Multimedia Technology Ch 5

[Multimedia Technology]

Zeeshan Bhatti

BS(IT) Part-III & P-IV Second Semester Chapter 5

[Lecture Handout] [Chapter 5] [Sound/ Digital Audio]

[Institute of Information and Communication Technology, University of Sindh, Jamshoro]

Chapter 5: Sound

Chapter 5: SOUND 5.1 Basics of Digital Audio 5.1.1 Application of Digital Audio — Selected Examples Music Production • Hard Disk Recording • Sound Synthesis • Samplers • Effects Processing Video – Audio Important Element: Music and Effects Web — Many uses on Web • Spice up Web Pages • Listen to Cds • Listen to Web Radio Many More Uses — try and think of some?

5.1.2 Advantages of Sound

 Sound makes a silent presentation come alive and is very stimulating to audiences.  Sound can evoke emotions in audiences that sway their perception and reception of the message.  Advertising companies have long discovered the power of sound in a presentation and are making full use of it in their commercials.

5.1.3 Advantages of Digital Audio over Analog Audio Digital audio has several advantages over analogue audio: • Digital recordings do not degrade with re-recording. Each copy is an exact reproduction of the original. Where as the analog sound recorded on the magnetic tape based cassettes looses their quality and original sound with every copy. • The recording performance is independent of the recording medium. Digital audio bit streams retrieved from the CD or hard disk will sound the same every time they are played back because the bit streams are exact copies. This is not the case with analogue recordings, where the characteristics of the recording medium can influence the reproduced sound and will degrade with time. It means that when the CD is played on the same system for 1000th time, it will sound the same as when it was played the first time. With a cassette tape, the quality will have changed due to degradation. • Digital audio is easy to process because the signal processing can be performed by mathematical algorithms.

5.1.4 Advantages of Digital Audio over Analog Audio By: Zeeshan Bhatti

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Chapter 5: Sound Like most things in life, nothing is for free. Digital audio is not perfect, it has several disadvantages of its own compared to analogue audio. • It requires two conversion stages: One to convert the analogue signals into a digital format and a second to convert the digital signals to analogue. • These conversion processes can introduce their own types of distortion, noise and defects. • With low sampling rate, the original sound quality can be greatly reduced.

5.2

Analog Sound

 Analogue sound is the sound which we hear everyday such as birds chirping, music playing from your cassette player and even the conversations we have with one another.  Analogue sound travels in continuous waves which are characterized by their frequencies and perceived loudness.  Frequency of sound measures the number of cycles a sound wave make in one second.

Frequency and Amplitude of Sound

 The unit of measurement of frequency is Hertz or Hz.  The more cycles a sound makes in a second the faster it is moving.  The perceived loudness of sound is called Amplitude and its unit of measurement is in Decibels or dB.  The louder the sound, the more vibrations it makes and the greater the decibels.

5.2

Digitization of Sound

Let us first analyze what a sound actually is: • Sound is a continuous wave that travels through the air • Sound is energy • When something vibrates in the air by moving back and forth (such as the cone of the loudspeaker), it creates the pressure. These waves spread like the ripples from a pebble tossed into a still pool, and when they reach your eardrums, you experience the changes of pressure, or vibrations, as sound. • The wave is made up of pressure differences. Sound is detected by measuring the pressure level at a location. • Sound waves have normal wave properties (reflection, refraction, diffraction, etc.).  Digital sound is made up of discrete values of 0s and 1s and is the approximation of analogue sound.  The digital nature of sound makes it possible for digital sound files to be edited, processed and stored in the PC. Sound Source — variety of sources generates Sound • Air Pressure changes • Electrical — Loud Speaker • Acoustic — Direct Pressure Variations The destination receives (sensed the sound wave pressure changes) and has to deal with accordingly: By: Zeeshan Bhatti

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Chapter 5: Sound Destination — Receives Sound • Electrical — Microphone produces electric signal • Ears — Responds to pressure hear sound Sound is required input into a computer: it needs to sampled or digitized: • Human Beings, Speakers, Video Cameras (Magnetic tape based) produce analog signals (continuous-valued voltages) as illustrated in Fig 5.1

Figure 5.1: Continuous Analog Waveform • To get audio or video into a computer, we have to digitize it (convert it into a stream of numbers) Need to convert Analog-to-Digital — Specialised Hardware • So, we have to understand discrete sampling (both time and voltage) DIGITAL RECORDING (Sampling and Sample)  To be able to replicate the analogue sound on the PC, it has to be captured and digitalized.  Using the sound card & microphone, sound like hat coming from an audiotape or even your voice can be recorded.  During the recording, these analogue waves are converted into digital signals through the process called Sampling. • Sampling – divide the horizontal axis (the time dimension) into discrete pieces. Uniform sampling is ubiquitous. • Quantization – divide the vertical axis (signal strength) into pieces. Some-times, a non-linear function is applied. – 8 bit quantization divides the vertical axis into 256 levels. 16 bit gives you 65536 levels.  The sampling process entails chopping up the analogue waves into many parts and taking every other part (called samples) at regular intervals to approximate the original sound.  Sampling process or sampling rate measures the number of samples taken per second.  Sampling rate ranges from 11,025 to 44,100 samples per second and is measured in Hertz or Hz.  It is usually measured in KHz (Kilohertz). An example is 11 KHz which is 11,025 Hz samples per second.  The more samples that can be taken in a second, the more accurate the approximate will be and the better the quality of the digital sound.  Sound sampled at 11 KHz is comparable to sound from the FM radio.  Sound sampled at 44.1 KHz is comparable to sound from audio CD.  Higher the sampling rate, the more bytes have to be stored in the PC’s hard drive. By: Zeeshan Bhatti Page 4 of 9

Chapter 5: Sound

Figure 5.2: Sampling a Waveform at different discrete frequencies affects the perceived waveform

By: Zeeshan Bhatti

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Chapter 5: Sound

5.3

Digitizing Audio That is the basic idea of digitizing a sound unfortunately things are (practically speaking) not so simple. • Questions for producing digital audio (Analog-to-Digital Conversion): 1. How often do you need to sample the signal? 2. How good is the signal? 3. How is audio data formatted?

