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LINE CODINGLINE CODING LINE CODINGLINE CODING
- Line coding Vol D1, ch 5, rev 1.0 -
- PREPARATION.................................................................................
- why line coding?
- the modules
- terminology ..............................................................................................
- available line codes
- NRZ-L......................................................................................................
- NRZ-M.....................................................................................................
- UNI-RZ ....................................................................................................
- BIP-RZ.....................................................................................................
- RZ-AMI ...................................................................................................
- Biฯ-L........................................................................................................
- DICODE-NRZ .........................................................................................
- band limiting
- duobinary encoding..................................................................................
- EXPERIMENT
- procedure...................................................................................
- TUTORIAL QUESTIONS
42 โ D1 Line coding
LINE CODINGLINE CODING^ LINE CODINGLINE CODING
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
PREPARATIONPREPARATIONPREPARATIONPREPARATION
This โexperimentโ 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 what ever 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 codingwhy line codingwhy line codingwhy 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:
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. bit clock recovery can be simplified. 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). error detection capabilities. bandwidth usage ; the possibility 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
44 โ D1 Line coding
terminologyterminology^ terminologyterminology
- 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 โ1โ and โ0โ, as appropriate.
- unipolar signalling : where a โ1โ is represented with a finite voltage V volts, and a โ0โ with zero voltage. This seems to be a generally agreed-to definition.
- those who treat polar and bipolar as identical define these as signalling where a โ1โ is sent as +V, and โ0โ as -V. They append AMI when referring to three-level signals which use +V and -V alternately for a โ1โ, and zero for โ0โ (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:
- polar signalling : where a โ1โ is represented with a finite voltage +V volts, and a โ0โ with -V volts.
- bipolar signalling : where a โ1โ is represented alternately by +V and -V, and a โ0โ by zero voltage.
- the term โRZโ is an abbreviation of โreturn to zeroโ. This implies that the particular waveform will return to zero for a finite part of each data โ1โ (typically half the interval). The term โNRZโ is an abbreviation for โnon-return to zeroโ, and this waveform will not return to zero during the bit interval representing a data โ1โ.
- the use of โLโ and โMโ would seem to be somewhat illogical (or inconsistent) with each other. For example, see how your text book justifies the use of the โLโ and the โMโ in NRZ-L and NRZ-M.
- two sinusoids are said to be antipodal if they are 180 0 out of phase.
available line codesavailable line codesavailable line codesavailable line codes
For a TTL input signal the following output formats are available from the LINE- CODE ENCODER.
NRZ-LNRZ-L NRZ-LNRZ-L
Non return to zero - level (bipolar) : this is a simple scale and level shift of the input TTL waveform.
NRZ-MNRZ-M NRZ-MNRZ-M
Non return to zero - mark (bipolar): there is a transition at the beginning of each โ1โ, and no change for a โ0โ. The โMโ refers to โinversion on markโ. This is a
Line coding D1 - 45
differential code. The decoder will give the correct output independently of the polarity of the input.
UNI-RZUNI-RZ UNI-RZUNI-RZ
Uni-polar - return to zero (uni-polar): there is a half-width output pulse if the input is a โ1โ; no output if the input is a โ0โ. This waveform has a significant DC component.
BIP-RZBIP-RZ BIP-RZBIP-RZ
Bipolar return to zero (3-level): there is a half-width +ve output pulse if the input is a โ1โ; or a half-width -ve output pulse if the input is a โ0โ. There is a return-to-zero for the second half of each bit period.
RZ-AMIRZ-AMI RZ-AMIRZ-AMI
Return to zero - alternate mark inversion (3-level): there is a half-width output pulse if the input is a โ1โ; no output if the input is a โ0โ. This would be the same as UNI- RZ. But, in addition , there is a polarity inversion of every alternate output pulse.
BiBi BiBiฯฯฯฯ-L-L-L-L
Biphase - level (Manchester): bipolar ยฑV volts. For each input โ1โ there is a transition from +V to -V in the middle of the bit-period. For each input โ0โ there is a transition from -V to +V in the middle of the bit period.
DICODE-NRZDICODE-NRZ DICODE-NRZDICODE-NRZ
Di-code non-return to zero (3-level) : for each transition of the input there is an output 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.
Line coding D1 - 47
EXPERIMENTEXPERIMENTEXPERIMENTEXPERIMENT
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.
2.083 kHz bit clock
ext. trig.
8.333 kHz from MASTER SIGNALS
TTL out re-timed bit clock
change polarity
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 (ยฑ2 volt pp). In normal applications this is assured, since it will obtain its input from the DECISION MAKER.
procedureprocedureprocedureprocedure
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):
- you read the TIMS Advanced Modules User Manual for more details of the LINE-CODE ENCODER and LINE-CODE DECODER modules than is included here.
- you select a short sequence from the transmitter message source
48 โ D1 Line coding
- at least initially you synchronize the oscilloscope to show a snapshot of the transmitter sequence. Later you may be interested in eye patterns?
- examine each code in turn from the encoder, confirming the transformation from TTL is as expected. On the other hand, and far more challenging, is to determine what the law of each transformation is without help from a Textbook or other reference.
- of significant interest would be an examination of the power spectra of each of the coded signals. For this you would need data capturing facilities, and software to perform spectral analysis.
- (^) and so on .............
resettingresettingresettingresetting
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. For more details refer to the TIMS Advanced Modules User Manual.
TUTORIAL QUESTIONSTUTORIAL QUESTIONS TUTORIAL QUESTIONSTUTORIAL 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. When is this desirable?
