# Presentation: 015748

## Advanced Audio Tutorial: Part 2Cabling & Mixing 

## Quality Challenges

- Echo Canceller

- Echo Canceller

- Echo Canceller

- Mixer

- Network Audio
- Codec / Computer

## Audio standards

- Balanced audio
- 3-wire cable: 2 wires for signal, one for ground
- Differential signal
- Excellent noise rejection
- Professional audio gear (Gentner, Midiman audio codec)
- Unbalanced audio
- 2-wire cable: one wire for signal, one for ground
- Little noise rejection
- Consumer-grade audio gear (standard PC audio codec)

## A Digression on Units

- Each mode of sound has a different unit of measurement
- In air, we measure Sound Pressure Level, SPL, in Newtons per square meter
- On the wire, we measure Volts
- In the computer, we measure a fixed-point value at some resolution

## Due to the large range of human hearing (6 orders of magnitude) a logarithmic scale is used
A decibel represents a ratio:

A doubling in intensity is a 3dB increase

- A decibel represents a ratio:
- A doubling in intensity is a 3dB increase

- Decibels

## Decibel measures

- Since dB is a relative measure, we need absolute reference points
- dB SPL reference is 0.000002 Newtons per square meter, the threshold of hearing
- dBV relative to 1V RMS
- dBu relative to .775V RMS (unloaded)
- Equivalent to dBv
- dBm relative to 1 mWatt
- dBFS relative to full scale representation (so all values are negative)

## Why the dBu and dBV?

- Professional gear uses a +4dBu voltage reference
- Consumer gear uses –10dBV reference
- Why? Just a choice; also stronger signal at +4dBu provides more protection against noise fields

## Conversions

- Professional audio gear: balanced, +4dBu reference
- Consumer gear: unbalanced, -10dBV reference
- To connect the two, need level and balance conversion hardware
- Matchmaker MM100, lots of others
- Also typically convert connectors (XLR on professional side, RCA on consumer side)

## Cabling issues

- Simple answer: buy quality cables, don’t make your own
- Connectors:
- XLR for balanced connections
- RCA for consumer unbalanced
- Mini-plug for computer soundcards
- 1⁄4” TRS (tip-ring-sleeve) for some balanced audio applications

## Quality Challenges

- Echo Canceller

- Echo Canceller

- Echo Canceller

- Mixer

- Network Audio
- Codec / Computer

## Review of Requirements

- Natural high quality sound
**Full Duplex**
**Hands free – microphones open**

## Full duplex

- In order to support free communication, must be able to speak and hear at the same time
- Implies bidirectional path through the entire hardware and software infrastructure

## Hands-free Operation

- For the tech to vanish, no click to talk
- This introduces a major problem: **Echo**

- Audio

- Capture

- PC

- Mixer

- Speaker

- Microphone

- Multicast

- Network

- Acoustic

- Coupling

## Methods for eliminating echo

- Gain control
- Separation of speaker and microphone
- Room treatment
- Click-to-talk
- Active echo cancellation

## Echo Canceller

- Computes estimate of echo return signal from speaker to microphone
- Subtracts echo return estimate from microphone audio
- Echo-free audio sent to output.

## Commercial echo cancellation hardware (first generation)

- Single echo canceller per device
- Coherent/Biamp Voicecrafter
- Intrusive whitenoise-burst for calibration
- Polycom Soundpoint
- Integrated mic/speaker
- Drift over time
- Multiple microphones required external mixer
- Roaming (wireless) microphones posed problems

## Commercial echo cancellation hardware (second generation)

- Common devices:
- Gentner AP series (AP400, AP800)
- ASPI EF1210
- Echo cancellation per microphone input
- “distributed echo cancellation”
- Gentner adds matrix mixer, automixing
- Microphones only gate on when sound present
- Aids in feedback, echo control, phase problems
- ASPI EF1210 requires external automixer
- But has noise cancellation

## Digression: Matrix Mixers

- Matrix of Inputs to Outputs
- Controlled by settings at crosspoints
- On / Off / Gate
- Some allow gain or attenuation adjustment at crosspoints
- If not, must equalize levels at edges
- Gain adjustment point:
- In upcoming diagrams, we draw a matrix mixer as distinct mixer per output, tightly bound to the device at the output

- G

- G

## Echo cancellation per input vs. External Mixer

- Why can’t we just plug lots of microphones into a mixer, then plug the mixer into the EC box?
- The distributed echo cancellation on each input has to work harder and cannot converge quickly.
- When the microphones are gated "off" by the external mixer, the echo canceller has nothing to train to within the room.
- It may appear to work, but under stress (multiple speakers, ambient noise, local reinforcement) it will fail
- EC hardware works best when used as designed
- Money spent on doing it right is money well spent
- Driving non-microphone inputs through an external mixer to the EC box is not a problem (VCR, CD player, ...)

