baseband audio

8/11/2019 BASEBAND AUDIO

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What Is Analog Audio?

Analog audio is a representation of a soundthat is analogous to the air pressure waves

of the sound. That may sound complicated,

but it is actually very easy to understand.

Sound is waves of air molecules. Analog

audio is a representation of the intensities of

those waves in a different form, such as

voltages on a wire or magnetized particleson a cassette tape.


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The Basics of SoundSound is nothing more than the movement of air.

For example, the sound of a firecracker goingoff is actually an experience of shock waves thatmove through the air due to the rapid expansionof the firecracker. The firecracker expands, themolecules of air get pushed towards you, yourears sense the movement of the air moleculesand your brain interprets that as sound. Whenthe air molecules move, there are points in thewave when they are all bunched up together

(this is high pressure) and there are points whenthere are relatively few air molecules (this is lowpressure).


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HOW SOUND WAVES ARE

RECORDEDTo record analog audio of a sound today, we use

a microphone, which has a diaphragm.The

diaphragm in a microphone is connected to a

device called a transducer. When the diaphragmis vibrated by the sound of a voice, the

transducer converts those vibrations into

electrical energy. The portions of the sound that

are high pressure cause the transducer to create

positive voltage, and the portions of the sound

that are low pressure cause the transducer to

create negative voltage.


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TRANSDUCER

IT IS A DEVICE WHICH CONVERTSONE FORM OF ENERGY INTO

ANOTHER FORM.

A MICROPHONE CONVERTS SOUNDPRESSURE WAVES INTO AN

ELECTRICAL SIGNAL.

A SPEAKER CONVERTS ELECTRICALSIGNAL INTO SOUND PRESSURE

WAVES.


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MICROPHONE INVENTION

When people wish to communicate to alarge crowd or when they want to recordsound, a microphone is a valuable piece ofequipment. The microphone (sometimes

called a "mike") was first invented by EmileBerliner in 1877. However, AlexanderGraham Bell was the first person to developit and make it useful. Today,

microphones are used in telephones,hearing aids, radios, tape recorders, andtelevision broadcasting.


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What is a microphone?

A device for turning acoustic power intoelectric power is called a microphone.

Acoustic power is real sound waves. In

other words, it changes sound into anelectric signal. These signals are usually

sent to an amplifier or recording device.

There are many different brands and also

different types. Still, they all have one thing

in common: they all use a diaphragm.


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This diaphragm is a thin part, sometimes made

of metal, that vibrates when sound goes to themicrophone. When the diaphragm shakes, it

causes the other parts of the microphone to

create signals.


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TYPES OF MICROPHONES

DYNAMIC MICROPHONE

CARBON MICROPHONE

CONDENSER MICROPHONE

ELECTRET MICROPHONES RIBBON MICROPHONE

GUN MICROPHONE

LIP MICROPHONE

LAPEL MICROPHONE

WIRED AND RF


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DYNAMIC MICROPHONE

The dynamic microphone has a thindiaphragm that is hung by suspension

wires. It can break if the wires become

damaged. One bad thing about this kind isthat because it is less sensitive, it does not

pick up sounds as well. But it is great for

live performances and recordings that want

to sound like live performances.


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CARBON MICROPHONE

Carbonis the next kind of microphone. It isan old fashioned design, used on the first

telephones. It uses carbon dust which is

compressed into the middle of thediaphragm. It makes electrical signals when

the sound waves pass through the dust. It is

still used on many telephones.


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CONDENSER MICROPHONE

Basically, the heart of any capacitor mic is apair of conducting plates, one fixed and theother in the form of a moving diaphragm.When the spacing between the plates

changes (as it does when the diaphragmvibrates) the capacitance varies, and if afixed electrical charge is applied to thecapacitor, an electrical signal is produced

which faithfully represents the diaphragmvibration.


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ELECTRET MICROPHONES There is another type of capacitor microphone, known as

the electret mic. Despite inauspicious beginnings, thesehave now been developed to the point where they can rivaltrue capacitor quality for a much lower price. Instead ofapplying an electrical charge to the microphone capsule viaan external power source, electret mics use a diaphragm

made from an insulating material that has a permanentelectrical charge. A preamplifier is still needed, but thiscan be built very cheaply, and will run from a battery insome cases.

Electret mics made in this way don't offer any realadvantage over dynamic mics, because the diaphragmshave to be quite heavy in order to carry the permanentelectrical charge .


