Experiment 6
Contents
1. EMBEDFS V PCMDATA 7 CLK i Figure 4 Front panel layout of the PCM DECODER PCM EE 400L Coding University of Nevada Las Vegas Experiment 4 Decoding Department of Electrical and Computer Engineering 2 Sbreak Fl P ROUT PCM EE 400L Coding University of Nevada Las Vegas Experiment 4 Decoding Department of Electrical and Computer Engineering EXPERIMENT 1 Sampling with Sample and Hold There is a stand alone SAMPLE AND HOLD sub system in the INTEGRATE amp DUMP module This will be used in the present experiment TI acquire an INTEGRATE amp DUMP module This is a multi purpose module Within it is a sub system which performs sample and hold operations Before plugging it in set the on board switch SWI to the S amp H 1 position 0 Analog signals connected to the input socket labeled I amp D 1 will now undergo a sample and hold S amp H 1 operation the result appearing at the I amp D 1 output socket Ignore the duplicate S amp H 2 option available at the I amp D 2 sockets T2 patch up the module according to Figure 6 below MASTER INTEGRATE SIGNALS and DUMP 10CkHz sing 1OOKHz cos 103kHz TTL sorpk TIL message sine Figure 6 TIMS model For a stable view of both input and output it is convenient to use a message which is a submultiple of the sample clock frequency Thus use the 2 03 kHz message sine wave from the MASTER S2. 0000 so only the embedded alternating 0 and 1 bits for remote FS in the LSB position should be seen Confirm this They should be 1920ms apart Confirm this both by measurement and calculation PCM EE 400L Coding University of Nevada Las Vegas Experiment 4 Decoding Department of Electrical and Computer Engineering T37 vary the DC output and show the appearance of new patterns on CH1 A When finished return the DC to its maximum negative value control fully anti clockwise The PCM signal is now ready for transmission In a later experiment the PCM signal will be sent via a noisy band limited channel For the present it will be connected directly to a TIMS PCM DECODER module Receiver decoder T38 use the front panel toggle switch to select the 4 bit LINEAR decoding scheme to match that of the transmitter T39 steal an 8 333 kHz TTL clock signal from the transmitter and connect it to the CLK input T40 in the first instance steal the frame synchronization signal FS from the transmitter by connecting it to the frame synchronization input FS of the receiver At the same time ensure that the FS SELECT toggle switch on the receiver is set to EXT FS T41 ensure both channels of the oscilloscope are set to accept DC set their gains to 1 volt em With their inputs grounded set their traces in the center of their respective halves of the screen Remove the grounds T42 conne
3. PCM EE 460L Coding University of Nevada Las Vegas Experiment 4 Decoding Department of Electrical and Computer Engineering PREPARATION 1 Sampling with Sample and Hold Before it is possible to transmit analog information via a digital system the analog signal must first be transformed into a digital format The first step in such a transformation typically involves a sampling process Natural sampling Natural sampling of an analog waveform message is examined in the experiment entitled sampling and TDM Natural sampling takes a slice of the waveform and the top of the slice preserves the shape of the waveform Flat top sampling A very common and easily implemented method of sampling of an analog signal uses the sample and hold operation This produces flat top samples Flat top sampling takes a slice of the waveform but cuts off the top of the slice horizontally The top of the slice does not preserve the shape of the waveform Figure 1 below contrasts the two methods Figure 1 Natural sampling above and flat top below Message reconstruction by low pass filtering In the experiment entitled sampling and TDM a simple analysis showed that there was no distortion of the message when reconstruction was implemented by low pass filtering PCM EE 460L Coding University of Nevada Las Vegas Experiment 4 Decoding Department of Electrical and Computer Engineering It will now be declared as a
4. Yes and no This would indeed be wide enough to pass the message but it would not be wide enough to pass any harmonic distortion components But the filter passband could not be made wider than half the sampling frequency else the Nyquist criterion would be violated and that is not much more than the current message frequency So something has to be changed Is a synchronous message necessary Not any longer after having seen the stationary sample and hold waveform So why not use an AUDIO OSCILLATOR set to its lowest frequency about 300 Hz and the 3 kHz LPF within the HEADPHONE AMPLIFIER module This would give plenty of room for any distortion components to appear at the output However unless they are of significant amplitude they may not be visible on the oscilloscope PCM EE 400L Coding University of Nevada Las Vegas Experiment 4 Decoding Department of Electrical and Computer Engineering T6 do as suggested above Use the oscilloscope to view both the input and output sine waves simultaneously Synchronize the oscilloscope externally to the source of the message As an engineering estimate if the distortion is not obvious then one could say the signal to distortion ratio is better than 30 dB probably better than 40 dB As well as one can judge the two waveforms are identical Could you estimate the amount of distortion introduced by the reconstruction process If there was visible distortion
