2-25
2.5.4 Basic principle of Viterbi detection
Recording
signal
1V
-1V
0V
Threshold level
"1"
"0"
"-1"
PB
signal
"1" "0" "0" "1" "0"
Usual detection
(Hard decision)
Viterbi decoder
0.01.0 -0.4 0.8 0.0 (V)
ERROR !!
A/D converter
"1" "0" "0" "1" "0"
"1" "0" "-1" "1" "0"
Select the most
reliable line
point A
Threshold level
Fig. 2-5-4 Basic principle of Viterbi detection
Fig. 2-5-4 is a conceptual chart showing the basic principle of Viterbi decoding method. Decoding means a
ternary decision that judges differential waveform at the identification point by the ternary criteria when
NRZI-recorded signal is played back.
The previous detection method is based on the ternary criteria of the preset identification level, and this
method is called the hard decision because of the fixed identification level. By this method, for example,
the identification value at the point "A" (in Fig.2-5-4) is "0", which represents an error occurrence.
On the other hand, the Viterbi decoding adopts the soft decision method. In the Viterbi decoding, playback
signal is converted from analog to digital data and then the signal level is read. If the signal level is 0.4V at
the point "A" by ways of example, the previous method judges it as "0", but the Viterbi decoding method
detects a possibility that it may be "0" or "1" and it assumes two kinds of bit strings of "10010" and "10110".
Next, the Viterbi method introduces another criterion in decision. In the NRZI recording, there is a
regularity in the recording signal and playback waveform. That is to say, there is a fall point between two
rise points in the recording signal. This means that there must be "
−
1" between "1" and "1". According to
this principle, the bit string of "10010" is theoretically non-existent, and "10110" is consequently selected.
As mentioned above, the Viterbi decoding method utilizes the regularity between bits or the redundancy of
NRZI-recorded signal for error correction. The above explanation of the Viterbi decoding method is just a
conceptual description, and a high degree of data processing system such as to select the most possible
bit string from a great deal of probabilities is introduced in the actual Viterbi decoding.
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