Cover lesson · Computer Science · Year 10 · Intermediate

Calculating file sizes for a digital archive

DCF: Interacting and collaborating → Storing and sharing

Name: Class: Date:

Cover teacher: you need no subject knowledge

What this practises: GCSE Computer Science Unit 1: Understanding Computer Science, Section 1.2.1 Data types, including representation, storage and compression: calculating the storage requirements for graphics and sound, base 2 storage units, lossy and lossless compression, and compression ratios. This is examination content.

Print: this sheet, one per pupil, and the stimulus sheet “Lleisiau’r Cwm: the digital archive brief”, one between two. Calculators are needed. No computers are needed.

Run: 7 min read the brief and do Step 1 · 12 min Step 2, image sizes · 10 min Step 3, sound sizes · 10 min Step 4, compression · 13 min Step 5, the storage plan · 3 min pupils tick the success criteria they have met. Total 55 minutes.

Collect: this sheet. The worked example has every answer if you want to read out the Step 2 answers at the halfway point.

What you are doing

A volunteer group in the Cynon Valley is turning old photographs and recorded interviews into digital files. They have a 15 GB cloud account and no idea whether everything will fit. You are going to work out how big each file is, what compression does to it, and then write them a storage plan they can follow. Show your working every time: in the exam, the working earns marks even when the final answer slips.

Step 1: storage units warm-up

Use section 2 of the brief. Remember: 1,024, not 1,000.

Question	Answer
How many bits are in 1 nibble? In 1 byte?
How many bytes are in 1 KB?
How many KB are in 1 GB?
How many different colours can a pixel show with a colour depth of 8 bits? (Hint: 2 to the power of 8.)

Step 2: how big is each scan?

Use the image formula in section 3 and the items in section 4. Give KB to 2 decimal places where needed and MB to 2 decimal places.

Item	Working (bits)	Bytes	KB	MB
P1
P2
P3
P4

(e) P3 is stored with a colour depth of 1 bit. Explain what would happen to the quality and to the file size if the volunteers scanned it at 8 bits instead. Why might they do that for a handwritten page?

Step 3: how big is each recording?

Use the sound formula. Change the length into seconds first.

Item	Seconds	Working (bits)	Bytes	KB	MB
A1
A2
A3

(d) A3 is much smaller per minute than A1. Name the two settings that make it smaller, and say what this does to the sound.

Step 4: what compression did

Use section 5 of the brief. Write each ratio as something : 1. Give percentages to 1 decimal place.

File	Compression ratio	Reduction in size	Use it for the master copy, the web copy, or both? Why?
P2 as PNG
P2 as JPEG
A1 as FLAC
A1 as MP3

Step 5: the storage plan

Use section 6 of the brief. Give GB to 2 decimal places.

(a) How much space would the whole collection take uncompressed? Will it fit in 15 GB?

(b) How much space would the master copies take if every photo is saved as PNG and every interview as FLAC?

(c) The web copies are JPEG and MP3. How much space do they take? Is there room for the masters and the web copies in 15 GB? How much is left over?

(d) Write Gareth a plan of about 80 words. Say which format to use for the masters and which for the web copies, and why. Say where the second copy of the masters should be kept. Suggest a file name pattern so a volunteer can find any item, for example by type, year and place.

Step by step

7 min Read the brief. Answer Step 1.
12 min Step 2: four image sizes, then question (e).
10 min Step 3: three sound sizes, then question (d).
10 min Step 4: ratios, reductions and choices.
13 min Step 5: totals for the collection and the plan for Gareth.
3 min Tick the success criteria you have met.

Success criteria

I can calculate the storage needed for an image and a sound file, showing the working from bits to bytes, KB and MB.
I use base 2 units correctly: 8 bits in a byte and 1,024 for every step after that.
I can calculate a compression ratio and the reduction in file size.
I can use compression as a file management technique, choosing lossless or lossy for a reason.
I can plan how files are stored and shared, including a second copy and a file naming pattern.

If you finish early

A2 is re-recorded in stereo (2 channels). Work out the new size in MB. What do you have to add to the formula?
A shop advertises a “1 TB” drive that holds 1,000,000,000,000 bytes. How many GB is that in base 2 units? Why does a computer show less than 1 TB?
Some families have asked for their photos to be private. Suggest one way the group could share the cloud folder with volunteers only, and one reason to encrypt the drive in the vestry.