signal theory assignment1
TRANSCRIPT
-
7/31/2019 Signal Theory Assignment1
1/5
BIOLOGICAL SIGNAL
Interpersonal communication has historically been limited by the ways in which
our bodies can sense the world. Beneath the skin, however, our bodies generate
dynamic electrical and chemical signals that carry information about our senses,
thoughts, emotions and responses to the world. Modern embedded and off-theshelf
technologies make it readily possible to measure biological signals and use these
signals to create new forms of expression and communication.
SYSTEM:
These will include circuits that (1) Measure and display the galvanic skin response
(GSR, a common component of lie detector tests) revealing heightened emotional
responses. (2) Measure heart rate (EKG) easily and cheaply using off-the-shelf
exercise equipment. (3) Detect and analyze brain activity (EEG) using a micro
-controller that is pre-programmed to perform a Fourier analysis to separatedifferent types of brain activity for display on RGB LEDs.
Here is a schematic of the electrocardiogram (a.k.a. ECG or EKG) signal,
identifying the physical events that correspond to each of the observed electrical
deflections as recorded from two electrodes places on right and left sides of the
chest.
-
7/31/2019 Signal Theory Assignment1
2/5
Amplification:
The first step is to amplify and buffer signal into a range that can be detected with
common circuitry. Below is a common circuit with a driven right leg (DRL),
commonly used to reduce noise using common-mode rejection.
Here is a similar amplifier circuit soldered together. Note that this is just the analog
portion of the circuit, amplifying the signal. The circuit works quite well, athough
-
7/31/2019 Signal Theory Assignment1
3/5
can be overrun by really heavy noise created by other electronics (especially el-
wire).
Detection:
Once the signal is amplified, we must devise a method to detect the beats. This is
easy if the signal is exactly like the schematic above, but consider the following
Complications:
1) Electrodes are reversed and the large peak is negative.
(2) The signal size can vary by a factor of 10 depending on individual differences
and electrode placement.
AUDIO SIGNAL:
Audio signals are voltages or currents in electronic circuits that vary in time
directly in proportion to a transduced sound pressure.
Waves of air pressure are converted by a microphone or other transducer into
electrical signals that can be manipulated and stored. In order to deal with these
signals, we need some way of describing and measuring them.
INFORMATION:
Most people are familiar with the VU meter, the indicator of signal amplitude so
commonly found on audio equipment. The meter tells the observer how large the
signal is relative to some reference level, thereby conveying information about the
loudness of the sound that generated the electronic signal. But audio signals also
convey information about pitch by varying the frequency of the electrical signal
-
7/31/2019 Signal Theory Assignment1
4/5
oscillation, and this is not directly reflected in the readings of a VU meter. There
are many aspects of audio signal measurement that must be understood if one is to
fully comprehend the operation of audio circuitry.
Sound is the phenomenon we experience when our ears are excited by vibrations
in the gas that surrounds us. When an object vibrates it sets the surrounding air in
motion, sending waves of compression and rarefaction radiating outward from the
object. Sound information is transmitted by the intensity and frequency of the
vibrations, where intensity is experienced as loudness and frequency as pitch.
Sound waves are longitudinal waves of compression and rarefaction in which the
air molecules move back and forth parallel to the direction of wave travel centered
on an average position, resulting in no net movement. When these waves strike
another object, they cause that object to vibrate.
CHARACTERISTICS:
Audio signals have frequency components that are complex In other words, most
audio signals are made up of several frequencies, combining to make the sound we
recognize The standard for human voice is taken to vary from about 100 Hz to
3000 Hz
Piano: Concert A above Middle C is 440 Hz
Lowest audible frequency for humans is around 20 Hz (A low, rumbling bass note)
Highest audible frequency is 20 kHz (beyond the range of most humans, but can be
heard by dogs)
Amplitude measurement: The range of loudness perceived by the ear varies over 6
orders of magnitude, making linear measurements of loudness difficult. Therefore,
a logarithmic scale of loudness has been adopted: the decibel (dB).
dB (power) = 10 log(P1/P2), where P is signal power
dB (voltage, current, SPL) = 20 log(V1/V2)= 20 log(Vmeas/Vref)
Since these measures are a ratio of two values, standard (reference) values are used
for comparison:
0 dBm1 milliwatt (power, not voltage measurement)
0 dBV1 Volt (dBA is similar only frequency-weighted to hearing)
0 dBu or (0dBv).775 Volts ( = 1 mW in 600 ohm circuit)
0 VUvariable reference, nominal operating level
-
7/31/2019 Signal Theory Assignment1
5/5
SYSTEM:
An electro-mechanical transducer like a microphone can convert these vibrations
into an electrical signal that oscillates in proportion to the air pressure variations.
Once the information content of the sound is transferred to an electrical signal, itcan then be stored and manipulated in the process we know as sound recording.
Most of the recording process involves the electrical analog of the original sound
pressure waves since sounds are impossible to store directly, so an understanding
of the function of electronic devices is fundamental to mastering the art of sound
recording.
Equipment including recorders, amplifiers, mixers, equalizers, compressors and
other signal processing gear can be used to alter the electrical information in
specific ways, giving an engineer the ability to dramatically alter the reproductionof the sounds that were recorded. While these devices may be complex, their
operation is based on a few elementary concepts of analog electronics. Analog
electronics refers to a system where the electrical signal amplitude varies
continuously in direct proportion to the intensity and frequency of the mechanical
vibrations that were transduced. Until recently, this was the only method available
for processing audio data. Digital electronics now allow discrete measurements of
the transduced signal to be created, stored, and manipulated by computers at a very
high rate of speed, producing a fast but non-continuous numerical representation of
the original sound.