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Back-off Language Model Compression

Boulos Harb
Proceedings of Interspeech 2009, International Speech Communication Association (ISCA), pp. 325-355
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With the availability of large amounts of training data relevant to speech recognition scenarios, scalability becomes a very productive way to improve language model performance. We present a technique that represents a back-off n-gram language model using arrays of integer values and thus renders it amenable to effective block compression. We propose a few such compression algorithms and evaluate the resulting language model along two dimensions: memory footprint, and speed reduction relative to the uncompressed one. We experimented with a model that uses a 32-bit word vocabulary (at most 4B words) and log-probabilities/back-off-weights quantized to 1 byte, respectively. The best compression algorithm achieves 2.6 bytes/n-gram at ≈18X slower than uncompressed. For faster LM operation we found it feasible to represent the LM at ≈4.0 bytes/n-gram, and ≈3X slower than the uncompressed LM. The memory footprint of a LM containing one billion n-grams can thus be reduced to 3–4 Gbytes without impacting its speed too much. See the presentation material from a talk about this paper.