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Digital Speech Processing

Transmission Channel Characterization & Simulation

A critical component in measuring the performance of a communication system is to understand how well it operates under less than ideal channel conditions. These channel conditions are most often simulated in terms of bit or burst errors that corrupt the data bitstream. Non-repeatable, generic degradations can take the form of random bit errors at a given rate (e.g., 1.0%) or Burst errors, whereby a given number of bits will have a given chance of being corrupted at a given rate (e.g., a 0.5% chance that 50% of the next 54 bits will be corrupted). More specific and repeatable channel simulations can be generated with the use of error patterns; an independent, usually file based, stream designed to model a specific channel condition and be combined with the data bitstream (e.g., by use of Exclusive OR logic).

ARCON has implemented methods to simulate a wide variety of channel conditions for both realtime and non-realtime vocoders in an integrated testbed solution. A realtime speech coder testbed designed by ARCON is available for Host Laboratory and Subjective Test & Evaluation functions. The testbed uses a central test fixture that allows access to two speech coders bitstreams during realtime operation to allow examination and error insertion. ARCON designed a realtime channel simulator PC application that interfaces to the central test fixture. The channel simulator modifies both bitstreams using either randomly generated errors that may effect single bits or designated blocks based on density. A constant channel delay can also be applied. The simulator also reads error patterns from a file. This allows the emulation of a variety of channel conditions such as HF, UHF, VHF and Satellite.