Download Tetrahedral Arrangement Soundfield Microphone

Tetrahedral Arrangement Soundeld Microphone
Thomas Allen u4123966 Simeon Baker-Finch u3938976
Riley Doust u4114397
October 24, 2006
Abstract
This report presents the design and testing of a soundeld microphone subsystem specic to the ambisonics
surround sound format. The design incorporates four uni-directional microphones in a tetrahedral array with
the aim of reproducing a sound image that accurately reects a real audio experience. The four input signals
are passed through a pre-amplier stage and are then fed into the mixing module that formats the signals
according to the trigonometric relationships dened by spherical harmonic theory. The four output signals,
W, X, Y and Z comprise the B-format that is processed further to accommodate for a specic loudspeaker
arrangement. Testing of the design was carried out in PSPICE, while the hardware was rst constructed on
a breadboard and later implemented on a printed circuit board (PCB).
Contents
1 Introduction
3
2 Theory
4
2.1
2.2
Ambisonics and the B-Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
2.1.1
Encoding a Field of Sound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
2.1.2
Decoding the B-Format to a Speaker Array
. . . . . . . . . . . . . . . . . . . . . . . .
5
Analogue Electronics Implementation Theory . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
2.2.1
Operational Amplier
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
2.2.2
Dierence Amplier
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
2.2.3
Summing Amplier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
2.2.4
Dierential Amplier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
3 Design
9
3.1
Microphone Capsules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2
Microphone Preamplier Stage
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
3.3
B-Format Formation Stage (Mixing Stage) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
4 Implementation
9
12
4.1
PSpice Simulation
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
4.2
Breadboard Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
4.2.1
Signal Interference and Breadboarding . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
4.2.2
Capacitive Interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
4.2.3
Inductive Interference
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
4.3.1
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
Printed Circuit Board Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
4.4.1
The PCB Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
4.4.2
Design Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
4.3
4.4
4.5
Noise Cancellation and Filtering
Hybrid Final Implementation
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5 Results
5.1
5.2
5.3
14
General Observed Behaviour
5.1.1
14
Amplier gain
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
Results of PSpice Simulation
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
5.2.1
The Preamplier Stage
5.2.2
Mixing Stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Results of a Streamlined Breadboard/PCB Implementation
. . . . . . . . . . . . . . . . . . .
15
16
6 Conclusion
16
7 Appendix A: Circuit Schematic
18
1
8 Appendix B: Photos
19
9 Appendix C: PSpice Schematics and Simulation Results
22
9.1
Preamplier Stage test Circuits and Results . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
9.2
Mixing Stage Test Circuits and Results
25
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10 Appendix D: Results of Hybrid Implementation.
28
11 Appendix E: Eagle PCB Layout
31
12 Associated Data Sheets
32
2
1
Introduction
Attempts to incorporate directional information into a sound image date back to the early 19th century.
Multiple telephones were spaced out across a concert hall in France in an attempt to recreate the concert
hall sound elsewhere.
The sound was transmitted over separate wires to the same number of telephone
receivers. [11] As microphone and loudspeaker technologies developed, researchers made various unsuccessful
attempts at surround recording and playback. Today, surround sound playback technology is commonplace in
home entertainment, cinema sound, live music venues and elsewhere. Usually, the kind of sound played back
through these systems has been recorded by a number of mono-channel microphones, and professionally
mixed into a format which mimics reality when played through a specic speaker conguration.
True
surround sound recording recreates the sound space which a human experiences - we can sense sounds from
the left, right, high, low, far away and nearby, placing each sound at a particular point in the space around
us.
The modern soundeld microphone concept was originally developed in the 1970s to originate ambisonic
sound material. Ambisonics is a method of recording information about a soundeld and reproducing it
over some form of loudspeaker array so as to produce the impression of hearing a true three dimensional
sound image. [11] Particularly important in the development of ambisonic theory is the characterisation of
the B-Format sound signal.
