AUDIO VIDEO SYSTEMS

Transcription

1 AUDIO VIDEO SYSTEMS PROF. PRATIKGIRI GOSWAMI ASSISTANT PROFESSOR ELECTRONICS & COMMUNICATION DEPARTMENT, L. D. COLLEGE OF ENGINEERING

2 PHYSICS OF SOUND Sound is simply an airborne version of vibration. The air which carries sound is a mixture of gases. As Figure shows, the innumerable elastic collisions of these highspeed molecules produce pressure on the walls of any gas container. If left undisturbed in a container at a constant temperature, eventually the pressure throughout would be constant and uniform.

3 PHYSICS OF SOUND Sound disturbs this simple picture. Figure (left) shows that a solid object which moves against gas pressure increases the velocity of the rebounding molecules, whereas in Figure (right) one moving with gas pressure reduces that velocity. The average velocity and the displacement of all the molecules in a layer of air near a moving body is the same as the velocity and displacement of the body. Movement of the body results in a local increase or decrease in pressure of some kind. Thus sound is both a pressure and a velocity disturbance.

4 PHYSICS OF SOUND When the volume of a fixed mass of gas is reduced, the pressure rises. The gas acts like a spring; it is compliant. However, a gas also has mass. Sound travels through air by an interaction between the mass and the compliance. The speed of sound is proportional to the square root of the absolute temperature. The speed of sound experienced by most of us is about 1000 ft per second or 344 m per second.

5 WAVELENGTH Sound can be due to a one-off event known as percussion, or a periodic event such as the sinusoidal vibration of a tuning fork. The sound due to percussion is called transient, whereas a periodic stimulus produces steady-state sound having a frequency f. Because sound travels at a finite speed, the fixed observer at some distance from the source will experience the disturbance at some later time. In the case of a transient sound caused by an impact, the observer will detect a single replica of the original as it passes at the speed of sound.

6 WAVELENGTH In the case of the tuning fork, a periodic sound source, the pressure peaks and dips follow one another away from the source at the speed of sound. For a given rate of vibration of the source, a given peak will have propagated a constant distance before the next peak occurs. This distance is called the wavelength lambda.

7 SOUND AND THE EAR

8 THE COCHLEA

9 MENTAL PROCESSES

10 LEVEL AND LOUDNESS At its best, the ear can detect a sound pressure variation of only 2x10-5 Pascals root mean square (rms) and so this figure is used as the reference against which the sound pressure level (SPL) is measured. The sensation of loudness is a logarithmic function of SPL; consequently, a logarithmic unit, the decibel, was adopted for audio measurement. The dynamic range of the ear exceeds 130 db, but at the extremes of this range, the ear either is straining to hear or is in pain. The frequency response of the ear is not at all uniform and it also changes with SPL. The subjective response to level is called loudness and is measured in phons.

11 RMS MEASUREMENTS

12 DECIBEL

13 THANK YOU PROF. PRATIKGIRI GOSWAMI

14 AUDIO VIDEO SYSTEMS PROF. PRATIKGIRI GOSWAMI ASSISTANT PROFESSOR ELECTRONICS & COMMUNICATION DEPARTMENT, L. D. COLLEGE OF ENGINEERING

15 VELOCITY OF SOUND Distance travelled in one second 344 m/s at 20 o C and 332 m/s at 0 o C Relationship between velocity and temperature

16 INTENSITY OF SOUND Sound Pressure of 20x10-6 Pa Just audible sound Threshold of Hearing. Pertains 1 pico-watt/m 2 of sound intensity 63 Pa 10 watt/m 2 Threshold of Pain

17 INTENSITY OF SOUND

18 LOUDNESS AND PHON Loudness : intensity of sound as judged by user. Ear is less sensitive at lower frequencies than at higher frequencies. Hence, it needs higher intensities at lower freq than at high freq to impart sensation of same loudness. The loudness of intensity of sound in db over threshold of hearing as perceived by the ear at 1000 Hz is called Phon. Intensity at 1000 Hz is 0 db zero phon loudness If it is 40 db loudness is 40 phon If intensity of 60 db at 40 Hz gives the same loudness as 0 db intensity at 1000 Hz loudness at 40 Hz is zero phon not 60 phon So loudness in phons at 1000 Hz intensity of sound in db