5. 1 pattern embedded as the LSB of each frame It is enabled by use of the FS SELECT front panel toggle switch Currently this is set to EXT FS 22 PCM EE 460L Coding University of Nevada Las Vegas Experiment 6 Decoding Department of Electrical and Computer Engineering T53 change the FS SELECT switch on the front panel of the PCM DECODER module from EXT FS to EMBED Notice that frame synchronization is re established after a short time Could you put an upper limit on this time 23 PCM EE 460L Coding University of Nevada Las Vegas Experiment 6 Decoding Department of Electrical and Computer Engineering TUTORIAL QUESTIONS Q1 assuming a sine wave is accompanied by a small third harmonic component how large would this have to be before its presence could be detected using only an oscilloscope This question would not please the purists because it raises more questions than it asks But attempt an answer You could even set up the signal using TIMS and demonstrate your reply Q2 define the slot bandwidth of a low pass filter Redefine the Nyquist criterion in terms of practical filter characteristics Q3 sample and hold flat top sampling can be shown to introduce distortion of the message if it is reconstructed by using a low pass filter alone From your general reading or otherwise is it possible to eliminate this distortion by further message processing hint key words are
6. then one should check the 3 kHz LPF reconstruction filter does it introduce its own distortion Compare the message shape before sampling but via this filter as well as after reconstruction Could you attempt to measure the mount of distortion The unwanted components will probably be hidden in the noise level meaning the signal to distortion ratio is much better than 40 dB Two tone test signal Testing for distortion with a single sine wave is perhaps not demanding enough Should you try a two tone test signal With the 3 kHz LPF as the reconstruction filter and an 8 333 kHz sample rate there should be no sign of aliasing distortion To demonstrate aliasing distortion T7 replace the 8 333 kHz sampling signal from the MASTER SIGNALS module with the TTL output from a VCO Monitor the VCO frequency with the FREQUENCY COUNTER Starting with the VCO set to its highest frequency on the LO range about 15 kHz slowly reduce it while watching the reconstructed message wave shape As soon as distortion is evident note the VCO frequency Knowing the reconstruction filter amplitude characteristic how does this agree with the Nyquist criterion PCM EE 400L Coding University of Nevada Las Vegas Experiment 4 Decoding Department of Electrical and Computer Engineering 2 PCM Encoding The only module required for this part of experiment is a TIMS PCM ENCODER It is not necessary for this experiment to become involv
7. would need more sophisticated equipment than is assumed here a digital analyzer a storage oscilloscope ability to capture a single frame and so on to deduce the coding and quantizing scheme from such an input signal PCM EE 400L Coding University of Nevada Las Vegas Experiment 4 Decoding Department of Electrical and Computer Engineering 3 PCM Decoding Transmitter encoder A suitable source of PCM signal will be generated using a PCM ENCODER module This module was examined in pervious part You should set it up before patching up the demodulator T31 before plugging in PCM ENCODER module set the toggles of the on board SYNC MESSAGE switch SW2 Set the left hand toggle DOWN and the right hand toggle UP This selects a 130 Hz sinusoidal message which will be used later Now insert the module into the TIMS system T32 use the 8 333 kHz TTL signal from the MASTER SIGNALS module for the CLK T33 select with the front panel toggle switch the 4 bit LINEAR coding scheme T34 synchronize the oscilloscope externally to the frame synchronization signal at FS Set the sweep speed to 0 5 ms cm say This should show a few frames on the screen T35 connect CH1 A of the SCOPE SELECTOR to the PCM OUTPUT of the PCM ENCODER T36 we would like to recognize the PCM DATA out signal So choose a large negative DC for the message from the VARIABLE DC module From previous work we know the corresponding code word is
