UNIT III: Multimedia Audio

Nature of sound waves, characteristics of sound waves, psychoacoustics, elements of audio systems: Microphone, amplifiers, speakers,

synthesizer, MIDI, digital audio, CD formats.

Audio File Format: WAV, AIFF, VOC, AVI, AVO, MPEG-I, MPEG-II, MPEG-III, MPEG-IV, RMF, WMA, MPC.

Acoustics

Sound is a form of energy similar to heat and light. Sound is generated from vibrating objects and can flow through a material medium from one place to another. During generation, the kinetic energy of the vibrating body is converted to sound energy.

Eg. spreading wave over the surface of water.

Acoustics is the branch of science dealing with the study of sound and is concerned with the generation, transmission and reception of sound waves.

Nature of sound waves,

Sound is an alteration in pressure, particle displacement or particle velocity propagated in an elastic material

Characteristics of sound

1. Amplitude – it is the maximum displacement of a particle in the path of a wave and is a measure of the peak to peak height of the wave. The physical meaning of amplitude is the intensity of energy of the wave. For sound waves this corresponds to the loudness of sound (measured in decibel – dB).

Larger the energy of the sound wave, more is its amplitude and louder its perception in our ears.

Amplitude

Characteristics of sound

2. Frequency measures the number of vibrations of a particle in the path of a wave, in one second. The physical meaning of frequency of a sound wave is the pitch of sound. A high pitched sound (whistle) has higher frequency than a dull flat sound (drum). Frequency is measured in Hertz (Hz)

3. Waveform indicates the actual shape of the wave when represented pictorially. Shapes of waves can be sinusoidal, square, triangular. Complex sounds can be arbitrary or irregular in shape. Physical meaning of waveform is quality of sound (guitar or violin).

Frequency

Waveform

Characteristics of sound

4. Speed depends on the medium through which the sound travels and the temperature of the medium but not the pressure.

psychoacoustics

Psycho acoustics is the branch of acoustics which deals with the human auditory perception ranging from the biological design of the ear to the brain's interpretation of the aural information.

1. Decibel (dB) – level of loudness is logarithmic.2. Sound Measurement – 2 popular methods.

Direct Method (measuring a set of environmental factors like temperature, humidity, viscosity etc to compute sound energy levels)

Indirect Method (measuring sound pressure level of a reference sound of known energy levels and comparing those pressure levels with the sound measured.)

Psycho acoustics...

Sound pressure is the amount of pressure the sound waves exert on a surface eg diaphragm of a microphone and provides a way to compute its energy content. (SPL)

SPL (dB) = 20log10 (Pressure A / Pressure B)

3. Hearing Threshold and Masking2 fundamental phenomenon that govern human hearing are the

minimum hearing threshold(least audible sound) and the amplitude masking(occurs because an audible sound has a tendency to distort the threshold curve and shift it upwards).

Elements of Audio System

Microphone – input of sound to the audio system. Converts sound energy to electrical energy. Sound pressure exists as patterns of air pressure. The microphone changes this information into patterns of electrical current. There are 2 types of microphones :–

1. moving coil type Consists of thin metallic diaphragm and an attached coil of wire.

A magnet produces a magnetic field which surrounds the coil. As sound impinges on the diaphragm attached to the coil, it causes movement of the coil producing a proportionate intensity current.

2. condenser typeThe diaphragm is actually the plate of a capacitor, which moves

on the receipt of sound by changing the capacitance and generating a voltage.

Microphones

Based on directional properties, microphones are of 3 types :1. Omni Directional Microphone – sensitive to sound coming from all directions. Eg – environmental sounds in a wild life video clip.

2. Bi Directional Microphone – sensitive to sound coming from two directions ie front and rear. Eg – 2 persons sitting on opposite sides of a table.

3. Uni Directional Microphone – records sound from a single source.

Note : Polar Plot (cardiod) of a microphone is a graph plotting the output level of the microphone against the angle at which the incident sound is produced.

90 90 90

Polar Plots of omni directional, bi directional and uni directional microphones

Amplifiers

It is a device in which varying signal controls a flow of energy to produce an output signal that varies in the same way but has a larger amplitude. Input signal may be current, voltage, mechanical motion etc. output signal is usually of the same nature.

Audio amplifiers are electronic in nature and use a series of transistors as their principal components connected in a printed circuit board. Amplification is not 100% due to noise and produces some distortion of the signal.

The ratio of the output amplitude to the input amplitude is known as the gain of the amplifier.

Amplifier circuits are designated as A, B, AB and C (for analog designs) and D and E (for digital designs)

Elements of Audio System...

Amplifiers...

Class A - use 100% of the input cycle for generating the output, although only 50% efficiency is obtained. The amplifying transistor is biased such that the device is always conducting, due to which more power is wasted.

Adv : it gives the best quality sound due to the fact that the transistor operates on the most linear portion of its characteristic curve which produces minimum distortions of the output wave.

Disadv : Generation of large amount of heat.

Amplifiers...

