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Codes Two of the codes the Navy uses are found in manual telegraphy and in teletypewriter operation. One is very easy to understand while the other is more complex. Let's look at these two types and how they work. MANUAL TELEGRAPHY. - In manual telegraphy, the most widely used code is the Morse code. In this code, two distinctive signal elements are employed-the dot and the dash. The difference between a dot and a dash is its duration, a dash being three times as long as a dot. Each character is made up of a number of dots and/or dashes. The dot and dash elements making up any character are separated from each other by a time interval equal to the duration of one dot. The time interval between the characters for each word is equal to the duration of three dots. The interval between words is equal to seven dots. (A signal-man uses the Morse code to send visual flashing-light messages. The radioman uses the Morse code to send messages electrically.) TELETYPEWRITER MESSAGE TRANSMISSION. - In teletypewriter operation, the code group for each character is of uniform length. Since the Morse code is an uneven length code, it cannot be used in teletypewriter operation without additional code converters. The FIVE-UNIT (five-level) CODE has been the most commonly used in modern printing telegraphy and is universally used in teletypewriter operation. This is also known as the Baudot code. The mechanical sending device in the teletypewriter divides the sending time for each character into five short code elements (impulses) of equal duration. The five-unit code is an example of what is called an even length or constant length code (one in which the number of signal elements for a character is the same for every character and the duration of each element is constant). In the five-unit code, each character consists of a combination of five signal elements; each element may be either a mark or a space. A total of thirty-two combinations of signal elements are possible with this arrangement. The thirty-two possible combinations available from the five-unit code are insufficient to handle the alphabet and numbers since twenty-six combinations are required for the letters of the English alphabet alone. This leaves only six combinations for numerals, symbols, or nonprinting functions. This number of combinations is obviously inadequate; therefore, two of the thirty-two combinations are used as shift signals. The shift signals are often referred to as case-shift signals (one case is a letter shift, and the other a figure shift.) These two shift signals permit the remaining code combination to be used as letter-shift signals for letters and as figure-shift signals for numerals, function signs, and so forth. When a letter shift is transmitted, it sets the receiving instrument in a condition to recognize any letter signal combination. It will recognize letter combinations until a figure shift is received. Then the receiving instrument sets itself in a condition to recognize any figure signal combination received. The interpretation of a signal combination is determined by the previous shift signal. This plan enables 30 of the 32 available combinations to have two meanings. Q.14 There are not enough combinations of the five-unit code to handle the alphabet,
symbols and so forth. What is used to increase the number of available code combinations? The two basic modes of teletypewriter operation are ASYNCHRONOUS (start-stop) and SYNCHRONOUS. The most common mode used in teletypewriter operation is the start-stop mode. Synchronous operation is used more in high-speed data systems. Let's examine their differences. ASYNCHRONOUS. - In the start-stop mode of operation, the receiving device is allowed to run for only one character. It is then stopped to await the reception of a start signal indicating the next character is about to start. In this manner any difference in speed between the transmitting and receiving devices can accumulate only during the duration of one character. However, you should note that a penalty must be paid for this advantage. The length of each character must be increased to include a unit (element) to start the receiving device and another to stop it. The start unit precedes the first intelligence unit and is always a space signal. Its purpose is to start the receiving machine. The stop unit follows the last code unit and is always a mark signal. Its purpose is to stop the receiving machine in preparation for receiving the next character. The start unit must be equal to at least one unit of the code. The standard mode uses a stop unit that is 1.42 times the length of one intelligence unit. It is common practice to refer to a code unit as an element and to use the terms interchangeably. You will also hear duration of a unit referred to as the unit interval. The length of time required to transmit the entire character is called the CHARACTER INTERVAL. Character interval becomes very important in some transmissions because certain items of equipment are character length conscious or code conscious. Stop unit intervals of various lengths are used or produced by various equipment (1.0, 1.27, 1.5, 1.96, 2.0, and so forth). Basically, the only difference between them is the length of time required to transmit one character. SYNCHRONOUS. - Synchronous teletypewriter