8. IGNALS module together with the 8 333 kHz TTL clock PCM EE 400L Coding University of Nevada Las Vegas Experiment 4 Decoding Department of Electrical and Computer Engineering T3 select a sweep speed to show two or three periods of the message say 0 1 ms cm Set equal gains of both channels say 1 volt cm With the patching shown in Figure 6 you might expect to obtain oscilloscope displays similar to that of Figure 2 Try it T4 note the output from the socket labeled READY Sketch it with respect to the clock and output signal showing time relationships There is a processing delay within the sample and hold sub system As a result the two displays will be shifted relative in time The ready signal occurs within the time during which the sample is available and could be used to signal analog to digital A D circuitry to start a conversion Message reconstruction Now that you have seen a sample and hold operation you are ready to reconstruct the message from it This is a low pass filtering operation T5 use a TUNEABLE LPF module to reconstruct the message Decide on then set a suitable bandwidth Report your findings Then read on To what passband width did you set the filter Remember you are looking for any possible distortion components introduced by the sample and hold operation and then the reconstruction process Since the message is at 2 03 kHz a passband of 3 kHz would be wide enough
9. Vegas Experiment 4 Decoding Department of Electrical and Computer Engineering 4 bit data format From measurements made so far you should be able to answer the questions e what is the sampling rate e what is the frame width e what is the width of a data bit e what is the width of a data word e how many quantizing levels are there e are the quantizing levels uniformly linearly spaced 7 bit linear encoding T24 change to 7 bit linear encoding by use of the front panel toggle switch It would take a long time to repeat all of the above Tasks for the 7 bit encoding scheme Instead T25 make sufficient measurements so that you can answer all of the above questions in the section titled 4 bit data format above Making one or two assumptions such as you Should be able to deduce the coding scheme used Companding This module is to be used in conjunction with the PCM DECODER in next part As a pair they have a companding option There is compression in the encoder and expansion in the decoder In the encoder this means the quantizing levels are closer together for small input amplitudes that is in effect that the input amplitude peaks are compressed during encoding At the decoder the reverse action is introduced to restore an approximate linear input output characteristic It can be shown that this sort of characteristic offers certain advantages especially when the message has a high peak to average amplitude chara
10. and Computer Engineering e Vin the analog signal to be encoded e PCM DATA the output data stream the examination of which forms the major part of this experiment e CLK this is a TTL red input and serves as the MASTER CLOCK for the module Clock rate must be 10 kHz or less For this experiment you will use the 8 333 kHz TTL signal from the MASTER SIGNALS module TIMS PCM time frame Each binary word is located in a time frame The time frame contains eight slots of equal length and is eight clock periods long The slots from first to last are numbered 7 through 0 These slots contain the bits of a binary word The least significant bit LSB is contained in slot 0 The LSB consists of alternating ones and zeros These are placed embedded in the frame by the encoder itself and cannot be modified by the user They are used by subsequent decoders to determine the location of each frame in the data stream and its length The remaining seven slots are available for the bits of the binary code word Thus the system is capable of a resolution of seven bits maximum This resolution for purposes of experiment can be reduced to four bits by front panel switch The 4 bit mode uses only five of the available eight slots one for the embedded frame synchronization bits and the remaining four for the binary code word in slots 4 3 2 and 1 3 PCM DECODING The signal that you will generate using the PCM ENCODER modul
11. aperture effect sinx x correction Q4 from your knowledge of the PCM ENCODER module obtained during preparation for the experiment calculate the sampling rate of the analog input signal Show that it is the same for both the 4 bit and the 7 bit coding schemes What can you say about the bandwidth of an input analog signal to be encoded Q5 define what is meant by the data frame in this experiment Draw a diagram showing the composition of a frame for a The 4 bit coding scheme b The 7 bit coding scheme 24 PCM EE 460L Coding University of Nevada Las Vegas Experiment 6 Decoding Department of Electrical and Computer Engineering Q6 it is possible to transmit each frame at a much slower rate than it was produced and of course recover the original message as well Explain how this might be done When might this be an advantage Q7 explain why a DC message gives a stable oscilloscope display of the PCM DATA output Why is the display unstable when a sine wave for example is the message Q8 two PCM signals can be combined to produce a time division multiplexed PCM TDM signal With the measurements so far performed this does not seem and indeed is not possible with two PCM ENCODER modules Why is this so Suggest what changes could be made to the module to implement PCM TDM Q9 in the present experiment a stolen clock signal was used Why would transmission of the PCM signal via a band limit
