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

3.1.3 Previous method of head clog detection

The previous head clog detection system (for the models of GR-DVL9800 and before) is based on the
count of sync blocks as the criterion. The count of sync blocks that a head plays back per frame is:
NTSC: ( Audio 17 + Video 152 ) × 5 Track = 845
PAL: ( Audio 17 + Video 152 ) × 6 Track = 1014
Strictly explaining, number of sync blocks of the ITI sector and sub code sector must be added to the
above count. However, these additional counts cannot be detected by the system of the third generation
models (GR-DVX7 and after).
If some sync block is not detected, the data in the sync block is treated as an error that is impossible to
correct. If the status that the count of sync blocks in the AUDIO/VIDEO sector per 1 frame is lower than
240 of the threshold level continues in the short-playback mode just after start of recording for a certain
period (more than 0.5 second), the system judges that the head is clogged. In other words, the system
recognizes the head clog when the quantity of playback data is one-fourth as little as the normal.
Therefore, if the system detects head clog in recording, it recognizes the recording part as impossible for
playback (regards as no-signal recording).
The short-playback mode just after restart (resuming) of recording is the status that the tape is rewound
for 1.5 second (back-space) according to the absolute track number that is memorized as recording is
suspended (by pause operation). When recording is resumed (restarted), the tape is transported in the
play mode first and then recording is actually resumed with the point of the memorized track number. This
period is called the short-playback period (mode). Head clog detection is not started at the first start of
recording but done at every resuming of recording for the second time, third time, and so on.
REC start
R E C
1.5 sec
Short PB
Fig. 3-1-6 Short Play Back
REC stop
Back Space
Track No. Finding
R E C
3-4