Data Representation 1 am/pm Time allowed: 22 minutes

Transcription

1 High Weald Academy GCSE COMPUTER SCIENCE 8520/DR1 Paper DR1 Data Representation 1 am/pm Time allowed: 22 minutes Materials There are no additional materials required for this paper. Instructions Use black ink or black ball point pen. Use pencil only for drawing. Answer all questions You must answer the questions in the spaces provided. Some questions will require you to shade a lozenge. If you make a mistake cross through the incorrect answer. Do all rough work in this book. Cross through any work that you do not want to be marked. You must not use a calculator. Information The marks for questions are shown in brackets. The maximum mark for this paper is 18. You are reminded of the need for good English and clear presentation in your answers. Please write clearly, in block capitals, to allow character computer recognition. Centre number Candidate number Surname Forename(s) Candidate signature

2 Answer all the questions in the spaces provided 0 1 Images can be represented in a computer s memory as a bitmap Explain how an image can be represented as a bitmap. The image is divided into pixels. [3 marks] Each pixel is given a binary code to represent the colour of that pixel. The bit pattern will represent all the pixels of the image, starting at the top left corner of the image, and defining each row of the actual image. The image file may also have metadata; extra data that is not used to recreate the image but carries more information about the image (such as when it was taken, where it was taken, the total number of colours in the image) A bitmapped image with a colour depth of one can represent images that use two colours. How many more colours can be represented in an image if the colour depth is increased from one to four? One bit has two possible choices (permutations). Two bits has double this (), three bits will have double that (8) and bits would have double that 16. Therefore, bits offers 1 more colours than 1 bit does (16-2).

3 0 2 One way of representing sound digitally is to take samples of the original sound Define sampling rate. The sampling rate is the number of samples taken of an analogue signal per second Define sampling resolution. The sampling rate is the number of bits used by each sample to represent the analogue signal To calculate the storage requirements that will be needed to store sound samples you need to know both the sampling rate that will be used and the sampling resolution that will be used. Explain how to calculate the storage requirements for the sampled sound data. First you need to multiple the sampling rate by the sampling resolution. You then need to multiple the result from this calculation by the number of seconds the recording is for. So, if an audio signal is recorded at 16 bits per sample, sampled at 10 times per second and lasts for 1 minute, the storage required would be 16 x 10 x 60 (60 seconds is 1 minute). This would give a result of 9600 bits, or 1200 bytes (9600 8). 6 6

4 State the denary representation of the binary number = State the hexadecimal representation of the denary number 125. You must show your workings. First I will convert 125 into binary: = is the binary. Split this into 2 nibbles: 0111 = 7, 1101 = 13 = D So, 125 in denary = 7D in hex Give one reason why programmers often use hexadecimal, instead of binary, to represent numbers. Because it is easier for humans to understand and read than binary The ASCII character set uses seven bits to encode any character. What is the total number of characters that can be encoded in ASCII? 1 bit has 2 choices (permutations), each new bit doubles this. So, 2 bits has, 3 bits has 8 and so on. 7 bits has 128 permutations, so ASCII can encode 128 characters.

5 0 The following image represents a bitmap image where a black pixel is represented using the bit pattern 00 and a white pixel is represented using the bit pattern 01. The binary coding of each row is shown next to the image Which one of the following images has the correct encoding? Image Encoding Tick One Box ü 0. 2 State the maximum number of different colours that can be encoded when using two bits per pixel. Two bits has permutations, so this would offer a maximum of different colours State the minimum number of bits needed to encode 32 different colours. 1 bit has 2 choices (permutations), 2 bits has, 3 bits = 8, bits = 16 and 5 bits = 32. So, you would need a minimum of 5 bits to encode 32 different colours. 0. State one factor, other than the number of bits used to represent individual colours, that can affect the quality of a bitmap image. One factor that might impact upon the quality of an image file is if it is compressed using a lossy method. Another would be to lower the number of pixels being used (reduce the resolution).