12. bly be recovered from this waveform But it would be difficult to predict with what accuracy Low pass filtering of the waveform at the output of the decoder will reconstruct the message although theory shows that it will not be perfect It will improve with the number of quantizing levels What amplitude characteristic is required for the reconstruction filter If any distortion components are present they would most likely include harmonics of the message If these are to be measurable visible on the oscilloscope in the present case then they must not be removed by the filter and so give a false indication of performance So we could look for harmonics in the output of the filter But we do not have conveniently available a spectrum analyzer An alternative is to use a two tone test message Changes to its shape especially its envelope are an indication of distortion and are more easily observed with an oscilloscope than when a pure sine wave is used It will be difficult to make one of these for this experiment because our messages have been restricted to rather low frequencies which are outside the range of most TIMS modules 21 PCM EE 460L Coding University of Nevada Las Vegas Experiment 6 Decoding Department of Electrical and Computer Engineering But there is provided in the PCM ENCODER a message with a shape slightly more complex than a sine wave It can be selected with the switch SW2 on the encoder
13. circuit board Set the left hand toggle UP and the right hand toggle DOWN See the Appendix to this experiment for more details A message reconstruction LPF is installed in the PCM DECODER module version 2 and above If you do not have such a module then bypass the next two Tasks T50 change to the complex message from the PCM ENCODER as described above T51 connect PCM DECODER output to tunable LPF Set tunable LPF to low cutoff frequency and observe the reconstructed message Make comparisons between the 4 bit linear and the 7 bit linear coding schemes Try different message amplitudes into the PCM ENCODER Can you observe any distortion Record your observations If you think the LPF itself might have introduced some distortion you could check by connecting the complex message to its input direct and observing the output Companding It is now time to verify the companding algorithm installed in the encoder T52 use the front panel toggle switches on both modules to select 4 bit companding Use both low and high level messages into the PCM ENCODER Check the quantizing characteristic Record your observations and comment upon them Frame synchronization In all of the above work the frame synchronization signal FS has been stolen from the encoder as has been the clock signal This was not necessary The PCM ENCODER has circuitry for doing this automatically It looks for the alternating 0 and
14. ct CH2 A to the sample and hold output of the PCM DECODER DC message You are now ready to check the overall transmission from transmitter input to decoder output The message is a DC signal T43 connect the PCM DATA output signal from the transmitter to the PCM DATA input of the receiver PCM EE 460L Coding University of Nevada Las Vegas Experiment 6 Decoding Department of Electrical and Computer Engineering T44 slowly vary the DC output from the VARIABLE DC module back and forth over its complete range Observe the behavior of the two traces The input to the encoder moves continuously The output from the decoder moves in discrete steps These are the 16 amplitude quantizing steps of the PCM ENCODER You are observing the source of quantizing noise The output can take up only one of 16 predetermined values T45 draw up a table relating input to output voltages You can now see the number of quantizing levels at the transmitter and their values T46 compare the quantizing levels just measured with those determined in the experiment entitled PCM encoding T47 reset the coding scheme on both modules to 7 bit Sweep the input DC signal over the complete range as before Notice the granularity in the output is almost un noticeable compared with the 4 bit case There are now 2 rather than 2 steps over the range Periodic message It was not possible when examining the PCM ENCODER in the experiment
