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# Chapter 3

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### Chapter 3

1. 1. Chapter 3 - Sampling Converting the Real World to the Digital World…
2. 2. We know that analogue signals constantly change and digital signals have a value of 1 or 0.
3. 3. So how do we get digital signals? <ul><li>Digital signals are produced from the ‘real’ analogue waveform </li></ul><ul><li>Digital codes represent analogue signals and therefore have to be reconstructed back into analogue signals. </li></ul>Original Reconstructed 0110101100010101
4. 4. Sampling <ul><li>The value of an analogue signal at a point is called a sample. </li></ul>Sample
5. 5. Variables in sampling <ul><li>The following can be altered when sampling a analogue waveform: </li></ul><ul><ul><li>Sampling frequency – this is how many times a sample is taken every second. </li></ul></ul><ul><ul><li>Resolution – the number of bits used to record the value of a sample </li></ul></ul>
6. 6. Sampling Frequency <ul><li>The number of times that a sample is taken per second will affect the quality of the digitised information. i.e…. </li></ul>
7. 7. Sampling Frequency and Waveforms <ul><li>The sampling rate needs to be more than double the highest frequency component of the waveform </li></ul><ul><li>If not, aliasing can occur – this means that the reconstructed wave is not representative of the original </li></ul>
8. 9. Hearing Sampling…
9. 10. Resolution <ul><li>The resolution is the number of ‘bits’ used to represent a value of a sample. </li></ul><ul><ul><li>1 bit has 2 values, 0 or 1 = 2 1 </li></ul></ul><ul><ul><li>2 bits has 4 values 0, 1, 2, 3 = 2 2 </li></ul></ul><ul><ul><li>3 bits has 8 values, 0, 1, 2, 3, 4, 5, 6, 7 = 2 3 </li></ul></ul><ul><li>The more values, the more accurate the digital sample. Note that 8 bits = a byte </li></ul><ul><li>Draw a table of Number of Bits, Number of Levels and power of 2 – see page 65 for help. </li></ul>
10. 11. 2 1 6 65536 16 … … … 2 9 512 9 2 8 256 8 2 7 128 7 2 6 64 6 2 5 32 5 2 4 16 4 2 3 8 3 2 2 4 2 2 1 2 1 2 n Number of Levels Number of Bits
11. 12. Redundancy <ul><li>To prevent errors in sending digital signals, more information is sent. </li></ul><ul><li>This can be seen in a CD with a scratch or hole in it… </li></ul>Jarrow Sings the Blues MFI Records 1934