Class B – Class B amplifiers only use half of the input cycle for amplification. They are used in RF power amplifiers where distortion is unimportant. They are mostly used in pairs in a push pull configuration ie each element is used to amplify the opposite half of the input wave after which they are joined together in the output. This produces cross over distortion.

Disadv : the produce lot of distortion

Adv : they are efficient compared to Class A as the amplifying element is switched off during half of the cycle.

Amplifiers...

Class AB – In order to avoid cross over distortion, the elements are kept on even after the corresponding half cycles are over. The elements are biased in such a way that they operate over a linear portion of their characteristic curve during a half cycle, but still conduct a small amount of current in the other half.

Uses – mostly used for audio amplifiers.

Amplifiers...

Class C – they use less than half of the input cycle for amplification. They are used for filtering or suppressing certain unwanted harmonics from the input wave.

Adv : Very efficient.

Disadv : huge amount of distortion.

Amplifiers...

Negative Feedback

Distortions (noise) in the output wave is an inherent problem due to the non linearity of amplifying elements. There is a limit to reducing the distortion. Negative feedback helps in reducing the distortion by feeding back a portion of the output back to the input so that it is subtracted from the original input.

The negative distortion combine with the positive distortion to make the output signal more or less linear.

Amplifiers...

Class D – they use a series of transistors as switches. The input signal is sampled and converted to digital pulses using an ADC. The pulses are then used to switch the transistors on and off (therefore more efficient).

The sampling rate must be at least double that of the highest input frequency value (called the Nyquist frequency) otherwise it produces low frequency distortions called aliasing.

Quantization error results due to the finite number of transistor switches.

Amplifiers...

Class E –They use Pulse Width Modulation (PWM) to produce output waves whose widths are proportional to the desired amplitudes. This requires a single transistor for switching and a fast clock to enable the switching operation. Generally used for controlling small DC motors.

Adv : cheap compared to others.

Disadv : abrupt switching off / on can produce severe distortion.

Speakers

It is a device that converts electrical energy back to acoustic energy. There are 2 types :

1. Dynamic Loudspeaker – made of wire coil and diaphragm and a permanent magnet.

2. Woofers and tweeters – loudspeaker is divided into - smaller units called woofers to handle low frequencies - mid range for handling middle frequencies and - tweeters for handling high frequencies.

Synthesizer

They are electronic instruments which allow us to generate digital samples of sounds of various instruments synthetically ie without the actual instrument being present. The core of a synthesizer is a special purpose chip or IC for producing sound.

Types of Synthesizers :1. FM Synthesizer – generate sound by combining elementary sinusoidal tones (using Fourier transform) to build up a note having the desired waveform.

2. Wavetable Synthesizer – produce sound by retrieving high quality digital recordings of actual musical instruments from memory and playing them on demand.

Characteristics of a Synthesizer

Polypony – refers to its ability to play more than one note at a time. Previously synthesizers were monophonic ie they could play only one note at a time.

A synthesizer is said to be multitimbral if it is capable of producing 2 or more different instrument sounds simultaneously.

MIDI-Musical Instrument Digital Interface

It is a protocol or set of rules for connecting digital synthesizers to each other or to digital computers. 2 synthesizers communicate via MIDI.

MIDI information tells a synthesizer in its most basic mode, when to start and stop playing a specific note.

MIDI information can also be more hardware specific. It can tell a synthesizer to change sounds, master volume, modulation devices and even how to receive information.

It can also be used to indicate the starting and stopping points of a song.

digital audio

Digital form is a degraded version of the analog form because of quantization error, however quality can be improved by increasing sampling rate. The upper value of sampling rate is limited by the hardware / software capabilities and economic considerations, while the lower value is decided by the Nyquist theorem.

Important parameters for digital audio.1. Practical Audio Sampling Parameters – bit depths of 8 bits

and 16 bits are sufficient.2. Aliasing – if audio frequency > Nyquist frequency => errors3. Bit Rate and File Size4. Streaming Audio – for downloading files on internet.5. High Fidelity (HiFi) – reproduction of sound and image almost identical to the original in quality.

CD formats

A compact disc is an optical disc used to store digital data, originally developed for storing digital audio.

1. CD-Digital Audio (CD DA)2. CD-ROM Digital Data (CD ROM)3. CD-Interactive (CD-i)4. CD-ROM Extended Architecture (CD-ROM XA)5. Photo-CD6. Video-CD (VCD)7. CD-Recordable (CD-R)8. CD-Rewritable (CD-RW)9. CD+Graphics (CD+G)

Audio File Format

WAV – Waveform AudioAIFF – Audio Interchange File FormatVOC – VoiceAVI – Audio Video InterleaveAVO – more commonly referred to as AVIMPEG-I : Motion Pictures Experts Group IMPEG-II : Motion Pictures Experts Group IIMPEG-III : Motion Pictures Experts Group IIIMPEG-IV : Motion Pictures Experts Group IVRMF – Rich Music FormatWMA – Windows Media AudioMPC – Musepack