Compression

1 State two reasons why files are compressed. 2

One mark for each of the following, to a maximum of 2.

• Uses less storage space
• Faster to transmit or download because less data is sent

Model answer. Files are compressed so that they take up less storage space on a device, allowing more files to be stored. They are also compressed so that they can be transmitted more quickly across a network, because there is less data to send, which reduces both the download time and the bandwidth used.

2 Explain the difference between lossy and lossless compression. 4

One mark for each of the following, to a maximum of 4.

• Lossy permanently removes some of the data from the file
• The original file cannot be recovered after lossy compression
• Lossless reduces size without removing any data
• The original file can be perfectly restored when decompressed

Model answer. Lossy compression reduces file size by permanently removing some of the data, typically detail that the human eye or ear is unlikely to notice, such as very high frequencies in audio or subtle colour differences in an image. Because the data has been deleted, the original file cannot be recovered from the compressed version. Lossless compression reduces file size without removing any data at all, instead recording the information more efficiently by identifying repetition and encoding it compactly. When a losslessly compressed file is decompressed, the original is restored exactly, byte for byte.

3 A company needs to email a large spreadsheet containing financial records to its accountant. Explain which type of compression should be used and why. 4

One mark for each of the following, to a maximum of 4.

• Lossless compression should be used
• Every value in a financial spreadsheet is significant
• Lossy compression would permanently remove data, corrupting the records
• Lossless still reduces the size so it is quicker to email, and the file opens exactly as it was

Model answer. The company should use lossless compression, for example by placing the spreadsheet in a ZIP archive. In a financial spreadsheet every single figure is significant, and the accountant needs the file to be exactly as it was created. Lossy compression works by permanently discarding data that is judged unimportant, and applied to a spreadsheet this would destroy or alter values, producing records that are inaccurate and legally worthless. Lossless compression still makes the file meaningfully smaller, so it attaches and sends more quickly and is less likely to hit an email size limit, but when the accountant decompresses it the spreadsheet is restored exactly as it was, with every value intact.

4 Describe how lossless compression is able to reduce the size of a file without losing any data. 3

One mark for each of the following, to a maximum of 3.

• It identifies repeated data or patterns within the file
• The repetition is recorded more efficiently, for example storing a value once with a count
• The original can be reconstructed exactly from this record

Model answer. Lossless compression works by finding repetition within the data rather than throwing anything away. The algorithm identifies patterns and sequences that occur more than once, and records them more efficiently, for example by storing a repeated value a single time together with a count of how many times it should appear, or by building an index of common sequences and replacing each occurrence with a short reference to that index. Because the record contains everything needed to rebuild the original, decompression reconstructs the file exactly as it was, so no data is lost.

5 A video streaming service compresses all of its films using lossy compression. Evaluate this decision. 6

One mark for each of the following, to a maximum of 6.

• Lossy gives a much greater reduction in file size than lossless
• Smaller files use less bandwidth, allowing streaming without buffering
• Less storage is needed on the servers, reducing cost
• Some quality is permanently lost
• The removed data is chosen to be least noticeable to viewers, so the impact is limited
• Concludes that lossy is appropriate here, with a caveat such as keeping lossless masters

Model answer. Using lossy compression is the right decision for a streaming service. Lossy compression achieves a far greater reduction in file size than lossless could, often reducing a film to a small fraction of its original size, and this directly determines whether streaming is viable at all. Smaller files require far less bandwidth per viewer, which means films start quickly and play without buffering even on slower home connections, and it also means the service can serve many more simultaneous viewers from the same infrastructure. Storage costs fall too, since a library of thousands of films takes up a fraction of the space it otherwise would. The cost is that quality is permanently reduced and the original cannot be recovered from the compressed copy. However, lossy video compression is designed to discard the information the human visual system is least sensitive to, so on a typical television or phone screen most viewers notice little or no difference. On balance the decision is correct, because without lossy compression streaming at scale would be impossible. The service should, however, retain uncompressed or losslessly compressed master copies of each film, so that higher quality versions can be produced in future as bandwidth improves, rather than being permanently limited by today's compressed files.