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Size differences between NL and SL data

Comparing the pure size of stored NL and SL data reveals a great quantitative difference. There are two reasons why SL data require orders of magnitude more storage space than written language corpora. The first one is simply the difference in coding between text and speech. Whereas the ASCII string of a word like and needs only three bytes, many more bytes are required as soon as the phonemes  of this word are transformed into an acoustic output for storing the AD-converted data. If in the given example we assume that in clear speech the utterance of a three-phoneme-syllable  takes about half a second and if we apply an amplitude quantisation  of 16 bits and a non-stereo hi-fi sampling rate  of 48 kHz, the NL/SL ratio amounts to approximately 1:16000.
The second reason follows from the great variability in the phonetic forms of spoken words. As pointed out above, any written text must be reproduced by many speakers in more than one speaking style  (at least at slow, normal and fast speeds with low, normal, high voice, etc.), if the corpus is intended to reflect some common sources of variability.

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