# Presentation: 402148

## Audio in the Access Grid

- AG Tutorial
- May 24, 2000

## Ultimate Goals

- Hands free
- No click to talk
- No searching for microphones
- Never have to think about microphones
- Interactive
- Interrupt other speakers
- Multiple conversations at the same time
- Clear
- No background noise
- Rich-sounding speech
- No feedback
- No echo

## Audio Principles

- Sound: fluctuating pressure levels in our ears
- Analog audio: fluctuating voltages on a wire
- Digital audio: numbers representing the voltages
- Network audio: digital audio encoded for transport over the network
- The purpose of the audio equipment and software we use in the AG is to move between these domains.

## Transducers

- Microphone: sound to analog audio
- Speaker: analog audio to sound
- PC audio capture adapter
- Capture side: analog audio to digital audio
- Playback side: digital audio to analog audio
- Rat
- Digital audio to and from network audio
- Control of the PC audio capture adapter
- What’s the Gentner do?
- Manipulates analog audio
- (Internally, conversions to and from digital audio)

## Sound  Pressure Level or SPL, measured in Pascals (Pa)

0 dB is the level of the lowest noise a healthy person can hear
Conversation at 3’ about 60dB
Sound pressure meter

- 0 dB is the level of the lowest noise a healthy person can hear
- Conversation at 3’ about 60dB
- Sound pressure meter

- Measuring sound: in the air

## Measuring sound: Analog audio

- dBu measures voltage on the wire relative to 0.775V reference
- dbV relative to a 1.0 V reference
- VU meters, Gentner level meters
- Pro audio gear: +4dBu (1.8 dBV) reference
- Consumer audio gear: -10dBV (-7.8 dBu)reference
- 11.8 dB offset between pro and consumer
- cf. http://www.rane.com/par-d.html#decibel

## Measuring sound: Digital Audio

- dBFS measures level compared to full-scale representation of audio
- 0dBFS is equivalent to full-scale audio.
- In other words, VERY LOUD

## Level mappings

- At each conversion stage, there is some mapping between measured levels
- Microphone
- SPL measurement to dBV measurement
- Analog to digital conversion
- dBV measurement to dBFS measurement
- Calibration defines reference conversions
- Music industry defines mapping between SPL and digital level
- -20dBFS maps to 85dB for cinema/theatre work, 80-85dB for television and 85-95dB for music
- Convenient to center reasonable levels about 0dBu

## Digital audio encoding

- Default AG Audio
- 16kHz sampling rate with 16 bit samples
- No compression
- 256kbps
- CD quality audio
- 44kHz sampling rate with 16 bit samples
- No compression
- 704kbps
- Various compression schemes available (rates for 16 kHz sample rate)
- G.726-40 (80 kbps)
- GSM (26.4 kbps)
- LPC (5.6 kbps)
- law (128 kbps)

## Equipment: Microphones

- Characteristics
- Pickup patterns
- Omnidirectional
- Cardioid
- Hypercardioid
- Boundary
- Dynamic range
- Phantom power requirements
- Governed by Inverse Square law
- Money spent on pro-quality speakers is money very well spent

## Equipment: Speakers

- Pretty much what you expect
- Again, money spent on quality speakers is money well spent
- Pair of Genelec speakers very easily filled the Grand Ballroom at HPDC

## Equipment: Cabling

- XLR is standard connector for pro-audio gear
- Balanced connections
- Signal, signal return, ground
- (X = ground, pin1. L = live or +, pin 2. R = return or –, pin 3)
- Provide noise rejection

## Equipment: cabling, cont.

- For microphone lines, phantom power piggybacks on the signal lines
- Conversion between balanced and unbalanced
- Ideally, via an active device designed for this
- Often needs a level conversion as well between pro and consumer levels
- If no device available:
- Balanced to Unbalanced: connect + to signal, ground to ground
- Unbalanced to balanced: connect signal to +, ground to ground and –

## Equipment: mixers

- Purpose: combine multiple inputs into fewer outputs
- Level adjustment at inputs and outputs
- Fader: level adjustment
- Often provides phantom power for microphones
- May provide balanced/unbalanced conversion
- Generally use pro audio level reference (0 dBu)
- Bus: device that combines audio signals together

## Equipment: echo cancellation

- Problem:
- Network audio sent to speakers is picked up by microphones and retransmitted
- Solutions:
- Keep gains down on mics
- Separate mics and speakers
- Optimal solution:
- Use digital signal processing techniquest to *subtract* the audio from the speakers from the audio picked up by the microphones
- Requires detailed knowledge of the acoustics of the space
- Modern digital echo cancellers work very well