15. cteristic as does speech and where the signal to noise ratio is not high PCM EE 400L Coding University of Nevada Las Vegas Experiment 4 Decoding Department of Electrical and Computer Engineering This improvement will not be checked in this part But the existence of the non linear quantization in the encoder will be confirmed In the next part entitled PCM decoding it will be possible to check the input output linearity of the modules as a compatible pair T26 change to 4 bit companding by use of the front panel toggle switch T27 the TIMS A4 companding law has already been selected first Task Make the necessary measurements to determine the nature of the law Periodic messages Although the experiment is substantially complete you may have wondered why a periodic message was not chosen at any time Try it T28 take a periodic message from the SYNC MESSAGE socket This was set as the second Task T29 adjust the oscilloscope to display the message Record its frequency and shape Check if these are compatible with the Nyquist criterion adjust the amplitude if necessary with one of the BUFFER AMPLIFIERS T30 now look at the PCM DATA output Synchronize the oscilloscope as previously to the frame FS signal Display two or three frames on CH1 A and the PCM DATA output on CH2 A You will see that the data signal reveals very little It consists of many overlaid digital words all different One
16. duration of the frame under examination PCM EE 460L Coding University of Nevada Las Vegas Experiment 6 Decoding Department of Electrical and Computer Engineering 6 Message reconstruction can be achieved albeit with some distortion by low pass filtering A built in reconstruction filter is provided in the module Encoding At the encoder the sample and hold operation before encoding is executed periodically It produces a rectangular pulse form Each pulse in the waveform is of exactly the same amplitude as the message at the sampling instant But it is not possible to recover a distortionless message from these samples They are flat top rather than natural samples Call this the sampling distortion At the encoder the amplitude of this waveform was then quantized It is still a rectangular pulsed waveform but the amplitude of each pulse will in general be in error by a small amount Call this waveform s t Decoding The voltage at Vou of the decoder is identical with s t above The decoder itself has introduced no distortion of the received signal But s t is already an inexact version of the sample and hold operation at the encoder This will give rise to quantization distortion as well as the sampling distortion already mentioned You should read about these phenomena in a Text book TIMS PCM DECODER module A TIMS PCM DECODER module will be used for decoding The front panel of this mod
17. e will be decoded in this experiment by TIMS PCM Decoder Module A clock synchronization signal will be stolen from the encoder In the PCM DECODER module there is circuitry which automatically identifies the location of each frame in the serial data stream To do this it collects groups of eight data bits and looks for the repeating pattern of alternate ones and zeros placed there embedded by the PCM ENCODER in the LSB position It can be shown that such a pattern cannot occur elsewhere in the data stream provided that the original band limited analog signal is sampled at or below the Nyquist rate When the embedded pattern is found an end of frame synchronization signal FS is generated and made available at the front panel PCM EE 400L Coding University of Nevada Las Vegas Experiment 4 Decoding Department of Electrical and Computer Engineering The search for the frame is continuously updated Why Under noisy conditions not relevant for this particular experiment the reliability of the process will depend upon the size of the group of frames to be examined This can be set by the on board switch SW3 of the PCM DECODER module See Table A 2 in the Appendix to this experiment for details Stolen Frame synchronization can also be achieved of course by stealing the synchronization signal FS from the PCM ENCODER module Use of this signal would assume that the clock signal to the PCM DECODER is of the c
18. ed channel necessitate phase adjustment of this stolen clock signal to the PCM DECODER Q10 sketch the waveforms at the output Vou from the decoder for the 4 bit and the 7 bit linear encoding scheme and a large amplitude sinusoidal synchronous message at the encoder A sketch might show these as being the same but a more accurate drawing would show more clearly the difference Explain Q11 two sources of distortion of the reconstructed message have been identified they were called sampling distortion and quantizing distortion a Assuming a sample and hold type sampler what can be done about minimizing sampling distortion b What can be done about minimizing quantizing distortion Q12 quantizing distortion decreases with the number of quantizing levels available There is usually a price to be paid for such an option What would this be Was that apparent in the present experiment Explain 25 PCM EE 460L Coding University of Nevada Las Vegas Experiment 6 Decoding Department of Electrical and Computer Engineering APPENDIX For a MASTER CLOCK of 8 333 kHz Table A 1 below gives the frequencies of the synchronized message at the SYNC MESSAGE output for the setting of the on board switch SW2 For other clock frequencies the message frequency can be calculated by using the divide by entry in the Table These messages are periodic but not necessarily sinusoidal in shape The term sinuo
