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Giacintec T.C.E. Mesure, Télémesure, Equipement Télémesure, Capteurs, Electronique

PCM encoding

Introduction

Shaping measurement signals for wire or radio transmission can be achieve different wise. Before entering PCM, it is interesting to have an overlook over different encoding methods. A simple one is the conversion of the signal into a frequency that will be called subcarrier and can be frequency multiplexed. The multiplex is carrying all measurements simultaneously and without de-phasing . Another solution is consisting in phase modulation of each measurement but it is not in used too much these days. Other thechnics called pulse analogic modulations can help shapping a message.

PAM or Pulse Amplitude Modulation

PDM or Pulse Duration Modulation

PPM Pulse Position Modulation

PFM Pulse Frequency Modulation

PCM ( Pulse Code Modulation ) data are transmitted as a serial bit stream of binary-coded time-division multiplexed words. A standard define pulse strain structure and system design characteristics for the implementation of PCM Telemetry formats

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See underneath the PCM Code definitions.

Two classes of PCM format exisct : the basic and simplier Class 1 and the more complex, class 2. Class 1 as a fixed format when Class 2 has a changing format.

Data is represented as a word and the message as an association of several words. Message has to be referenced to determine start up and end, as well, each information has to be referenced in time. A PCM message starts with a synchronisation word or several words that will recognize start and end of the message.

To process PCM message, beside synchronisation, processing must be done versus the clock. The clock is also a frame compared continuously with the message frame.

Military and spatial PCN clocks uses a pulse frame 1/0 with fixed pulse durations. This allows a very good accuracy but is complicated at level of demodulation and, therefore, very expensive.

Other types of clocks exist and particularly the one used in our Microcode. This clock is based on PDM and processing this pulse duration clock is easier and less expensive.



Giacintec T.C.E. Mesure, Télémesure, Equipement Télémesure, Capteurs, ElectroniqueObviously, the accuracy is not identical to the one of strong PCM clocks but is, anyway sufficient for a lot of Military and civilian applications.

Bi and NRZ-L

Microcode gives a frame as a Bi-L signal with a maximum speed of 1 Mb/s along with an NRZ-L signal and a clock allowing reccording.

Signal Bi-L or Bi-phase level ( split phase ) makes the level change occurs at center of every Bit, one is represented by one level with transition to the zero level and zero is represented by a zero level with transition to one level.

For NRZ-L, One is represented by level 1 and zero by the other level.

When words are encoded, a reference message or synchronisation is sent, then the words on the frame and the clock.

Mesurements and encoding

Analogic measurement signals are conditionned and amplified to match the digital conversion. 6, 12, 18 measurement signals ( up to 48 48 ), and digital channels ( on/off ) as


Here is a PCM frame structure. Max. frame length for PCM Class 1 is 8192 Bits or 512 words. A word for a PCM Class 1 has a length of minimu 4 and maximum 16 Bits.


Giacintec T.C.E. Mesure, Télémesure, Equipement Télémesure, Capteurs, Electroniquewell as a frame counter on 24 Bits are used to date signals and pick up measurements following a stocked program and then fit onto the PCM frame.

Conditionners are to supply input levels for batches of 6 channels. A group is compatible with +/- 10 V ; +/- 5 V ; 0 to 10 V and 0 à 5 V, B group +/- 4,096 V ; +/- 2,048 V ; 0 to 4,096 V and 0 to 2,048 V. Obviously, choice of level depend upon measurement sensitivity.

On off channels operate in 0 – 5 V.

Conditionners take signals output of the sensor, for some with and depending upon power supply and achieve, depending of the sensor a cold junction compensation, an amplification / linearisation ( Thermocouples ) or power supply and amplification for strain gages.

Analog signal has, inside, all the errors due to the sensor as well as the noise. Coded message correspond the the analogic signal processed and amplified.

Analog signal is translated into digital and will, therefore be protected from pollutions. In other words, a signal with an x accuracy will be transmitted and seen with the same accuracy.

In terms of bandwidth, sampling frequency is given by the components and the limitation is the 1 Mb/s, frame length including synchro message and the value of each word in between 4 and 16 Bits ; The 24 Bits counter will allow, beside other things to date 16 million frames.

Programmation will allow to sample quicker or slower different channels depending upon theyr nature. One would expect large bandwidth for accelerometers and low bandwidth for temperatures.

Why a ‘ low cost ‘ PCM

Some aeronautical applications requests few channels and the low cost PCM will fullfill the need. For instance, R.P.V. applications or ground tests are the first concerned. Transposition to industrial needs is evident. When the cost of a classical PCM encoder / Decoder will never be acceptable for industrial applications, this little device will answer with a better ration quality / price to an hybrid need in between Mil and RS 232 or Pseudo PCM. Data are then from analogical, converted and transmitted on a PCM Frame and the access on a simple PC become evident.


Here the PCM encoder as a PC board with spece in the housing to install conditionners and transmitter.

Max PCM speed is adjusted from 15.625 to 1000 Kbits/s ( with options when changing the quartz. ).


Giacintec T.C.E. Mesure, Télémesure, Equipement Télémesure, Capteurs, ElectroniqueWorst part of a PCM system is the demodulation. Whatever can try to do, this remain of a Military level ans consequently has a cost

The card is plug in a PC and will allow analogic and digital output of the data. The card can process up to 20 Mb/s, Signal demodulation, timing and processing are included.

Processing

Analogical processing is allowed also.

Analogical processing make possible instantaneous reaction and for process controls and regulation this is a main concern.

Regulation loops can be operated, even remotely and why not wireless.

Hybrids systems

For some applications, PCM message can be along with analogical messages so the very high bandwidth signals can be transmitted in same time.


Processing allow operation directly in engineering units with possibilities of motions displays, bargrapfs, histograms and so on.

Exportation under ASCII and much more are made possible.

Other sophisticated processing and calculations offers multiple asdvantages.


Giacintec T.C.E. Mesure, Télémesure, Equipement Télémesure, Capteurs, ElectroniqueFor others applications, the video signal has to be along with the measurement message. These hybrids systems are used for testing of vehicles or part of vehicles ( trains, Mil vehicles, RPV’s and so on ).

Conclusions

Choice of a telemetry system and of the ‘ encoding ‘ of measurements depend on several factors and upon the type of application.

One has to keep in mind these industrial systems are far from mass market products. Even if we have been looking deep into Bluetooth and wiFi solutions ( not cheap Bluetooth and wiFi or Zigbee ), even if we still carry low cost telemetries, we focuss on real industrial solutions answering to applications were telemetry needs to be adapted to the process and where PCM is one of the solution.

If analogical systems suppose specific design and adaptations to answer the applications like ( high speed, high temperature, large bandwidth and small volumes … ), if our Micritrad system is matching the needs of rotating ovens or tunnels in extreme conditions, if our Bluetooth and WiFi systems are adressing few applications, our PCM telemetry is completing the large range of our products.

Microcode is used in aeronautics for some applications where the usual PCM is too expensive, sometimes over the cost of the total application.

Our task is to help out the user in order to make the best choice and succeed.

We are always developping new solutions for applications and we keep developping new telemetry concepts to remain ahead on the market.

See our PCM Telemetry spec sheet.

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