## Commercial echo cancellation hardware (third generation)

- Common devices:
- Gentner XAP series (XAP400, XAP800)
- ASPI Vortex EF2280
- More advanced EC
- Noise cancellation
- Intelligent autogain
- Effects processing (filtering, compression, etc)

## ASPI EF1210 Block Diagram

## ASPI EF1210 Channel Processor

## AP400 Block Diagram

## XAP400 Block Diagram

## ASPI Vortex Block Diagram

## Gentner Channel Processors

- XAP series

- AP series

## Gentner XAP Processing Unit

## ASPI EF2280 Channel Processor

## Issues in EC setup

- EC Reference signal is key
- Typically bound to a particular output on the EC device
- EC Ref will be subtracted from the microphone inputs
- Hence, do not route microphones to the EC reference output
- If reference is correct, and levels are configured properly, EC will take care of itself!

## Local Reinforcement

- What if I want to feed my speakers from the microphones?
- EC reference must not contain microphone inputs
- Typical solution involves routing incoming network audio to an unused output for EC reference

## Normal Configuration

- Transmit to
- Network

- 

- Recv from
- Network

- G

- G

- G

- EC Reference Signal

## Local Reinforcement Configuration (BAD)

- EC
- Ref

- G

- G

## Why is that bad?

- No independent adjustment of microphone levels
- Same level goes to codec as to local speakers
- Echo cancellation will be wrong
- Generated audio will be patchy
- Local audio will be patchy
- Level adjustments will be difficult -- much less gain than you expect to have

## Local Reinforcement(distortion fixed, still bad)

- 

- nothing

- EC
- Ref

- G

- G

## Why?

- We’ve fixed our EC problem by configuring a “ghost” output as the EC reference.
- We still have the microphone level problem, which will likely cause big problems in real life
- Feedback! The level at which the codec is being fed is likely far higher than the level you want fed to the speakers
- Want to be able to just “crack open” the microphone to the local speakers

## Local Reinforcement(practical)

- Mic

- 

- nothing

- EC
- Ref

- 
- Subbus

- G

- G

- G

## Second improvement

- We are taking advantage of a *subbus*
- Subbus is an independent mixing bus that can be hooked into the main mix matrix
- We route all microphones that we will use for reinforcement to the subbus
- Subbus level adjustment used to control amount of local reinforcement
- Subbus mute used to enable/disable local reinforcement

## Local reinforcement with crosspoint adjustments

- 

- nothing

- EC
- Ref

- G

- G

- G

- G

- G

## Crosspoint Adjustment

## Local reinforcement with crosspoint adjustments & virtual reference (XAP)

- Virtual
- Reference

- EC
- Ref

- G

- G

- G

- G

- G

## Crosspoint Adjustments

- If the matrix mixer allows adjustments at the crossing points of the matrix, not just the outputs...
- Then we don’t necessarily need the subbus.
- Individual microphones can be adjusted for best reinforcement levels
- More complex to maintain, however

## Network / Telephone Bridging

- Commonly used configuration for telco/network bridge
- Connect AG session to telecon
- Bridge in AG nodes with broken network
- Balancing levels can be twitchy
- Microphone to network and microphone to telco
- Telco to speakers and telco to network

## Telephone Balancing Act

- Out

- 

- CODEC
- In

- EC
- Ref

- G

- G

- 

- Telco

- G

- G

- G

## Balancing solutions

- Normalize levels
- Adjust incoming gains so that levels within the matrix are nominally 0 dBu
- Then all output levels should be happy
- This can be twitchy to set up and maintain
- Doesn’t always do the right thing
- Again, matrix crosspoint adjustments help a lot

## Telephone balancing with crosspoint

- 

- CODEC

- EC
- Ref

- G

- G

- 

- Telco

- G

- G

- G

- G

- G

- G

- G

- Telco to Network

- Telco to Speakers

- Network to Telco