The first experiment for the laboratory of Digital Communications is simply an introduction to the course. For this experiment, we were expected to be familiarized with the basic principles and concepts in digital communication particularly with the concept of information – how it is represented and transmitted. The experiment is divided into two parts: Introduction to Digital Communications and PCM Encoding. For the first part of the experiment, we were asked to monitor the output signal coming from an oscilloscope after setting the PCM level and PCM bandwidth of the equipment to maximum.
These settings were done because, as discussed in theory, extremely wide bandwidth is required to produce sharp-edged square waveform. Based on the signal produced by the oscilloscope from Rate 2 and Rate 3, we were able to deduce that the output is a binary coded information because it was a square wave, which means there are only two possible states, either high/low or 1/0. For the signal produced from Rate 1, it was only a continuous straight line.As was discussed in theory, for a signal to contain information it must at least contain a change in state, thus the output produced in Rate 1 contains no information. A continuation of the first part of the experiment, we used the spectrum analyzer.
For this part we had to observe the information transfer rate and its relationship with the specific spectra coming from Rates 1, 2, and 3. From Rate 1, since it does not contain any information, it has no bit duration and bit rate. Bit duration is the time during which the bit lasted and bit rate is measured as bits per seconds.When we analyzed the spectra for each rates, we only considered the significant frequency components which has adequate amplitude level because the frequency spectrum of a square wave could go up to infinity and thus it would be impractical to include other frequencies that has no ample amplitude level. In this part, we have deduced that a waveform with fast edges has higher bandwidth than that with rounded edges since more harmonics are included.
Finally, we were given seven pulse trains each containing a different 8-bit pattern to transmit.It was observed that the frequency spectrum varies for the seven patterns, which means that the spectrum of the pulse train is dependent on the bit pattern. There were patterns that have the same gain at certain frequencies and same frequency components because their patterns were almost the same, only that the highs and lows were interchanged. The greatest bandwidth was occupied by the pulse train with bit patterns changing state the most. For the second part of the experiment, we were asked to sketch the code representation for the letters E and Q in Morse, CCITT, ASCII, and EBCDIC.Based on the sketches, Morse has shorter length for code representation compared to CCITT, ASCII and EBCDIC where the three has the same length but different representation.
After observing the code representations, we transmitted different characters without start and stop bits over the transmission line and observed that the character received from the receiver is different from the one that was sent. This was because the receiver does not know where the bit pattern started and ended.So, obviously, this problem can easily be fixed by adding the start and stop bits, but the negative side of doing so decreases the efficiency of the transmission of information because there were added bits that does not contain information. The last part of the experiment is the observance of the two serial communication schemes. The addition of start and stop bits when the bit pattern is transmitted is designated as asynchronous transmission while the addition of start-of-frame and end-of-frame characters is synchronous transmission.
A sync word is transmitted along with the information for synchronization. Conclusion Digital communications is a system of communication where the information to be send over the transmission line is digitally encoded before transmission. The information takes the shape of a square wave, which has an extremely wide bandwidth, and is represented as a binary code where there are only two possible states, binary 0 and binary 1. For a signal to contain information, a change in state is required, thus a continuous straight line has no information.
Since the waveform is a square wave, the frequency spectrum could go up to infinity, but for analysis, only those harmonics with significant amplitude is considered. Bit duration is the times during which a certain bit lasts and the bit rate is easily described as bits per second. The frequency components of a spectrum of a pulse train vary depending on the bit pattern the pulse train contains. The greater the times the bits change states, the greater bandwidth it occupies.
There are a lot of ways to represent a character in codes. It can be in Morse, CCITT, ASCII, andEBCDIC. Each of those representations represents a character differently. The Morse code has a shorter length of representing a character, while the CCITT, ASCII, and EBCDIC have a longer length of representing a character and they all have the same length.
Serial communications is a process in digital communication wherein data is sent one bit at a time over a communication line. There are two schemes designated for serial communication that is synchronous and asynchronous. Asynchronous transmission adds start and stop bits to its data prior to transmission.This causes a low efficiency for transmission because there are additional bits sent, reducing the time used for the actual information to be sent. The counterpart of asynchronous is the synchronous transmission.
Synchronous transmission uses start-of-frame and end-of-frame characters. It has a higher efficiency since synchronization bits are not transmitted as frequent as it was sent in asynchronous. Also in synchronous, the bit clock recovery is important because t is used as timing reference. The clock signal synchronize transmitter and receiver.