19 SONE used to determine the increase in loudness Loudness sensation produced by 1000 Hz sound of 40 db (i.e. 40 phon) is called 1 sone. Increase in sone value in db is given by (P-40)*log2, where P is the loudness in phons. Relationship between sones (L) and phons (P) is given by 10 log L = (P 40) log2

20 OVERTONES AND TIMBRE Speech and musical instruments not pure Harmonics Frequencies other than the fundamental are called Overtones. Proportion of tones and overtones form the special characteristic particular sound can be recognized Recognizing who is speaking Timbre related to proportion in which overtones are present in the sound,

21 INTERVALS, OCTAVES, HARMONICS Interval : ratio of two frequencies. Interval of 1:2 is called an Octave. e.g. one octave of 100 Hz is 200 Hz, two octaves of 100 Hz is 400 Hz. Mathematically, an octave for two freq f 1 and f 2 is defined by Harmonic : integer ratio between two frequencies e.g. 100 Hz freq of 200 Hz is 2 nd Harmonic but one octave 100 Hz 400 Hz is 4 th Harmonic but two octaves 100 Hz 145 Hz is 0.5 octave

22 PITCH Pitch : characteristic of sound mainly related to frequency sound consists of pure tone i.e. no harmonics pitch is determined by freq alone But in speech and music, pitch depends on freq as well as intensity. unit of pitch MEL One mel = tone of 1000 Hz and 40 db intensity (i.e. intensity of 1 sone) Relationship between pitch and freq is not linear the reason sensation of ear to all freq is not linear

23 RESONANCE EFFECT IN SOUND SYSTEMS Sound transducers mechanical parts capable of vibrating all these systems have natural resonant freq depending on the mass and compliance of the vibrating system absorbing and reflecting surfaces can have a natural resonant freq. Resonant freq of such systems is given by

24 EXAMPLES

25 EXAMPLES

26 THANK YOU PROF. PRATIKGIRI GOSWAMI

27 AUDIO VIDEO SYSTEMS PROF. PRATIKGIRI GOSWAMI ASSISTANT PROFESSOR ELECTRONICS & COMMUNICATION DEPARTMENT, L. D. COLLEGE OF ENGINEERING

28 MICROPHONES Transducer converts sound pressure variations into electrical signals of the same freq and phase and of amplitudes in the same proportion as in pressure variations. Quality determined by following characteristics: Sensitivity SNR Freq Response Non-Linear Distortion Directivity Output Impedance

29 MICROPHONES Sensitivity: SNR: output in mv (or in db below 1 volt) for the sound pressure of 1 micor-bar (or 0.1 Pa) at 1000 Hz. noise due to resistance of circuit, built-in transformer etc. ratio in db of output to the output in the absence of sound. Distortion: Non-linear : distorts amplitude produces harmonics other than original input sound should not be more than 5% (for quality microphones) should not be more than 1% (for hi-fi system) Phase distortion : relative path difference due to multiple microphones

30 MICROPHONES Freq Response: the BW of audio freq in the output of microphone within ±1 db of the output at 1000 Hz. microphone which gives flat response within ±1 db for freq 40 to Hz is considered good for hi-fi systems.

31 MICROPHONES

32 MICROPHONES Directivity: angle for half power points in a polar diagram represents directivity of microphone Mathematically,

33 MICROPHONES Directivity: Omni-Directional (pressure µp) Figure of Eight (ribbon µp) Cardioid or Heart Shaped (series combination of pressure and ribbon)

34 MICROPHONES Output Impedance: to determine which type of matching transformer would be needed to transfer the power efficiently from µp to the transmission line and then to the amplifier. if output imp quite low (e.g. dynamic µp) built-in step-up transformer to match line impedance.

