1. Data representation – Sound.

2. 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

3. 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

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

5. 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

6. 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

7. Analogue to Digital Conversions

8. 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, …)

9. 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)

10. Digital Mono Storage Requirement
16 bits per 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 = Kb

11. Digital Stereo Storage Requirement
16 bits per 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 * * *
= Kb/Sec
How Much Storage For A 15 Second Clip?
Kb/Sec * 15 Sec = Kb

12. 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

13. 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.

14. Exam Question…