19. ed with how the PCM ENCODER module achieves its purpose What will be discovered is what it does under various conditions of operation Before plugging the module in T8 select the TIMS companding A4 law with the on board COMP jumper in preparation for a later part of the experiment T9 locate the on board switch SW2 Put the LEFT HAND toggle DOWN and the RIGHT HAND toggle UP This sets the frequency of a message from the module at SYNC MESSAGE This message is synchronized to a sub multiple of the MASTER CLOCK frequency Patching up To determine some of the properties of the analog to digital conversion process it is best to start with a DC message This ensures completely stable oscilloscope displays and enables easy identification of the quantizing levels Selecting the 4 bit encoding scheme reduces the number of levels 2 to be examined T10 insert the module into the TIMS frame Switch the front panel toggle switch to 4 BIT LINEAR i e no companding T11 patch the 8 333 kHz TTL SAMPLE CLOCK from the MASTER SIGNALS module to the CLK input of the PCM ENCODER module T12 connect the Vininput socket to ground of the variable DC module T13 connect the frame synchronization signal FS to the oscilloscope ext synch input PCM EE 400L Coding University of Nevada Las Vegas Experiment 4 Decoding Department of Electrical and Computer Engineering T14 on CH1 A display the frame synchronization signal FS Ad
20. ent of Electrical and Computer Engineering Knowing 1 The number of slots per frame is 8 2 The location of the least significant bit is coincident with the end of the frame 3 The binary word length is four bits 4 The first three slots are empty in fact filled with zeros but these remain unchanged under all conditions of the 4 bit coding scheme then T18 identify the binary word in slots 4 3 2 and 1 Quantizing levels for 4 bit linear encoding You will now proceed to determine the quantizing encoding scheme for the 4 bit linear case T19 remove the ground connection and connect the output of the VARIABLE DC module to Vin Sweep the DC voltage slowly backwards and forwards over its complete range and note how the data pattern changes in discrete jumps T20 if you have a WIDEBAND TRUE RMS METER module use this to monitor the DC amplitude at Vin otherwise use the oscilloscope CH1 B Adjust Vin to its maximum negative value Record the DC voltage and the pattern of the 4 bit binary number T21 slowly increase the amplitude of the DC input signal until there is a sudden change to the PCM output signal format Record the format of the new digital word and the input amplitude at which the change occurred T22 continue this process over the full range of the DC supply T23 draw a diagram showing the quantizing levels and their associated binary numbers PCM EE 400L Coding University of Nevada Las
21. entitled PCM encoding to see the sample and hold waveform within the encoder But you have just been looking at it assuming perfect decoding at the output of the decoder With a periodic message its appearance may be more familiar to you T48 turn back to 4 bit mode Change to a periodic message by connecting the SYNC MESSAGE of the PCM ENCODER via a BUFFER AMPLIFIER to its input Vin An amplitude of 2 Vpp is suitable Slow down the oscilloscope sweep speed to 1 ms cm Observe and record the signal at CH2 A 20 PCM EE 460L Coding University of Nevada Las Vegas Experiment 6 Decoding Department of Electrical and Computer Engineering When you agree that what you see is what you expected to see prepare to make a change and predict the outcome Currently the encoding scheme is generating a 4 bit digital word for each sample What would be the change to the waveform now displaying on CH2 A if at the encoder the coding scheme was changed from 4 bit to 7 bit Sketch your answer to this question show the waveform before and then after the change T49 change the coding scheme from 4 bit to 7 bit That is change the front panel toggle switch of both the PCM ENCODER and the PCM DECODER from 4 bit to 7 bit Observe record and explain the change to the waveform on CH2 A When satisfied proceed Message reconstruction You can see qualitatively that the output is related to the input The message could proba