In this format, three reference channels (x, y and z) locate sound sources in
space, and a fourth (w) channel senses the air pressure changes that describe sound. [11]
Research continues among various academic groups who seek improvements to the ambisonic system, and
applications for a three-dimensional soundeld. Academics in music look to true surround sound in order
to reproduce concert hall sound, as well as to extend the boundaries of current musical practice. Auditory
display and data sonication oer a new tool in interpreting information; surround sound oers further
variables on which to map such data. New media artists see this technology as an opportunity to build truly
immersive environments, particularly for interactive installations. As music composers, particularly those
involved in electronics-based genres, have ventured into this area, and engineers have been pushed to meet
the demands for newer, better, and more interesting sound spatialisation and diusion technologies. [12]
Ambisonic technology has found a niche in the business community also. Modern teleconference applications and services attempt to make communication between participants at remote sites more eective by
presenting information in a manner which is perceived as much like natural, physical interaction as possible. [4] Recording using a soundeld microphone employs technologies which have the potential to increase
such realism.
Studio engineers agree that even when the soundeld is not reproduced using a large number of loudspeakers, sound quality is advanced. There's no question that sounds recorded ambisonically make better
stereo. . . There are no deadspots with ambisonics. [2]
3
2
Theory
2.1 Ambisonics and the B-Format
2.1.1 Encoding a Field of Sound
Systems that attempt to produce a sound experience equivalent to that experienced by a human in reality
include wave-eld synthesis, holophony and ambisonics. All three systems attempt to recreate the wavefronts
in a sound eld, based on the Huygens Principle. [12] this physical principle is based on the fact that waves
add constructively in the propagation direction. Of the three systems, only ambisonics can record an playback
a sound image equivalent to the real experience.
In rst-order ambisonics, sound information is transported via four channels: W, X, Y and Z. This signal
conguration is known as the B-format. Each letter represents a zero order or rst order spherical harmonic
component of three dimensional space.
The channels of the B-format are based on the dissection of the space around the microphone into spherical
harmonics (see gure 1). Many physical functions follow Laplace's equation. These include such values as
incompressible uid ow, electrostatic potential, gravitational potential and the displacement of an elastic
membrane. A solution of Laplace's equation, which is similar to both the wave equation and Schrödinger's
equation, in three dimensions is called a spherical harmonic. This complex physical phenomenon will not be
discussed further here see
[9] [1] for more information.
The zero order harmonic (W) represents the sound pressure which is incident to the microphone, whilst the
rst order harmonics, X, Y and Z are three components of the gradient of the incident pressure. That is,
the rst order harmonics are related to the velocity of air particles representing a sound pressure wave. The
information from these three signals precisely locates the origin of a sound in the space surrounding the
microphone as in gure 1 and gure 2.
Figure 1: The directional gain patterns of full-sphere B-Format signals.
The role of the analogue electronic circuit built for this project is to convert sound pressure information from a
sphere surrounding the tetrahedral array of microphones into an ambisonic B-Format signal. Discussion of the
4
Figure 2: Top view of directional gain patterns of full-sphere B-Format signals.
The Z-channel spherical
harmonic pattern points out of the page.
particulars of the electronics implementation follows. In principle, the role of the circuit is to appropriately
weight and combine voltage signals which represent the sound pressures incident at each of the four elements
of the tetrahedral array, so that the output represents the four channels of the B-Format.
In particular,
when:
L = lef t,
B = back ,
F = f ront,
R = right,
D = down,
U = up,
and the four microphone input signals come from the directions:
LF U,
LBD,
RBU,
RF D,
the following relations must be reected in the circuit output:
W
X
Y
Z
= −(LF U + LBD + RBU + RF D)
√
= 2 2(−LF U + LBD + RBU − RF D)
√
= 2 2(−LF U − LBD + RBU + RF D)
√
= 2 2(−LF U + LBD − RBU + RF D)
2.1.2 Decoding the B-Format to a Speaker Array
A number of loudspeaker congurations will allow the ambisonic B-Format signals to be reproduced as air
pressure dierentials in a three dimensional sound space.
