Chap65

268 views

Published on

Published in: Technology, Business
0 Comments
0 Likes
Statistics
Notes
  • Be the first to comment

  • Be the first to like this

No Downloads
Views
Total views
268
On SlideShare
0
From Embeds
0
Number of Embeds
2
Actions
Shares
0
Downloads
9
Comments
0
Likes
0
Embeds 0
No embeds

No notes for slide

Chap65

  1. 1.  Explain the theory of audio.
  2. 2. In This Chapter, you’ll learn on:  What is Audio?  What is Analog and Digital Audio?  What is Bandwidth?  How to digitize analog audio?  Sampling Theory of digital audio.
  3. 3.  The range of frequencies detectable by the human ear — approximately 20Hz to 20kHz. 20Hz is the lowest-pitched (bassiest) sound we can hear, 20kHz is the highest pitch we can hear.  Audio work involves the production, recording, manipulation and reproduction of sound waves. To understand audio you must have a grasp of two things: 
  4. 4.  1. Sound Waves:  Sound waves exist as variations of pressure in a medium such as air. They are created by the vibration of an object, which causes the air surrounding it to vibrate. The vibrating air then causes the human eardrum to vibrate, which the brain interprets as sound.  Sound waves travel through air in much the same way as water waves travel through water. In fact, since water waves are easy to see and understand, they are often used as an analogy to illustrate how sound waves behave. 
  5. 5.  Sound Waves: 
  6. 6.  2. Sound Equipment:  A piece of audio equipment is any device designed principally to reproduce, record or process sound. This includes microphones, radio receivers, AV receivers, CD players, tape recorders, amplifiers, mixing consoles, effects units, and loudspeakers.  Audio theory is simpler than video theory and once you understand the basic path from the sound source through the sound equipment to the ear, it all starts to make sense.
  7. 7.  The Field of Audio Work  The field of audio is vast, with many areas of specialty. Hobbyists use audio for all sorts of things, and audio professionals can be found in a huge range of vocations. Some common areas of audio work include: Studio Sound Engineer Live Sound Engineer Musician Music Producer DJ Radio technician Film/Television Sound Recordist Field Sound Engineer Audio Editor Post-Production Audio Creator
  8. 8.  What is Analog Audio?   Analog audio is a representation of a sound that is analogous to the air pressure waves of the sound. 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 particles on a cassette tape. In audio, an analog signal is a smooth-flowing representation of music or sound. Like a bow on a violin string, analog sound is marked by a continuously flowing sound.
  9. 9.  What is Analog Audio? Examples of analog audio recording are:, Compact Cassette magnetic tape recorder Gramophone record (also known as a phonograph record, vinyl, etc.)
  10. 10.  What is Analog Audio?  In an analog audio system, sounds begin as physical waveforms in the air, are transformed into an electrical representation of the waveform, via a transducer (for example, a microphone), and are stored or transmitted. To be re-created into sound, the process is reversed, through amplification and then conversion back into physical waveforms via a loudspeaker.  Although its nature may change, analog audio's fundamental wave-like characteristics remain the same during its storage, transformation, duplication, and amplification.  Analog audio signals are susceptible to noise and distortion, unavoidable due to the innate characteristics of electronic circuits and associated devices. In the case of purely analog recording and reproduction, numerous opportunities for the introduction of noise and distortion exist throughout the entire process.
  11. 11.  What is Digital Audio?  Digital audio has emerged because of its usefulness in the recording, manipulation, mass-production, and distribution of sound. Modern distribution of music across the Internet via on-line stores depends on digital recording and digital compression algorithms. Distribution of audio as data files rather than as physical objects has significantly reduced the cost of distribution.  When audio is digitized, distortion and noise are introduced only by the stages that precede conversion to digital format, and by the stages that follow conversion back to analog.
  12. 12.  What is Digital Audio?  Digital audio takes advantage of some peculiarities of acoustics and the human ear. When sound is converted from analog into digital audio, the hardware "samples" the level of the waveform at a specific interval. For CD audio, this interval is 1/44,100th of a second. In other words, 44,100 times each second a special chip calculates a value for analog input and sends it off for use or storage. This process is called "digitizing" a sound.
  13. 13.  What is Digital Audio?  The result, if we were to graph it as we have with analog waveforms, would look quite different. Instead of smooth, gradual changes we would see stair steps as the line jerked from sampling data point to sampling data point. Here’s a picture showing the difference:
  14. 14.  What is Digital Audio?  There are two useful terms here- the "sampling rate" and the "sample size".  The sample rate is the number of times per second that the analog signal is measured.  The sample size tells us what number is associated with the maximum value.  The maximum value of the analog data doesn’t change- if you try to add power past a certain point you just start blowing up hardware.  But if the range is from 0 to 1000 the values stored will represent the analog data more closely than if they range from 0 to 10.
  15. 15.  What is Digital Audio?  Still, if analog data is smooth and digital data is made up of stair steps, why doesn’t digital audio sound bad? The answer is fairly technical, but what it boils down to is that, as long as the samples are taken often enough, the noise created by the stair stepping is too high in frequency for us to hear. According to the theory, the frequency of this noise will always be at least twice the sampling frequency. This is called the Nyquist Limit.  Very few if any humans can hear above about 20,000 cycles per second. Note that the speed chosen for audio CDs is 44,100 cycles per second. It is no coincidence that CD sampling rate is just over twice what our ears can
  16. 16.  What is Digital Audio?  The conversion of analog data to digital and back to analog is accomplished by special chips. A chip that converts analog to digital is called an ADC- an Analog to Digital Converter. The ADC measures the amount of current at each sampling interval and converts it to a binary number. This is called "digitizing" the sound. On the other end is a chip called a DAC- a Digital to Analog Converter. This chip takes a binary number and converts it to an output voltage.
  17. 17.  What is Digital Audio?  Here's what happens if you record your voice using a microphone plugged into your computer, then edit it and play it back over your speaker system: the microphone generates an analog waveform corresponding to the compression and rarefaction cycles generated by your voice. This smooth analog waveform is converted into a series of binary values by the ADC which are then transferred into the memory of your computer. Once you are done editing (if you’ve ever tried editing analog tape you’ll appreciate how much easier digital editing is!) the computer sends the resulting series of binary numbers to the DAC, which converts them to a (relatively) smooth analog waveform that drives your speaker.
  18. 18.  What is Digital Audio?  Digital, or digitized, sound is easier to reproduce and manipulate without loss in quality. Some question whether the quality is quite as good as analog sound, but it can be very good indeed, and CDs don’t wear out like records used to. Digital audio can also be compressed much more easily than analog, which is why MP3 is a digital format.  Digital is not necessarily better, but it is different, and offers advantages to engineers and end users that will increase its dominance in the coming years.
  19. 19.  What is Bandwidth?  In computer networks, bandwidth is often used as a synonym for data transfer rate - the amount of data that can be carried from one point to another in a given time period (usually a second). This kind of bandwidth is usually expressed in bits (of data) per second (bps). Occasionally, it's expressed as bytes per second (Bps). A modem that works at 57,600 bps has twice the bandwidth of a modem that works at 28,800 bps. In general, a link with a high bandwidth is one that may be able to carry enough information to sustain the succession of images in a video presentation.
  20. 20.  What is Bandwidth?  In electronic communication, bandwidth is the width of the range (or band) of frequencies that an electronic signal uses on a given transmission medium. In this usage, bandwidth is expressed in terms of the difference between the highest- frequency signal component and the lowest-frequency signal component. Since the frequency of a signal is measured in hertz (the number of cycles of change per second), a given bandwidth is the difference in hertz between the highest frequency the signal uses and the lowest frequency it uses. A typical voice signal has a bandwidth of approximately three kilohertz (3 kHz); an analog television (TV) broadcast video signal has a bandwidth of six megahertz (6 MHz) -- some 2,000 times as wide as the voice signal.
  21. 21.  How to Digitize Analog audio  The term digitization is often used when diverse forms of information, such as text, sound, image or voice, are converted into a single binary code. Digital information exists as one of two digits, either 0 or 1. These are known as bits (a contraction of binary digits) and the sequences of 0s and 1s that constitute information are called bytes.[3]  Analog signals are continuously variable, both in the number of possible values of the signal at a given time, as well as in the number of points in the signal in a given period of time. However, digital signals are discrete in both of those respects – generally a finite sequence of integers – therefore a digitization can, in practical terms, only ever be an approximation of the signal it represents.