35 MICROPHONES Requisites of a good µp : high sensitivity high SNR flat freq response over most of the audible freq range natural resonant freq outside audible range low distortion correct output impedance required directivity

36 MICROPHONES Categories from point of view of impedance : Low Impedance (fraction of an Ω to about 50 Ω) Ribbon and moving coil Medium ( Ω) Carbon High (75 k Ω and higher) Crystal and condenser

37 MOVING COIL µp also called Dynamic µp uses the principle of Electromagnetic Induction sound pressure moves coil placed in magnetic field change in flux emf is produced forms output of µp

38 MOVING COIL µp Magnet permanant POT Type gives uniform magnetic field in the gap Diaphragm thin circular sheet of nonmagnetic material is of light weight slightly domed for extra rigidity fixed with the help of springs springs provides compliance to the motion of diaphragm mass of the diaphragm and coil assembly provide inductive effect protective cover metal grill to save delicate diaphragm and coil assembly from being mishandled

39 MOVING COIL µp Sound strikes the diaphragm coil moves in and out in magnetic field changes flux through the coil results in emf being produced in coil value of emf depends on the rate of change of flux and hence on the motion of coil displacement of coil depends on sound pressure Pressure Microphone B : flux density in tesla (or Wb/m 2 ) l : length of coil in meters v : velocity of diaphragm in m/s Then induced voltage :

40 MOVING COIL µp Characteristics : Sensitivity : 30 µv for SPL of 0.1 Pa SNR : 30 db Freq Response : 60 Hz to 8000 Hz for ±1dB Distortion : less than 5% Directivity : basically Omni-directional O/P Impedance : quite low about 25 ohm to match line of 200 ohm step up transformer of about 3:1 turns ratio is built

41 MOVING COIL µp

42 MOVING COIL µp Other Features: magnet is quite heavy heavy protective case very robust and reliable does not need external bias can be spoken into from distance of 25 cm cost is not high lower cost than ribbon and capacitor higher than crystal and carbon Applications : most widely used suitable for PA systems and broadcast studios

43 THANK YOU PROF. PRATIKGIRI GOSWAMI

44 AUDIO VIDEO SYSTEMS PROF. PRATIKGIRI GOSWAMI ASSISTANT PROFESSOR ELECTRONICS & COMMUNICATION DEPARTMENT, L. D. COLLEGE OF ENGINEERING

45 RIBBON MICROPHONE Moving coil high freq response is poor due to mass of the diaphragm. Mechanical mass is equivalent to inductance attenuates higher freq. To increase freq response light weight aluminum ribbon is used in place of diaphragm and coil assembly. Ribbon acts as a conductor (placed in a magnetic field) as well as a diaphragm.

46 CONSTRUCTION

47 CONSTRUCTION

48 RIBBON MICROPHONE Magnet horse shoe magnet with extended pole pieces provides strong magnetic field Ribbon light aluminum foil corrugated at right angles to its length to provide greater surface area Main feature lightness of ribbon 0.2 mg a few microns thick and about 3 mm wide whole unit is enclosed in a circular or rectangular baffle

49 WORKING Sound Pressure Ribbon (an electric conductor) placed in magnetic field is made to move at right angles to magnetic field change in magnetic flux through ribbon emf is induced across ribbon emf is proportional to the rate of change of flux i.e. proportional to sound waves striking the ribbon Driving mechanical force is proportional to the difference of the pressures acting on two sides of ribbon particle velocity of sound waves Pressure Gradient or Velocity Microphone

50 CHARACTERISTICS Freq Response : mass is very small low mechanical inductance results in good high freq response up to Hz stiffness of the suspension of ribbon is also small results in good low freq response overall freq response is 20 to Hz Directivity : force of sound can be applied to the ribbon from the front as well as from the rear side bi-directional Figure of Eight

51 CHARACTERISTICS

52 CHARACTERISTICS Impedance : low fraction of ohms (0.25 ohm) Sensitivity : less than that of moving coil type because length of the ribbon is less than the length of coil wire of moving coil type about 3 µv for SPL of 0.1 Pa SNR : less background noise higher about 50 db more suitable for hi-fi systems Distortion : low about 1%

53 OTHER FEATURES requires careful handling as ribbon is very light DC current through ribbon from a battery of meter can strain the ribbon resistance or continuity should not be measured with a meter should not be blown into causes strain in ribbon sensitive to breathing too kept at about 50 cm or more away from the source of sound slightest wind causes rumble noise in the output excellent transient response does not need external bias cost is high

