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ADAPTIVE DELTA MODULATION
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1. the two BUFFER amplifiers is near unity it might be prudent to remove them Small gain adjustment could be introduced by the scheme illustrated in Figure 4 This scheme has not been included in any of the Tasks If you elect to use it then modify the instructions accordingly Adaptive delta modulation
2. ADAPTIVE DELTA MODULATION PREPARA TION PT 142 BAG Or GN aries sts asa neta e enlace sot EE E 142 TIMS adaptive delta modulatot sss sss see 142 the voltage controlled amplifier VCA 143 EXPERIMENT T ereenn Sides ads etn Sas ee ens Ra as 144 the adaptive control Voltage sese eee eee 144 VCA Calibration nanc ant cena atiedncne nani 144 Manual cantio losene e eE EEEE EE EEE 145 stability renn e eee 145 Eh STT 145 demodulation isnan ieee A E ene 146 TUTORIAL QUES TIONS var ceased e E uoeeds 147 APPENDIX Arrese an a N R A eet canna E R R 148 loop stability eseese n e a EE 148 Adaptive delta modulation Vol D1 ch 15 rev 1 0 141 ADAPTIVE DELTA MODULATION ACHIEVEMENTS introduction to a variation of the basic delta modulator which adjusts the step size according to the slope of the signal being sampled PREREQUISITES completion of the experiments entitled Delta modulation and Delta demodulation in this Volume ADVANCED MODULES DELTA MODULATION UTILITIES DELTA DEMOD UTILITIES WIDEBAND TRUE RMS METER optional PREPARATION background It is assumed that you have been introduced to the principles of adaptive delta modulation in your course work TIMS adaptive delta modulator The basic delta modulator was studied in the experiment entitled Delta modulation It is implemented by the arrangement shown in block diagram form in Figure 1 ts SPa LIMITER modulate F mss signal message z output INTEGRAT
3. OR L clock Figure 1 basic delta modulator You will remember that with this modulator there was a conflict when determining the step size A large step size was required when sampling those parts of the input waveform of steep slope But a large step size worsened the granularity of the sampled signal when the waveform being sampled was changing slowly A small step size is preferred in regions where the message has a small slope 142 D1 Adaptive delta modulation This suggests the need for a controllable step size the control being sensitive to the slope of the sampled signal This can be implemented by an arrangement such as is illustrated in Figure 2 o gt message In V delta LIMITER SAMPLER modulated signal output INTEGRATOR clock Figure 2 an adaptive delta modulator The gain of the amplifier is adjusted in response to a control voltage from the SAMPLER which signals the onset of slope overload The step size is proportional to the amplifier gain This was observed in an earlier experiment Slope overload is indicated by a succession of output pulses of the same sign The TIMS SAMPLER monitors the delta modulated signal and signals when there is no change of polarity over 3 or more successive samples The actual ADAPTIVE CONTROL signal is 2 volt under normal conditions and rises to 4 volt when slope overload is detected The gain of the amplifier and hence the step s
4. TIES module has a socket labelled ADAPTIVE output The signal from this socket is at a level of either 2 or 4 volts The lower output is what might be called the normal level If at any time the delta modulated signal contains three or more consecutive samples of the same size then this signal goes to the higher 4 volt level Three or more consecutive samples of the same level indicates slope overload When including the VCA in the feedback path you must ensure that at no time will either of the inputs to the MULTIPLIER exceed its safe ie linear operating range say 5 volts absolute maximum VCA calibration Before setting up the delta modulator it is wise to familiarise yourself with the operation of the VCA T1 set up a VCA according to the block diagram of Figure 3 Use the VARIABLE DC module as a control signal and a sinewave as input Connect each via a BUFFER amplifier so that the values of V max and Vingx y can be determined These are the overload levels for each of the inputs They are likely to be similar Select DC coupling with the front panel switch of the MULTIPLIER T2 measure the VCA gain for a control voltage of 2 volt This is the normal output from the ADAPTIVE CONTROL of the DELTA MODULATOR UTILITIES module Your measurements should have shown that the MULTIPLIER can accept inputs considerably in excess of the TIMS ANALOG REFERENCE LEVEL before overload sets in 144 pi l more detai
5. UMMER These should be exactly the same as observed during Task T4 The slope overload should therefor be apparent as before stability There are now three amplifiers in the feedback loop At the best of times this could be a cause for concern the stability of the whole system could be compromised Refer to the Appendix to this experiment for further comment adaptive control The VCA is now set up in the feedback loop but is currently in a passive mode You are now ready to implement adaptive control of the loop gain by replacing the fixed control voltage V with the adaptive control voltage from the modulator What you will want to observe is the reduction of the length of the period of the slope overload T8 while watching the length of the slope overload portion of the sawtooth waveform from the INTEGRATOR replace the DC voltage from the VARIABLE DC supply to the VCA with the ADAPTIVE CONTROL voltage from the modulator Adaptive delta modulation pi 145 T9 replace the DC control voltage with the ADAPTIVE CONTROL from the SAMPLER Notice that the slope overload remains as for the conditions of the previous Task when the control voltage was 4 volt but that the granularity at the extrema of the message has not been worsened To change between adaptive and non adaptive operation move the patch cord from the ADAPTIVE CONTROL output socket of the SAMPLER to the preset V1 output of the VARIABLE DC module T10 spend so
