On 02/08/2012 06:43 AM, David Rowe wrote: > Hello List, > > I am also intrigued by the wideband possibilities for Codec 2 over > narrow HF digital radio channels. I was brainstorming this with Bruce > and Jean-Marc recently. > > As Jean-Marc points out, there is some more spectral information above > 4kHz. For example a few more peaks and troughs in the speech spectrum > in the range of 4-8kHz. This would require a higher LPC order, and over > the channel a few more LSPs. The good news is that the ear is not very > sensitive to this information, so it can be encoded fairly coarsely at a > low bit rate. That's a misleading way to express things. The ear is *very* sensitive to the energy between 4kHz and 7kHz, which is why so many sounds, especially unvoiced ones, can't be distinguished on a narrow band phone call. It is true to say the ear is not very sensitive to the precision with which this energy is reproduced. Jean Marc has a really neat piece of software to stretch narrowband voice to wideband, and produces a very pleasant result. However, its creating something from nothing, so it cannot actually recreate the lost vocal information and restore the speech clarity lost with a narrow band voice. > No extra bits are required to encode more harmonics or voicing for > wideband audio - we would just extend the harmonics out to 8kHz, setting > the amplitudes based on the LPC envelope. > > So for a few hundred bit/s more we could probably get a Codec 2 running > at a 16 kHz sample rate: i.e. an "AM broadcast" audio bandwidth of > between 50 and 7000 Hz. It wouldn't sound as good as an audio codec > like Opus (or wideband Speex). But booming bass out of a Ham radio > might be an interesting. > > Curiously, this could all be sent over a regular HF Ham radio SSB > "channel" using say 2kHz of RF spectrum, and might only require 1dB more > power. >
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