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RIBBON MICROPHONE

The ribbonis another variety ofmicrophone. Ribbon microphones use avery thin metal wire that is suspended in amagnetic field. When the sound waves or

vibrations hit the ribbon, the ribbon vibratesand sends waves through the magnetic fieldcreating electric signals. Ribbonmicrophones are very sensitive and usuallyused for special recordings. They are veryfragile and can break easily when dropped.


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GUN MICROPHONE

IT IS SUITABLE FOR OUT DOORCOVERAGES LIKE REPORTING,

SHOOTINGS ETC.

LIP MICROPHONEIT IS A HIGHLY DIRECTIONAL

MICROPHONE.IT IS USED IN

MATCH COVERAGES BYCOMMENTRATORS.


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LAPEL MICROPHONE

A lapel microphone is a very handy tool for those

speaking in public. It attaches to clothing andtakes the place of a more static-positionmicrophone. The lapel microphone can amplify avoice and ensure quality communication with verylittle extra equipment.

By its very nature, a lapel microphone is quitesmall. It has a powerful reach and yet is smallenough to be held between two fingers. It doesnthave an unlimited range, however, and must beattached to clothing within a foot or two of themouth. A lapel microphone is most often placedon the flap of a button-down or Polo Shirt or to thetop part of a blouse or other kind of shirt or top.


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What is Impedance? Impedanceis an electronics term which measures

the amount of opposition a device has to an ACcurrent (such as an audio signal). Technicallyspeaking, it is the combined effect of capacitance,inductance, and resistance on a signal. The letter Z

is often used as shorthand for the word impedance,e.g. Hi-Z or Low-Z.

Impedance is measured in ohms, shown with theGreek Omega symbol . A microphone with thespecification 600 has an impedance of 600ohms.


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Microphone Impedance

When dealing with microphones, one

consideration which is often misunderstood or

overlooked is the microphone's impedance rating.

Perhaps this is because impedance isn't a "critical"

factor; that is, microphones will still continue tooperate whether or not the best impedance rating

is used. However, in order to ensure the best

quality and most reliable audio, attention should

be paid to getting this factor right. If you want the short answer, here it is: Low

impedance is better than high impedance.


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What is Microphone Impedance? All microphones have a specification referring to their

impedance. This spec may be written on the mic itself(perhaps alongside the directional pattern), or you mayneed to consult the manual or manufacturer's website.

You will often find that mics with a hard-wired cableand 1/4" jack are high impedance, and mics with

separate balanced audio cable and XLR connector arelow impedance.

There are three general classifications for microphoneimpedance. Different manufacturers use slightlydifferent guidelines but the classifications are roughly:

Low Impedance(less than 600)

Medium Impedance(600 - 10,000)

High Impedance(greater than 10,000)


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Which Impedance to Choose?

High impedance microphones are usually quitecheap. Their main disadvantage is that they do notperform well over long distance cables - afterabout 5 or 10 metres they begin producing poorquality audio (in particular a loss of highfrequencies). In any case these mics are not a goodchoice for serious work. In fact, although notcompletely reliable, one of the clues to amicrophone's overall quality is the impedance

rating.

Low impedance microphones are usually thepreferred choice.


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Matching Impedance with OtherEquipment

Microphones aren't the only things with impedance. Otherequipment, such as the input of a Sound Mixer, also has anohms rating. Be aware that what one system calls "lowimpedance" may not be the same as your low impedancemicrophone - you really need to see the ohms value toknow exactly what you're dealing with.

A low impedance microphone should generally beconnected to an input with the same or higher impedance.If a microphone is connected to an input with lowerimpedance, there will be a loss of signal strength.

In some cases you can use a line matching transformer,which will convert a signal to a different impedance formatching to other components.


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Impedance MatchingIn the early days of high fidelity musicsystems, it was crucial to pay attention tothe impedance matching of devices sinceloudspeakers were driven by outputtransformers and the input power of

microphones to preamps was something thathad to be optimized. The integrated solidstate circuits of modern amplifiers havelargely removed that problem, so this

section just seeks to establish someperspective about when impedancematching is a valid concern.