22. formly for linear sampling within the range 2 0 volts the TIMS ANALOG REFERENCE LEVEL These are the system quantizing levels 5 Each quantizing level is assigned a number starting from zero for the lowest most negative level with the highest number being L 1 where L is the available number of levels 6 Each sample is assigned a digital binary code word representing the number associated with the quantizing level which is closest to the sample amplitude The number of bits n in the digital code word will depend upon the number of quantizing levels In fact n log L 7 The code word is assembled into a time frame together with other bits as may be required described below In the TIMS PCM ENCODER and many commercial systems a single extra PCM EE 460L Coding University of Nevada Las Vegas Experiment 4 Decoding Department of Electrical and Computer Engineering bit is added in the least significant bit position This is alternately a one or a zero These bits are used by subsequent decoders for frame synchronization 8 The frames are transmitted serially They are transmitted at the same rate as the samples are taken The serial bit stream appears at the output of the module 9 Also available from the module is a synchronizing signal FS frame synch This signals the end of each data frame PCM ENCODER module front panel features vA MASTER FS N PCM DATA Figure 3 Front
23. just the sweep speed to show three frame markers These mark the end of each frame T15 on CH2 A display the CLK signal T16 record the number of clock periods per frame Currently the analog input signal is zero volts Vin is grounded Before checking with the oscilloscope consider what the PCM output signal might look like Make a sketch of this signal fully annotated Then T17 on CH2 B display the PCM DATA from the PCM DATA output socket Except for the alternating pattern of 1 and 0 in the frame marker slot you might have expected nothing else in the frame all zeros because the input analog signal is at zero volts But you do not now the coding scheme There is an analog input signal to the encoder It is of zero volts This will have been coded into a 4 bit binary output number which will appear in each frame It need not be 0000 The same number appears in each frame because the analog input is constant Your display should be similar to that of Figure 7 below except that this shows five frames too many frames on the oscilloscope display makes bit identification more difficult FS end of frame marker aam Peeeee Rees eee bee I LESENE FS frame synch brid 11 PCM data out clock periods LSB at end of frame time Figure 7 5 frames of 4 bit PCM output for zero amplitude input PCM EE 400L Coding University of Nevada Las Vegas Experiment 4 Decoding Departm
24. n obvious fact if message reconstruction by low pass filtering of natural samples results in no distortion then there must be distortion when flat top pulses are involved Analysis of the distortion for flat top pulses will not be attempted here Instead some observations will be made and you can draw your own conclusions Sample width An important observation must be made The pulse width determines the amount of energy in each pulse and so can determine the amplitude of the reconstructed message But in a linear and noise free system the width of the samples plays no part in determining the amount of distortion of a reconstructed message Sample and hold sampling The sample and hold operation is simple to implement and is a very commonly used method of sampling in communications systems In its simplest form the sample is held until the next sample is taken So it is of maximum width This is illustrated in Figure 2 below Figure 2 Sampling by sample and hold for full sample width In the above example the sampling instant is coincident with the rising edge of the clock signal In practice there may be a processing delay before the stepped waveform is presented at the output This is the case in the sub system being examined in this experiment PCM EE 460L Coding University of Nevada Las Vegas Experiment 4 Decoding Department of Electrical and Computer Engineering 2 PCM Encoding This is an int
25. orrect phase This is assured in this experiment but would need adjustment if the PCM signal is transmitted via a band limited channel see Tutorial Question 0 Hence the embedded frame synchronization information Companding You should prepare by reading something about the principles of companding You will already be aware that the PCM ENCODER module can incorporate compression into its encoding scheme The PCM DECODER module can introduce the complementary expansion The existence of these characteristics will be confirmed but their effectiveness in intelligibility enhancement when speech is the message is not examined PCM decoding The PCM DECODER module is driven by an external clock This clock signal is synchronized to that of the transmitter For this experiment a stolen clock will be used The source of frame timing information has been discussed above Upon reception the PCM DECODER 1 Extracts a frame synchronization signal FS from the data itself from the embedded alternate ones and zeros in the LSB position or uses an FS signal stolen from the transmitter see above 2 Extracts the binary number which is the coded and quantized amplitude of the sample from which it was derived from the frame 3 Identifies the quantization level which this number represents 4 Generates a voltage proportional to this amplitude level 5 Presents this voltage to the output Vow The voltage appears at Vou for the
