Continuous Time Signals A signal represents the evolution of a physical quantity in time. Example: the electric signal out of a microphone. At every time t the signal has a value Volts (say) t x(t ) x(t ) t Digital Processing of Continuous Time Signals Signals can be processed numerically by a digital computer or using a DSP chip. We need: 1. Analog to Digital Converter (ADC): convert the signal to a numerical sequence 2. Digital to Analog Converter (DAC): convert it back to analog, if we need to. ADC DAC DSP Analog to Digital Converter (ADC) It performs Sampling and Quantization. x[n] Qx(nTs ) x(t ) ADC Fs 011 001 000 101 Parameters: Ts Fs NB Sampling interval (sec) Sampling frequency (Hz=1/sec) Number of Bits per Sample 110 Ts Digital to Analog Converter (DAC) It converts a signal back to Continuous Time by holding the value within the sampling interval. x[n] x(t ) DAC t Fs t Energy of a Signal A signal represents a physical quantity, like a Voltage, a Current, a Pressure … We define its total Energy as: EX x(t ) 2 dt Example: x(t ) 5.0V 2 .0 t (m sec) EX (5.0)2 2.0 103 50103Volts2 sec Power of a Signal A signal represents a physical quantity, like a Voltage, a Current, a Pressure … We define the Average Power: T / 2 1 2 PX lim x(t ) dt T T T / 2 In particular if the signal is a periodic repetition of a pulse: T0 period 1 PX T0 x(t ) period 2 dt Example Take a square wave. Suppose it is a voltage and the values are in Volts: x(t ) 0 .5 0 1 .5 3 .0 t (m sec) 0.52 1.5 103 2 PX 0 . 125 Volts 3 103 Its square root is called the Root Mean Square (RMS) value: X RMS PX 0.35 Volts Relative Power: deciBells (dBs) In many problems we are interested in the relative power, with respect to the power of a reference signal. For example, suppose the reference has a power PXref 0.01Volts2 Then, in the previous example: PX dB PX 0.125 10log10 10log10 10.97dB PXref 0.01 You could use the RMS values and obtain the same result: PX dB X RMS PX 10log10 20log10 PXref XrefRMS Some Typical Values for Acoustic Signals Take the air pressure of an audio signal. Let the reference be the threshold of hearing. For a typical person this Pref 1012 Watts / m2 Pref dB 1012 10 log10 12 0dB 10 Watts / m2 dB Threshold 1012 0 ppp 108 40 p 106 60 f 104 80 fff 102 100 pain 1 120 Signal to Noise Ratio Usually all the signals we don’t want we call them “noise”. This can be caused by actual background noise, interference from another source (someone talking during the movie) or any other undesired sources. w(t ) noise x(t ) signal y(t ) what we get The Signal to Noise Ratio (SNR) characterizes how “noisy” the signal is: PX SNR 10log10 PX PW dB PW dB Example 1. You hearing something at a level “f” (forte), and someone talks at level “ppp” (pianissimo), then the SNR is (refer to the table): SNR 80 40 40 dB 2. You hearing something at a level “f” (forte), and someone talks at level “p” (piano), then the SNR is (refer to the table): SNR 80 60 20 dB Quantization Noise Back to Discrete Time Signals. When we quantize a signal with a finite number of bits, we introduce errors which are perceived as noise. Problem: what is the relation between number of bits per sample and SNR? original quantized N B bits per sample 011 error 001 000 101 t Ts 110 For an average signal, statistically it can be shown that: Then Perror 1 Psignal 2NB 3 2 SNR 10log10 3 22 NB 4.77 6.02NB dB Example We want to determine the number of bits per sample to obtain a good SNR of at least 100dB. Then we need: 4.77 6.02N B 100 which yields N B 95.23/ 6.02 15.8 Then we need at least 16 bits per sample.