On Sat, Jun 11, 2011 at 07:40:54PM +0300, Patrik T. wrote:
As you all know, VCM is short for Variable Coded Modulation.
As I have read on your forum, the new DVBS-2 NOAAPORT transmission is variable.
So far I have noted that users use 8PSK demodulation and FEC?.
FEC?, due to users use AUTO most likely (search all HW supported FEC and lock
when found, I guess it is 9/10?).
VCM is a standardized optional mode of DVB-S2. It allows each
S2 frame of data to be sent in a specified and changeable modulation.
The actual modulation used is one of the standard set available in S2
ranging from QPSK to 32APSK… with any of the standard FECs from 1/2 to
All VCM implies is that rather than using the same modulation
all the time for everything it can be changed by the transmitting end on
the fly as conditions or requirements change.
Unfortunately for those in possession of normal demodulators
designed for CCM (Constant Coding Modulation) the ability to read and
understand the BPSK FEC 1/2 control messages and headers/sync words when
the demod is set to the actual modulation used for the data doesn’t
exist for the most part and such demods will treat such as errors and
perhaps drop lock or fail to recognize valid data frames properly even
though the data is actually in a format they understand. This applies
even if the data is ALWAYS in the same format.
A proper VCM demod chip will read the header and control message
and set up to the required modulation and FEC fast enough to recover the
frame transmitted in that format.
This can be used to partition traffic into a high and low
priority stream or streams with different modulation parameters, but
need not be.
A receiver receiving a VCM stream sees a series of frames one
after the other and separated by the BPSK control messages and special
sync headers, some it may be able to relaibly demodulate without
uncorrected errors, some not depending on Eb/No (carrier to noise).
If the data is partitioned into different classes sent with different
modulation a driver for such a chip would have to sort out the frames it
was interested in from those it wasn’t and forward them on to higher
layers of processing as needed. This might be implemented as multiple
separate streams sent to multiple separate endpoints, or as the whole
stream forwarded as received with frames that got garbled marked as such
for sorting out by upper layers.
As far as I currently understand it, NOAAPORT is not yet (or
maybe ever) transmitting different priority streams using different
modulations - but they could.
But as implemented currently the modulation used on all the
NOAAPORT streams can be changed from time to time as reception
conditions at critical sites deteriorate. Without VCM doing this
would require that all demods in the entire system be subject to being
switched by remote control from modulation to modulation dynamically by
some sort of control mechanism via some highly reliable path - with VCM
this reliable path is built into the signal as the BPSK FEC 1/2 control
messages are very robust and repeated regularly in the signal stream.
And such switches can be done without losing any data. Further a demod
that has been off line and comes on line will be able to immediately
determine the mode in use, rather than having to search or wait for some
kind of off line command message.
The downside is that VCM is not currently supported by cheap
DVB-S2 demod cards… though that is coming.
Dave Emery N1PRE/AE, [email protected] DIE Consulting, Weston,
"An empty zombie mind with a forlorn barely readable weatherbeaten
‘For Rent’ sign still vainly flapping outside on the weed encrusted pole
celebration of what could have been, but wasn’t and is not to be now