In audio (sound) production, jam sync is a mode of device synchronization using SMPTE time code in which a slave device can furnish its own timing during the time that a master device is temporarily unstable. Jam sync is usually an optional mode that the user can select.
In a basic SMPTE synchronization scenario, time code is fed from one device (the master) to a second device (the slave). The slave device follows the time locations as transmitted by the master device. If the master device goes to one hour and thirteen minutes (expressed as 01:13:00:00; Hours:Minutes:Seconds:Frames), the slave device follows it there.
The problem arises when the master device's SMPTE time code has an error in it, caused by a small bad portion of tape, for example. In most situations, the slave device would not know what to do for a split second since it has missing instructions from its host. It would stutter or stop completely.
By using jam syncing, the slave device can be instructed to ignore small dropouts of time code from the master, thus ensuring smooth synchronization. To do this, it generates its own time code whenever the master's time code is missing.