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As a general rule, the maximum power transferfrom an active device like an amplifier or antennadriver to an external device occurs when the

impedance of the external device matches that ofthe source. That optimum power is 50% of thetotal power when the impedance of the amplifier ismatched to that of the speaker. Improperimpedance matching can lead to excessive poweruse, distortion, and noise problems. The mostserious problems occur when the impedance of theload is too low, requiring too much power fromthe active device to drive the load at acceptable

levels. On the other hand, the prime considerationfor an audio reproduction circuit is high fidelityreproduction of the signal, and that does notrequire optimum power transfer.


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In modern electronics, the integrated

circuits of an amplifier have at their

disposal hundreds to thousands of activetransistor elements which can with

appropriate creative use of feedback make

the performance of the amplifier almostindependent of the impedances of the input

and output devices within a reasonable

range.


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On the output side, a loudspeaker may still

have a nominal impedance of something

like 8 ohms, which formerly would haverequired having an amplifier output stage

carefully matched to 8 ohms. But now with

the active output circuitry of audioamplifiers, the effective output impedance

may be very low. The active circuitry

controls the output voltage to the speaker so

that the appropriate power is delivered.


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To emphasize the over simplification involved in

the above model, it should be noted that the

loudspeaker is not a simple resistor - it contains a

coil or coils with significant inductance, and is

typically composed of two or three speakers with a

crossover network that has capacitance and

inductance. So the impedance of the loudspeakerwill inevitably vary with frequency.

Note that it is safer in terms of total power to go to

higher impedance speakers (series speakers), but

more typical practice is to put speakers in parallel,lowering the impedance.


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Matching Microphone to Input

While impedance matching of a microphone to an

audio amplifier is not the problem it was in the

early days of high fidelity sound reproduction,

there are some considerations that still apply.

In practical terms, the modern microphone needsto deliver optimal voltage to the preamplifier, and

not necessarily the optimum power that would

require impedance matching. Considering the

microphone as a voltage source, the voltagedelivered to the input of the preamplifier is given

by


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where Vsource is the signal generated by the

microphone mechanism, Ri the impedance of the

microphone and RL the input impedance of the

preamplifier. The actual signal power delivered to

the preamp can be expressed in decibels of loss

compared to the microphone's generated signal .Assuming a resistive circuit so that the power if

proportional to the square of the voltage:


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As long as the microphone has enough signal strength to

provide the minimum signal input to the mixer, it can be

an advantage to connect a low impedance microphone to

a moderately higher impedance input. From this point of

view, current practice for "low impedance" inputs toaudio mixers typically have impedances from 1000 to

2000 ohms.As a rule of thumb, a signal loss of 6dB is

acceptable.


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PICKUP PATTERNS OF A MICROPHONE

Omnidirectional. This kind of microphonepicks up sound from all directions. Theseare used for group vocals and recordings.

Unidirectionalmicrophones pickup sound

from only one direction. They are good forrecording single voices. This makes themgood for interviews in places that are loud.Because they can pickup from long

distances, they are also great forsurveillance.


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Bi-directional. It gets sound from twoplaces. It is great for recording two voices at

the same time.

Carotidis the last type of pickup pattern. Itis very unusual, because it picks up sound in

a heart shaped pattern. These are actually avery commonly used microphone. They aregreat for talk shows, because the audiencesound will not be picked up as much as the

people on stage. This also makes it verygood for live music performances.


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Directional microphones are referred to ascardioids because a graph of their sensitivity atdifferent angles looks 'heart-shaped'. The leastsensitive spot of a cardioid microphone is right

behind it, with the most sensitive spot being 'on-axis'. Cardioids (or the more tightly-focusedhypercardioids -- see below) are used extensively

in live performance, because of the need toprevent spill and acoustic feedback.

Hypercardioid mics, sometimes also known assupercardioids, are useful in situations wheresound leakage is a real problem, but in the studio,

they tend not to be used, as relatively smallphysical movements by the performer can causethe sound level to change significantly.


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PROXIMITY EFFECT The proximity effect comes into play when vocalists sing

very close to a mic, and the effect is that the level of bassin the recorded signal goes up enormously. The proximityeffect is all down to the laws of physics, and may be a

benefit or a problem depending on what you do with it --experienced live performers can use the proximity effect asa type of dynamic EQ, allowing them to alter the tone of

their voice as they sing, simply by varying the mic-to-mouth distance.

In the studio, mics tend to be used at a reasonable distancefrom the performer, usually with a pop shield in between,so the proximity effect doesn't affect the recording of

vocals. When recording electric guitar or bass, theproximity effect is often deliberately brought into play, tohelp create a more punchy sound.


