Flute Tone Investigations
- Appendix 3
“Audio Lab” Control Unit Specification
This paper details the
specification for a custom interface designed to facilitate audio and
sound testing using readily available software. My specific
purpose is flute acoustics, but the unit would have far wider
application. The specification is not yet complete and will be
updated as concepts firm and construction proceeds.
“Audio Lab” Control Unit
To interface the computer to the real world to
enable it to function as an efficient and comprehensive audio
laboratory. Uses and facilities will include:
Digital sound recording and playback
User-friendly connection and control of inputs, outputs and
Audio Equipment testing and calibration
Sound Level metering and analysis (FFT, etc)
Room acoustics measurement
Supply power to test rigs
Power control of audio equipment
Facilitate VOIP calls
The audio lab control unit will be custom built,
and take the form of a 2U rack type box under the computer monitor. It
will be used in conjunction with an external high end computer audio
interface, in my case the ProSonus FireBox. Only the line inputs and
outputs of the Firebox will be used, to give maximum flexibility and
control to the control unit. All circuitry will be designed or selected
to avoid compromising the quality of the interface. While it is
expected to be used largely at 44.1kHz sampling rate (for easy
consistency with other audio applications), circuits will be designed
with the possibility of using the FireBox’s 48kHz or 96kHz sampling
rates occasionally, should a wider frequency range be needed.
Wherever possible, readily available circuit
designs (eg mic preamps, power supplies, amplifiers, millivoltmeter,
oscilloscope interface, etc, kits) will be used to simplify design and
construction. Permanent connections will be via the rear panel,
temporary connections via the front panel. Incoming mains will be
filtered to reduce likelihood of interference. A Ground Lift switch
will be incorporated if needed to deal with earth loop induced hum.
A separate input selector for each channel will
provide for a convenient choice of inputs:
5. Other Channel (L->R, R->L)
6. Computer output (“Loopback”)
8. Spare input positions, eg for turntable (RIAA) input, Telco
Separate input selector switches - one for each
channel – will permit situations such as:
routing a mono source to both channels
singer on mic with guitar on line input
monitoring output on one channel and a
selected input on the other
type inputs (BNC) that can take attenuator probes for fossicking
around when faultfinding. Connects via suitable high Z preamp. Zin
For sound measurement & acoustic music input. Pair of XLR mic
connectors for quality mic input to low noise and distortion mic
preamp. Phantom P48 switchable for each mic.
Line inputs, including:
RCA phonos for attaching devices under test, line
1/4" mono jacks for flying test leads
3.5mm stereo connector for device under test or to
accept input from a portable recorder, cassette deck, etc.
3.5mm “stereo” connector for a headset mic for VOIP, narration etc -
connects to self powering mic preamp. Headset Mic level control.
“Other” channel allows either channel to be sourced (in mono) from
Link to computer output for diagnostic self-test (loopback) and to
meter and analyse output side levels for before & after testing
Calibration signals (see Calibration section below)
Spare input positions will allow for future expansion, eg other
specialist mics, phono preamp for disc-to-CD conversion, telco
The various software packages offer control of level where needed,
but this may not always be enough, or convenient to use, so some
additional control will be offered:
A switched attenuator for each channel will offer
sensitivity steps of 10dB over the range +20 to – 20dB, or 10V to 100mV
on the line inputs and oscilloscope input.
A level control will offer fine control of gain. CCW is zero;
the CW extreme of the switch is the calibrated position.
A line out side would bring the output of the sound
interface box via an amp and an attenuator to ensure adequate and
adjustable drive levels:
jacks for test leads
RCAs for device under test
3.5mm stereo jack for device under test,
the CAL terminal for multimeter and oscilloscope probes.
The attenuator would cover the range 10V to 10mV
(20dBu to –40dBu)
Fine adjustment would cover the range 0 (CCW) to
The unit will be calibrated to the real world using
the following approach:
output will be taken from some suitable software
The inputs will be
switched to Computer Output
output will be monitored with an external AC Digital Voltmeter
The output will be
adjusted for a convenient level, eg 1V RMS at a frequency well
within the bandwidth of the AC voltmeter (eg 50-60Hz)
controls on the Audio Lab Interface will be set so that the input
metering on the software reads the same level.
calibration signal will be available for routine calibration checks.
The Calibration is intended as a convenient check
if the PC mixer gets tweaked by any of the software. A selector switch
will offer a range of convenient calibration and check sources:
Sine (from an oscillator). 1kHz at 1VRMS.
Square (ditto). 1kHz @ 1V Pk-to-pk
5% THD - a signal with 5% THD
5% EHD - a signal with 5% Even Harmonic Distortion
Pos pulse - narrow positive-going 1V pulses to enable absolute phase
Computer out (Internal oscillator turned off to avoid interference)
As well as appearing at the outputs, the calibrator
signal will appear at a front panel terminal convenient for clipping on
oscilloscope probes and a multimeter during calibration set up and
Level measurement and monitoring will be provided
on-screen by software emulating either the VU meter or the European PPM
ballistic characteristic or choice thereof, and calibrated through the
LEDs for each channel will warn of impending
overload to the preamps that would cause clipping.
excursions will be limited to just over 0dBfs to protect Firebox input.
Sound quality monitoring
Switch speakers to monitor Input, Output or Off
Monitor Speaker level control
Headphone output with level control for the headset
headphones for Skype or when narrating.
Power outputs and control
A mains socket on the rear will provide power for
powered loudspeakers and the Firebox, so that these are also switched
off when the Audio Lab is not in use.
On power up, output will be muted until the Firebox
has had time to synch.
Consideration will be given to an automatic power
down of the audio chain in the absence of sound for a set period, in the
manner analogous to a screen power-saver
A separate power circuit will also control a 3pin
mains socket on the back to switch off little-used devices on the desk (eg
A mains socket on the side provides convenient
power for devices under test.
A multipin output connector to provide 48V, +15V, -15V, 5V and 0V
for circuit board and breadboard testing..
A "hum output", approximately 1V RMS of mains hum
for testing Common Mode Rejection Ratio and output impedance balance of
low-level inputs and outputs.
Stereo ¼" jacks to XLR for testing pro gear,
Stereo ¼" jacks to RCA for testing domestic
Stereo ¼" jacks to croc clips for circuit
testing and faultfinding
Oscilloscope probes for circuit testing,
Mic test box – to test Level, Freq. response, CMRR, Noise of
mics and mic inputs
Dummy mic capsules to permit testing of
internal microphone electronics
"Soundproof box" to establish noise floor of
Sound level meter to calibrate absolute SPL
Measurement microphone - omni pressure mic for
free-field level measurements.
Low self-noise mic, for examining noise and
Probe mics, and associated balanced adapters and calibrators.
Vibration sensors, for determining nature of
Calibrated Lazy Susan to test microphone polar
Dummy load 4/8/16 ohms for amplifier testing
Breadboard, for circuit development
Cake tin, for electrostatic shielding
breadboard and other test setups
Magnetic pickup coil for detection of strong
magnetic interference fields.
Audio 1:1 isolation transformer for floating
sources or loads
Artificial flute blowing machine
Artificial face for flute blowing machine