operation does not in all cases have to rely upon elements of the transmitted character to maintain proper position in relation to the receiving device. External timing signals may be used that allow the start and stop elements to be discarded. You will then see only the elements necessary to convey a character. Synchronous systems have certain advantages over asynchronous systems. The amount of time taken to transmit stop and start elements is made available for information transmission rather than for synchronizing purposes. Only the intelligence elements are transmitted. In start-stop signaling, the ability of the receiving device to select the proper line signal condition is dependent upon signal quality. For example, suppose the stop-to-start transition arrives before it should; then, because of atmospherics, all subsequent selection positions in that character will appear earlier in time in each code element. A synchronous system has a higher capability for accepting distorted signals because it does not depend on a start-stop system for synchronization. Modulation Rate Several terms are used to refer to teletypewriter modulation rates or signaling speeds. These include BAUD RATE, BITS PER SECOND, and WORDS PER MINUTE. Baud is the only term that is technically accurate. The other terms are either approximations or require explanation. The word baud by definition is a unit of modulation rate. You will sometimes see it used to refer to a signal element, but this reference is technically incorrect. Baud rate is the reciprocal of the time in seconds of the shortest signal element. To find the modulation rate of a signal in bauds, you must divide the number 1 by the time duration of the shortest unit interval present in the signal. For example, 22 milliseconds (.022 seconds) is the time interval of the shortest unit in the five-unit code at 60 words per minute. To find the number of bauds corresponding to 60 words per minute, divide 1 by .022. Rounding off the result of the division gives us the number 45.5, which is the baud equivalent of 60 words per minute. Each increase in words per minute will correspondingly decrease the signal unit time interval. (The defense communications system standard speed for teletypewriter operation is 100 words per minute or 75 baud.) Words per minute is used only when speaking in general terms for an approximation of speed. The term 100 words per minute means 100 five letter words with a space between them can be transmitted in a 60-second period. However, you can obtain this nominal words-per-minute rate in several systems by varying either modulation rate or the individual character interval (length). For this reason, the modulation rate (baud) method of reference rather than words per minute is used. Formula for baud rate and words per minute are as follows
BIT is an acronym for the words binary digit. In binary signals, a bit is equivalent to a signal element. Because of the influence of computer and data processing upon our language, modulation rate is sometimes expressed in bits per second. When you understand all signal elements being transmitted are of equal length, then the modulation rate expressed in bits per second is the same as the modulation rate expressed in baud. Dc Circuits You were told the two conditions mark and space may be represented by any convenient means. The two most common are NEUTRAL and POLAR operation. In neutral, current flow represents the mark, and no current flow represents the space; in polar operation, current impulses of one polarity represent mark, and impulses of the opposite polarity of equal magnitude represent the space. NEUTRAL. - Neutral circuits make use of the presence or absence of current flow to convey information. A neutral teletypewriter circuit is composed of a transmitting device, a battery source to supply current, a variable resistor to control the amount of current, a receiving device, and a line for the transmission medium. POLAR. - Polar operation differs from neutral operation in two ways. Current is always present in the polar system, and it is either positive or negative. A polar teletypewriter circuit contains the same items as a neutral circuit plus an additional "battery" source. The battery referred to here is not an actual battery but is a solid-state dc power supply. It provides variable current to the teletypewriters. The reason for having an extra battery source is because polar circuits use positive battery for marks and negative battery for spaces. You will find in polar operation that the distortion of a signal is almost impossible through low line currents, high reactance, or random patching of signal circuits or equipment. In polar signaling when you experience a complete loss of current (a reading of zero on a milliammeter), you know you have line or equipment trouble; whereas the same condition with neutral signaling may indicate a steady space is being transmitted. This gives us a condition called RUNNING OPEN. Under this condition, the teletypewriter appears to be running because the machines is decoding the constant space as the Baudot character blank and the type hammer continually strikes the type box but there is no printing or type box movement across the page. Q.15 What are the two teletypewriter modes of operation? |
 
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