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82 01-0870-401B 01/03
Demodulator Application Notes
Data Inversion / Spectral Inversion
In QPSK modulations, every symbol consists of two bits of data. These two bits
are commonly referred two as the I and Q samples. Data inversion occurs when
both I and Q are inverted. Spectral inversion occurs when either I or Q are
inverted. Spectral inversion is caused by selecting the lower sideband instead of
the upper sideband in the mixing and RF filtering process.
All Tiernan demodulators automatically check for and compensate for
data/spectral inversion.
Information Rate/
Symbol Rate/Channel
Spacing
This section provides rate definitions and relationships as well as information on
channel spacing.
Definitions
Information Rate (Bits/Sec)
Information rate is the rate of data leaving the demodulator measured in bits per
second. For Tiernan systems, the information rate is generally equal to the
aggregate transport stream rate.
For the TDR6, the information rate is the data rate parameter.
Channel Rate (Bits/Sec)
Channel rate is the rate of data in the demodulator, before the FEC decoding. The
channel rate equals the information rate plus the FEC encoding overhead
measured in bits per second.
Symbol Rate (Symbols/Sec)
Symbol rate is the rate of symbols being transmitted through the channel
measured in bits per second. For QPSK modulation, there are 2 bits per symbol.
You can query the TDR6 symbol rate using the
DMD.
x
SR command. There is no
front panel equivalent for the SR command.
Aggregate Rate (Bits/Sec)
Aggregate data rate is the rate of the encoder output transport stream measured in
bits per second. The transport stream can be either a single transport stream or
several multiplexed transport streams. The aggregate rate is generally the
information rate going into the modulator or coming out of the demodulator.
Relationships
Information Rate = Aggregate Rate
where the code_rate can be 1/2, 2/3, 3/4, 5/6, 7/8.
The minimum required E
b
/N
0
level varies depending on the code rate. For a table
showing the relationship between the E
b
/N
0
level and the code rate, refer to the
chapter on troubleshooting.
Maximum Rates
Symbol Rates
The minimum and maximum symbol rates are:
■ Minimum = 1 Mbaud
■ Maximum = 30 Mbaud
Symbol Rate 12⁄ * 204 188⁄()* 1 Code Rate⁄()*Information Rate=