- Differential Manchester 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. Either code can be interpreted with the clock half-period either before or after the data half-period.
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; this makes synchronization even easier. 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. All these positive aspects are achieved at the expense of doubling clock frequency.
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. BMC is used as the encoding method in AES3 and S/PDIF. Many magnetic stripe cards also use BMC encoding, often called F2F (frequency/double frequency) or Aiken Biphase. That standard is described in ISO/IEC 7811. SMPTE time code also uses BMC. BMC is also the original "frequency modulation" used on single-density floppy disks, before being replaced by "double-density" modified frequency modulation.
- Watkinson, J. (1994) The Art of Digital Audio, 2nd edition. Oxford: Focal Press. ISBN 0-240-51320-7
- Introduction to magnetic stripe technology
Line coding (digital baseband transmission) Main articles Basic line codes Extended line codes Optical line codesCarrier-Suppressed Return-to-Zero · Alternate-Phase Return-to-Zero
Wikimedia Foundation. 2010.
Look at other dictionaries:
Manchester code — In telecommunication and data storage, Manchester code (also known as Phase Encoding, or PE) is a line code in which the encoding of each data bit has at least one transition and occupies the same time. It therefore has no DC component, and is… … Wikipedia
Differential coding — In digital communications, differential coding is a technique used to provide unambiguous signal reception when using some types of modulation. It makes data to be transmitted to depend not only on the current bit (or symbol), but also on the… … Wikipedia
Bipolar encoding — An example of bipolar encoding, or AMI (Alternate mark inversion). In telecommunication, bipolar encoding is a type of line code (a method of encoding digital information to make it resistant to certain forms of signal loss during transmission).… … Wikipedia
Delay encoding — In telecommunications, delay encoding is the encoding of binary data to form a two level signal such that (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 … Wikipedia
Differentieller Manchester-Code — Codierung des Bitsstrings 10100111001 mittels des differentiellen Manchester Codes Der differentielle Manchester Code, im Englischen auch als Conditional DePhase Encoding (CDP) bezeichnet, ist ein Leitungscode, welcher zur Übertragung von… … Deutsch Wikipedia
Line code — An example of coding a binary signal using rectangular pulse amplitude modulation with polar non return to zero code … Wikipedia
Conditioned Diphase — (CDP) is a line code for digital baseband transmission. Conditioned diphase uses a line encoding technique that encodes the digital data to be transmitted with a clock signal. Conditioned diphase is also known as Differential Manchester encoding … Wikipedia
Biphase mark code — The biphase mark code is a type of encoding for binary data streams. When a binary data stream is sent without modification via a channel, there can be long series of logical ones or zeros without any transitions which makes clock recovery and… … Wikipedia
Comparison of synchronous and asynchronous signalling — Synchronous and asynchronous transmissions are two different methods of transmission synchronization. Synchronous transmissions are synchronized by an external clock, while asynchronous transmissions are synchronized by special signals along the… … Wikipedia
Методы кодирования цифровых сигналов — В этой статье не хватает ссылок на источники информации. Информация должна быть проверяема, иначе она может быть поставлена под сомнение и удалена. Вы можете … Википедия