10725” LCD monitor that works for him.

Matt found a sufficiently passive DVI switch that can be run “backwards” (with one video source (namely the Linux output) feeding either of two monitors instead of the usual one monitor being fed by either of two video sources).

If you want to use the Classroom Linux machine and are not Jonathan Abel we recommend you select Normal Monitor.

Any time you log in or out of the Linux machine it re-scans the monitor and will detect the current display and set its video resolution appropriately. So even if the login screen looks weirdly low-res, just log in anyway and it should correct itself.

Most people should press the Normal monitor button. (Actually most of the time it should already be in this state.)
The Classroom Linux machine using the “Normal” monitor (mounted on a swivel arm)
Only Jonathan Abel should press this button
Only Jonathan Abel prefers this low-res monitor

This page of CCRMA documentation last committed on Thu Jan 4 19:15:55 2024 -0800 by Matthew James Wright. Stanford has a page for Digital Accessibility.