702T and 744T Time-of-Day Accuracy
The 702T and 744T recorders excel in applications requiring precise time code synchronization. To achieve precision time code, these units have extremely accurate internal clocks to generate the time code.
The 702T and 744T have two independent internal clock circuits—a high-accuracy, temperature-compensated voltage-controlled crystal oscillator (TCVCXO) for time code, and a standard watch-crystal-type oscillator for time-of-day (TOD).
TCVCXO – Temperature-Compensated Voltage-Controlled Crystal Oscillator
The TCVCXO is part of the high-precision time code circuity, providing clocking precision better than 0.2 PPM (parts per million). This oscillator is tunable—it can be manually tuned up and down, the “VC” of the TCVCXO mouthful—with an Ambient Controller to match other time code devices. The unit is factory-tuned to a rock-stable house reference. Because of this topology, 7-Series recorders can match other precision time code equipment with very high accuracy.
The time code clock oscillator is temperature compensated (the “TC” part of the acronym). This is important because all crystals (the heart of the oscillator) are are subject to changes in operation with changes in temperature. When temperatures fluctuate, accuracy is reduced. The 702T/744T’s TCVCXO ensures that no matter what the operating temperature is the time code precision is rock solid. The downside of a TCVCXO is that it consumes relatively high current (milliamps instead of microamps).
Because our design allows users the convenience of being able to turn the 702T/744T off (for a few hours) and still maintain accurate time code, the 7-Series internal AA battery can run the time code circuitry for about two hours while still maintaining its high-accurate clock. If the recorder is never powered down, the time code can run indefinitely without losing any of its precision.
TOD – Time of Day Clock
The TOD clock inside the recorder is a much less precise clock and is only used to keep time of day. This clock runs off of the same AA battery and can run for weeks on a fully charged AA battery. The crystal used for this clock’s oscillator is not temperature compensated and can have significant drift. This is due, again, to crystal’s temperature dependence. Temperatures inside the recorders can vary widely based on whether the recorder is on or off, and the ambient temperature. This is unlike the relatively stable temperatures many TOD clocks experience when they are sitting in an equipment rack, placed directly on a user’s wrist, or are inside a household appliance.
The AA battery which powers both of these clocks is a rechargeable NiMH battery. It is charged when:
- the unit is on and
- when the unit is powered off with external DC is connected to pins-2 and -3 of the 4-pin Hirose DC connector.
The AA battery requires ~12 hours to fully charge. The AA battery should be changed every 12 months if the unit is used weekly.
Because of the two type of clocks in these units, a user needs to recognize that the time-of day-clock should not be used for synchronization. It is best practice to regularly charge the AA battery and verify the time and date setting weekly.