Information Processes and Technology

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Digitising sound

The real world that we live in has an infinite variation of sound and light. This infinite variation is referred to as analog. For the sound to be used in a computer it must be digitised.

The elements covered in this summary are:

Outcomes

This material addresses aspects of the following syllabus outcomes:

A student:

H1.1 applies and explains an understanding of the nature and function of information technologies to a specific practical situation
H2.2 explains and justifies the way in which information systems relate to information processes in a specific context
h2.1 analyses and describes a system in terms of the information processes involved.
H2.2 develops and explains solutions for an identified need which addresses all of the information processes
H3.1 evaluates and discusses the effect of information systems on the individual, society and the environment
H3.2 demonstrates and explains ethical practice in the use of information systems, technologies and processes
H4.1 proposes and justifies ways in which information systems will meet emerging needs
H5.2 assesses the ethical implications of selecting and using specific resources and tools, recommends and justifies the choices
H7.1 implements and explains effective management techniques
H7.2 uses methods to thoroughly document the development of individual and team projects.

Source: Board of studies NSW, Stage 6 Information Processes and Technology, Preliminary and HSC Courses (2007)

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Analog to digital conversion

Whether the sound comes from an old LP, cassette tape or recording of the natural world all sound that is not generated electronically is analog.

The first step in the digitisation of any audio is to determine how the audio will be used. For example, a digital audio recording for Internet streaming has very different requirements to a recording for an audio CD. When analog files are saved to a computer there are many different formats that the file can be saved as along with many different settings. Each file format has a slightly different purpose. The .wav and .aif file types are uncompressed digital audio and are best suited for creating Audio CDs or master files while audio formats such as .mp3, .mpg, .mpeg, .rm, and .wma, are best for computer and internet presentations because of their compression.

To convert analog sound to a digital format we require an analog-to-digital converter (ADC). This function is normally built into a sound card.

Activity 1

  1. Why does the use of the audio file need to be determined before digitisation should take place?
  2. Explain the purpose of an analog to digital converter and what is the difference between this and a digital to analog converter.
  3. Research methods used to digitise sound.

Answers

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Sampling of sound(and Sample rate)

The method used to digitise sound is called sampling. When a sound file is sampled we take a slice of sound at discrete intervals. The more often this slice is recorded the better the quality. Imagine you are in an experiment and have been asked to listen to a conversation. In this experiment you can only listen fortwo seconds at a time before the sound is cut off. The more often the sound is turned on the more of the conversation you pick up. In fact if we were able to turn the sound back on really quickly you would miss very little of the conversation. This is the way sampling works and the number of slices taken during a period of time is called the sample rate. The higher the sample rate the better the quality of the sound being recorded. So the larger the sample rate the better the quality because more data has been retainedand the larger the file size. Lets have a look at a table with sample rates.

Sample Rate (the larger the number the more often the sound is sample in a fixed timeframe) Quality and Use
11 kHz low sample rate but good for speech
22 kHz satisfactory music playback
32 kHz broadcast quality
44 kHz Commercial audio CD quality
48 kHz digital audio tape

Sample rate for digitisation of sound. Higher sample rate the higher the quality It can be seen from the illustration on the left that data is lost during the process of digitisation. It is unavoidable. However the loss is so small that it is negligable and often the final product can be enhanced anyway.
image courtesy of bettscomputers.com

Activity 2

  1. What is the difference between sampling and sample rate.
  2. Research the following terms and explain them with the use of a drawing:
    • amplitude
    • wavelength
    • frequency

Answers

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Audio file formats

Although many file formats are used to store digitised sound, music, and even instrument or synthesizer tracks on computers in reality the choices become more limited. Limitations occur as a function of the output device being used. For example most DVD and CD players will not accommodate most of the sound formats.

Other factors that need to be considered include sound quality, the types of equipment used for playback, and trade-offs between file sizes (smaller files mean more music) and listening quality (more compression normally means lower audio fidelity).

Some compression methods are "lossy," which means that data is lost during compression. This is a one-way, irreversible process. Other compression methods are "lossless", which means that no data is lost during compression and an uncompressed version of data can be generated from the compressed form. This is a two-way, reversible process. Lossy compression schemes usually result in smaller files.

Another key ingredient in the audio compression process is a special piece of software called a codec, short for the type of "compression/decompression" algorithm that's used to translate uncompressed data into a compressed form and to decompress for access or playback. Codec Central (external website) is a great source of information on codecs of all kinds.

Common digital audio formats

table 1 provides a list of many common digital audio file formats and some details about them

table 1

Name Ext Date Type Cmp Notes
Advanced Audio Coding aac 1997 Lossy Yes Also known as MPEG-4 AAC. Supports various compression rates, commonly used for iPods.
Audio Interchange File Format aiff 1999 Lossless No Original, uncompressed Mac sound format. Not cross platform and not used in devices. format is not supported by all web browsers.
Compact Disk Digital Audio (CD-DA) cda 1979 Lossless No Standard digital audio CD format, uncompressed but not used in many devices except CD players.
Musical Instrument Digital Interface .mid or .midi 1982 lossless no but not required The MIDI (Musical Instrument Digital Interface) is a format for communicating music information between electronic sound devices like synthesizers and PC sound cards. MIDI format is flexible and can be used for basic to professional music.

However NOTE The downside of MIDI is that it cannot record sounds (only notes). MIDI files do not contain sampled sound, but a set of digital musical instructions (musical notes) that can be interpreted by a PC's sound card. The best part of the MIDI format is that MIDI files can be extremely small. For example a 5 minute file might only be around 23K.
MPEG-1 Level 3 (MP3)

Moving Pictures Experts Group
.mp3 or .mpga 1993 Lossy Yes Uses perceptual coding to compress signals at various compression rates.

MPEG format was originally developed for video by the Moving Pictures Experts Group. MP3 files are the sound part of the MPEG video format. MP3 is very popular format. MP3s combine good compression rates (small files) with high quality.
OGG Vorbis ogg 2000 Lossy Yes Ogg Vorbis is used to store and play digital music in much the same way as MP3, VQF, AAC does at present. It is different from these other formats because it is completely free, open, and unpatented. It has been designed to completely replace all proprietary, patented audio formats. (supported on some devices).
QuickTime Audio mov 1997 Lossy Yes Same MPEG-4 technology as AAC, but limited usability because not used in devices.
Real Audio Media rma 1995 Lossy Yes Proprietary, compressed audio format but limited usability because not used in devices. The RealAudio format was developed for the Internet by Real Media. The format also supports video including streaming.
Waveform sound files wav 1991 Lossless+ No Used for system sounds on Windows PCs (not used for high quality music or in devices).
Windows Media Audio wma 1997 Lossy+ Yes Offers various rates of compression or no compression (supported on most devices).

Activity 3

  1. Use the Windows sound recorder to record sounds and make a list of the various settings such as sample rate. Make a recording and then use sound recorder to convert to various formats with a variety of different settings. Examine the difference in file size and quality.

  2. Examine the table above and then explain and justify the different criteria that needs to be examined when choosing a sound format. For example what file type would you record in if you were:
    • a professional musician with voices
    • a professional instrumentalist
    • a web page designer
    • about to record to CD for playback in a standard CD player

Answers

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