The following explanation is not central to the
microphone design and build process, but gives perspective to the practical applications of a system incorporating both recording and playback technologies. Decoding methodology exists for sets of loudspeakers
placed at the vertices of regular polyhedrons and cuboids, as well as for diametric congurations (2n speakers
arranged in diametrically opposed pairs with respect to the listener). [6] Perhaps the easiest to understand
is the
Cuboid Decoder Theorem, proposed by a leader in ambisonic theory, Michael Gerzon. [6, 7, 8]
For eight speakers placed at the vertices of a cube surrounding the listener, and where:
5
L = lef t,
B = back ,
F = f ront,
R = right,
D = down,
U = up,
the following relations are true:
LBD
=
LBU
=
LF D
=
LF U
=
RF D
=
RF U
=
RBD
=
RBU
=
1
√ (W
2 2
1
√ (W
2 2
1
√ (W
2 2
1
√ (W
2 2
1
√ (W
2 2
1
√ (W
2 2
1
√ (W
2 2
1
√ (W
2 2
− X + Y − Z)
− X + Y + Z)
+ X + Y − Z)
+ X + Y + Z)
+ X − Y − Z)
+ X − Y + Z)
− X − Y − Z)
− X − Y + Z)
Figure 3: The 8 speakers forming an ambisonic cube.
Decoding the recorded B-Format allows the listener to experience the complete spatial sound experience
which was incident upon the tetrahedral array microphone at recording time.
6
2.2 Analogue Electronics Implementation Theory
2.2.1 Operational Amplier
The internal block diagram of an op-amp (gure 4) shows that the input stage consists of a dierential
amplier. A gain is applied to the output of this stage. Stemming from the identication of the op-amp
input stage as a dierential amplier is following basic relationship:
vout = Aol (v+ − v− ).
(1)
Figure 4: Operational amplier internal arrangement.
When a feedback network is introduced (when the op-amp output is connected, possibly through a number
of electrical components, to one of the two op-amp inputs), the closed loop gain,
Acl
can be calculated. The
closed loop gain depends on the nature of the feedback network, and the op-amp characteristic equation
becomes:
In this case,
vout
= Acl .
vin
vin
(2)
is used, since in most common congurations, one of the two op-amp input terminals is
grounded.
2.2.2 Dierence Amplier
A dierence amplier employs an opamp in dierential mode to obtain an output proportional to the difference between two scaled inputs. [3] For the conguration shown in gure 5, the following relationship
represents the ideal circuit characteristic equation.
vout =
R2
(v1 − v2 ).
R1
Figure 5: The dierence amplier. [3]
7
(3)
2.2.3 Summing Amplier
A summing amplier can be used to calculate a weighted sum of a number of input voltages. For example,
for the three input conguration shown in gure 6, the following relationship exists between the output
voltage and the inputs.
vout = −(
RF
RF
RF
vin1 +
vin2 +
vin3 ).
R1
R2
R3
(4)
Figure 6: The summing amplier. [3]
2.2.4 Dierential Amplier
A dierential amplier circuit (gure 7) can provide high voltage gain and common-mode rejection. [5]
Figure 7: The dierential amplier. [5]
Considering applications in which a non-zero signal is applied to both inputs, the dierential amplier is said
to be in either
dierential
or
common-mode
conguration. The characteristics of both modes stem from the
key concept that the dierential amplier outputs an amplied signal representing the dierence between
the two inputs. For dierential mode, the two inputs are the same, but perfectly out of phase. In this case,
the output is a signal with the same frequency, but twice the amplitude of the input. In the common-mode
case, both inputs are identical, thus no signal is amplied and the output of the dierential amplier is zero.
This is known as common-mode rejection. The role of the dierential amplier in the soundeld microphone
circuit is explained later.
8
3
Design
Initial design was chosen from a website (see [13]) which oers circuit schematics for a range of unique,
interesting analogue electronics applications. First, the circuit was modelled using
PSpice
software. Some
design faults were noted at this stage - particularly the presence of circuit noise at a frequency of between
1 and 2MHz at the output for a clean input signal. Changes were made in both modelling and prototyping
stages to enhance the performance of the circuit. Making such changes allowed the design team to be more
involved in the analogue electronics at the heart of the device, and to learn about real world limitations in
terms of component performance. The team noted that sometimes
bandaid solutions
were the best way to
confront a challenge in electronics implementation given restraints in both budget and time.
