Composite Video Signal (CVS) - Horizontal sync details

Make sure you read the CVS in the order given below. Links to different CVS is given at end of this explanation.

There are 4 types of CVS viz.:

1. CVS for pedestal height

2. CVS for blanking pulses

3. CVS for horizontal sync detail

4. CVS for vertical sync detail

If in exam, a question comes as ‘Explain CVS’ then depending on the marks, explain anyone that you want to explain. For 5 marks, write any from first three. I’d prefer first one as it’s easy whereas if the question is for 10 marks, go for the 4th one. The one that I've explained below is the third one.

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The total time required for horizontal trace and retrace is 64 µ seconds. Out of this 64 µ sec, 12 µ sec is line blanking period which means it takes 12 µ sec for retrace. During this 12 µ sec, a synchronizing pulse is added which is used to sync the horizontal scanning oscillator. This CVS explains how this 12 µ sec is actually divided.

The horizontal line blanking period is divided into three parts:

1. Front porch of 1.7 µ seconds.
2. Line synchronizing pulse of 4.7 µ sec.
3. Back porch of 5.8 µ sec.

1.7 µ sec + 4.7 µ sec + 5.8 µ sec = 12 µ sec

Front porch:

This is known as front porch because it is added in front of the picture information. This front porch allows the receiver circuitry to settle down from whatever level exist at end of picture detail, to the blanking level before the synchronizing pulse occurs (the sync pulses are always added at the blank level). Saw tooth waveform is used for trace and retrace purpose. In the diagram below, the red line shows the time taken for the receiver circuitry to go to the blank level.

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Line sync pulse:

After the front porch ends, the retrace begins. These line sync pulses are inserted in the CVS from transmitter’s side and are separated at the receiver. The main function of these pulses is to keep the transmitter and distant receiver in precise synchronization. The nominal time duration of this pulse is 4.7 µ sec. Apart from providing synchronizing, the horizontal scanning oscillator uses this pulse to complete its retrace. During this pulse, the beam completes its fly back and comes to the extreme left of the raster (screen) so that it can trace the next line.

Back porch:

This is known as the back porch because it is added the back side of the next picture information. It is nothing but an extension to the line sync pulse because this period of 5.8 µ sec allows plenty of time for the fly back to complete. Apart from this, it gives the horizontal scanning oscillator enough time to reverse the direction of current. During retrace, the current flows from left to right but for tracing the next line, the current should flow from right to left. For this, reversing the direction of current flow is needed.

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The timing of back and front porch are so set that it creates black bars on right and left side of the raster.

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