New Method Of Head Clog Detection - JVC GR-DVA10 Technical Manual

3.1.4 New method of head clog detection

The new head clog detection system (for this DVC series and after) performs detection in the normal
(usual) playback mode besides the short-playback mode just after resuming of recording as well as the
previous system. The criterion of the new detection system is not the count of sync blocks but number of
A/V inner errors per frame.
1) Detection in normal playback
Only when sync data recorded on both channels or one channel is read in the normal playback mode
(after detection of non-signal part), the new system judges that the head is clogged and warns the
user about it if number of A/V inner errors per frame continuously exceeds the threshold level for 7
seconds (for 210 frames: 30 x 7).
If errors less than the threshold level are continuously detected for 2 seconds after that, the system
judges the head as not clogged and cancels the warning indication. If the playback is suspended or
discontinued in the head clog status, the warning indication remains as it was until the system
detects no clog, or Eject or Power Off operation is performed.
2) Detection in short playback
In the period of short-playback just after recording is resumed, the servo controls the track position
according to the self-recording just before recording is suspended and the detection system judges
the head clog and warns the user about it if the status that number of A/V inner errors per frame
exceeds the threshold level continues for 6 frames (for about 0.2 second) after the capstan phase
was locked.
If recording is suspended (by pause operation) as the head is clogged, the warning indication
remains until the system detects no clog in the short-playback mode or Eject or Power Off operation
is performed.
3) Setting of threshold level
The error threshold level is set by the EEPROM as follows.
5 low-order bits in 8 bits: Threshold level in normal playback (0 ~ 31)
3 high-order bits in 8 bits: Coefficient in short-playback (0 ~ 7)
The actual threshold level in the normal playback is 10 times as high as the standard setting value,
and that in the short-playback is several times as high as the actual threshold level in the normal
playback because of the short detection time. If this detection level is converted into number of A/V
inner errors per second (the error rate), it approximates to 10,000. In other words, the system judges
that the head is clogged when number of A/V inner errors per second exceeds 10,000. If the
threshold level of the previous sync block count system is converted into the error rate, it
approximates to 1,000,000. As compared with the sync block count system, the detection capacity of
the new detection system is improved by 20 dB or so.
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