Tascam PCI-822 User Manual page 23

When one device (the slave) gets its sample rate from another device
(the master), we say that the slave device is locked to the master. In
contrast, when two devices are operating at independent sample
rates, they are slipping relative to each other. When multiple sample
rates are detected, some devices mute, some generate clicks and
pops, and some exhibit completely erratic behavior.
Some devices (such as CD players and DAT machines when they're
playing) are automatically set to "Internal" sync, and therefore must
be the master device. To transfer audio from a DAT machine or CD
player to the computer, you must select "SPDIF" as the clock source
in the PCI-822 control panel.
Some devices (such as a DAT machine when it's recording) must be
slaved to another device. To transfer audio from the computer to a
DAT machine, you must set the PCI-822 clock source to "Internal."
Other devices (such as most digital mixers and multitrack tape
machines, and the PCI-822) can be either the master or a slave. In
each usable system, exactly one device is set to "Internal" sync, and
all other devices are slaved to that device.
REMEMBER! Any digital audio system must always have exactly one
sample clock master, which should determine the sample rate for all
devices in that system. If the sample rates aren't locked, you'll hear
glitches, dropouts, or nothing at all, and the PCI-822 control panel will
display slip errors in the Digital Input Status section.
20 PCI-822 User's Guide