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HDTA 614 C ASI
2.7 e
xPl anation
of
the
ter m
"
symbol
r ate
"
Modulation schemes such as QPSK and QAM transmit multiple bits simultane-
ously. These are referred to as symbols. In addition to the user data flow which
transmits video and audio information, error correction bits are transferred.
The FEC number states the ratio of user bits to the complete transmitted bits.
The output symbol rate is calculated as follows:
256-QAM:
SR (A)
= FEC x
1
/
4
x SR (E)
128-QAM:
SR (A)
= FEC x
2
/
7
x SR (E)
64-QAM:
SR (A)
= FEC x
1
/
3
x SR (E)
32-QAM:
SR (A)
= FEC x
2
/
5
x SR (E)
16-QAM:
SR (A)
= FEC x
1
/
2
x SR (E)
4-QAM:
SR (A)
= FEC x
1
/
1
x SR (E)
Example:
Output symbol rate SR (A) 64-QAM, FEC=
3
/
4
,
Input symbol rate SR (E) = 27,500 kSymb/s
SR (A)
=
3
/
4
x
1
/
3
x 27,500 kSymb/s =
6,875 kSymb/s
—>
If no "FEC" is stated in the station lists, it can be assumed to be "FEC
=
3
/
4
".
t
r ansPonder
with
a
very
low
symbol
r ate
(sCPC
station
)
The extremely low data rate means that the output symbol rate is very low. If
there are reception problems with different digital receivers, set output symbol
rate to a higher value.
d
efined
symbol
r ates
Some cable operators specify a fixed symbol rate (e.g. 6,900 kSymb/s).
