Unit 2
Representing sounds
Lesson 1 What do I need to know? Main activity 2 introduces the idea of ‘sampling’, ‘s ampling’, taking a reading of the pressure wave that is the sound and recording it as a numeric value. Typically CD-quality audio uses 44 100 or 48 000 samples per second. The free software tool Audacity (audacity.sourceforge.net ) allows users to zoom in and see each sample individually. Controlling the sample rate is a key way of reducing the size, and quality, of a sound file in much the same way that a 20-megapixel photograph is a much more accurate reproduction than a 0.3-megapixel photograph. This will be explored by students in future lessons.
Learning objectives MUST: Understand uncompressed file size Understand that a bigger file takes longer to download
Unit 2
Representing sounds
Key terms
Bandwidth Waveform
Teaching notes Starter: File size, Interactive 2.1A Begin the lesson by asking students to complete 2.1.1 Think-IT. This is supported and extended by Interactive 2.1A. By putting the list in order students should be able to demonstrate an understanding of the key measures of file size. Main activity 1: Can we stream audio? Worksheet 2.1A, Worksheet 2.1B, Worksheet 2.1C Introduce the challenge for this unit, helping a record label open an independent music store that allows users to stream live to their mobile devices over 3G by devising a compression strategy that will allow them to keep the file size down without compromising on sound quality. Make sure that students understand the different between streaming (downloading songs second by second, playing them and then
Unit 2
Representing sounds
insightful students may spot that it is possible to reduce the number of data points in the graph).
Unit 2
Representing sounds
Differentiation and extension In Main activity 1, most will be able to put many of the files in size order in 2.1.1 Think-IT and will appreciate the issues surrounding streaming music and bandwidth. Many will be able to put all the files in order of size and complete the calculations in 2.1.2 Think-IT. Some will be able to relate bandwidth and download times and will appreciate issues related to streaming of music other than bandwidth. In Main activity 2, most will be able to record or import a sound file into a soundediting program and modify some of the settings. Many will be able to apprecia te how the waveform relates to volume and pitch.
Homework Ask students to complete Worksheet 2.1D. Note that podcasts are typically of lower quality because they generally focus on the spoken word rather than music and tend to be longer than music tracks. Suggested next lesson
Unit 2
Representing sounds
Answers 2.1.1 Think-IT B A tiny text file, which can be defined as a file of 10,000 characters or 150 lines D
A 6-page Word document
E
A 4-minute song saved as an MP3
A
A DVD film
C
A BluRay film
2.1.2 Think-IT / Worksheet 2.1A / Worksheet 2.1B a) 130 KB 130 kilobytes × 1024 bytes = 133 120 bytes 133 120 bytes × 8 bits = 1 064 960 bits b) 8 MB
Unit 2
Representing sounds
a) 6 MB = 48 Mb. 48/56 = 0.85 seconds, or more typically 48/28 = 1.7 seconds b) 48/1 = 48 seconds c) 48*1024/80 = 614 seconds or just over 10 minutes
Unit 2
Representing sounds
2.1.5 Think-IT The discussion should relate to the fact that downloading files is not time dependent, whereas streaming is. Once a track has been downloaded it can be played without a connection to the internet, but streaming requires real-time responses and slow downloads will interrupt the playback of the file. As an ext ension, you could tell students that CD-quality MP3 files are stored at a bitrate of 320 kbps, whereas many download services offer files that are stored at 192 kbps or lower. 2.1.6 Compute-IT The waveform gets taller as the volume increases. Notes with a higher pitch rise and fall more quickly; the waves are more compressed horizontally.
Unit 2
Representing sounds
Assessment grid Unit 2 Lesson 1 National Curriculum Programme of Study statement
Progression Pathway attainment statement
Learning objectives (Must, Should, Could)
Activity or resource reference
Reporting statement
KS3, Bullet Point 6: Understand how instructions are stored and executed within a computer system; understand how data of various types (including text, sounds and pictures) can be represented and manipulated digitally, in the form of binary digits
Data and data representation Knows that digital computers use binary to represent all data. Understands how bit patterns represent numbers and images. Knows that computers transfer data in binary. Understands the relationship between binary and file size (uncompressed). Defines data types: real numbers and Boolean. Queries data on one table using a typical query language.
MUST understand uncompressed file size
2.1.1 Think-IT 2.1.2 Think-IT Worksheet 2.1A Worksheet 2.1B Answers on p. 24
Has some understanding of file size
KS3, Bullet Point 6 (see above)
Data and data representation (see above)
MUST understand that a bigger file takes longer to download
2.1.4 Think-IT 2.1.5 Think-IT Worksheet 2.1C Answers on p. 25
Understands that a bigger file takes longer to download
SHOULD understand that a sound file can be displayed as a graph
2.1.6 ComputeIT Answers on p. 25
Understands that a sound file can be displayed as a graph
Information technology Evaluates the appropriateness of digital devices, internet services and application software to achieve given goals. Recognises ethical issues surrounding the application of information technology beyond school. Designs criteria to critically evaluate the quality of solutions, uses the criteria to identify improvements and can make appropriate refinements to the solution. KS3, Bullet Point 6 (see above)
Data and data representation Understands how numbers, images, sounds and character sets use the same bit patterns. Performs simple operations using bit patterns e.g. binary addition. Understands the relationship between resolution and colour depth, including the effect on file size. Distinguishes between data used in a simple program (a variable) and the storage structure for that data.
Unit 2
KS3, Bullet Point 6 (see above)
Representing sounds
Data and data representation (see above)
COULD know different media formats and understand their typical file sizes
2.1.1 Think-IT 2.1.2 Think-IT 2.1.4 Think-IT 2.1.6 ComputeIT Worksheet 2.1A Worksheet 2.1B Worksheet 2.1C Answers on pp. 24 –25
Has a sound understanding of different media formats and their typical file sizes