In telecommunication, bipolar encoding is a type of
return-to-zero
Return-to-zero (RZ or RTZ) describes a line code used in telecommunications signals in which the signal drops (returns) to zero between each pulse. This takes place even if a number of consecutive 0s or 1s occur in the signal. The signal is s ...
(RZ)
line code
In telecommunication, a line code is a pattern of voltage, current, or photons used to represent digital data transmitted down a communication channel or written to a storage medium. This repertoire of signals is usually called a constrained c ...
, where two nonzero values are used, so that the three values are +, −, and zero. Such a signal is called a duobinary signal. Standard bipolar encodings are designed to be
DC-balanced
In signal processing, when describing a periodic function in the time domain, the DC bias, DC component, DC offset, or DC coefficient is the mean amplitude of the waveform. If the mean amplitude is zero, there is no DC bias. A waveform with no DC ...
, spending equal amounts of time in the + and − states.
The reason why bipolar encoding is classified as a
return to zero (RZ) is that when a bipolar encoded channel is idle the line is held at a constant "zero" level, and when it is transmitting bits the line is either in a +V or -V state corresponding to the binary bit being transmitted. Thus, the line always returns to the "zero" level to denote optionally a separation of bits or to denote idleness of the line.
Alternate mark inversion
One kind of bipolar encoding is a
paired disparity code, of which the simplest example is alternate mark inversion. In this code, a binary 0 is encoded as zero volts, as in
unipolar encoding
Unipolar encoding is a line code. A positive voltage represents a binary 1, and zero volts indicates a binary 0. It is the simplest line code, directly encoding the bitstream, and is analogous to on-off keying in modulation.
Its drawbacks are t ...
, whereas a binary 1 is encoded alternately as a positive voltage or a negative voltage. The name arose because, in the context of a
T-carrier
The T-carrier is a member of the series of carrier systems developed by AT&T Bell Laboratories for digital transmission of multiplexed telephone calls.
The first version, the Transmission System 1 (T1), was introduced in 1962 in the Bell System ...
, a binary '1' is referred to as a "mark", while a binary '0' is called a "space".
"alternate mark inversion (AMI) signal", ''ATIS Telecom Glossary 2000'', last updated 28 February 2001, retrieved 25 January 2007
Voltage build-up
The use of a bipolar code prevents a significant build-up of DC, as the positive and negative pulses average to zero volts. Little or no DC-component is considered an advantage because the cable may then be used for longer distances and to carry power for intermediate equipment such as line repeater
In telecommunications, a repeater is an electronic device that receives a signal and retransmits it. Repeaters are used to extend transmissions so that the signal can cover longer distances or be received on the other side of an obstruction. Some ...
s.["T1 Fundamentals", Revision 1.0, dated 23 January 1997, by Digital Link, retrieved on 25 January 2007](_blank)
The DC-component can be easily and cheaply removed before the signal reaches the decoding circuitry.
Synchronization and zeroes
Bipolar encoding is preferable to non-return-to-zero
In telecommunication, a non-return-to-zero (NRZ) line code is a binary code in which ones are represented by one significant condition, usually a positive voltage, while zeros are represented by some other significant condition, usually a negati ...
whenever signal transitions are required to maintain synchronization between the transmitter and receiver. Other systems must synchronize using some form of out-of-band communication, or add frame synchronization
In telecommunication, frame synchronization or framing is the process by which, while receiving a stream of framed data, incoming frame alignment signals (i.e., a distinctive bit sequences or syncwords) are identified (that is, distinguished fro ...
sequences that don't carry data to the signal. These alternative approaches require either an additional transmission medium for the clock signal or a loss of performance due to overhead, respectively. A bipolar encoding is an often good compromise: runs of ones will not cause a lack of transitions.
However, long sequences of zeroes remain an issue. Long sequences of zero bits result in no transitions and a loss of synchronization. Where frequent transitions are a requirement, a self-clocking encoding such as return-to-zero
Return-to-zero (RZ or RTZ) describes a line code used in telecommunications signals in which the signal drops (returns) to zero between each pulse. This takes place even if a number of consecutive 0s or 1s occur in the signal. The signal is s ...
or some other more complicated line code
In telecommunication, a line code is a pattern of voltage, current, or photons used to represent digital data transmitted down a communication channel or written to a storage medium. This repertoire of signals is usually called a constrained c ...
may be more appropriate, though they introduce significant overhead.
The coding was used extensively in first-generation PCM networks, and is still commonly seen on older multiplexing
In telecommunications and computer networking, multiplexing (sometimes contracted to muxing) is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource - a ...
equipment today, but successful transmission relies on no long runs of zeroes being present.["All You Wanted to Know About T1 But Were Afraid to Ask", Bob Wachtel, retrieved on 25 January 2007](_blank)
/ref>
No more than 15 consecutive zeros should ever be sent to ensure synchronization.
