The System Time Clock (STC) uses a 27 MHz oscillator with a worst case tolerance of +/- 810 Hz.  [Note that this is a looser tolerance than that of the NTSC color subcarrier.  The ATSC standard therefore contains a tighter constraint on the transmitted data rate so that it may be used if desired to regenerate the NTSC color carrier in equipment intended to feed an NTSC receiver.  As discussed in the section Timing Model , the data and STC frequencies are independent.] There is a means provided to signal to the receiver the actual accuracy of the system time clock if it is better than the minimum allowed.

The STC first divides the the 27MHz by 300, giving a 90kHz clock that is counted by a 33 bit counter to give the base STC value.  The remainder is taken as a 9-bit value that may be used as an STC extension.

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