26-07-2012, 04:03 PM
Digital Transmission
[attachment=29121]
Delay Encoding (by Miller)
In telecommunications, delay encoding is the encoding of binary data to form a two-level signal where a- A "0" causes no change of signal level unless it is followed by another "0" in which case a transition to the other level takes place at the end of the first bit period. b- A "1" causes a transition from one level to the other in the middle of the bit period. Delay encoding is used primarily for encoding radio signals because the frequency spectrum of the encoded signal contains less low-frequency energy than a conventional non-return-to-zero (NRZ) signal and less high-frequency energy than a biphase signal. Delay encoding is an encoding using only half the bandwidth for biphase encoding but features all the advantages of biphase encoding. It is guaranteed to have transitions every other bit, meaning that decoding systems can adjust their clock/DC threshold continuously. One drawback is human readability (e.g. on an oscilloscope). Delay encoding, also called Miller encoding, is similar to Manchester encoding, except that a transition occurs in the middle of an interval only when the bit is 1, which allows higher data rates...
Differential encoding: (Differential Manchester encoding)
also called biphase mark code (BMC) or FM1, is a line code in which data and clock signals are combined to form a single 2-level self-synchronizing data stream. It is a differential encoding, using the presence or absence of transitions to indicate logical value. It has the following advantages over some other line codes: A transition is guaranteed at least once every bit, allowing the receiving device to perform clock recovery. Detecting transitions is often less error-prone than comparing against a threshold in a noisy environment. Unlike with Manchester encoding, only the presence of a transition is important, not the polarity. Differential coding schemes will work exactly the same if the signal is inverted (wires swapped). (Other line codes with this property include NRZI, bipolar encoding, coded mark inversion, and MLT-3 encoding). If the high and low signal levels have the same voltage with opposite polarity, coded signals have zero average DC voltage, thus reducing the necessary transmitting power and minimizing the amount of electromagnetic noise produced by the transmission line. The symbol rate is twice the bitrate of the original signal. Each bit period is divided into two half-periods: clock and data. The clock half-period always begins with a transition from low to high or from high to low. The data half-period makes a transition for one value and no transition for the other value. One version of the code makes a transition for 0 and no transition for 1 in the data half-period; the other makes a transition for 1 and no transition for 0. Thus, if a "1" is represented by one transition, then a "0" is represented by two transitions and vice versa, making Differential Manchester a form of frequency shift keying. Differential Manchester is self-synchronizing since there is a change in the signal at least every two bits. It is not necessary to know the polarity of the sent signal since the information is not kept in the actual values of the voltage but in their change: in other words it does not matter whether a logical 1 or 0 is received, but only whether the polarity is the same or is different from the previous value. Finally, if the high and low states have the same voltage with opposite polarity, coded signals have zero average DC voltage, thus reducing the necessary transmitting power and minimizing the amount of electromagnetic noise produced by the transmission line. Applications: Differential Manchester is specified in the IEEE 802.5 standard for token ring LANs, and is used for many other applications, including magnetic and optical storage.
[attachment=29121]
Delay Encoding (by Miller)
In telecommunications, delay encoding is the encoding of binary data to form a two-level signal where a- A "0" causes no change of signal level unless it is followed by another "0" in which case a transition to the other level takes place at the end of the first bit period. b- A "1" causes a transition from one level to the other in the middle of the bit period. Delay encoding is used primarily for encoding radio signals because the frequency spectrum of the encoded signal contains less low-frequency energy than a conventional non-return-to-zero (NRZ) signal and less high-frequency energy than a biphase signal. Delay encoding is an encoding using only half the bandwidth for biphase encoding but features all the advantages of biphase encoding. It is guaranteed to have transitions every other bit, meaning that decoding systems can adjust their clock/DC threshold continuously. One drawback is human readability (e.g. on an oscilloscope). Delay encoding, also called Miller encoding, is similar to Manchester encoding, except that a transition occurs in the middle of an interval only when the bit is 1, which allows higher data rates...
Differential encoding: (Differential Manchester encoding)
also called biphase mark code (BMC) or FM1, is a line code in which data and clock signals are combined to form a single 2-level self-synchronizing data stream. It is a differential encoding, using the presence or absence of transitions to indicate logical value. It has the following advantages over some other line codes: A transition is guaranteed at least once every bit, allowing the receiving device to perform clock recovery. Detecting transitions is often less error-prone than comparing against a threshold in a noisy environment. Unlike with Manchester encoding, only the presence of a transition is important, not the polarity. Differential coding schemes will work exactly the same if the signal is inverted (wires swapped). (Other line codes with this property include NRZI, bipolar encoding, coded mark inversion, and MLT-3 encoding). If the high and low signal levels have the same voltage with opposite polarity, coded signals have zero average DC voltage, thus reducing the necessary transmitting power and minimizing the amount of electromagnetic noise produced by the transmission line. The symbol rate is twice the bitrate of the original signal. Each bit period is divided into two half-periods: clock and data. The clock half-period always begins with a transition from low to high or from high to low. The data half-period makes a transition for one value and no transition for the other value. One version of the code makes a transition for 0 and no transition for 1 in the data half-period; the other makes a transition for 1 and no transition for 0. Thus, if a "1" is represented by one transition, then a "0" is represented by two transitions and vice versa, making Differential Manchester a form of frequency shift keying. Differential Manchester is self-synchronizing since there is a change in the signal at least every two bits. It is not necessary to know the polarity of the sent signal since the information is not kept in the actual values of the voltage but in their change: in other words it does not matter whether a logical 1 or 0 is received, but only whether the polarity is the same or is different from the previous value. Finally, if the high and low states have the same voltage with opposite polarity, coded signals have zero average DC voltage, thus reducing the necessary transmitting power and minimizing the amount of electromagnetic noise produced by the transmission line. Applications: Differential Manchester is specified in the IEEE 802.5 standard for token ring LANs, and is used for many other applications, including magnetic and optical storage.