Greg Hill wrote:
My first thought was an RC filter, too. But I'd suggest that 500 Hz is too
high a cutoff, because a note like a middle C is 256 Hz. I don't think
it's uncommon for a voice (especially a male voice) to be in that range
frequently. Although (in English, at least) vowels generally have a low
frequency and sharp consonants (like a t) have a high frequency. It's the
consonants which do the most for understanding a word, so maybe having
those low frequencies attenuated wouldn't be so bad after all.
The telephony codecs all filter hard below 300Hz and above 3400Hz, so
your middle C gets attenuated quite a lot on any normal phone line. The
F0 (basic pitch contour) of my voice can go below 50Hz. However,
filtering at 300Hz clearly doesn't screw that up, since all phones do
that and voices still sound OK :-) The 3400Hz limit has more effect on
voices, especially a trill woman's voice. 7kHz telephony makes 's', 'f'
and other unvoiced sounds much easier to distinguish. That has
suprisingly little effect on overall intelligibility, though.
Regards,
Steve
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