John,
D-STAR has a very specific standard for the on air digital voice
signal. The signal rate must be 4800 bps (to achieve 6.25khz.
bandwidth). Any variance from that rate and you have broken the
standard and would be incompatible with D-STAR. The specification
also requires the use of AMBE vocoder technology. The specification
also requires the use of the 3600 bps encoding (2400 bps voice + 1200
bps forward error correction or FEC) and any other values would be
incompatible even though the AMBE vocoder can be run at other rates
and voice to FEC ratios. After you use 3600 bps for voice+FEC, there
is an additional 1200 bps that is used for protocol addressing and
various user defined data components (Icom has added some useful
extensions in that space, GPS, Short Text, some protocol redundancy).
To move outside of these standards is certainly within the realm of
amateur radio and amateur radio experimentation but D-STAR is very
defined, by the JARL, as to what can be done. If one wants a higher
fidelity voice signal, then one is free and encouraged to experiment,
but it won't be D-STAR and won't be able to take advantage of the D-
STAR infrastructure. Certainly the manufacturer of AMBE technology
works to improve the encoding and decoding of voice and could possibly
come up with new generations of chips to fit the D-STAR parameters,
but they would have to be backward compatible for acceptance in the
market.
The final sound of the AMBE signal after decoding is not an exact,
high fidelity, reproduction of the original voice encoding, but for
the sake of communicating intelligence it works very well (assuming
the speaker is speaking intelligently :) ). Different people have
different perspectives on that audio, for example, one local ham finds
the AMBE processed voice easier to listen to due to his specific
hearing ability.
There are some in the hobby that would go for FM broadcast quality
fidelity, but it is terribly inefficient from a spectrum point of
view. Higher encoding bit rates could provide higher fidelity, for
example Sirius digital satellite radio uses AMBE as well, but again it
would be at the sacrifice of bandwidth for what is a communications
service that does not need the fidelity and musicality of an
entertainment service.
AMBE is a registered trademark of Digital Voice Systems, Inc.
D-STAR is a trademark of the JARL (in Japan) and a registered
trademark of Icom in the US and several other markets.
On Sep 1, 2010, at 11:26 AM, n2gyn wrote:
Thank you all for you reply and comments.
Let me make myself clearer.
I would like to see the audio quality of D-Star be improved. To MY
ears' everyone sound like a robot. I thought this was due to the low
bit rate. I am NOT impressed with the digital voice mode. I want to
hear a more natural sounding voice. My telephone sounds better.
How could this be achieved if not by bit rate?
John
John D. Hays
Amateur Radio Station K7VE
PO Box 1223
Edmonds, WA 98020-1223 VOIP/SIP: j...@hays.org
