[ct-user] DVP white noise

[Bill Claypool]

I did a little more digging into the problem I had with white noise on
playback.  There seems to be a problem when CT copies files, either
from the hard disk (my C:) to the RAM disk (my D:), or in the other
direction.  In particular, bytes equal to hex 1A (control Z - end of
file in text mode) or 0D (carriage return) did not copy.  In my test
file, I had one of each, so the copied file was two bytes shorter than
the original.  There may be other codes that don't copy - I didn't
test them all.
 
Since the 12-bit audio takes two bytes to record one sample, missing a
byte shifts the sample by 8 bits, scrambling the audio into white
noise.  When the second missing byte is skipped, the audio is back on
track.  Therefore, the white noise can come and go several times
during the playback, but always at the same times during the playback.

There is a workaround.  I make the recordings with CT, which writes
good files to the RAM drive.  I then exit CT and copy files using the
DOS COPY command to a subdirectory on my C: drive (NOT \CT\DVP).
Then, I make sure there aren't any DVP files in my \CT\DVP directory.
Then, each time I boot up, I need to copy the DVP files to the D:
directory myself using COPY.  Then when CT starts, as long as it
doesn't find any DVP files in the \CT\DVP directory, it won't
overwrite the good files in the RAM drive.

I didn't have any problems until I got a new microphone.  The old one
didn't generate the same level of audio, and I never got the bad codes
in the audio.  As long as you never exceed about 89% of full scale
audio level, the noise won't happen.

Bill
NU0Q

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