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line coding - LINE CODING 2007 Mobasseri ECE 3770 1...

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©2007 Mobasseri ECE 3770 1 LINE CODING
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©2007 Mobasseri ECE 3770 2 Statement of the Problem On the way from transmitter to the receiver, we are now here… Two questions: How to pick pulse sequence ? How to pick pulse shape ?s Source encoder Analog message 1 0 0 1 1... ?
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©2007 Mobasseri ECE 3770 3 Picking a pulse sequence: line coding There are numerous ways we can convert a string of logical 1’s and 0’s to a sequence of pulses. But what are the ground rules? Example: convert 1 0 1 1 0 1 1 1 1 1 0 0 1 0 1 1 0 1 OR
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©2007 Mobasseri ECE 3770 4 Line coding concerns Here are what we have to know before making a selection Presence or absence of DC level Spectrum at DC Bandwidth Built-in clock recovery Built-in error detection Transparency
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©2007 Mobasseri ECE 3770 5 DC level Some line codes have a DC level some don’t. DC levels are inherently wasteful of power. Also, their DC levels will be stripped over links with transformers or capacitor coupled repeaters 1 1 1 1 0 0 1 0 1 1 0 1 DC level 0 DC
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©2007 Mobasseri ECE 3770 6 Spectrum at DC This is related to the DC level. Generally we want to have the spectrum to be 0 at f=0. Finite spectrum at f=0, indicates partial presence of a DC level
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©2007 Mobasseri ECE 3770 7 Bandwidth A most critical component. We must be able to compare the bandwidth taken up by different line codes while transmitting a fixed number of pulses per second. B B
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©2007 Mobasseri ECE 3770 8 Clock recovery Knowing the beginning of pulses at the receiver is critical in bit detection. Otherwise you don’t know where you are or what you are looking at. Can you tell where a pulse ends and where the next one begins?
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