3.1 Microphone Capsules
The most basic requirement for building an eective tetrahedral array soundeld microphone is four unidirectional, cardoid pattern microphone capsules. A good signal to noise ratio characteristic is desirable for
recording purposes, and the microphones must be small in size, since they need to be mounted as close as
possible to the others in the array for best ambisonic performance. All breadboard testing was done using
cheap, omnidirectional capsules, available from any hardware store (see data sheet
[15]). These capsules
were not mounted in the correct tetrahedral conguration. For the nal implementation, low-noise, unidirectional capsules (see datasheet [16]) were obtained from Ariose Technologies (Taiwan) via Crest Technologies
(Nunawading, Victoria). The capsules were mounted on a somewhat primitively fashioned tetrahedron on a
small stand (see gure 8).
Figure 8: The microphone and stand.
According to the datasheet for the electret microphone inserts 16, a certain conguration of resistor and
capacitor must be connected at the output terminal for proper operation.
heavily ltered by the combination of
2.2kΩ
resistor and
9
100µF
The
+bat
supply voltage is
capacitor. These leftmost components on
the schematic layout also act to provide the required
1.5V
bias to each capsule, rather than the
9V
rail
voltage.
3.2 Microphone Preamplier Stage
Each of the four microphone capsules which make up the tetrahedral array are attached to a preamplier
circuit.
Preampliers are commonplace in circuits using microphones, because the low voltage output of
the microphone must be converted into a relatively high voltage input to a recording device. Contemporary
microphone amplier design can be categorised into four types, based on active elements: discrete semiconductor, vacuum tube, integrated circuit, and hybrid.
Within these categories are found FETs, BJTs, IC
function modules, IC opamps, discrete opamps, among other components. [10]
The circuit implemented in this project incorporates four separate preamplifer stages one for each microphone capsule. Each uses a simple discrete transistor dierential amplier connected to an op-amp with
feedback. This gives low noise from using good quality cheap discrete transistors congured to provide some
gain, and good linearity provided by the high open-loop gain of the op-amp with overall feedback. [13]
The TL074 is a low-noise, JFET-input operational amplier, designed to have low input bias and oset
currents, and fast slew rate. [17] Unfortunately, the input noise of this IC is too high to be used alone as a
microphone preamplier, and must be coupled with the discrete transistor dierential amplier to achieve
the best possible performance in minimising circuit noise.
The original design [13] called for the use of 2N4403 BJTs in the dierential amplier stage. 2N2905 PNP
switching transistors were chosen based on availability, and proximity to the characteristics of the preferred
component. See ( [18] and
[19]) to conrm that the two transistors are interchangeable for the purposes of
the soundeld microphone circuit.
The trimpot variable resistors in each preamplifer stage can be used to adjust the voltage gain of each
microphone input signal. Some microphones are more sensitive than others. The trimpots can be varied to
ensure equal level inputs. In advanced usage, the shape of the soundeld being recorded can be changed
by altering the gain pattern which is directly controlled by the four variable resistances.
See
[7, 13] for
discussion of advanced topics.
3.3 B-Format Formation Stage (Mixing Stage)
The ambisonic B-format is realised as the circuit output according to a weighted sum of the inputs, following
the relationships described above for encoding soundeld information.
essentially an opamp set up in an arithmetic conguration.
The mixing stage of the circuit is
A combination of summing and dierence
amplier is used so that both negative and positive weightings can be applied to the input waveforms. Note
that the capacitors in parallel with the feedback resistors for each opamp in the mixing stage have very small
capacitance. They are present to form an active low pass lter, which eliminates any high frequency noise
generated by the circuit from the output. These capacitors do not have a signicant eect on the opamp
summer or dierence congurations, except that they limit the circuit frequency response to an appropriate
range.
Mixing together the four amplied signals for the W component of the B-format is relatively simple. We
10
need:
W = −(LF U + LBD + RBU + RF D).
(5)
Summing amplier theory suggests that for the conguration shown in gure 9,
voutW = −
20kΩ
(vLF U + vLBD + vRBU + vRF D ).
10kΩ
(6)
Figure 9: The mixing stage for the W channel of the B-Format.
