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De la lista de Asterisk.
Autor: Steve Underwood.
These days, you can achieve far better quality than a normal phone call at rates much lower than 64kbps.
The main quality issue with normal phone calls is they are limited to 4kHz bandwidth. This is insufficient for good quality speech. 8kHz bandwidth really improves things. It lets you distinguish things "f" from "s", which is almost impossible on a normal phone line. In the 1980s ISDN was introduced with the promise of 8kHz bandwidth (actually specified as 7.1kHz), using a codec called G.722. This uses 48, 56 or 64kbps, and is dramatically better than a normal phone call. Because fully digital end-to-end connections never became common, G.722 never became common either. These days, modern codecs do much better than G.722, but even clunky old G.722 at 48kbps is clearly better than u-law or A-law at 64kbps.
So, forget these weird notions many people have about the magic of A-law and u-law phone calls:
- Normal phone calls have too limited bandwidth for good quality. One of the benefits of VoIP should be to break the 100 year old model of <4kHz bandwidth calls.
- They are not uncompressed - u-law and A-law are lossy compression schemes, which start at 96kbps, and compress this to 64kbps. They use a very simple, but very obsolete way of doing that.
- Modern compression doesn't have to be about achieving indifferent quality at super low bit rates (e.g. G.729). It can be about achieving really good quality at medium bit rates in the 30-64kbps range.
Regards
Autor: Steve Underwood.
These days, you can achieve far better quality than a normal phone call at rates much lower than 64kbps.
The main quality issue with normal phone calls is they are limited to 4kHz bandwidth. This is insufficient for good quality speech. 8kHz bandwidth really improves things. It lets you distinguish things "f" from "s", which is almost impossible on a normal phone line. In the 1980s ISDN was introduced with the promise of 8kHz bandwidth (actually specified as 7.1kHz), using a codec called G.722. This uses 48, 56 or 64kbps, and is dramatically better than a normal phone call. Because fully digital end-to-end connections never became common, G.722 never became common either. These days, modern codecs do much better than G.722, but even clunky old G.722 at 48kbps is clearly better than u-law or A-law at 64kbps.
So, forget these weird notions many people have about the magic of A-law and u-law phone calls:
- Normal phone calls have too limited bandwidth for good quality. One of the benefits of VoIP should be to break the 100 year old model of <4kHz bandwidth calls.
- They are not uncompressed - u-law and A-law are lossy compression schemes, which start at 96kbps, and compress this to 64kbps. They use a very simple, but very obsolete way of doing that.
- Modern compression doesn't have to be about achieving indifferent quality at super low bit rates (e.g. G.729). It can be about achieving really good quality at medium bit rates in the 30-64kbps range.
Regards
