DELTA DEMODULATION. distortion - SNDR measurement TUTORIAL QUESTIONS speech Delta demodulation Vol D1, ch 14, rev 1.

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1 DELTA DEMODULATION PREPARATION delta demodulation methods EXPERIMENT test signal the demodulator distortion - a qualitative look speech distortion - SNDR measurement TUTORIAL QUESTIONS Vol D1, ch 14, rev

2 DELTA DEMODULATION ACHIEVEMENTS: introduction to the demodulation of a delta modulated signal; measurement of quantization distortion at the receiver; listening test on speech. PREREQUISITES: completion of the experiment entitled Delta modulation in this Volume. ADVANCED MODULES: DELTA MODULATION UTILITIES; DELTA DEMOD UTILITIES; WIDEBAND TRUE RMS METER PREPARATION delta demodulation methods You should refer to your text book and course work for background information regarding delta demodulation methods, and the likely sources of distortion. For this experiment you will supply your own delta modulated signal, using the modulator examined in the experiment entitled Delta modulation. The TIMS DELTA DEMOD UTILITIES module will be used for demodulation (the receiver). It contains a SAMPLER and an INTEGRATOR. The SAMPLER uses a clock stolen from the modulator (the transmitter). The SAMPLER accepts TTL signals as input, but gives an analog output for further analog processing - for example, lowpass filtering. The principle of the demodulator is shown in block diagram form in Figure 1 below. It performs the reverse of the process implemented at the modulator in the vicinity of the sampler and integrator. delta modulation IN SAMPLER clk INTEGRATOR message out stolen message (for distortion measurement) stolen clock Figure 1: a demodulator for delta modulation D1

3 The sampler, which is clocked at the same rate as the one at the modulator, outputs a bi-polar signal (±V volts). The integrator generates a sawtooth-like waveform from this. This is an approximation to the original message. Having the same time constant as that at the modulator, and with no noise or other signal impairments, it will be identical with the corresponding signal at the modulator. However, it is not the message, but an approximation to it. The sawtooth waveform contains information at the message frequency, plus obvious unwanted frequency components (quantizing noise). The unwanted components which are beyond the bandwidth of the original baseband message are removed by a lowpass filter. Those unwanted components which remain are perceived as noise and distortion. Unlike ideal sampling of an analog signal, and ideal reconstruction with a lowpass filter (refer to the experiment entitled The sampling theorem within Volume A1 - Fundamental Analog Experiments), the reconstruction of the message from a delta modulator is not perfect. You will find that the SNDR 1 is relatively poor, and certainly a lot worse than the signal-to-noise ratio capabilities of the TIMS system (typically better than 40 db). Thus the SNDR that you will be measuring will be entirely due to the imperfections of the delta modulator itself. However, do not then declare that delta modulation has no practical applications. You will find, in the experiment entitled Adaptive delta modulation, in this Volume, that there are means of implementing improvements. With further refinement in the circuitry, a higher clock speed, and sophisticated adaptive algorithms 2, delta modulation can perform remarkably well. It is used extensively in the field of digital audio. EXPERIMENT test signal T1 set up a delta modulator of the type examined in the experiment entitled Delta modulation. Set it up initially for what you consider to be the best approximation to the message (compare the two inputs to the SUMMER). 1 signal-plus-noise-and-distortion ratio 2 see the experiment entitled Adaptive delta modulation in this Volume D1-137

4 the demodulator For this demodulator you will use the DELTA DEMOD UTILITIES module. This contains a LIMITER, a clocked SAMPLER, and an INTEGRATOR of the type in the DELTA MODULATION UTILITIES module. T2 obtain and examine a DELTA DEMOD UTILITIES module. Read about it in the TIMS Advanced Modules User Manual. T3 model the demodulator of Figure 1. Set the time constant of the INTEGRATOR to the same value as selected in the modulator. Use the RC LPF in the DELTA DEMOD UTILITIES for the output filter. Note the SAMPLER accepts a TTL signal from the modulator, as well as a stolen clock. For oscilloscope triggering use the message signal, also stolen from the modulator. Set the front panel clock switch to match that at the modulator. T4 confirm that the signals at each of the INTEGRATOR outputs are similar. T5 confirm that the output of the demodulator lowpass filter is a reasonable copy of the original message. distortion - a qualitative look At the modulator you can change the sampling rate (100 khz, 50 khz, and 25 khz with the front panel switch), and the step size (RC time constants). You can also control the amount of slope overload. All of these have their influence on the measured SNDR. T6 introduce various mal-adjustments at the modulator (observed at the output of the modulator INTEGRATOR), and observe their effect at the demodulator output. Use both a sinusoidal message, and a complex message 3. speech If you have bandlimited speech available at TRUNKS, or from a SPEECH module, you can make many interesting listening tests. How would you describe speech when distorted by slope overload? T7 make qualitative assessments of the effect of the various mal-adjustments, at the modulator, on the demodulated speech. 3 as defined in the experiment entitled Delta modulation in this Volume D1

5 distortion - SNDR measurement For quantitative signal-to-noise-and-distortion ratio measurements (SNDR) you can model the scheme illustrated in Figure 2. message plus noise and distortion rms volts `stolen` sinusoidal message Figure 2: noise and distortion measurement Recall the experiment entitled Modelling an equation (within Volume A1 - Fundamental Analog Experiments), where the technique of signal cancellation in an adder was first introduced. You can use the WIDEBAND TRUE RMS METER to measure the distortion components. The principle is to cancel the wanted sinusoidal message from the adder output, leaving only the unwanted components (noise-plus-distortion). Having obtained a minimization of the message from the adder output, then removal of the messageplus-noise from the adder leaves the (stolen) message, which will give the reference with which to compare the noise-plus-distortion. A model of the measurement system is illustrated in Figure 3. Remember to set the on-board switch of the PHASE SHIFTER to LO. stolen message from delta demodulator Figure 3: SNDR measurement T8 as before (when making qualitative observations), introduce various maladjustments at the modulator, and observe their effect at the demodulator output. Use a sinusoidal message (refer to Tutorial Question Q4). D1-139

6 TUTORIAL QUESTIONS Q1 the term granular noise is often used in the context of delta modulation. Explain where this term comes from. Describe the compromise which has to be made when determining a step size in a delta modulator. Q2 describe the procedure used when measuring SNDR with the scheme of Figure 2. Q3 what was the effect upon output noise and distortion of an increase of: a) step size b) sampling rate c) slope overload? Q4 the noise-and-distortion measurement scheme of Figure 2 was used when the message was a single sine wave. Would it be effective for measurement with a more complex message? Explain. Q5 you were advised to set the time constant of the INTEGRATOR in the demodulator to be the same as that in the modulator. Was this essential? Describe the consequences of using a different time constant at the demodulator. Q6 could the message be recovered from the delta modulated signal using only a lowpass filter? Explain D1