For the X,Y and Z channels, both sum and dierence operations are required. Focusing on the X channel,
theory suggests that the electronic circuit must reect:
√
X = 2 2(−LF U + LBD + RBU − RF D).
(7)
Combining summing and dierence amplier theory, for the conguration shown in gure 10, note that the
inputs to the opamp (+) and (-) terminals are rst summed, and then the dierence is taken between them.
So:
voutX =
56k
[(vLBD + vRBU ) − (vLF U + vRF D )].
10k
Figure 10: The mixing stage for the X channel of the B-Format.
11
(8)
4
Implementation
4.1 PSpice Simulation
Before construction was initiated, the soundeld microphone circuit was simulated using PSpice software.
Figure 11 details the structure of the top level schematic. Detail for each block is attached, appendix C.
Figure 11: Top level schematic of PSpice implementation.
4.2 Breadboard Implementation
The rst physical realisation of the soundeld microphone circuit was achieved on a breadboard. Various difculties were encountered during this process, as detailed below. A photo of the breadboard implementation
is attached, appendix B.
4.2.1 Signal Interference and Breadboarding
Throughout the whole implementation process it was realised that achieving hi-delity results from the preamp stage of the design was very much dependent on the circuit layout on the breadboard. Over the course
of the prototyping stage of the circuit two critical factors had to be addressed to achieve the desired output.
4.2.2 Capacitive Interference
Capacitive interference will propagate through a circuit when wires at dierent voltages are placed closed
to each other. When voltage changes in one wire, a voltage can be induced in the neighboring one through
capacitive coupling. Higher frequency source interference results in larger capacitive interference. Through
research it was determined that the best way to prevent capacitive interference is to shield all wires and
components with a Gaussian surface. [14] The shield would be connected to ground, protecting the inner
signal carrying conductor from the interference. However, for our purposes, having signal carrying wires and
devices placed at distance signicantly apart generally suced to eliminate the problem.
12
4.2.3 Inductive Interference
Inductive interference is the result of collapsing and reforming magnetic elds associated with time varying
currents carried by a conductor. The magnetic elds created by the alternating signal induce similar currents
in neighboring wires. [14] It is not always easy to prevent this interference during breadboard implementation.
Preventative measures that were taken to avoid inductive interference included keeping the breadboard design
compact, and ensuring that no wires were running parallel to each other; running wires perpendicular to
each other eectively nullies the potential for inductive interference due to the direction of the magnetic
eld vector.
4.3 Debugging
4.3.1 Noise Cancellation and Filtering
Despite the meticulous bread-boarding techniques that were followed, high frequency noise in the order of 2
MHz was being propagated through three out the four pre-amps initially constructed. This noise was most
likely a result of capacitive coupling of the transistors used in the pre-amp stage of the design. The outer
metal casing of each BJTs was not electrically isolated from the device (the casing was found to be the
collector of the BJT 19). We found that the noise was eliminated when we physically touched the casing
of the BJTs. The body's capacitive eects de-coupled the devices, eliminating the high frequency gain. To
replicate this eect, capacitors were placed in parallel with the
18kΩ
collector resistors. This limited the
gain of the 2MHz signal. A more eective method was to directly place a capacitor between the collectors
of the BJTs. A low pass lter was implemented on the output to eliminate any other noise.
The specications of all of the lters that were implemented were garnered from the equation:
f=
1
2πRC
(9)
The cut-o frequency of the lters was always set to 20 kHz, the upper limit of the audible frequency
spectrum of the human ear. Being an audio application, all frequencies greater then 20 kHz are necessarily
unwanted noise.
The introduction of lters and noise minimisation measures during the implementation of the soundeld
microphone circuit on breadboard resulted in a more robust design. Resolving circuit anomalies throughout
the course of prototyping is an eective method for increasing circuit performance.
4.4 Printed Circuit Board Implementation
4.4.1 The PCB Design
Designing the printed circuit board (PCB), after observation of the circuit analog electronic theory and
breadboard implementation, further considers physical and practical considerations. Like most analog circuits, the microphone pre-amp serves to manipulate signals.
