The MIDISPORT 8x8/s is capable of reading and writing SMPTE User Bit
groups U1 through U8. The values of the User Bits output during writing may be
set with the Remote Control software. When reading User Bits from an incoming
source, a MIDI User Bits message is sent to the application each time sync is
acquired and whenever a change in User Bits is detected.
SMPTE Offset
An “offset” is the starting time of the first frame of time code and may be set to
any time value in the range 00:00:00:00 through 23:59:59:29. Usually, you can set
your offset to any value in this range, but sometimes the production
environment will dictate the offset requirements. The MIDISPORT 8x8/s has a
default write offset of 01:00:00:00. If you need to change the offset time, you may
do so via the supplied Windows Remote Control software.
IMPORTANT: Try to avoid high offsets that may wrap around (from 23:59:59:29
back to 00:00:00:00). Some programs become confused by this wrap-around.
Choose an offset that will allow your entire session to be striped without rolling
over the 24:00:00:00 time code value.
At times, you may be given a tape with SMPTE already on it. The offset time on
the tape you receive will affect the start time that you set for your MIDI
sequencing. For example, if the time code on the tape starts at 02:00:00:00 (a 2-
hour offset) and you need to start your sequence playing 3 minutes, 30 seconds
into the work, you should set the sequencer start time to 02:03:30:00.
Because MTC requires 8 quarter-frame messages to send a full frame of time
code, a minimum of 2 to 4 frames will be needed for a sequencer to first sync up
to a running SMPTE. Therefore, when syncing a MIDI sequencer or any program
to time code, it is a good idea to set your sequence start time to a few seconds
later than the offset of the time code on tape. In other words, give your
sequencer some time to acquire sync and stabilize before it begins playing back.
For example, if your tape starting offset is 01:00:00:00, then set the sequence start
time to 01:00:05:00, which is 5 seconds later. This is also known as a 5-second
pre-roll. Some sequencers will actually allow you to directly set the “pre-roll”
time to a specific number of seconds. These programs will automatically sync up
to an incoming MTC source for that pre-roll amount, and then start playing.
Flywheeling
Due to the nature of magnetic tape, sometimes drop-outs occur on the tape and
leave gaps in the recorded time code. When a drop-out or other media
corruption occurs at the sync source and there is a resulting period of corrupted
time code, the MIDISPORT 8x8/s continues to “flywheel,” outputting time code
from the last properly received frame and incrementing at the proper frame rate.
This flywheeling continues until valid time code reappears on the MIDISPORT
8x8/s SMPTE input or until a preset time-out occurs. The length of this time-out
period is called the “Flywheel Time.” The MIDISPORT 8x8/s flywheel time may
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