DVB-S Measuring Technology - Digital Video and Audio Broadcasting Technology - page 270

• Pre-Reed-Solomon bit error ratio
• Post-Reed-Solomon bit error ratio
The most interesting bit error ratio providing the most information about
the transmission link is the pre- Viterbi bit error ratio. It can be measured
by reapplying the data stream after the Viterbi decoder to a convolutional
coder with the same configuration as that of the transmitter. If then the
data stream before the Viterbi decoder is compared with that after the con-
volutional coder (Fig. 15.1.) (taking into consideration the delay of the co-
der), the two are identical if there are no errors. A comparator for the I
branch and for the Q branch then determines the differences, and thus the
bit errors.
The bit errors counted are then related to the number of bits transmitted
in the corresponding period, resulting in the bit error ratio
BER = bit errors / transmitted bits;
The range of the pre-Viterbi bit error ratio is between 1•10 -4 to 1•10 -2 .
This means that every ten-thousandth to hundredth bit is errored.
The Viterbi decoder can only correct a proportion of the bit errors.
There is thus a residual bit error ratio remaining before the Reed-Solomon
decoder. Counting the correction processes of the Reed-Solomon decoder
and relating them to the number of bits transmitted within the correspond-
ing period of time provides the pre-Reed-Solomon bit error ratio. The limit
pre-Reed-Solomon bit error ratio is about 2•10 -4 . Up to there, the Reed -
Solomon decoder can repair all errors. At the same time, however, the
transmission is "on the brink". A little bit more interference, e.g. due to too
much attenuation due to rain, and the transmission will break down and the
picture will start to show "blocking".
But the Reed-Solomon decoder, too, cannot correct all bit errors, result-
ing in errored transport stream packets which are then flagged in the TS
header (transport error indicator bit = 1). If the errored transport stream
packets are counted, the post-Reed-Solomon decoder bit error ratio can be
calculated.
If very low bit error ratios (e.g. less than 10 -6 ) are measured, long meas-
uring times in the range of minutes or hours must be selected to detect
these with some degree of accuracy. Since there is a direct relation be-
tween bit error ratio and the carrier/noise ratio, this can be used for deter-
mining the latter (see diagram in Section 14.6 “Influences affecting the
satellite transmission link”, Fig. 14.24.). Virtually every DVB-S chip or
DVB-S receiver contains a circuit for determining the pre-Viterbi bit error