54 APPLICATIONS suitable for dramas due to figure of eight pattern side by side two dead sides dead area on stage

55 CARDIOID MICROPHONES Ribbon and Moving coil both are mounted on the same housing connecting them into series Cardioid Pattern results suitable for orchestras and court scenes in dramas where large number of persons are present

56 CRYSTAL MICROPHONES based on the principle of Piezo Electric Effect Difference of potential between the opposite faces of some crystals is produced when these are subjected to mechanical pressure. crystals : Quartz,Tourmaline, Rochelle salt and ceramic. Rochelle Salt : high piezo electric effect but is susceptible to moisture cannot withstand with high temperature of 50 o C Quartz and Tourmaline : low piezo electric effect Ceramic : most suitable for crystal microphones as it is not susceptible to moisture and can withstand high temp up to 100 o C

57 CONSTRUCTION

58 CRYSTAL MICROPHONES Crystal is cut along certain planes to form a slice. Metallic foil electrodes are attached to two surfaces to carry the potential difference to the output terminals. Two thin crystal slices suitably cut are placed in an insulating holder with an air space between them. Large number of such elements are combined to increase emf. Diaphragm : made of Aluminium, is attached to the crystal surface through a push road. The whole unit is enclosed in a protective case.

59 FUNCTIONING sound wave compression compresses crystal rarefaction converse takes place crystal is extended and is under tension due to this compression and extension varying potential difference is generated which is proportional to the mechanical pressure applied to the crystal by the sound waves Pressure Microphone

60 CHARACTERISTICS Sensitivity : good about 50 mv for 0.1 Pa SNR : high about 40 db Freq Resp : Hz Distortion : low about 1% Directivity : Omni-directional Out Imp : high about 1 MΩ

61 OTHER FEATURES can be spoken into at close range high imp requires relatively short leads to the input circuit of the amplifier to prevent loss of higher audio frequencies, or pick of hum. mixer circuit will load it and cause severe loss of bass hence cannot be used in multi microphone system does not need a bias supply should not be exposed to direct sun light for long time cost is low

62 APPLICATIONS Home recording systems Amateur Communication Mobile Communication

63 CAPACITOR/CONDENSER MICROPHONE When capacitance changes, the charge on the capacitor tends to remain the same, and hence voltage changes in accordance with V=Q/C. If C increases,v will decrease and vice versa.

64 CAPACITOR/CONDENSER MICROPHONE Diaphragm acts as one plate of capacitor. The other plate Back Plate is fixed. Sound Pressure moves the diaphragm in C increases moves out C decreases Why? Change in C Change in V Pressure Microphone

65 CONSTRUCTION Plates are near each other and form a capacitance of a few microfarads. Fixed DC voltage of about 50 to 100 volt is applied between plates. Plates are insulated from each other. Capacitance of this microphone is about 30 pf.

66 CHARACTERISTICS Sensitivity : output is very low built-in amplifier output is then about 3mV at SPL of 0.1 Pa SNR : high about 40 db Freq Resp : excellent 40 Hz to 15 khz standard microphone against which other microphones and loudspeakers are tested used in sound level meters. Distortion : low about 1% Directivity : Omni-directional Output Imp : High about 100 MΩ

67 APPLICATIONS used as standard mic for calibrating other mic used in sound level meters used in professional hi-fi systems

68 THANK YOU PROF. PRATIKGIRI GOSWAMI

69 AUDIO VIDEO SYSTEMS PROF. PRATIKGIRI GOSWAMI ASSISTANT PROFESSOR ELECTRONICS & COMMUNICATION DEPARTMENT, L. D. COLLEGE OF ENGINEERING

70 ELECTRET MICROPHONE Capacitor mic costly and unsuitable for field work due to external bias Electret Mic capacitor mic but it has built-in charge.

71 ELECTRET MICROPHONE Insulating Material Teflon can trap large quantity of fixed charge and can retain it indefinitely. Back Plate coated with thin layer of Teflon charged negatively at the time of manufacturing this charge remains trapped for long period induces positive charge on the diaphragm. +ve charge on diaphragm, -ve charge on Teflon establish an electric field across the gap of capacitor plates sound pressure C changes, Q remains constant same characteristics as capacitor mic except that it does not need external bias and less costly. sensitive to temperature and humidity which cause leakage of charge. used as tie clip mic for lecturers and as wireless mic in sports meet.