6. introduced This is a qualitative measurement but nonetheless very instructive The setting up procedure at the demodulator will be somewhat similar to that used at the modulator Of interest will be a measurement to resolve the question is it necessary to make the demodulator adaptive in the same manner as at the modulator Is there a penalty for not doing so 146 D1 Adaptive delta modulation T12 set up a demodulator Use a complex message Observe recovered waveforms under various conditions TUTORIAL QUESTIONS QI make a positive statement about how your observations at the modulator confirmed that the ADAPTIVE CONTROL improved the performance of the modulator Adaptive delta modulation pi 147 148 pi APPENDIX loop stability You are working with a feedback loop At the best of times these can run into instability if the loop gain is too high Some of this instability can be caused by un planned for phase changes round the loop SAMPLER outpure gt x gt to SUMMER ADAPTIVE CONTROL DC for mean voltage ie gain control Figure 4 gain modification using VCA The presence of two BUFFER amplifiers in cascade does not help the situation These were placed there in the non adaptive modulator as a convenient method of changing the loop gain Now there is an extra source of phase change introduced by the VCA and also a new source of gain adjustment If it turns out that the gain provided by
7. ize is made proportional to this control voltage Provided the slope overload was only moderate the approximation will catch up with the wave being sampled The gain will then return to normal until the sampler again falls behind Much work has been done by researchers in this area and sophisticated algorithms have been developed which offer significant improvements over the simple system to be examined in this experiment the voltage controlled amplifier VCA The VCA can be modelled with a MULTIPLIER This is shown in Figure 3 input y t gt x k V y t output k multiplier constant Figure 3 the voltage controlled amplifier The control in Figure 3 is shown as a DC voltage This may be set to any value in the range V max Beyond Vmax the MULTIPLIER will overload However the control voltage need not be DC but can be time varying Under these conditions the arrangement is more likely be called a modulator Adaptive delta modulation pi 143 You have met the MULTIPLIER constant k in earlier experiments of Part I where it was defined and measured The block diagram of Figure 1 was modelled in the experiment entitled Delta modulation Refer to that experiment for details The adaptive delta modulator of Figure 2 differs only by the addition of a voltage controlled amplifier VCA modelled as described above with a MULTIPLIER the adaptive control voltage The DELTA MODULATION UTILI
8. ls in the TIMS Advanced Modules User Manual Adaptive delta modulation Likewise the INTEGRATOR input can under some conditions be subject to quite large input signals but it is robust and can also handle input amplitudes well in excess of the TIMS ANALOG REFERENCE LEVEL You will notice that except for the presence of the MULTIPLIER in the feedback loop the modulator is the same as that studied in the experiment entitled Delta modulation You should use the same setting up procedure as in that experiment with the adaptive control inhibited This is done by connecting 2 volt in place of the ADAPTIVE CONTROL voltage to the MULTIPLIER manual control T3 model the block diagram of Figure 1 This is not the adaptive modulator Refer to the experiment entitled Delta modulation for details The amplifier in the feedback loop is modelled with two BUFFER amplifiers in cascade T4 observe the two inputs to the SUMMER Adjust the feedback gain so that the sawtooth shows some evidence of ie moderate slope overload T5 observe the control voltage from the ADAPTIVE CONTROL output socket It will be alternating between V no slope overload and V2 volt following the onset of and coincident with the slope overload Record the value of V about 2 volt T6 insert the VCA between the SAMPLER and the BUFFER AMPLIFIER Set the control voltage to the VCA to V volts obtained from the VARIABLE DC supply T7 observe the two inputs to the S
9. me time examining the waveforms at the various interfaces As necessary replace the ADAPTIVE CONTROL voltage with the manual DC voltage control Don T forget to monitor the ADAPTIVE CONTROL voltage itself In other words make sure you make enough observations to appreciate what is happening T11 use a complex message as described in the experiment entitled Delta modulation and compare results by visual inspection of the INTEGRATOR output waveform with and without the adaptive feedback operating You should now be reasonably confident from your observations at the modulator transmitter that the adaptive feedback control will improve the performance of the system as observed at the demodulator receiver Thus it might be agreed that the object of the experiment has been achieved For positive verification however it is necessary to build a demodulator and make some further observations demodulation It is essential that you have already completed the experiment entitled Delta demodulation This introduced methods of noise and distortion measurement which are required now You should now model a delta demodulator as described in the experiment entitled Delta demodulation Whilst absolute measurement of signal to noise plus distortion ratio SNDR measurements are of interest of greater interest in the present situation is to observe the change to the demodulated waveform which happens when the adaptive feedback is
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