Why is Manchester encoding preferred to PIE(Pulse Interval Encoding) for applications were data is transferred through inductive coupling? For transmission of data and power through wireless medium (other than inductive coupling) will PIE be better suited?
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1The only thing that comes immediately to mind is that the transmit time for Manchester encoding is a strict function of the data length, whereas with PIE, the data content also plays a role. – Connor Wolf Oct 28 '12 at 08:06
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Also Manchester encoding/decoding is very simple to implement - it only takes a few gates, which is/was signficant given its long history (invented circa 1948 ? 1950 ?) – Paul R Oct 28 '12 at 09:05
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Manchester is the simplest scheme that works, there are many many others. – starblue Oct 28 '12 at 09:10
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Manchester encoding is designed so that the percentage of time that the signal is in one state is exactly equal to the percentage of time that the signal is in the other state. This means that there is no accumulating bias of the average level of the signal while it is transferred through what ever communications medium is in use. Pulse Interval Encoding on the other hand does not have this property.
The average level of zero for signals being sent through a channel is important for any medium that cannot be DC coupled or for where an accumulating bias toward one state or the other would would cause the receivers used in the path to saturate toward one side or the other.
Manchester encoding extracts a price for its use because it requires two times the bandwidth of the actual data rate being sent through the communications channel. There are a number of other serial type protocols that help to lower this "cost" by being designed for less than the 2X bandwidth requirement. The various schemes add compensating patterns to the data flow that so that the average level of the signal stays at zero over a longer period of time than the one bit interval used by Manchester.
Pulse interval encoding is commonly used for IR encoding with remote controls. They get by with this at the receiving end in one of two ways. The messages sent are short compared to the repeat time between messages which allows the receiver to relax to the center of its detection range between messages. And some IR encoding protocols include both the true and inverted copies of the data in the message packet sent which helps to average the receiver. It is interesting to note here that Manchester encoding is actually used in some IR remote protocols including the Philips RC5 encoding.
Michael Karas
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Can Baseline/DC wander be a problem if the PI encoded data is modulated using a modulation scheme like ASK? Also, is baseline wander a problem if a digital decoder is used instead of an analog one? – Mike George Oct 29 '12 at 06:20
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@MikeGeorge - Sure you can use ASK but that could apply to any scheme of encoding, Manchester, PIE and others, that use the encoding as a frequency shift selection envelope. ASK can certainly solve the DC average level of the signal through the communications channel. With respect to receiver/decoder - All receivers are inherently analogue at the front end and even the simplest ones use a comparator to convert the input signal to the digital domain. It is in this initial stage where DC level offsets can be a problem for signalling that does not have zero average level. – Michael Karas Oct 29 '12 at 08:00
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Will a comparator be required, if I use OOK(on-off keying) with envelope detection to demodulate the encoded data? – Mike George Oct 29 '12 at 10:54
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@MikeGeorge - The concept here is too general to be able target specific design questions. So much depends upon factors including usage model, isolation requirements, distance, transmission medium, bit rate, acceptable bit error rate, available node processing bandwidth and product BOM cost requirements. – Michael Karas Oct 31 '12 at 11:44
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@MichaelKaras: Another example of a situation where Manchester encoding helps is when using automatic-gain-control circuits to receive data. With Manchester coding, one can simply say "when signal is below threshold, increase threshold; when signal is above threshold, decrease it". When the threshold is below the noise level, the input will read high all of the time it's supposed to be high, and some of the time it should be low; since the duty cycle would be over 50%, the threshold would increase until that was no longer the case. When the threshold is above (signal-noise), then... – supercat Apr 03 '13 at 16:24
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the input will read low all the time it's supposed to be low, but some of the time it should be high. This will thus reduce the threshold. If the signal level exceeds the noise level, the threshold will tend toward a zone allowing reliable reception. Using Manchester, the signal could only read over the threshold e.g. 40% of the time and below it 60% if threshold is too high. With PIE, such a balance occer whether the threshold was too high, too low, or just right. – supercat Apr 03 '13 at 16:30
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@supercat - No argument from me. I agree with what you wrote. I like Manchester encoding more than sliced bread!! Also with Manchester and using a robust decoding scheme it is possible to pull valid data out of the input stream with rather high percentage pulse width distortion. Such scheme requires a state machine type design that checks currently detected pulse widths against an expected range for 1x and 2x wide pulses. Some applications I used tolerances of as much as +/- 30% from nominal. – Michael Karas Apr 03 '13 at 16:50
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@MichaelKaras: Pulse-width distortion can be corrected in both Manchester and PIE encodings, though with Manchester encoding if the pulses are somewhat "ramped", the pulse-width distortion may be corrected in hardware. – supercat Apr 03 '13 at 19:38
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I don't know what "PIE" encoding is, but when manchester encoding is used it is usually for one of these reasons:
- The average level is 1/2 even over short intervals. Since each bit is 1/2 bit high and 1/2 bit low differing only in which comes first, every bit averages to 1/2. This is useful to automatic level control of data slicing circuits, which in turn helps with signal to noise ratio.