72 CARBON MICROPHONE When fine carbon granules enclosed in a case are subjected to pressure variations, the resistance of granules changes. when connected in series with a load through DC supply current changes according to changes in resistance.

73 CARBON MICROPHONE Sound waves diaphragm compression presses carbon granules R decreases current through circuit increases vive versa for rarefaction absence of sound steady current flows Net resistance of carbon granules is given by, r = R 0 + δr r = Net resistance in ohms R 0 = Steady resistance for no sound δr = Variation of resistance due to sound pressure.

74 CHARACTERISTICS Sensitivity : high about 100 mv SNR : Poor Freq Resp : 200 to 5000 Hz Distortion : High 10% Directivity : Omni-directional but above 300 Hz falls beyond an angle of 40 o from front Out Imp : about 100 ohm

75 OTHER FEATURES mechanically very rigid prone to moisture and heat small in dimensions cost of mic is the lowest in all other mic Applications: only in telephones sometimes used in portable radio communication sets

76 LOUDSPEAKERS Transducer which converts electrical signals of audio frequency into sound waves of same frequency. Characteristics : Efficiency Noise Freq Resp Distortion Directivity Impedance

77 CHARACTERISTICS Efficiency : Defined as ratio of output sound power to the input audio (electrical) power. Depends on proper matching of the impedance. Frequency Response: It indicates loudspeakers response for the audible frequency range of sound. Response of loudspeaker should be flat within ± 1 db for the frequency range of 16 to 20 KHz

78 CHARACTERISTICS Distortion: Any change in frequency, phase and amplitude of the output sound as compared to input audio signal is called distortion. Frequency and phase distortion results due to mass and compliance effect. Non-linear distortion will result due to non-uniformity in magnetic field in which coil moves. Noise: Unwanted sound, not contained in the input signal but present in output of loudspeaker is called noise. Vibration of mechanical parts casing noise.

79 CHARACTERISTICS Directivity: Ratio of actual sound intensity at a point in the direction of maximum sound intensity to the sound intensity that would have been available there, had the loudspeaker been omnidirectional. Impedance: Input impedance of loudspeaker represented ohms. It has to match with the source amplifier for maximum power transfer. Long leads fixed line voltage is specified 100 volt in India amplifier at the peak output power will give 100 volt to the loudspeaker line connected to it. For such case, impedance R = (100 x 100) / P

80 MOVING COIL CONE TYPE LS Works on principle of interaction between magnetic field and current. A coil placed in uniform magnetic field and audio current passes through it, resulting in force. Generated force is proportional to the audio current and hence causes vibratory motion in the coil, which makes conical diaphragm to vibrate resulting in sound waves.

81 MOVING COIL CONE TYPE LS Force on coil due to interaction current and magnetic field is given by F = B * l * i * sinα F = Force in newton B = Flux density in tesla l = length of coil wire in m. α = Angle between coil and field Normally, α = 90 and hence, F = B * l * I

82 CONSTRUCTION

83 CONSTRUCTION Because of use of permanent magnet it also called permanent magnet type speaker voice coil single layer winding of fine enameled wire wound on cardboard or fiber cylinder Paper Cone conical diaphragm made of paper or parchment

84 FUNCTIONING Audio current flows through voice coil force (= B*l*i) acts on the coil and moves in to and fro paper cone attached to coil also moves causes compression and rarefaction cycles in the air Two transformations : Electro-mechanical and Mechano-acoustical also called Direct Radiating Type

85 CHARACTERISTICS Efficiency : quite low due to fact that it acts as a direct radiator complete mismatch between the low acoustic load of large volume of air and high mech load of coil and cone assembly SNR : 30 db or better Freq Resp : restricted to mid freq only 200 Hz to 5000 Hz woofer and tweeter can be designed woofer (up to 40 Hz), tweeter (up to 10 khz) Distortion : non-linear due to non-uniformity of magnetic field about 10% Directivity : basically omni-directional less behind baffles and enclosures Impedance and Power : varies from 2 to 32 ohm about 25 watt

86 ELECTRODYNAMIC LS To provide very strong magnetic field for high wattage speakers Electro magnet is used instead of permanent magnet. Working principle is same as that of permanent magnet type.

