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This is about error correction. The probes add a redundant convolutional code to their signal. Decoding this is easy as long as the error rate is low, a compute
by NohatCoder 3y ago
This is about error correction. The probes add a redundant convolutional code to their signal. Decoding this is easy as long as the error rate is low, a computer program can simply guess what bits have flipped. The issue becomes harder with a higher error rate, and a Viterbi decoder is computationally expensive, but can correct higher error rates than other constructions.
Since the signal strength degrades with distance to Earth, error correction naturally becomes much more of an issue later in the mission. I guess that the probes may have switched between different levels of redundancy through the mission, as the transmission error rate rises. But there was never a point where the convolutional code wasn't useful, it just became slightly more useful with a better decoder.
- kqr 3y ago> a Viterbi decoder is computationally expensive, but can correct higher error rates than other constructions. Higher than others at the time, or higher than turbo codes or low-density parity checks?
- NohatCoder 3y agoWhat I read is that it is the best theoretically possible error correction mechanism, given a convolutional code as input, and thus also the highest cost mechanism that one would consider. This doesn't mean that it is universally the best way of doing error correction, other ways of generating redundancy may provide a better set of tradeoffs. Also, convolutional code is a system that can be configured in many ways, the complexity of the code generation feeds back into the decoding, so a simple convolutional code would be Viterbi decodable at the time, but a more complex system would overall provide better error correction, even though choosing such a system meant that Viterbi would be computationally infeasible.