26. panel layout of the PCM ENCODER SLAVE lt lt SELECT CODING SCHEME NN A Aa The front panel layout of the module is shown in Figure 3 Technical details are described in the TIMS Advanced Modules User Manual Note and understand the purpose of each of the input and output connections and the three position toggle switch Counting from the top these are e SLAVE not used during this experiment Do not connect anything to this input e MASTER not used during this experiment Do not connect anything to this output e SYNC MESSAGE periodic synchronized message Either sinusoidal or sinusoidal like sinuous its frequency being a sub multiple of the MASTER CLOCK being any one of four frequencies selected by an on board switch SW2 A message synchronized to the system clock is convenient for obtaining stable oscilloscope displays Having a recognizable shape but being more complex than a simple sine wave gives a qualitative idea of distortion during the decoding process See Table A 1 in the Appendix to this experiment for more details e SELECT CODING SCHEME a three position toggle switch which selects the 4 bit or 7 bit encoding scheme of the analog samples or together with an on board jumper connection the companding scheme e FS frame synchronization a signal which indicates the end of each data frame PCM EE 460L Coding University of Nevada Las Vegas Experiment 4 Decoding Department of Electrical
27. roductory experiment to pulse code modulation PCM The experiment will acquaint you with the PCM ENCODER which is one of the TIMS Advanced Modules This module generates a PCM output signal from an analog input message In this experiment the module will be used in isolation that is it will not be part of a larger system The formatting of a PCM signal will be examined in the time domain After in this experiment we will illustrate the recovery of the analog message from the digital signal PCM Encoder The input to the PCM ENCODER module is an analog message This must be constrained to a defined bandwidth and amplitude range The maximum allowable message bandwidth will depend upon the sampling rate to be used The Nyquist criterion must be observed The amplitude range must be held within the 2 0 volts range of the TIMS ANALOG REFERENCE LEVEL This is in keeping with the input amplitude limits set for all analog modules A step by step description of the operation of the module follows 1 The module is driven by an external TTL clock 2 The input analog message is sampled periodically The sample rate is determined by the external clock 3 The sampling is a sample and hold operation It is internal to the module and cannot be viewed by the user What is held is the amplitude of the analog message at the sampling instant 4 Each sample amplitude is compared with a finite set of amplitude levels These are distributed uni
28. ule is shown in Figure 4 Technical details are described in the TIMS Advanced Modules User Manual Note and understand the purpose of the input and output connections and the toggle switches Counting from the top these are e SLAVE not used during this experiment Do not connect anything to this input e MASTER not used during this experiment Do not connect anything to this output e SELECT CODING SCHEME a three position toggle which selects the coding scheme used by the signal to be decoded PCM EE 400L Coding University of Nevada Las Vegas Experiment 4 Decoding Department of Electrical and Computer Engineering e FS SELECT a two position toggle switch which selects the method of obtaining the frame synchronization signal FS either external at EXT FS or derived internally from the embedded information in the received PCM itself EMBED FS e EXT FS connect an external frame sync signal here if this method of frame synchronization is to be used e EMBED FS if the frame synch signal is derived internally from the embedded information it is available for inspection at this output e PCM DATA the PCM signal to be decoded is connected here e Vour the decoded PCM signal e CLK this is a TTL red input and serves as the MASTER CLOCK for the module Clock rate must be 10 kHz or less For this experiment you will use the 8 333 kHz TTL signal from the MASTER SIGNALS module FS SELECT EXT FS
29. us means sine like LH toggle RH toggle divide freq with approx ampl clock by 8 333kHz and waveform clock Table A 1 The PCM DECODER module has built in circuitry for locating the position of each frame in the serial data stream The circuitry looks for the embedded and alternating 0 and 1 in the LSB position of each frame The search is made by examining a section of data whose length is a multiple of eight bits The length of this section can be changed by the on board switch SW3 Under noisy conditions it is advantageous to use longer lengths The switch settings are listed in Table A 2 below left toggle right toggle groups of eight bits e es ee Dow s DONN DOWN DOWN Table A 2 Synchronization search length options 26
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