# Lesson video

In progress...

Hi, I'm Dharini.

Welcome to lesson 4 of Data Representation, Going Audio Visual.

For this lesson, you will need to instal a free audio editing software, Audacity.

So be sure to ask a parent or carer's permission before installing the software on your device.

You will also need pen and paper to make notes.

Remove as many distractions as you can.

And when you're ready, let's get started.

In this lesson, you will recall that sound is a wave.

You will be able to explain the function of microphones and speakers as components that capture and generate sound.

You will be able to define key terms such as sample, sampling frequency or rate, sample size.

You will be able to describe how sounds are represented as a sequence of bits.

The form of sound.

The nature of sound.

Sound is a wave.

Vibrations can set particles in motion, generating variations in density that is pressure points.

So whenever a moving object or vibrating objects, they can set the particles of air in motion generating the variations in air pressure.

On the right you can see the particles in motion which generates the low density or the low pressure points.

I can see only few dots and draw some high density or high pressure points that you see more dots within that specific point.

Here's a Scratch programme to see an animated illustration of the nature of the sound wave and how it links to variations in air pressure.

Here is a Scratch animation which illustrates the nature of sound in wave form, and how it links to the variations in air pressure.

So when I switch on the microphone, see what happens.

So, the microphone which captures the sound in the form of air pressure points, and it converts them to electric voltages.

The variations in electric voltages which mirrors the air pressure points.

That's what you see on screen in the wave form.

Components for sound.

Which competent is used to capture sound? What do you think? Did you guess it right? Brilliant.

Well done.

The microphone, it captures the sound.

So microphones allow devices to capture sound as electricity.

So the input to microphone is the air pressure.

We saw that in the Scratch animation.

So microphone's input is the air pressure point, the variations in air pressure points.

And what microphone does is convert variations in pressure to variations in electric voltage.

So it's input is air pressure but its output is electric signals, the electric voltages.

And those variations in electric voltages mirrors the variations in air pressure.

But the digital devices represent these wave forms as sequence of binary digits.

So which component is used to generate sound? We know microphone is used to capture sound, so which component is used to generate the sound? What do you think? Why do you hear the sound? Speakers.

Did you get it right? Well done! Speakers allow devices to generate sound from electricity.

So speakers is the opposite of microphones.

So it generates sound from the electric voltages.

So speakers input is the voltage and the output is the air pressure.

So speakers converts the electric voltages into air pressure points, the variations in air pressure.

Whereas the microphones they convert variations in air pressure to variations in electric voltage.

But the digital devices produce these wave forms as a sequence of binary digits.

Task 1, the form of sound.

Use Audacity to examine how different sounds have different forms. You will need audio files.

So make sure to download all the audio files you have been provided with, to examine the different forms of sounds.

Pause this video to complete your task, and resume once you're finished.

Analogue to digital.

Analogue and digital.

These wave forms on the right are analogue.

They are continuous streams. Individual points can have any value.

Now the challenge, how can they be digitised? That is, how can they be represented as a sequence of binary digits? So how are we going to represent the wave form to sequence of binary digits? What do you think? You're going to do an unplugged activity which explains a bit more about how to convert an analogue wave form to digital, that is a sequence of binary digits.

Carrie is going hiking on a 10-hour trail on Mount Digi.

Okay, so that's what you see on the bottom of the graph, 00 to 10 hours.

So she goes hiking on a 10-hour trail.

And this is an example of her altitude that is how high she is, which may change along the route.

Taking samples at regular intervals.

She promises her worrying parents that she will send hourly updates with her altitude, that is how high she is at the moment so that they can trace how she is doing.

Carrie's parents say that they don't need the details.

So they don't want to know a lot about it.

All they need is a 2-bit code for how high she is.

So they agree that a rough estimate will be efficient, so they don't need the exact details.

So all they need is a 2-bit code to represent how high she is.

So she has to send these details every hour to her parents.

So she will be sending a 2-bit code for how high she is.

How many possible altitude ranges for 2 bits do you think? Did you guess this right? Yes, it's four.

Two power two, two squared.

That's four.

So when you're sending 2- bit code or when Carrie is sending 2- bit code to her parents, she will have only the four possible altitude ranges.

That will be 00, 01, 10, and 11.

So for this scenario, we can keep 00 as low altitude range.

01 as a medium altitude range, and 10 as a high altitude range.

And 11 as a very high altitude range.

Analogue to digital.

So the first message.

What do you think she will send the first message as? Is she in 00, 01, 10, or 11 altitude range.

What do you think? So her first message to her parent is 01 because she is in the altitude range 01.

And again, these are approximate values.

So what do you think her second message to her parents will be? 10.

Yes, you guessed it right.

10.

So her first message is 01.

Her first message is 01.

