SV: [speex-dev] Speex modes
steveu at coppice.org
Sun Oct 13 03:18:04 PDT 2002
Pontus Carlsson wrote:
>Btw, have you tried using SBR-technology or similar with speech codecs? That
>might be a good idea I thought.. But I don't know if it produces as good
>quality with speech codecs as it does for music codecs. Do you know if there
>is any open source variant of SBR?
SBR exploits a limitation of your ears. At high frequencies (like over
10kHz) you cannot determine pitch with any accuracy. You hear up to
15kHz to 20kHz (depending on age and other factors), but you really
cannot identify pitch at these frequencies. You cannot even determine if
content above about 10kHz is properly harmonically related to the lower
pitched fundamentals which usually give rise to them.
I don't know of any voice specific coder that even attempts to capture
energy above 10kHz. SBR just isn't relevent. Most wideband speech coding
captures only 7kHz to 8kHz bandwidth. The key improvement that gives
over the 3kHz to 4kHz most mainstream voice coders capture is to clean
up unvoiced sounds. fffff, sssss, and other unvoiced sounds appear
almost the same at telephone bandwidth. At 7kHz bandwidth they have
enough character to make them more distinguishable. The basic
intelligibility improvement you get is usually small. However, the voice
is rather more pleasant and less tiring to listen to. That brings
considerable intelligibility improvements in a long discussion. Adding
energy up to the limit of hearing adds more to the pleasantness of the
voice, but it isn't usually considered enough to get people excited
about commiting extra bits per second to it.
<p><p>--- >8 ----
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