Data representation – Sound.

Recap How many colours could be represented per pixel in a 4 bit colour pallet The more bits per pixel the greater the colour depth, better image quality Resolution is the number of pixels per inch (PPI) or (DPI) As we know, a bitmapped image when enlarge does not gain more pixel, these pixels are merely stretched

Learning Objectives - 3.2.3 Understand how analogue data [sound, temperature, light intensity] is represented in binary 3.2.4 Understand the limitations of binary representation of data [quantisation, sampling frequency] and how bit length constrains the range of values that can be represented

Definitions Type Definition Other Fact Analogue Continuously variable Music Speech Activation Sensor Digital Binary pattern Can be stored Can be processed

When To Use Which? Analogue Playing Music Telephone Call Analogue Music stored on USB Key Fibre optic transmission A song being edited in Audacity Electricity along a copper wire Analogue Digital Digital Digital Analogue

How do we store sound on computers? Sample the sound at set intervals. Max possible Value Max possible Value +128 +128 time time -127 -127 Min Possible Value Min Possible Value

Analogue to Digital Conversions

Sampling Sampling Resolution = “quantisation” The number of bits needed to store one sound sample. The height of the wave is called the AMPLITUDE! – This is the aspect that is sampled. Sampling Frequency = The rate at which samples are taken The height of the wave is sampled at regular intervals The height is represented by a code (011, 111, …)

What affects the quality / file size? How often you take a sample…(sample rate) 44100 Hz 16000 Hz… The Sample Resolution How many bits you use for each sample 16 bits per sample gives 65,536 possible levels. (216)

Digital Mono Storage Requirement 16 bits per sample @ 44.1KHz 44100 sample X 2 / 1024 44100 Samples Sec 16 Bits Sample / 1 Byte 8 Bits / 1 Kb 1024 Bytes * How Much Storage For A 5 Second Clip? = 86.1 Kb/Sec 86.1 Kb/Sec * 5 Sec = 1033.2 Kb

Digital Stereo Storage Requirement 16 bits per sample @ 44.1KHz by 2 channels 44100 sample X 4 / 1024 1 Byte 8 Bits 1 Kb 1024 Bytes 44100 Samples Sec 16 Bits * 2 Sample * * * = 172.27 Kb/Sec How Much Storage For A 15 Second Clip? 172.27 Kb/Sec * 15 Sec = 2584.05 Kb

Thinking! - Sampled Levels How many different levels could be sampled using 1 bit 2 bits 3 bits 16 bits If positive and negative values were required, then we could go from – 032,768 to +32,767 including 0. (this is the two’s compliment method) Where have you seen this kind of question before? Gives just 2 levels 0 and 1 = 21 Give 4 levels: 00,01,10,11 = 22 Gives 8 levels = 23 Give 216 or 65,536 different possibilities 12

Open Audactiy Plug headphones / mic in before you open application. Once you have a sample Using a Microphone provided record your sample Start with the sample rate set to 44,100 Export the file Repeat the exercise with the sample rate set to 16,000 Explain the difference in file size Show your working.

Exam Question…