  22. 22.  Digitization occurs in two parts:  Discretization  The reading of an analog signal A, and, at regular time intervals (frequency), sampling the value of the signal at the point. Each such reading is called a sample and may be considered to have infinite precision at this stage;  Quantization  Samples are rounded to a fixed set of numbers (such as integers), a process known as quantization.  In general, these can occur at the same time, though they are conceptually distinct.  A series of digital integers can be transformed into an analog output that approximates the original analog signal. Such a transformation is called a DA conversion. The sampling rate and the number of bits used to represent the integers combine to determine how close such an approximation to the analog signal a digitization will be.
  23. 23.  When we digitize sound, we need to consider some parameters that determine the amount of information stored in a file and the quality of the digital sound. Sampling rate:  A sampling rate is the number of times the analog sound is sampled during each period (one complete waveform) and converted into digital information. The most common sampling rates are 44.1, 22.05, and 11.025 kHz(kilo-Hertz). A sampling rate of 44.1 kHz means that 44,100 samples of the analog audio will be taken. Therefore, the more samples taken, the closer the digital version will approximate the original analog version.
  24. 24. Bits per sample:  Bits per sample describes how much information (number of amplitudes) in each sample the computer is collecting. It involves a process of converting a sampled sound into an equivalent digital value. The number of individual sounds that can be represented depends on the number of bytes used to  store the digital values. Naturally, more bytes, more possible sound and higher quality of digitized sound.
  25. 25.  Mono vs. stereo:  Mono describes a system where all the audio signals are mixed together and routed through a single audio channel. Stereo sound systems have two independent audio channels, and the signals are reproduced by two channels separated by some distance. It is difficult to decide which one is the best. The two sound channels give the illusion that the sound is coming from a certain location.  However, going to mono will reduce the file size by half. Sometimes a welldesigned mono system is better than a low quality stereo system.  Note: It is important to balance the sampling rate, bits per sample to come up with an acceptable quality of the sound with the minimum file size. The following comparison is provided by Michael Kennedy (2000):
  26. 26. Sound quality Sampling rate (kHz) Bit Depth File size (MB) Stereo/Mono Compact disc quality 44.100 16 30.2 stereo good quality 44.100 16 15.1 mono 44.100 8 15.1 stereo 22.050 16 15.1 stereo 22.050 16 7.5 mono 22.050 8 7.5 stereo low quality 11.025 8 3.7 stereo 8.000 8 1.3 mono
  27. 27.  How to digitize sound?  1st step: connect the output source to the computer: This can be done by connecting a mini audio cable from the headphone output on the tape recorder to the sound input on the computer. (DAT and CD work in essentially the same way.)  2nd step: connect headphones or speakers for monitoring into the computer’s headphones jack.  3rd step: open up software, such as Sound Edit 16 or CoolEdit.  4th step: Start playing the tape and listen to the sound. Observe the sound level and make sure that it does not go into the red zone.  5th step: control the sound level with the tape recorder volume controls.  6th step: after digitizing the sound, save it in an appropriate format.
  28. 28.  Sampling Theory of digital audio.  Sounds from the real world can be recorded and digitized using an analog-to-digital converter (ADC). As in the diagram below, the circuit takes a sample of the instantaneous amplitude (not frequency) of the analog waveform. Alternatively, digital synthesis software can also create samples by modeling and sampling mathematical functions or other forms of calculation. A sample in either case is defined as a measurement of the instantaneous amplitude of a real or artificial signal.  Frequencies will be recreated later by playing back the sequential sample amplitudes at a specified rate. It is important to remember that frequency, phase, waveshape, etc. are not recorded in each discrete sample measurement, but will be reconstructed during the playback of the stored sequential amplitudes.
  29. 29.  Sampling Theory of digital audio. Samples are taken at a regular time interval. The rate of sample measurement is called the sampling rate (or sampling frequency). The sampling rate is responsible for the frequency response of the digitized sound.

×