87 ELECTRODYNAMIC LS Advantages : higher power better freq response Disadvantages : power supply needed for field coil heavier in weight costlier

88 HORN TYPE LS instead of radiating acoustic power directly in open space of listeners area, power is first delivered to the air trapped in fixed non-vibrating tapered or flared horn and from here to the air in the listeners area. Indirect Radiating Loudspeaker horn does acoustically what the cone does mechanically horn acts as an acoustic transformer allows better impedance-matching results in increased efficiency 30-50%

89 HORN TYPE LS Air chamber is lined with sound absorbing material cross sectional area increases logarithmically horn acts as a high pass filter Cut-off Freq f c = (C A)/(2 π V) C = velocity of sound A= Area of cross section of throat V = Volume of air chamber In terms of diameter of mouth, lowest frequency can be produced by horn is f = (170/d), d = diameter of mouth in m.

90 HORN TYPE LS low freq resp is improved by wide mouth and high freq resp is improved by small throat To improve low freq response large size horn unwieldy horn structure is folded back in itself to conserve physical space contains cone loudspeaker with a horn Horn for high fidelity

91 CHARACTERISTICS Efficiency : 30-50% SNR : 40 db Freq Resp : Hz Distortion : low, less than 5% Directivity : about 90 o differs from low freq to high freq (concentrated in a narrow cone about axis of the horn) Impedance : 16 ohms Power : about 100 watts

92 BAFFLES & ENCLOSURES Baffle : rigid flat material used to extend the edges of a LS cone increases the effective length of the acoustical transmission path between the front and the back of radiator

93 BAFFLES & ENCLOSURES air on both sides of the cone, front as well as back cone radiates on both the sides compression in front rarefaction in back vice versa sound waves produced in rear are of opposite phase

94 BAFFLES & ENCLOSURES waves from rear leak or diffract round the sides and meet the waves in front if path difference is small compared to (lambda/2) two waves will almost cancel each other response drops off sharply to increase the path difference by physical barrier baffle made of a good sound insulating material soft to check rattle soft wood or cellotex

95 TYPES OF BAFFLES Finite Baffle Infinite Baffle Enclosure Bass-Reflex Baffle

96 FINITE BAFFLE Wooden cabinet as in radio receivers and TV receivers being of finite size, are not very effective cause some loss of low audio freq due to diffraction

97 INFINITE BAFFLE ideally which has infinite lateral dimensions not feasible baffle dimensions are so large that the freq at which the path length is far below the lowest freq to be used infinite baffle e.g. speaker fixed in a hole of in the wall of house

98 ENLOSURES speaker mounted in a closed box with an opening in the front box serves the purpose of infinite baffle material 18 mm thick plywood or particle board internal reflections cause sound energy to reach cone changes vibrations of cone itself causes irregularities reduced by padding inner surfaces with sound absorbents

99 BASS REFLEX ENLOSURES radiation from the back is used to strengthen the front radiation port or cut is designed in such a way that it permits the flow of air from rear to front with additional path difference of 180 o phase difference resulting into total 360 o reinforce front radiation

100 ACOUSTIC LABYRINTH low freq bass reflex encl will have large dimensions solution encl is divided into parts lengthens the path of wave causes additional path difference of 180 o

101 Hi-Fi ENCLOSURE

102 MULTI-WAY SPEAKER SYSTEM single loudspeaker cannot have flat resp for whole audio freq range single speaker cannot produce both, the good solid bass and the smooth crisp treble solution spectrum is divided into at least 2 and preferably 3 parts lower audio freq 16 Hz to 1000 Hz Woofers higher audio freq Tweeters mid audio freq 500 to 5000 Hz Squawker in this case woofer covers up to 500 Hz and tweeter from 5000 Hz

103 THANK YOU PROF. PRATIKGIRI GOSWAMI