[libdv-dev] video export sensitivity to disk performance

[Alastair M. <am...@pe...>]

Following up to my post yesterday about export to a camcorder vs to
a DV-bridge, I've found that a lot of the timing/sync problems seem
to be due to disk performance -- although there's never a complaint
about dropped frames.

The DV-bridge (Dazzle's Hollywood) still works better than my Sony
TRV103 camcorder, but I found a major difference between the two
PCs I was testing on.   Curiously, the one with the better CPU and
more memory does *worse* -- a file that exported perfectly to the
DV-bridge on the "slower" (CPU) machine gave repeated interruptions and
lost data on the "faster" (CPU) one.

I ran 'hdparm -t' and 'hdparm -T' to find the disk timing, and that
seems to be the critical difference.   The numbers:

                      Export works         Export fails/scrambled
Buffer-cache read      43.4 MB/sec          20.0 MB/sec
Buffered disk read      8.3 MB/sec           6.6 MB/sec

The timing on both is bad enough that exporting to the camcorder is
pointless, but I do get a few more frames (although scrambled) on
the machine with faster disk.    (In both cases the file is larger
than available RAM).

Neither Kino nor dvconnect complain about transfer problems.

I do have a faster machine I can try after I move the 1394 card
(102 MB/sec cache reads, 15.4 MB/sec disk reads), which I can also
put a SCSI drive on, but I won't have time to try that before I
go out of town for a week.  I'm hoping the camcorder works there.

Bottom line: camcorder is still more sensitive to timing than is
the DV bridge, but disk (and bus) performance has a *major* effect on
whether either is usable.

Hope this helps somebody,
  -- Alastair