Experiment 3: Line Coding - University of Wollongong
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1. There are no step by step Tasks for you to perform Instead it is left to you to ensure that in the approximate order indicated 1 You read the TIMS Datasheets for more details of the LINE CODE ENCODER and LINE CODE DECODER modules than is included here see website 2 You select a short sequence from the transmitter message source 3 At least initially you synchronize the oscilloscope to show a snapshot of the transmitter sequence This is done by triggering the oscilloscope on the SYNC output of the SEQUENCE GENERATOR In experiment 5 we will look at snapshots and eye diagrams if you want to read ahead 4 Examine each code in turn from the encoder confirming the transformation from TTL is as expected using the information available in the TIMS Datasheets You may also like to do some research and confirm that these techniques are the same or similar to those in your text books or references for this subject To do this setup one of the buffer amplifiers to have a gain of 1 Check each coding scheme without a buffer amplifier and with a buffer amplifier and comment on which coding schemes are un affected by the presence of an inversion 180 degree phase shift in the channel Also comment on what needs to be done to the received TTL signal to recover the effect of inversion eg do we need to logically invert the received TTL signal 5 Of significant interest would be an examination of the power spectra of each of the coded signals2. For this laboratory take the power spectra of the first two coding schemes NRZ L and NRZ M sketch these and compare the two plots In particular note the envelope of the spectra and the spacing between discrete components which should be related to the fundamental frequency of your line code signals ie the clock frequency resetting Resetting of the LINE CODE ENCODER and the LINE CODE DECODER after the master clock is connected or after any clock interruption is strictly not necessary for all codes But it is easier to do it for all codes rather than remember for which codes it is essential This means you have to press the reset button on the encoder followed by the reset button on the decoder You can also turn the TIMS off and then on again which will then mean that this will be required When doing that put the oscilloscope on the output of the SEQUENCE GENERATOR and the output of the LINE CODE DECODER and watch the lack of an output turn into a related signal trace there may be a phase delay For more details refer to the TIMS Advanced Modules User Manual Tutorial Questions Q1 why introduce the complications of line encoding in a digital transmission system Q2 apart from the inevitable delay introduced by the analog filter did you notice any other delays in the system You may need this information when debugging later experiments Q3 an important function of many line encoders is the elimination of the DC component Wh
3. Experiment 3 Line Coding Modified by Dr Peter Vial from EMONA original laboratory March 2011 ACHIEVEMENTS familiarity with the properties of the LINE CODE ENCODER and LINE CODE DECODER modules and the codes they generate PREREQUISITES an appreciation of the purpose behind line coding EXTRA MODULES LINE CODE ENCODER and LINE CODE DECODER PREPARATION This experiment is tutorial in nature and serves to introduce two new modules In your course work you should have covered the topic of line coding at whatever level is appropriate for you TIMS has a pair of modules one of which can perform a number of line code transformations on a binary TTL sequence The other performs decoding Why line coding There are many reasons for using line coding Each of the line codes you will be examining offers one or more of the following advantages e spectrum shaping and relocation without modulation or filtering This is important in telephone line applications for example where the transfer characteristic has heavy attenuation below 300 Hz e bit clock recovery can be simplified e DC component can be eliminated this allows AC capacitor or transformer coupling between stages as in telephone lines Can control baseline wander baseline wander shifts the position of the signal waveform relative to the detector threshold and leads to severe erosion of noise margin e error detection capabilities e bandwidth usage the possibilit
4. INECODE ENCODER NRZ L Non return to zero level bipolar this is a simple scale and level shift of the input TTL waveform NRZ M Non return to zero mark bipolar there is a transition at the beginning of each 1 and no change for a 0 The M refers to inversion on mark This is a differential code The decoder will give the correct output independently of the polarity of the input UNI RZ Uni polar return to zero uni polar there is a half width output pulse if the input is a 1 no output if the input is a 0 This waveform has a significant DC component BIP RZ Bipolar return to zero 3 level there is a half width ve output pulse if the input is a 1 or a half width ve output pulse if the input is a 0 There is a return to zero for the second half of each bit period RZ AMI Return to zero alternate mark inversion 3 level there is a half width output pulse if the input is a 1 no output if the input is a 0 This would be the same as UNI RZ But in addition there is a polarity inversion of every alternate output pulse Bid L Biphase level Manchester bipolar V volts For each input 1 there is a transition from V to V in the middle of the bit period For each input 0 there is a transition from V to V in the middle of the bit period DICODE NRZ Di code non return to zero 3 level for each transition of the input there is an output
5. en is this desirable Bibliography 1 Lender A Thue Duobinary Technique for High Speed Data Transmission IEEE Trans Comm Electron vol 82 pp 214 218 May 1963