## Equipment setup overview

- Place speakers
- Place microphones
- Install cabling
- Install Gentner
- Install computer connections
- Configure Gentner
- Set initial levels
- Balance levels

## Place speakers

- 2-4 speakers (we typically use 2)
- Front of room (where we expect remote speakers to speak)

## Place microphones

- Microphones should be closer to their users than any competing sound sources
- Typically 2-4 feet for desktop microphones
- Ceiling-mounted microphones discouraged

## Install cabling

- Buy high quality cables!
- Node setup at one Chautauqua was hampered by cheap cables, audio problems vanished when good cables installed
- Markertek is a good source
- Route cables away from power lines
- Keep cables as short as possible

## Install Gentner

- Install near computing gear, visible to node operator
- Gentner requires XLR to Phoenix cables
- I purchase 2-3’ M-F XLR cables and cut them in half
- Does anyone have a good tool for stripping coax ground shields?
- Standard setup:
- Output D to speakers

## Install computer connections

- RS-232 connection to control PC
- Gentner connects to PC using level conversion box
- Standard setup:
- Output A to conversion box to PC line in
- PC line out to conversion box to Input A

## Configure Gentner

- Read the Gentner manual. There’s good stuff in there.
- Set up basic routing
- Microphones gated to Output A (microphones to PC)
- Input A routed to Output D (PC to speakers)
- Set echo cancellation reference to Output D
- While you remember, configure the Gentner microphone labels to correspond to the placement of your microphones

## Gentner Configuration

## Set initial levels

- Set microphone levels to –25dB
- Set Input A, Output A, Input D level to 0dB

## Balance Levels

- This is an iterative process that takes practice
- Don’t be afraid to experiment
- Basic procedure:
- Set individual mic levels
- Set Gentner to PC level
- Set PC to Gentner level

## Setting microphone levels

- Set microphone for AGC, Echo cancellation, and phantom power if required
- Recruit a volunteer to sit as the user of the microphone, and have him read a book to you
- Gentner level meter should show levels peaking around 0 dB
- Repeat for each microphone in your setup

## Setting Gentner-to-PC level

- Run rat on the audio PC. Set it for line in, and set the capture level at its midpoint
- Have your volunteer read his book to you
- Adjust the Gentner level (Output A) such that the levels peak just in the red, and spend most of their time high in the green

## Running the reflector

- On the video machine, run
- 		reflector 20000 20001
- Type a value for the maximum delay. 2000-5000 (2-5 seconds) works well. Click the Apply button
- On the audio machine, run
- 	rat –allowloopback videomachine/20000

## Setting PC-to-Gentner level

- Start an audio reflector
- Start rat on the audio machine
- Set the rat speaker level to its midpoint
- Set the Gentner Input A level and the Output D level to 0dB
- Have your volunteer read his book again
- Adjust the speaker volume controls to a comfortable level.

## Advanced Gentner Setup

- Use of subbus for common microphone configuration
- Local reinforcement
- Feeds local microphones to speakers. Not useful for small groups, but commonly used in auditorium situation
- Important: Route audio from PC to an unused output. Set that output as the echo cancellation reference
- Route reinforced microphones to an unused subbus
- Route that subbus to the speakers
- This allows you to control microphone to speaker levels independently of microphone to network levels.
- Gentner chaining, telco operation, etc.

## Testing

- Local audio tests
- Headphones connected to Gentner output
- Headphones connected to PC output, and internal PC audio card loopback enabled
- Network audio tests
- Packet reflector
- Local machine configured with Linux to listen in on multicast group. (Can likely configure video machine with an audio card, if slots allow)
- Live remote testing
- Does anyone hear me??

## Telephone bridging

- Gentner AP400 / AP10 include telephone bridge
- Just another input/output pair
- Glitches
- Dialing (requires autoanswer, manual dialing, or use of the Gentner PC interface)
- Level adjustment (may need to be done for each call)
- Balancing
- Only one adjustment on input levels, has to feed both local speakers and audio machine
- Only one adjustment on output levels, has to be fed from local microphones and from audio machine

## Potential problems

- No record level in rat, or level too low
- Cable built wrong, or unplugged
- Two end rule!
- Input not gated to output in matrix
- Phantom power not enabled for microphones that require it
- Microphone input set for line level or adjusted too low
- Input muted
- Output muted
- PC cable built incorrectly
- Power meters not enabled in rat options
- Transmit not pressed

## Potential problems, cont.

- Echoes
- Echo cancellation reference set incorrectly
- Microphones too close to speaker
- Microphone plugged into non-microphone input
- Echo cancellation disabled for input
- Local feedback
- Microphone routed to local speaker
- Distorted audio
- Local reinforcement incorrectly configured
- Loudspeakers too close to microphones
- Low battery in wireless microphone