There are two popular ways to ensure that no more than 15 consecutive zeros are ever sent: robbed-bit signaling In communications systems, robbed-bit signaling (RBS) is a scheme to provide maintenance and line signaling services on many T1 digital carrier circuits using channel-associated signaling (CAS). The T1 carrier circuit is a type of dedicated circui ...
and bit stuffing
In data transmission and telecommunication, bit stuffing (also known—uncommonly—as positive justification) is the insertion of non-information bits into data. Stuffed bits should not be confused with overhead bits.
Bit stuffing is used for ...
.
T-carrier uses robbed-bit signaling: the least-significant bit of the byte is simply forced to a "1" when necessary.
The modification of bit 7 causes a change to voice that is undetectable by the human ear, but it is an unacceptable corruption of a data stream. Data channels are required to use some other form of pulse-stuffing, such as always setting bit 8 to '1', in order to maintain a sufficient density of ones. Of course, this lowers the effective data throughput to 56 kbit/s per channel.[Telecom Dictionary, retrieved 25 January 2007](_blank)
/ref>
If the characteristics of the input data do not follow the pattern that every eighth bit is '1', the coder using alternate mark inversion adds a '1' after seven consecutive zeros to maintain synchronisation. On the decoder side, this extra '1' added by the coder is removed, recreating the correct data. Using this method the data sent between the coder and the decoder is longer than the original data by less than 1% on average.
Error detection
Another benefit of bipolar encoding compared to unipolar is error detection
In information theory and coding theory with applications in computer science and telecommunication, error detection and correction (EDAC) or error control are techniques that enable reliable delivery of digital data over unreliable communi ...
. In the T-carrier example, the bipolar signals are regenerated at regular intervals so that signals diminished by distance are not just amplified, but detected and recreated anew. Weakened signals corrupted by noise could cause errors, a mark interpreted as zero, or zero as positive or negative mark. Every single-bit error results in a violation of the bipolar rule. Each such bipolar violation
A bipolar violation, bipolarity violation, or BPV, is a violation of the bipolar encoding rules where two pulses of the same polarity occur without an intervening pulse of the opposite polarity. This indicates an error in the transmission of the ...
(BPV) is an indication of a transmission error. (The location of BPV is not necessarily the location of the original error).
Other T1 encoding schemes
For data channels, in order to avoid the need of always setting bit 8 to 1, as described above, other T1 encoding schemes (Modified AMI code
Modified AMI codes are a digital telecommunications technique to maintain system synchronization. Alternate mark inversion (AMI) line codes are modified by deliberate insertion of bipolar violations. There are several types of modified AMI codes, ...
s) ensure regular transitions regardless of the data being carried. In this way, data throughput of 64 kbit/s per channel is achieved. B8ZS
Modified AMI codes are a digital telecommunications technique to maintain system synchronization. Alternate mark inversion (AMI) line codes are modified by deliberate insertion of bipolar violations. There are several types of modified AMI codes, u ...
is a newer format for North America, where HDB3
Modified AMI codes are a digital telecommunications technique to maintain system synchronization. Alternate mark inversion (AMI) line codes are modified by deliberate insertion of bipolar violations. There are several types of modified AMI codes, u ...
is the original line coding type used in Europe and Japan.
A very similar encoding scheme, with the logical positions reversed, is also used and is often referred to as pseudoternary encoding. This encoding is otherwise identical.
Historical uses
B-MAC
B-MAC is a form of analog video encoding, specifically a type of Multiplexed Analogue Components (MAC) encoding. MAC encoding was designed in the mid 80s for use with Direct Broadcast Satellite systems. Other analog video encoding systems inclu ...
, and essentially all family members of the Multiplexed Analogue Components
Multiplexed Analogue Components (MAC) was an analog television standard where luminance and chrominance components were transmitted separately. This was an evolution from older color TV systems (such as PAL or SECAM) where there was interferen ...
Television Transmission family used ''Duobinary'' to encode the digital audio, teletext, closed captioning and selective access for distribution. Because of the way Duobinary was coupled to the NICAM
Near Instantaneous Companded Audio Multiplex (NICAM) is an early form of lossy compression for digital audio. It was originally developed in the early 1970s for point-to-point links within broadcasting networks.Croll, M.G., Osborne, D.W. and Spi ...
like digital audio subsystems for the MAC family, up to 50% of data reduction was possible in both Stereo and Mono transmission modes. At least with some data transmission systems, duobinary can perform lossless data reduction though this has seldom been utilized in practice.
See also
* MLT-3 encoding
MLT-3 encoding (Multi-Level Transmit) is a line code (a signaling method used in a telecommunication system for transmission purposes) that uses three voltage levels. An MLT-3 interface emits less electromagnetic interference and requires less band ...
* Polar encoding
In telecommunication, a non-return-to-zero (NRZ) line code is a binary code in which ones are represented by one significant condition, usually a positive voltage, while zeros are represented by some other significant condition, usually a negati ...
References
{{Bit-encoding
Encodings
Line codes