6. ference You do not need to do this to complete this laboratory however duobinary encoding A duobinary encoder and decoder is included in the line code modules Duobinary encoding is also called correlative coding or partial response signalling The precoded duobinary encoding model implemented in the LINE CODE ENCODER module is described in the TIMS Advanced Modules User Manual see website Experiment Figure 3 shows a simplified model of Figure 1 There is no source encoding or decoding no baseband channel and no detection For the purpose of the experiment this is sufficient to confirm the operation of the line code modules change polarity ext trig SEQUENCE LIME CODE BUFFER AMPLIFERS 10 TTL out re timed bit clock 8 333 kHz from 2 083 kHz bit clock MASTER SIGNALS Figure 3 simplified model of Figure 1 When a particular code has been set up and the message successfully decoded without error the BUFFER should be included in the transmission path By patching it in or out it will introduce a polarity change in the channel If there is no change to the message output then the code in use is insensitive to polarity reversals Note that the LINE CODE DECODER requires for successful decoding an input signal of amplitude near the TIMS ANALOG REFERENCE LEVEL 2 volt peak peak In normal applications this is assured since it will obtain its input from the DECISION MAKER Procedure
7. ing decoder at the receiver These are included in the block diagram of Figure 1 which is of a typical baseband digital transmission system It shows the disposition of the LINECODE ENCODER and LINE CODE DECODER All bandlimiting is shown concentrated in the channel itself but could be distributed between the transmitter channel and receiver TTL 1 PA i LINE SOURCE MESSAGE SOURCE CODE CODE DECODER SOURCE ENCODER DECODER TRANSMITTER CHANNEL RECEIVER Figure 1 baseband transmission system The LINE CODE ENCODER serves as a source of the system bit clock It is driven by a master clock at 8 333 kHz from the TIMS MASTER SIGNALS module It divides this by a factor of four in order to derive some necessary internal timing signals at a rate of 2 083 kHz This then becomes a convenient source of a 2 083 kHz TTL signal for use as the system bit clock Because the LINE CODE DECODER has some processing to do it introduces a time delay To allow for this it provides a re timed clock if required by any further digital processing circuits eg for decoding or error counting modules Terminology e The word mark and its converse space often appear in a description of a binary waveform This is an historical reference to the mark and space of the telegraphist In modern day digital terminology these have become HI and LO or 1 and 0 as appropriate e unipolar signalling where a 1 is represented with a finite voltage V volt
8. pulse of opposite polarity from the preceding pulse For no transition between input pulses there is no output The codes offered by the line code encoder are illustrated in Figure 2 below These have been copied from the Advanced Module Users Manual where more detail is provided input DATA 0 1 0 00 O 141 4 1 0 0 141 90 5V TTL Figure 2 TIMS line codes The output waveforms apart from being encoded have all had their amplitudes adjusted to suit a TIMS analog channel not explicitly shown in Figure 2 When connected to the input of the LINE CODE DECODER these waveforms are de coded back to the original TTL sequence band limiting No matter what the line code in use it is not uncommon to bandlimit these waveforms before they are sent to line or used to modulate a carrier As soon as bandlimiting is invoked individual pulses will spread out in the time domain and interfere with adjacent pulses This raises the issue of inter symbol interference ISI A study of ISI is outside the intended scope of this text but it cannot be ignored in practice Bandlimiting by pulse shaping can be effected and ISI controlled by appropriate filter design called channel equalisers An alternative approach duobinary encoding was invented by Lender 1 As an exercise outside the laboratory you might go to the IEEE Explore electronic database available on the University of Wollongong Library website and briefly read this re
9. s and a 0 with zero voltage This seems to be a generally agreed to definition e those who treat polar and bipolar as identical define these as signaling where a 1 is sent as V and 0 as V They append AMI when referring to three level signals which use V and V alternately for a 1 and zero for 0 an alternative name is pseudoternary You will see the above usage in the TIMS Advanced Modules User Manual as well as in this text However others make a distinction Thus e polar signalling where a 1 is represented with a finite voltage V volts and a 0 with V volts e bipolar signalling where a 1 is represented alternately by V and V and a 0 by zero voltage e the term RZ is an abbreviation of return to zero This implies that the particular waveform will return to zero for a finite part of each data 1 typically half the interval The term NRZ is an abbreviation for non return to zero and this waveform will not return to zero during the bit interval representing a data 1 e the use of L and M would seem to be somewhat illogical or inconsistent with each other For example see how your text book justifies the use of the L and the M in NRZ L and NRZ M e two sinusoids are said to be antipodal if they are 1800 out of phase available line codes For a TTL input signal the following output formats are available from the L
10. y of transmitting at a higher rate than other schemes over the same bandwidth At the very least the LINE CODE ENCODER serves as an interface between the TTL level signals of the transmitter and those of the analog channel Likewise the LINE CODE DECODER serves as an interface between the analog signals of the channel and the TTL level signals required by the digital receiver The modules The two new modules to be introduced are the LINE CODE ENCODER and the LINE CODE DECODER You will not be concerned with how the coding and decoding is performed You should examine the waveforms using the original TTL sequence as a reference In a digital transmission system line encoding is the final digital processing performed on the signal before it is connected to the analog channel although there may be simultaneous bandlimiting and wave shaping Thus in TIMS the LINE CODE ENCODER accepts a TTL input and the output is suitable for transmission via an analog channel At the channel output is a signal at the TIMS ANALOG REFERENCE LEVEL or less It could be corrupted by noise Here it is re generated by a detector The TIMS detector is the DECISION MAKER module already examined in the experiment entitled Detection with the DECISION MAKER in this Volume Finally the TIMS LINE CODE DECODER module accepts the output from the DECISION MAKER and decodes it back to the binary TTL format Preceding the line code encoder may be a source encoder with a match
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