Signal Sampling

4fsc

Four times the frequency of SC (sub-carrier).

The digital sampling rate of a composite video signal with respect to the sub-carrier frequency of an NTSC or PAL analogue video signal.

The 4fsc frequency sample rate is typically:

14.3 MHz (28.6 MSPS) in NTSC.

17.7 MHz (35.4 MSPS) in PAL.

Sub-Carrier

A subcarrier is a sideband of a radio frequency carrier wave, which is modulated to send additional information.

Example:

VHS HiFi

Left 1.3Mhz / Right 1.7mhz

Video8 & High8 HiFi

Left 1.5Mhz / Right 1.7Mhz

Beta HiFi

Left is 1.38Mhz A head & 1.53Mhz B head

Right is 1.68Mhz A head & 1.83Mhz B head

Examples include the provision of colour in a black and white television system or the provision of stereo in a monophonic radio broadcast. There is no physical difference between a carrier and a subcarrier; the "sub" implies that it has been derived from a carrier, which has been amplitude modulated by a steady signal and has a constant frequency relation to it.

In simple terms, lets say you have a 5mhz signal, inside this you have audio at 1.2mhz for left and 1.8mhz for right 2.2mhz has a timecode signal and 4.5mhz has the video signal all these signals are modulated

Exurbs From Digital Video and HD Algorithms and Interfaces 2nd Edition (By Charles Poynton 2012-02-07)

Pages 162 to 180

The following pages give a clear explanation of what 4fsc, S-Video, and Chroma Sampling are, witch are the core surface concepts to understand the processing chain of software tape decoding and how analogue is presented in the digital domain.

(Editorial note to add pages once compiled)

Page 162