Particularly in this audio application, the
qualities of signals are an important consideration, and as was clear from the bread board implementation
results, an acceptable standard of signal quality was not achieved with the original circuit design. However
implementation of the circuit on PCB was expected to inherently improve the signal performance at the
time of designing the PCB the noise problem was thought to be primarily caused by the wiring, rather than
13
transistor case interference. Therefore the design for the PCB as sent to the manufacturer did not include
noise alleviation components.
A schematic of a circuit is laid out to maximise visual understanding of the component relationships. The
layout for the PCB therefore was dissimilar to the schematic optimised to create the shortest least
complicated path for signals and power supply and practically utilise available space. The rst and second
stages were realised on separate boards, with four signal wires joining each.
op-amps contained in a single TL074 IC. The tracks that carry
V + , V−
Each stage utilised the four
and earth are designed to be
thicker in order to carry more current with less resistance and potentially damaging heat dissipation. The
thickness of the track at a given point is proportional to the number of component pads that the 'rail' will
be sourcing/sinking current to. The earth track is thicker overall than the
V+
and
V−
tracks, because in the
implemented circuit, the single connection to earth must carry all current owing through
V+
and
V− .
4.4.2 Design Errors
The PCB was not designed perfectly to match the schematic the series of outputs to cable jacks that
connect the second stage to the recording device omits the ground connection. This was remedied in the
implementation by connecting an additional lead from the cable jack ground to the power ground. Additionally, the EAGLE library components nominated for the 100uF capacitors were much smaller than the
available capacitors, which caused diculty in tting the components to the PCB.
4.5 Hybrid Final Implementation
The nal implementation of the soundeld microphone was achieved using a mix of breadboard and PCB
circuit components. A tetrahedral structure was built to house the four unidirectional microphones. The
output of each microphone was attached to one circuit input.
The B-format output was taken from the
circuit and connected to two stereo wires for input into a computer. See appendix D for a photo and the
results of this implementation.
5
Results
The following details a number of tests carried out to characterise the operation of the soundeld microphone
analogue electronic circuit in both simulation (using
PSpice
software) and in physical reality modes. The
central goal of the experimental procedure dened below was to ensure that the microphone achieved proper
encoding of the surrounding sound pressure into the B-format. Noise performance is a key indicator in audio
applications much time was spent on eliminating unwanted circuit noise from the output voltage signals.
5.1 General Observed Behaviour
5.1.1 Amplier gain
The following voltage gains were observed for the two stages of the circuit design. The gain for the mixing
stage is chosen to be a multiple of the intended gain 1 for the W-channel and
14
√
2 2
for the other channels.
Gain stage PSpice simulated Breadboard observed
Preamplier
11.5
10 to 20
Mixing stage
2
1 to 3
5.2 Results of PSpice Simulation
For PSpice simulation, the circuit was subdivided into blocks. Note that the circuit block diagram (gure 11)
contains four identical preampliers, and four separate channel mixers.
5.2.1 The Preamplier Stage
Three variations of the preamplier stage were tested using transient analysis. The circuits and graphical
outputs are shown in appendix C. As mentioned above, the key performance parameter considered here was
the noise performance of the circuit.
The rst circuit to be simulated was a replica of the preamplier stage outlined in the design schematic (see
appendix A). A sine wave was applied at the microphone input terminal, and the output was plotted at the
right hand side of the
47µF
capacitor, across a load resistor. The key observation made from this simulation
was that a noisy signal appeared at the output. The noise oscillates at a frequency of around 1.5MHz. See
gure 18, appendix C.
For the second simulation, a capacitor was attached in parallel with each resistor which joins the collector
of a BJT with the negative power supply rail. This is intended to limit high frequency gain. Choosing a
capacitance of
44nF
eectively eliminates the circuit noise. Instability does appear at the output for the
initial stages of the transient response. See gure 20, appendix C.
As another, more simple approach to eliminate the high frequency noise initially found at the output, a single
capacitor (68nF ) was attached between collector terminals of the BJTs in the dierential amplier circuit.
This oered a more stable transient response than the solution discussed above. For detail, see gure 22,
appendix C.
Finally, a simple rst order lter, designed to have a cuto frequency of
20kHz
(ideal for an audio applica-
tion), was applied to the output of the preamplier. This had a signicant eect in noise reduction, but did
not fully eliminate the 1.5MHz circuit noise. See gure 24, appendix C.
