Chapter 7
Sound Recording and
Reproduction
Last Lecture:
• Electric Circuits
• AC and DC
• Alternating current and voltage
• Electric Current
• Resistance
• Voltage
• Ohm’s Law
• Series and Parallel Circuits
• Electric Energy and Power
Today:
*Audio amplification system
*Faraday’s Law of Induction
*Electric Guitar
*Microphone
*Loudspeaker
*Amplifiers
*AM-FM Tuners
*Tape Recorders
*Digital Sound
Audio Amplification and
Reproduction system
Signals from the signal sources are sent to a preamplifier, and then to a power amplifier, and finally to a
loud speaker.
Impedances need to be matched for efficient signal
transfer.
Electromagnetic Induction
(a) When there is no relative motion between the coil of wire and
the bar magnet, there is no current in the coil.
(b) A current is created in the coil when the magnet moves toward
the coil.
(c) A current also exists when the magnet moves away from the
coil, but the direction of the current is opposite to that in ( b).
Faraday's Law of Electromagnetic
Induction
Michael Faraday found experimentally that the magnitude
of the induced emf is proportional to the rate at which the
magnetic flux changed. Faraday’s law can be written as,

  N
;   B A.
t
where N is the number of turns in the loops, A is the area
of one loop, ξ is the induced emf, Φ is the magnetic flux,
and B┴ is the perpendicular component of the magnetic
field.
Electric Guitar
A Moving Coil Microphone
Condenser or Electrostatic
Microphone
A capacitor has two plates with a voltage between them. In the condenser mic,
one of these plates is made of very light material and acts as the diaphragm. The
diaphragm vibrates when struck by sound waves, changing the distance between
the two plates and therefore changing the capacitance. Changes in the
capaciatnce produces a corresponding audio electric signal. A voltage is required
across the capacitor for this to work.
From: http://www.mediacollege.com/audio/microphones/condenser.html
Loudspeaker
Loudspeakers use the
principle that a magnet
exerts a force on a
current-carrying wire to
convert electrical
signals into mechanical
vibrations, producing
sound.
Amplifiers
A preamplifier (preamp), or control amp is an electronic amplifier which
precedes another amplifier to prepare an electronic signal for further
amplification or processing. In general, the function of a preamp is to amplify a
low-level signal to line-level.
In a home audio system, the term 'preamplifier' may sometimes be used to
describe equipment which merely switches between different line level sources
and applies a volume control, so that no actual amplification may be involved.
In an audio system, the second amplifier is typically a power amplifier (power
amp). The preamplifier provides voltage gain (about: 10millivolts to 1volt) but
no significant current gain. The power amplifier provides the higher current
necessary to drive loudspeakers.
From: http://en.wikipedia.org/wiki/Preamplifier
Radio and Television; Wireless
Communication
This figure illustrates the process by which a
radio station transmits information. The audio
signal is combined with a carrier wave:
Radio and Television; Amplitude
Modulation
The mixing of signal and carrier can be done
two ways. First, by using the signal to modify
the amplitude of the carrier (AM):
Radio and Television; Frequency
Modulation (FM)
Second, by using the signal to modify the
frequency of the carrier (FM):
Radio and Television; Receiver
At the receiving end, the wave is received,
demodulated, amplified, and sent to a
loudspeaker:
Radio and Television; Tuner
The receiving antenna is bathed in waves of
many frequencies; a tuner is used to select the
desired one:
Tape Recorders
The Magnetic Playback Head of a
Tape Deck
Digital Sound Reproduction
Advantages of CD:
1. Smaller and lighter
2. No physical contact during reading, free from wear
3. No warpage
4. Portable
5. Good dynamic range, over 90 dB.
6. Free from noise, frequency variation, and distortion.
7. Random access is readily available
Binary Data
Digits in a binary system, either 0 or
1, are very simple. They can be very
simply represented in a computer.
Computers do their mathematical
operations in base 2.
Microphotograph of a CD
Pits are 0.5 micron (1 micron = 10-6 m) wide, 0.2 micron deep, and 1.6 micron apart.
Edges of the pits represent the “1s”, and smooth surface represents the “0s”.
Analog-to-Digital
Conversion
Laser Scanning of CD
Three-Beam Laser Tracking
Chap-7 Review
1. What is Ohm’s law of electrical circuits? Explain each symbol and give its unit.
2. Define electric power.
3. If a 100-Ω resistor is connected across the terminals of a 1.5-volt battery, what
electrical current will flow? Show this in a circuit diagram.
4. What electrical power does the resistor in (3) consume?
5. Describe the energy transformations take place in the above situation.
6. Describe the components of an audio reproduction system, and draw a block
diagram.
7. State Faraday’s law of magnetic induction in your own words.
8. Explain how a microphone and speaker works.
9. Convert the decimal (base 10) number 25 to binary.
10. Convert the binary number 0110 to base 10.
11. List 5 advantages for the use of a CD in sound reproduction.