1. Recap
In previous lessons we have looked at how numbers can be stored as binary.
We have also seen how images are stored as binary.
This lesson we are concerned with how sound is stored on a computer.

2. By the end of this lesson
I will be able to label a sound wave:
I will be able to describe the following terms: amplitude, frequency, ADC, sample rate.
Sample a sound wave in two different sample rates.

3. How sound is sampled and stored in digital form
Sound travels as a wave. In computer science, you need to be aware of the amplitude and frequency of a sound wave.

4. Computers store, process and interpret information in 1’s and 0’s, binary code.
In order to store the waves digitally on the computer, we need to convert the waveform into a numerical representation (binary)

5. How sound is sampled and stored in digital form
Sound waves are analogue, and as computer scientists we are concerned with how these analogue waves get stored digitally on a computer.
To store the sound in binary, we need to convert the analogue signal, to a digital, binary one.
This process is called analogue to digital conversion.

6. Video Time

7. ADC - Sampling
An ADC works by taking samples of a sound wave at regular intervals.
Sample

8. SAMPLING RATE 5 4 3 2 1 10 regular intervals = 10Hz -1 -2 -3 -4 -5
Analogue sound wave
SAMPLING RATE
Digital signal

9. Tasks You will now become a ADC!
Your task is to sample the following sound wave.
You must take a sample of the wave’s amplitutde every 5 milliseconds (ms)
Record each sample in the table below
Convert each sample into binary

10. ADC - Sampling Sample rate – the number of samples taken every second
The quality and size of a file is affected by two factors:
Sample rate – the number of samples taken every second
Bit depth – the number of bits used to store each sample.
The greater the frequency of samples (sample rate), the better the sound quality and file size.
Also, the more bits that are sampled, the greater the accuracy of the file, but also the greater the size.

11. Plenary
When using an ADC, as computer scientists, we care most about two things:
Size of the sound file (how many bits are required to store it)
Quality of the sound file (how closely the digital signal matches the original sound wave)
Which of the two tracks we listened to do you think was larger in file size?

12. Lesson 2

13. Lesson 2 – Calculating Audio File Size
Last lesson we looked at how computers use ADC’s to convert analogue sound waves to digital signals.

14. Aims By the end of this lesson I will:
Understand that the sample rate and bit depth have a direct impact on the size of a sound file.
Be able to calculate the file size of an audio file.

15. Three Factors to Consider
The sample rate , or the number of samples taken per second is measured in Hz (Hertz). This also impacts the size of a sound file.
Bit depth (also known as resolution) affects the size of a sound file. This is the number of bits required to store each sample.
The length in seconds of a recording in seconds is the third factor that impacts the file size.

16. Video Time

17. Channels (Mono vs Stereo)
True stereo sound is recorded with two or more microphones, which is then mixed into two different tracks, representing a unique 'right' and a 'left' channel. The placement of the mics and what they record can vary and be completely independent of each other... sometimes even recording in separate rooms! The end result is that you perceive the sound as being more lifelike.
It works because the right and left arrangement of the speakers in your room (or your headphones) means that the right and left channels of audio get more or less routed to each respective ear, tricking your brain into thinking that you are right there in the room with the musicians (ok not really, but it brings you closer to them).

18. Sample Rate in Hz
A common audio sample rate for music is 44,100 samples per second. The unit for the sample rate is hertz (Hz). 44,100 samples per second is 44,100 hertz or 44.1 kilohertz (kHz).
Telephone networks and VOIP services can use a sample rate as low as 8 kHz. This uses less data to represent the audio. At 8 kHz, the human voice can still be heard clearly - but music at this sample rate would sound low quality.

19. Bit depth in bits per sample
The bit depth is how many bits are required to store the result of each sample. More bits will be required to store greater and more accurate range of amplitudes.
Last lesson we used 4 and 5 bits per sample. This is extremely limited.
CD’s use 16 bits per sample.

20. file size (in bits) = sample rate (hz) * bit depth (bits) * seconds
Key Points
The formula you will need to remember is:
file size (in bits) = sample rate (hz) * bit depth (bits) * seconds
Sample rate (Hz) and bit depth (bits per sample) have direct impact on the quality and file size of the recorded sound.
They also has an impact on how accurately the computer can reproduce the original sound wave.

21. Activities
Complete the sound file size worksheets.