5.2.2 Mixing Stages
PSpice simulation was used to conrm that the arithmetic amplier immediately before circuit output was
designed correctly for each channel. Key outcomes of this simulation were that the circuit output the correct
signal according to the weighted sum and dierence equations described in the theory, above.
A sine wave signal was applied at each of the four inputs to each mixing stage. Each input was assigned half
the frequency of the last, in the order LFU, LBD, RBU, RFD. This way, the mixing stage is tested for a
large range of values in particular, note the similarity of this type of test input to a four-bit binary count.
A transient analysis was set up to display the changing waveform over time this is the key mode in which
a microphone is used (recording sound over time). The results of these tests conrmed the correct design of
the mixing stages for each channel. See appendix C for details of the simulation.
15
5.3 Results of a Streamlined Breadboard/PCB Implementation
A streamlined circuit was built in the latter stages of the project to achieve the best possible circuit operation
from the resources available. A combination of breadboard (used for the preamplier stages) and PCB (used
for the mixing stages) was connected to the tetrahedral array of four unidirectional microphones. The four
output channels, W, X, Y and Z were paired and connected to two 3.5mm jack stereo leads. These leads
were plugged into two separate sound cards in a single computer. Trial software (Adobe Audition) was used
to observe and record the B-format waveforms. Once recorded, one may use appropriate software to decode
the B-Format into a cuboid array of 8 loudspeakers.
The process of connecting the circuit to a computer allowed the group to conrm that the circuit was
operating correctly and accurately. The results of some recordings are shown in appendix D. It is interesting
to note the phase and amplitude dierences in the waveforms displayed in these results.
6
Conclusion
Recording audio information in a surround sound format is an interesting application of analogue electronics
that is prevalent in modern society. The team designed and built a subsystem specic to the ambisonics
surround sound format. Four uni-directional microphones in a tetrahedral array were constructed to replicate
the sound image that accurately represents a real audio experience. The four input signals are passed through
a pre-amplication stage, consisting of a dierential amplier and an operational amplier; the dierential
amplier being added to the design to reduce the output noise and to achieve gain distribution. A capacitor
was added to the design to eliminate the high frequency noise propagating through the circuit.
The signals X, Y and Z were outputs of the mixing module, formatted according to the trigonometric
relationships that describe the position of the sound according to the rst order spherical harmonic theory
mentioned in the report.
The remaining W channel conveys the actual compression wave characteristics
surrounding the microphones during recording.
This 4 channel output is called the B-format.
Further
processing is required to convert this audio format into information that can be reproduced through a
specied loudspeaker arrangement.
The implementation of audio processing circuits requires consideration of noise and interference characteristics that should inuence the circuit design at the schematic and PCB design stage. In this application it
was necessary to remedy problems that only became apparent when using the circuit for its intended purpose
- the audio application. PSPICE and EAGLE simulation is not sucient to gauge the extent of radiated
noise. The function generator signals were also more stable inputs than the inputs from the microphone.
All design considerations mentioned above mean that an analogue electronics application must be closely
monitored, tested and redesigned throughout the prototyping stage.
16
References
[1] Arfken, G. (1985) Spherical Harmonics, in
Mathematical Methods for Physicists, 3rd ed. Orlando,
Aca-
demic Press.
[2] Cromer, Ben (1992) Nimbus Brings Ambisonic Recording Home, in
[3] Durrani, S. (2006)
Pro Audio 104,4.
Lecture Notes for Analogue Electronics: ENGN3227. ANU, Canberra.
[4] Evans, Michael James (1997)
The Perceived Performance of Spatial Audio for Teleconferencing University
of York, England.
[5] Floyd, T.L & Buchla, D. (2002)
[6] Gerzon, M.A. (1992)
Fundamentals of Analog Circuits. Prentice-Hall, USA.
General Metatheory of Auditory Localisation, presented at the 92nd Convention of
the Audio Engineering Society. AES, USA.
[7] Gerzon, M.A. (1980)
Practical Periphony: the reproduction of Full-Sphere Sound, presented at the 65th
Convention of the Audio Engineering Society. AES, USA.