- Both high and low are visible frequently, which happens once per bit. This is useful for some other types of data slicers and to be able to track the strength of the signal against the background.
- It is self clocking. Only a single signal is required, such as you get over a simple RF transmission.
Olin Lathrop
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Pulse Interval Encoding can be considered self clocking as well. Such protocols either use a pulse between each bit interval or use a transition between bits to represent the bit boundary. – Michael Karas Oct 28 '12 at 13:25
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Why is Manchester encoding preferred to PIE(Pulse Interval Encoding) for applications where data is transferred through inductive coupling?
Inductive coupling is used for short distance communication to avoid connectors. Since this blocks DC and is not an ideal transmission line with constant impedance, flat frequency response, it has more group delay distortion than wired data links. Pulse Interval Encoding is asymmetric in pulse width for each symbol which is more sensitive to time delay noise, resulting in higher threshold of detection, lower bit rates or less distance. The best application of this modulation is low cost infrared remote control transmitter and receivers. They compromise the bit rate for signal integrity but is cheaper to decode without the need for synchronous or coherent receivers. Below is the signal for PIE code with OOK modulation where 1's and 0's are 2T, 1T durations of carrier ON followed by 1T duration of carrier off. THe long carrier durations are good for fast AGC normalization of carrier amplitude for easy AM detection.
Bi-Phase or Manchester baseband coding, although much less spectral efficient than other forms of phase coding, it is superior for noise immunity meaning it has a much improved bit error rate (BER) at low SNR's. "Raised Cosine" filtering can be used on this symmetrical data pattern which is preferred over PIE for data integrity and higher speed both at low cost.
For transmission of data and power through wireless medium, will PIE be better suited?
PIE is preferred for applications such as RFID passive transponders since to demodulate and decode this method is as "easy as PIE" ;) It can be very low power and low cost for the Rx can be passive or use energy stored from the carrier to power a short burst on its TX for RFID response.
THe challenge to store energy from a wireless signal requires the Rx to consume the least power, and acquire data cheaply, with reliable fast recovery of data. Thus optimal criteria of spectral efficiency and data rate must be compromised to yield the required data integrity, cost target, distance and sensitivity.
The requirements for charging devices through wireless medium with data are quite different than passive or active RFID's as very close coupling is needed as power requirement increases.
The best method is becomes one that consumes the least power to recover clock and data and with adequate sensitivity and data integrity to receive power and data.
PIE offers promise of low power but redundant coding like Bi-Ø which uses more spectrum due to the self clocking of redundancy of transmitting a full clock cycle per bit, can have better data integrity or BER.
Tony Stewart EE75
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1PIE is better at transferring power to a passive RFID tag from a RFID reader than Manchester encoding. – Mike George Oct 29 '12 at 11:59
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Yes PIE offers more energy to transmit carrier and Bi-Ø offers better reception on reader. Many different coding methods exist for RFID's 0,1 from 1/2T,2/3T to 7/8T,5/8T to 1T,7/8T with a wide range of AM levels and protocols. – Tony Stewart EE75 Oct 29 '12 at 14:34
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A very simple algorithm to generate Manchester code for the data in as under: 1. In accordance to IEEE802.3 it is original data XOR Clock= Manchester code, and 2. In accordance to G.E. Thomas it is ~(original data XOR Clock= Manchester code).
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This doesn't seem to be answering the question. It is also quite difficult to tell what exactly your point is. This is not a traditional forum. We expect answers here to actually answer the question and to stand on their own to a reasonable extent. – Olin Lathrop Apr 03 '13 at 13:36