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2.3.3 Explanation of the term “symbol rate”
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 error correction bits to user bits. The output sym-
bol 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 64-QAM, FEC=
3
/
4
,
Input symbol rate SR (E) = 27,500 kilosymbols per second
SR (A)
=
3
/
4
x
1
/
3
x 27,500 kilosymbols/s
SR (A)
=
6,875 kilosymbols/s
Note:
If no “FEC” is stated in the station lists, it can be assumed to be
“FEC =
3
/
4
”.
Reception from a transponder with a very low symbol rate
(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 QAM modula-
tion with stuffing to a higher value.
Defined symbol rates
Some cable operators specify a fixed symbol rate (e.g. 6,900 kilosymbols per
second).
