Dr. Clincy Professor of CS Chapter 4 Handout #4 Dr. Clincy Professor of CS Dr. Clincy Lecture

ANALOG-TO-DIGITAL CONVERSION We have seen in Chapter 3 that a digital signal is superior to an analog signal. The tendency today is to change an analog data to a digital signal. In this section we describe two techniques, pulse code modulation and delta modulation. Dr. Clincy Lecture

Components of PCM encoder PCM – Pulse Code Modulation 1st: analog signal is sampled 2nd: sampled signal is quantized 3rd: quantized values are encoded as bit streams (or codes) Analog signal is sampled every Ts seconds Sample rate is fs = 1/Ts Dr. Clincy Lecture

Three different sampling methods for PCM High-speed switch used – able to retain the shape of the signal Ideal but complex Sample-and-hold method that creates flat-top samples by using a circuit Sampling process also called pulse amplitude modulation (PAM) Dr. Clincy Lecture

Recovery of a sampled sine wave for different sampling rates Nyquist theorem states that the sampling rate must be at least 2 times the highest frequency of the signal Catches the essence of the signal Doesn’t improve the case Doesn’t capture the essence of the signal Dr. Clincy Lecture

Quantization and encoding of a sampled signal actual-amplitude/D actual amplitude Quantization steps: Determine Vmin and Vmax Divide range into L zones, each of height D D = [Vmin - Vmax]/L 3. Assign quantized values of 0 to L-1 to midpoint of each zone 4. Map the sample value to a quantized value Norm. Actual Error between actual and nornalized Quant. value for code Code that represents the voltage level Assume sample amplitudes between -20V and +20V Let L = 8 (levels) – therefore, D = [20 - -20]/8 = 5 Quantization error can contribute to Shannon’s SNR: SNRdB = 6.02nb + 1.76 where nb is bits per sample Bit rate = sampling rate x # of bits per sample = fs x nb Dr. Clincy Lecture

Components of a PCM decoder PCM decoder recovers the original signal Smooths out the staircase signal What is the minimum bandwidth of the filter the digitized signal will need ? Bmin = c x nb x 2 x Banalog x 1/r (nb = # bits per sample) If 1/r=1 and c=1/2, Bmin=nb x Banalog If the data rate and number of signal levels are fixed, minimum bandwidth is Bmin = N / [2 x log2 L] Dr. Clincy Lecture

The process of delta modulation PCM is more complex than Delta modulation PCM finds the amplitude of the signal; delta modulation simply finds the change in the signal from the previous sample Delta modulation doesn’t use codes – bits are sent one after another Positive changes are encoded as 1; negative changes are encoded as 0 Dr. Clincy Lecture

TRANSMISSION MODES The transmission of binary data across a link can be accomplished in either parallel or serial mode. In parallel mode, multiple bits are sent with each clock tick. In serial mode, 1 bit is sent with each clock tick. While there is only one way to send parallel data, there are three subclasses of serial transmission: asynch, syn and iso approaches (asynchronous, synchronous, and isochronous.) Dr. Clincy Lecture

Parallel transmission Dr. Clincy Lecture

Serial transmission Dr. Clincy Lecture

Asynchronous transmission In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or more stop bits (1s) at the end of each byte. There may be a gap between each byte. Asynchronous here means “asynchronous at the byte level,” but the bits are still synchronized; their durations are the same. Dr. Clincy Lecture

Synchronous transmission In synchronous transmission, we send bits one after another without start or stop bits or gaps. It is the responsibility of the receiver to group the bits. Dr. Clincy Lecture

Isochronous Transmission For realtime audio and video, uneven delays between frames is not acceptable – so synchronous transmission doesn’t work well The entire stream of bits must be synchronized – this is isochronous transmission Isochronous transmission guarantees data at a fixed rate Dr. Clincy Lecture