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Dear Bio-mechanist,
It was great to see the number of responses to Brad Lawrence's inquiry
regarding the synchronization of Ascension's magnetic trackers with A/D
data. Several of the responses suggested the use of TTL or other pulses to
synchronize readings from the Ascension trackers with readings from the A/D
board. Depending on how it is implemented, there can be pitfalls with this
approach when used with devices that rely on an Ethernet interface. Because
this is a common interface used in both the Ascension MotionStar and the
Polhemus StarTrack systems I thought that it warranted some additional
discussion.
TTL pulses can be used in a couple of ways. The first is to physically mark
two files with a signal that, in post processing, is used to align the two
files. This method is very reliable if one can be sure that the pulse is
recorded at the same time in both files. A direct TTL pulse to BOTH files is
one way to ensure that both are marked at the same instance in time. If
the file is marked twice, one can also determine the drift between the
clocks of the two systems.
A second method uses a common pulse as a trigger to initiate simultaneous
operation of two different systems. A variant of this approach is to use
the clock of one system to provide a repeating TTL that is used to trigger
individual measurements of the second system. As Brad pointed out, this is
the common approach with the trackers since there is no way to record a
direct TTL pulse in the Ascension system.
These two approaches differ in that the first synchronizes the data while
the second only synchronizes the measurement cycles. This second method can
be problematic because synchronized measurement cycles do not mean that the
readings of those measurements are synchronized. Using this approach, one
must also consider the "transport delays" between the issuance of a start or
read command and the receipt and processing of that command by the targeted
device. These delays can occur because of poor software architecture and the
communication interface used. If these delays are large relative to the
measurement cycle, a loss off synchronicity can occur. In the case of the
magnetic trackers, particularly when an Ethernet interface is used, the
"transport delay" can be significant resulting in the readings from the A/D
board and the magnetic trackers being from very different time periods.
Fortunately, it is possible to implement the first method with the Ascension
trackers although the explanation takes a bit of time. Regardless of the
approach used, systems should be tested by measuring a common physical
phenomena with devices attached to both data sources. I've posted two
tests on our web site at http://www.innsport.com/synch_tests.htm that we use
to assess how well systems are synchronized. I would welcome your
suggestions for additional tests.
________________________
Lee E. Johnson
ljohnson@innsport.com
tel: 773-528-1935
fax: 773-528-2149
Innovative Sports Training, Inc.
....The Total Solution in Motion Capture.
www.innsport.com
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It was great to see the number of responses to Brad Lawrence's inquiry
regarding the synchronization of Ascension's magnetic trackers with A/D
data. Several of the responses suggested the use of TTL or other pulses to
synchronize readings from the Ascension trackers with readings from the A/D
board. Depending on how it is implemented, there can be pitfalls with this
approach when used with devices that rely on an Ethernet interface. Because
this is a common interface used in both the Ascension MotionStar and the
Polhemus StarTrack systems I thought that it warranted some additional
discussion.
TTL pulses can be used in a couple of ways. The first is to physically mark
two files with a signal that, in post processing, is used to align the two
files. This method is very reliable if one can be sure that the pulse is
recorded at the same time in both files. A direct TTL pulse to BOTH files is
one way to ensure that both are marked at the same instance in time. If
the file is marked twice, one can also determine the drift between the
clocks of the two systems.
A second method uses a common pulse as a trigger to initiate simultaneous
operation of two different systems. A variant of this approach is to use
the clock of one system to provide a repeating TTL that is used to trigger
individual measurements of the second system. As Brad pointed out, this is
the common approach with the trackers since there is no way to record a
direct TTL pulse in the Ascension system.
These two approaches differ in that the first synchronizes the data while
the second only synchronizes the measurement cycles. This second method can
be problematic because synchronized measurement cycles do not mean that the
readings of those measurements are synchronized. Using this approach, one
must also consider the "transport delays" between the issuance of a start or
read command and the receipt and processing of that command by the targeted
device. These delays can occur because of poor software architecture and the
communication interface used. If these delays are large relative to the
measurement cycle, a loss off synchronicity can occur. In the case of the
magnetic trackers, particularly when an Ethernet interface is used, the
"transport delay" can be significant resulting in the readings from the A/D
board and the magnetic trackers being from very different time periods.
Fortunately, it is possible to implement the first method with the Ascension
trackers although the explanation takes a bit of time. Regardless of the
approach used, systems should be tested by measuring a common physical
phenomena with devices attached to both data sources. I've posted two
tests on our web site at http://www.innsport.com/synch_tests.htm that we use
to assess how well systems are synchronized. I would welcome your
suggestions for additional tests.
________________________
Lee E. Johnson
ljohnson@innsport.com
tel: 773-528-1935
fax: 773-528-2149
Innovative Sports Training, Inc.
....The Total Solution in Motion Capture.
www.innsport.com
---------------------------------------------------------------
To unsubscribe send SIGNOFF BIOMCH-L to LISTSERV@nic.surfnet.nl
For information and archives: http://isb.ri.ccf.org/biomch-l
---------------------------------------------------------------