[8] Gerzon, M.A. (1992)
Psychoacoustic Decoders for Multispeaker Stereo and Surround Sound, presented at
the 93rd Convention of the Audio Engineering Society. AES, USA.
[9] Groemer, H. (1996)
Geometric Applications of Fourier Series and Spherical Harmonics. New York, Cam-
bridge University Press.
[10] LaGrou, John. The Design of Microphone Preampliers, originally published in
http: // www. mil-media. com/ docs/ articles/ preamps. shtml
[11] Malham, D.G. (1998)
R-E-P Magazine. Online:
Last accessed: 20/10/06
Spatial Sound Mechanisms and Sound Reproduction. University of York, England.
[12] Malham, D.G. (2001) Toward Reality Equivalence in Spatial Sound Diusion, in
Journal. 25,4.
[13]
Make
your
own
ambisonic
soundeld
type
microphone.
Computer Music
(2003)
http: // homepage. ntlworld. com/ henry01/ cheap_ soundfield/ cheap_ soundfield. htm
accessed: 20/10/06
[14] Wolf, S. (1995)
Guide to Electronic Measurements and laboratory Practice. Prentice-Hall, USA.
[15]
AM4011-Electret Microphone Insert
[16]
Specication for Back Electret Condenser Micrphone
[17]
Low-Noise JFET-Input Operational Ampliers
[18]
PNP General Purpose Amplifer
[19]
PNP Switching Transistors
(Datasheet). Electus Distribution, Sydney.
(Datasheet). Ariose Electronics, Taiwan.
(Datasheet). Texas Instruments, Texas.
(Datasheet). Fairchild Semiconductor Corporation.
(Datasheet). Philips Semiconductors.
17
Online:
Last
7
Appendix A: Circuit Schematic
18
8
Appendix B: Photos
Figure 12: Breadboard implementation. The leftmost board houses two preamplier stages, the right and
second from right house one each.
The second board from the left is the mixing stage for the B-Format
channels.
19
Figure 13: Breadboard implementation of the mixing stage. TL074 opamp terminals are as shown in the
schematic. Inputs LFU, LBD, RBU, RFD are applied at the middle four connected lines, from bottom to
top.
Figure 14: Breadboard implementation of preamplier. The dierential amplier is characterised by the two
silver 2N2905 transistors.
20
Figure 15: PCB implementation. The board on the right is the PCB realisation of the four preamp stages.
On the left is the mixing circuit.
Figure 16: PCB implementation with collector-collector capacitors attached to limit high frequency gain.
21
9
Appendix C: PSpice Schematics and Simulation Results
9.1 Preamplier Stage test Circuits and Results
Figure 17: Preamplier: PSpice simulation of initial design.
Figure 18: PSpice simulation results for circuit of gure 17
22
Figure 19: Preamplier: PSpice simulation with capacitors added across collector resistors.
Figure 20: PSpice simulation results for circuit of gure 19
Figure 21: Preamplier: PSpice simulation with bridging capacitor between two collector terminals.
23
Figure 22: PSpice simulation results for circuit of gure 21
Figure 23: Preamplier: PSpice simulation of initial design with rst order lter on the output.
Figure 24: PSpice simulation results for circuit of gure 23
24
9.2 Mixing Stage Test Circuits and Results
Figure 25: W-channel output test circuit.
Figure 26: Results of simulation: w-channel.
25
Figure 27: X-channel output test circuit.
Figure 28: Results of simulation: x-channel.
Figure 29: Y-channel output test circuit.
26
Figure 30: Results of simulation: y-channel.
Figure 31: Z-channel output test circuit.
Figure 32: Results of simulation: z-channel.
27
10
Appendix D: Results of Hybrid Implementation.
28
Figure 33: The hybrid implementation of the soundeld microphone.
29
Figure 34: Adobe Audition recorded waveform: a nger click
Figure 35: Zoomed in on the nger click in gure 34
Figure 36: Adobe Audition recorded waveform: some ambient noise
Figure 37: Zoomed in version of gure 36
30
11
Appendix E: Eagle PCB Layout
31
12
Associated Data Sheets
32