questions","basePath":"/gcse/computer-science/aqa/data-compression/questions","formulaUrl":null,"initialIndex":27,"initialQuestionWindow":[{"id":"50b76925-b246-4448-b02c-6171db9fb754","revisionId":"88ce95b9-c0b9-44ac-be03-cf3026ea4edb","questionNumber":null,"questionType":"LA","specification":"A student was asked to describe how a Huffman tree could be created for the string in **Figure 1**.\n\nFigure 1\n```text\nBANANA SPLIT\n```\n\nHer response was:\n\n\"I would count the frequency of each character in the string and write them in a table sorted by their order of appearance. For example, the character 'N' has the highest frequency in this string. Then, I would take the two characters with the largest frequencies and combine them into a new parent node. The frequency of this new parent node is calculated by subtracting the smaller frequency from the larger frequency. I would then repeat this process with the remaining largest nodes until the entire tree is built.\"","difficulty":4,"difficultyBand":"medium","totalMarks":4,"sourceTitle":null,"calculatorUse":null,"parts":[{"id":"ec901338-cc7e-4c7a-8f93-a6e6d071715d","label":null,"content":"State **four** mistakes the student has made in her response.","markscheme":"- Identifies that the table should be sorted by frequency (usually ascending), not by the order of character appearance B1\n- Identifies that 'A' has the highest frequency in the string (appearing 3 times), not 'N' (which appears 2 times) B1\n- Identifies that the two nodes with the smallest/lowest frequencies should be combined, not the largest B1\n- Identifies that the parent node's frequency is calculated by adding/summing the frequencies of the child nodes, not subtracting them B1","marks":4,"order":0,"markingRubric":null,"explanation":null,"options":[]}],"options":[],"explanation":null,"firstAttemptFullMarks":null},{"id":"657f0bd1-f47b-4b66-89dd-4ddff2de9250","revisionId":"dd7ed690-4daa-4d47-8ae5-404f362df478","questionNumber":null,"questionType":"LA","specification":"","difficulty":4,"difficultyBand":"medium","totalMarks":3,"sourceTitle":null,"calculatorUse":null,"parts":[{"id":"4883bec6-6dfa-436f-b7b6-09f069d7e343","label":null,"content":"Using Huffman coding, the text `PETER PIPER PICKED A PEPPER` can be stored in 84 bits.\n\nCalculate how many additional bits are needed to store the same piece of text using 8-bit ASCII. Show your working.","markscheme":"- Counts 27 characters (including spaces) in the phrase `PETER PIPER PICKED A PEPPER` B1\n- Calculates the storage size in ASCII as $27 \\times 8 = 216$ bits (or $27 \\times 7 = 189$ bits if 7-bit ASCII is assumed) M1\n- Calculates the difference to get 132 additional bits (or 105 additional bits if 7-bit ASCII is assumed) A1","marks":3,"order":0,"markingRubric":null,"explanation":null,"options":[]}],"options":[],"explanation":null,"firstAttemptFullMarks":null},{"id":"b5189aa6-f190-4917-9b85-7fc80f2fee5b","revisionId":"51469b8b-a29a-42b6-b892-9deb3724ca02","questionNumber":null,"questionType":"MC","specification":"A microcontroller in an automated quality inspection system captures a $16 \\times 16$ pixel grayscale image, where each pixel is stored using $1\\text{ byte}$. One specific row of $16$ pixels has the following pixel values:\n\n`[255, 255, 255, 255, 0, 0, 0, 0, 0, 0, 128, 128, 128, 128, 128, 128]`\n\nThe system compresses this row using Run-Length Encoding (RLE). In this implementation, each run is encoded as a 2-byte packet: a 1-byte run count followed by a 1-byte pixel value.\n\nCalculate the percentage reduction in storage size achieved for this row after RLE compression.","difficulty":5,"difficultyBand":"medium","totalMarks":1,"sourceTitle":null,"calculatorUse":null,"parts":[],"options":[{"id":"7152253f-9f60-409e-8c88-27a2fc1ad328","content":"$$37.5\\%$","correct":false,"markscheme":"This is the ratio of the compressed size to the original size ($6/16$), representing the space occupied after compression rather than the percentage reducti

Data compression

Question 14

Data compression Questions

Data compression

Question 14

Data compression Questions