ARLA/CLUSTER: D-VAP - Como funciona um "Digital Voice Access Point"?

[João Costa > CT1FBF]

Ham Radio - How does the Digital Voice Access Point work?

The D-VAP used to connect a handheld transceiver to the International
D-star network uses G.M.S.K. technology, it stands for Gaussian
minimum shift keying, or to give the technology its correct
explanation Gaussian minimum filtered minimum shift keying.

G.M.S.K. is a form of modulation, modulation means to change, and in
this case we adapt the carrier wave in the signal by adding digital
tones to alter the carrier wave. Many communication systems use
G.M.S.K. it has an advantage over analogue systems G.M.S.K. is a more
efficient use of the radio spectrum. A problem occurs when using other
forms of phase shift keying, side-bands extend outward from the main
carrier wave; they can cause interference to other nearby
communication channel systems.

G.M.S.K. is an efficient modulation which conserves the radio spectrum
bandwidth. The most widely used communication system employing the
system is cellular radio or G.S.M. which stands for global system of
mobile, in other words the mobile telephone network, it is estimated
that over three billion people use the G.M.S.K. technology contained
inside the mobile phone.

Gaussian minimum shift keying is a derivative of M.S.K., which is
actually a form of continuous-phase shift frequency keying, one of the
problems with phase shift keying is that side-bands extend out from
the carrier wave, this problem can be overcome using minimum phase
keying and its derivative Gaussian minimum phase shift keying

Both M.S.K. and G.M.S.K. modulation technologies are classed as
continuous phase schemes; there are no phase discontinuities because
the frequency changes occur at the carrier zero crossing points. An
interesting point to note the unique factor of M.S.K. is that he
frequency difference between the logical one and logical zero states
is always equal to half of the data rate, this is expressed as the
modulation index and it is always equal to 0.5

The way the G.M.S.K. is modulated is to filter the modulated signal
with a Gaussian filter; this is then applied to a frequency modulator
where the modulation index is set to 0.5. This method is very simple
but effective, however there are drawbacks, the modulation index must
be exactly 0.5. 0.5 is difficult to achieve because the analogue
signal is not suitable because of drift that occurs between the
tolerances of components, thus making the index very difficult to set
exactly.

You can use an external aerial on your D-VAP to increase the
transmitted and receive range of the 10 mill watt transceiver which is
contained within the D-VAP, however avoid an amplifier to increase
transmitted range; the D-VAP was not designed as a stand-alone
transceiver and thus is not as clean as a conventional transceiver.

By
John Allsopp G4YDM
https://g4ydm.blogspot.co.uk/

Article Source:
http://EzineArticles.com/expert/John_Allsopp/1925417

Article Source:
http://EzineArticles.com/9631989