5.4

Computer Manipulation of Sound

Once Digitized processing the digital sound is essentially straightforward although it depends on the processing you wish to do (e.g. volume is easier to code than accurate reverb) Essentially they all operate on the 1-D array of digitized samples, typical examples include: • Volume • Cross-Fading • Looping • Echo/Reverb/Delay • Filtering • Signal Analysis Soundedit Demos • Volume • Cross-Fading • Looping • Echo/Reverb/Delay • Filtering

5.5

Sample Rates and Bit Size

Question: How do we store each sample value (Quantization)? 8 Bit Value (0-255) 16 Bit Value (Integer) (0-65535)  When sound is being digitalized, it can be digitalized as 8 bit or 16 bit.  Digitalizing sound at 8 bit results in the analogue sound being divided into 256 parts (28) per By: Zeeshan Bhatti

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Chapter 5: Sound second.  Sound digitalized at 16 bit divides the waves into 65536 parts(216) per second.  Sound recorded at 16 bit will have a more closer approximate to the original sound as more samples can be taken. Question: How many Samples to take? 11.025 KHz — Speech (Telephone 8KHz) 22.05 KHz — Low Grade Audio (WWW Audio, AM Radio) 44.1 KHz — CD Quality

Figure 5.3: A Sine Wave

Figure 5.4: Sampling at 1 time per cycle

Memory R equired for 1 Minute of Digital A udio 22.05KHz 44.1 KHz File Type

11.025KHz

16 Bit Stereo

10.1 Mb

5.05Mb

2.52 Mb

16 Bit Mono

5.05Mb

2.25Mb

1.26Mb

8 Bit Mono

2.25Mb

1.26Mb

630Kb

By: Zeeshan Bhatti

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Chapter 5: Sound

5.6 S ize of Data

There is a trade off between Audio Quality vs. Data Rate Some typical applications of sample bit size and sample rate are listed below:

• •

Telephone uses u-law encoding, others use linear. So the dynamic range of digital telephone signals is effectively 13 bits rather than 8 bits. CD quality stereo sound –> 10.6 MB / min.

5.7 T ypic al A udio F ormats

• P opular audio file formats include .au (Unix works tations ), .aiff (MAC , S G I), .wav (P C , DE C works tations ) • A s imple and widely us ed audio compres s ion method is Adaptive Delta P uls e C ode Modulation (ADP C M). B as ed on pas t s amples , it predicts the next s ample and encodes the difference between the actual value and the predicted value.  AIFF - Audio Interchange File Format  WAV  AU  MP3 - MPEG-1 Audio layer III  QuickTime  SND  MIDI

5.8 Introduc tion to MIDI (Mus ic al Ins trument Digital Interfac e)

Definition of MIDI: Definition of MIDI: “ a protocol adopted by the electronic mus ic indus try that enables computer, s ynthes izers , keyboards , and other mus ical device to communicate with each other.”  S tarted in E arly 1980s  C omputers ms t have s pecial MIDI interface , which us ually is incorporated in s ound cards .  MIDI is a s c ripting language – it codes “E vents ” that stand for the production of certain s ounds . By: Zeeshan Bhatti

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Chapter 5: Sound

5.8.1 C omponents of a MIDI S ys tem S ynthes izer: • It is a s ound generator (various pitch, loudnes s , tone colour). • A good (mus ician’s ) s ynthes izer often has a microproces s or, keyboard, control panels , memory, etc. S equenc er: • It can be a s tand-alone unit or a s oftware program for a pers onal computer. (It us ed to be a s torage s erver for MIDI data. Nowadays it is more a s oftware music editor on the computer.

•S tarted off as a special hardware device for storing and editing a sequence of mus ic events, in the form of MIDI data. • It has one or more MIDI INs and MIDI OUT s . T rac k: • T rack in sequencer is us ed to organize the recordings . • T racks can be turned on or off on recording or playing back. C hannel: • MIDI channels are us ed to s eparate information in a MIDI s ys tem. • T here are 16 MIDI channels in one cable. • C hannel numbers are coded into each MIDI mes s age. T imbre: • T he quality of the s ound, e.g., flute s ound, cello s ound, etc. • Multitimbral – capable of playing many different s ounds at the s ame time (e.g., piano, bras s , drums , etc.) P itc h: • mus ical note that the ins trument plays  i.e a C as opposed to G . V oic e: • V oice is the portion of the s ynthes izer that produces s ound. • S ynthes izers can have many (12, 20, 24, 36, etc.) voices . • E ach voice works independently and s imultaneous ly to produce s ounds of different timbre and pitch. P atc h: • the control s ettings that define a particular timbre.

By: Zeeshan Bhatti

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