And her second message is 10.

So what do you think will be her third message to her parents? It is the same, 10.

Do you think you can work out the rest of the messages based on her route? So you've got to identify what messages she will send from 02 to 10 hours.

Do you think you can do that? Then using the worksheet, work out the messages that Carrie will send to her parents every hour.

And once you have completed, resume this video.

Welcome back.

Task 2, analogue to digital solutions.

Let's see whether you managed to get all the answers correct for this one.

Because you got the route, all you need to identify is what message she will send to her parents, that is a 2-bit code.

So her first message is 01.

Second message is 10.

Third message is 10.

Fourth message is 10.

And fifth message is 10.

She's in the same range further and she's sending these four different messages in the same range, same altitude range.

And then she reaches the high altitude range which is 11.

And she goes back to 10, 10 again, and then goes lower from the 8th hour, which is 01, 9th hour it's 01 as well.

And in the 10th hour, she has reached 00.

So did you get all the answers correct? Well done.

So what you have done here is you have turned the continuous that is analogue wave, the continuous stream, you have digitised it into a sequence of binary digits.

Back to analogue.

We're reconstructing back to analogue.

So these are the messages that Carrie's parents have received.

01, 10, 10, 10, 10, 11, 10, 10, 01, 01, 00.

How can they retrace Carrie's route? All we need is an approximate route.

Because within every altitude range you can have a low, medium, or high within each individual altitude range.

But as long as it is within the altitude range and you have drawn it correctly, that's fine.

So how can they retrace the Carrie's route? Reconstructing back to analogue.

The first message they received is 01 and we don't know the exact altitude.

So we going to mark just the middle of 01 range.

So when you mark the altitude range either choose the middle, top or bottom.

But it should be within that attitude's range, that's all.

The second message they received was 10.

Do you think you can reconstruct the rest of the route? Yeah, you're looking for an approximate route as long as it is within the altitude range, that's perfectly fine.

So now it's your turn to complete the task 3, which is back to analogue.

So using the worksheet retrace Carrie's approximate route for hiking.

Yes, not the exact route because we don't have the exact altitude information.

So approximate route for hiking based on the messages received by her parents.

So make sure you look at the altitude range for that hour and mark a point within that altitude range.

Once you have completed, resume this video.

So far we have seen analogue to digital using 2 bits, so you can have four combinations of altitude ranges.

What about if you have 3 bits? If you're using 3 bits how many possible altitude ranges can you have? Eight, that's correct.

So here there are eight possible combinations using the 3 bits.

So here's an example of Carrie sending messages to her parents using 3 bits and for every 1/2 an hour.

Since she is using 3 bits through eight different altitude ranges, and you can see this graph is more accurate.

So at least we have more information whether it's at the bottom or the middle or the top.

Since we've got eight possible attitude ranges.

Digitising sound.

Digitising sound.

The challenge for you was to represent the analogue that is continuous stream of sound wave as a sequence of binary digits.

And that's what exactly you've done with the Carrie's messages to her parents using the altitude ranges.

You take the sample voltage at regular time intervals.

So that's the first step.

When you're digitising sound, you take the sample voltage at regular time intervals.

Similar to the Carrie's case, she sent messages every hour or every half an hour.

So you take the sample electric voltage at regular time intervals.

The number of samples taken per second is called the sampling rate.

So what is a sampling rate? It's the number of samples taken per second.

And its typical value for example is 44,100 samples per second.

The second step it's to record a sequence of bits for each sample.

Like this.

So the number of bits recorded per sample is called a sample size.

So the number of binary digits assigned per sample is called a sample size.

It's on the right hand side, you can see it has the sample size of 16 bits.

Sample rate is the number of samples taken per second, and the sample size is a number of binary digits assigned per sample.

Here is an example.

Look on the right hand side.

Each dot stands for a sample.

In this case, our sample size that's the number of bits assigned per sample is 16 bits.

So our sample size is 16 bits.

And the sampling rate that is number of samples taken per second, is 44,100 samples.

So it takes 44,100 of these 16-bit samples to represent a single second of sound.

With 90 samples in the image, we are looking at.

02 seconds of sound, only a part of the sound.

Because it's 90 samples out of the 44,100 samples taken in a second.

The big picture.

Any moving object can set particles of air in motion, generating variations in pressure or density.

And microphones capture the sound.

So microphone converts the variations in air pressure to variations in electric voltages.

And this is what you see in the wave form on screen.

And analogue representation of wave form is the continuous streams. Speakers generate sound.

And speakers convert electric variations in electric voltages to variations in air pressure.

And finally, we use 0s and 1s that's a sequence of binary numbers, to digitise analogue representation.

And sample rate is the number of samples taken per second.

Sample size is the number of binary digits recorded per sample.

I would love to see your work.