Decommutator
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[edit] Overview
'Decommutator, Decom, or Decommutation is a digitally-based term used in Telemetry to describe the process of removing multiple channels of encoded data from a frameworked and synchronized serial bit stream that acts as a carrier for the data. The core of the stream is data composed of physical measurements. Decommutation is the complex digital analogue of the demodulation process in analog-based electromagnetic signal transmission. For example, removal of music (data) from a radio carrier is analogous to the removal of measurement data from a telemetric serial bit stream. Whereas in the case of the radio signal you only remove one channel of music, in a telemetric serial bit stream you remove multiple channels (could be thousands) of measurements.
The reverse process of a Decommutator is a Commutator or Multiplexer, which adds multiple channels of data onto a framed and synchronized serial bit stream. Time-division multiplexing (TDM) and Pulse Code Modulation (PCM) are basic telemetric techniques of encoding or cleverly adding measured data to the serial bit stream. These techniques allow multiple channels to share the single carrier, thereby conserving bandwidth. The term "commutation" is derived from direct current electromechanical motors where switching devices (i.e. - "brushes") called commutators, reverse the electric current through a winding immersed in a magnetic field to produce mechanical torque.
There may be several levels of commutation in a telemetry system to be able to accommodate the efficient use of the serial bit stream when the various measurements to be made require different sampling rates. A slow-changing measurement will require less sampling than a fast-changing measurement. For example, ambient temperature and the transient produced by lighting, respectively. Hence, in this two channel example there might be a subcommutation level and a supercommutation level. Both having various kinds of “accounting” methods available to keep track of the synchronized data frames, including Subframe Identification, Frame Code Complement, and Unique Recycle Code. The subcommutation level may sample at one-millionth the rate of the supercommutation rate, depending on the fundamental sampling rate of the measurements as determined by their Nyquist frequency.
Realization of a decommutation process is achieved by implementing software, firmware, and hardware. Software decommutation is best for handling large varieties of complex IRIG formats since computer code is flexible. But because execution is relatively slow despite the ability to multi-thread measurands, the transmit rate of the serial bit stream is not exactly state-of-the-art. Often groups of distinct measurements (channels) are assigned to several hardware-based (DSP/FPGA) decommutators for the purpose of optimizing the transmit rate without exceeding the desired Bit error ratio (BER). It is possible to achieve a rate of one-in-a-million errors with a relatively large signal-to-noise input ratio.
