O-LevelComputer ScienceData Representation2210/12 · Oct/Nov 2019 · Q514 Marks

Audrey wants to send a sound file to Nico using email. The file is too large to attach to an email so Audrey decides to compress the file. She uses lossy compression to reduce the size of the sound file.

(a)Describe how lossy compression reduces the size of the sound file.[4]
(b)(i)State one advantage Audrey could give of using lossy rather than lossless to compress the sound file.[1]
(b)(ii)State one disadvantage Nico could give of using lossy rather than lossless to compress the sound file.[1]
(c)(i)Explain what is meant by a MIDI file.[4]
(c)(ii)MIDI uses serial data transmission. Explain two advantages of using serial transmission rather than parallel transmission. Advantage 1 Advantage 2[4]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

[1] (a) A compression algorithm is used
[1] (a) Discards any unnecessary sounds
[1] (a) ... using perceptual musical shaping
[1] (a) ... such as removing background noise / sounds humans can't hear // or other suitable example
[1] (a) Reduces sample size / resolution // by example
[1] (a) Reduces sample rate // by example
[1] (a) Sound is clipped
[1] (a) The data is permanently removed
[1] (b)(i) One from: The file size will be smaller than lossless; Requires less storage space; Requires less time to transmit
[1] (b)(ii) One from: The quality of the sound will be reduced; The original file cannot be restored
[1] (c)(i) Musical Instrument Digital Interface file
[1] (c)(i) Stores a set of commands / instructions for how the sound should be played
[1] (c)(i) Does not store the actual sounds
[1] (c)(i) Data in the file has been recorded using digital instruments
[1] (c)(i) Specifies pitch of the note // specifies the note to be played
[1] (c)(i) Specifies when each note plays and stops playing // Specifies key on/off
[1] (c)(i) Specifies duration of the note
[1] (c)(i) Specifies volume of the note
[1] (c)(i) Specifies the tempo
[1] (c)(i) Specifies the type of instrument
[1] (c)(ii) It uses a single wire
[1] (c)(ii) ... therefore, it is cheaper to manufacture / buy / install
[1] (c)(ii) ... therefore, less likely to have interference // no crosstalk
[1] (c)(ii) ... therefore, can be used over longer distances
[1] (c)(ii) Data is sent a bit at a time
[1] (c)(ii) ... therefore, less chance of data being skewed // data is received in order
[1] (c)(ii) Transmission can be synchronised
[1] (c)(ii) ... can reduce rate of errors