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WHAT TO USE: QUICK GUIDESVOCALSWhen it comes to recording vocals, there are nohard and fast rules about the type of mic to choose,

because all that matters is the end result. For thisreason, some top pop singers record usingrelatively inexpensive dynamic mics, rather than

capacitor models, because the dynamic mic givesthem a warmer, thicker, more punchy sound. Onthe other hand, a breathy, intimate voice can

benefit from the detailed high-end of a capacitormic. Not only do the different types of mic sound

quite different, but you'll also notice that evensimilarly-specified mics of the same type but fromdifferent manufacturers will also sound noticeablydifferent.


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DRUMSFor drum work, the most common setup is

to use dynamic mics for all the close mics(on the individual drums) and then augmentthese with capacitor mics for the overheadmics and the hi-hat. Dedicated kick drummics are available (such as the AKG D112,currently retailing at 216), and these willproduce a more powerful bass drum sound

than general-purpose dynamic models.Some engineers also prefer to use capacitormics on the snare drum for a crispersound.


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GUITAR

Electric guitar is less demanding on mics, because

a typical guitar cab has a very limited high-

frequency output, and in most instances, a

dynamic vocal or drum mic will do fine

Definitions of Wow and Flutter:

A measurement of speed instability in analog

equipment usually applied to cassette transports

and turntables. Wow is slow-speed variations, and

flutter is fast-speed variations. Lower percentagesare better.


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AUDIO SIGNAL IMPAIRMENTS

1 AMPLITUDE ERRORS2 FREQUENCY ERRORS/FREQUENCYRESPONSE

3 PHASE ERRORS

4 THD+N

5 NOISE OR S/N

6 CROSSTALK

7 INTER MODULATION

8 WOW & FLUTTER


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AUDIO PRE-AMPLIFIER:Audio Input : Microphone /-70dB

Input Impedance : 600 Ohms.Audio Output: 0dB

SPECIFICATION FOR STEREODISTRIBUTION AMPLIFIER:

1. GENERAL

The stereo distribution amplifier will be used forfeeding a stereo high quality programme tovarious destinations. The distribution amplifier

should be solid state audio amplifier having onestereo input and 4 separate individually adjustablestereo outputs.


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2. INPUT IMPEDANCE

Input impedance shall be >=10 k ohm (balanced).3. INPUT LEVEL

(a) Nominal : 0 dBu

(b) Maximum : +20 dBu

4. GAINShall have adjustable gain of 5 dB with respect tonominal setting.

5. OUTPUT LEVEL

(a) Nominal : 0 dBu

(b) Maximum ; +20 dBu6. OUTPUT IMPEDANCE

Output impedance shall be


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7. FREQUENCY RESPONSE

0.5 dB in frequency range of 40 Hz to 20 kHz.

8. TOTAL HARMONIC DISTORTION

Less then 0.3% at nominal level (1 kHz) and less than0.5% at maximum output level.

(Terminated into a load of 600 ohm) throughout the audiofrequency range of 40 Hz to 20 kHz.

9. SIGNAL TO NOISE RATIO AT NOMINAL

INPUT/OUTPUT, RMS UNWEIGHTED (22 Hz-22kHz)>= 90 dB

10. INTER OUTPUT LOADING

(a) If one of the outputs gets short circuited, the level on

the rest of the outputs shall not fall by more than 0.3 dB,(b) If two of the outputs got short circuited, the level oneach of the remaining outputs shall not fall by more than0.6 dB.


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11. INTER-CHANNEL PHASE DIFFERENCE

Not more than 5 degree in frequency range of 125

Hz to 10 kHz and 10 degree from 40 Hz to 20kHz.

12. INTER-CHANNEL LEVEL DIFFERENCE

Within O.5 dB, from 40 Hz to 20 kHz.

13. INTER-CHANNEL CROSS TALK

Equal to or better than 60 dB at 20 kHz at nominal

level.


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14. INPUT/OUTPUT CONNECTORS

Input and all outputs shall be on 3-pin XLR

connectors.15. POWER REQUIREMENT

230 V 10%, 48-52 Hz single phase ACsupply.

16. CONTROLSFollowing front panel controls shall beprovided:

(a) ON/OFF switch with ON LED

indicator and power fuse.(b) Screw driver control for level

adjustment for each output.


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