Chapter 7 Sound Recording and

Reproduction

Last Lecture: Today:• Electric Circuits *Audio amplification system• AC and DC *Faraday’s Law of

Induction• Alternating current and voltage *Electric Guitar• Electric Current *Microphone• Resistance *Loudspeaker• Voltage *Amplifiers• Ohm’s Law *AM-FM Tuners• Series and Parallel Circuits *Tape Recorders• Electric Energy and Power *Digital Sound

Audio Amplification and Reproduction system

Signals from the signal sources are sent to a pre-amplifier, 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,

.; ABt

N

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 ReproductionAdvantages of CD:

1. Smaller and lighter2. No physical contact during reading, free from wear3. No warpage4. Portable5. 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.