system time clock (MPEG-2) (STC)

The System Time Clock (STC) is an electronic component of the MPEG-2 decoder that synchronously decodes the transport streams and program streams of digital TV.

The system time clock generates the clock signal and synchronizes it with the synchronization signal of the program stream. For this purpose, the System Time Clock generates a frequency of 27 MHz, which forms the clock reference and which the decoder needs to synchronize the program stream. The 27 MHz is divided by 300 and the resulting frequency of 90 kHz is used by the decoder for the transport stream( TS).

In terms of circuitry, the System Time Clock is a feedback voltage controlled oscillator( VCO) or Voltage Controlled Crystal Oscillator( VCXO). This is controlled with a control voltage obtained from the System Clock Reference( SCR) or the Program Clock Reference( PCR). The control voltage is underlaid with an additional voltage generated by a counter from the frequency deviation between the reference frequency of the transport stream and the feedback oscillator frequency. The output signal of the System Time Clock is a highly stable 27 MHz signal synchronized with the transport stream.