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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Composite Video Signal (CVS) - Blanking Pulse

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 second one.

Click the image for full view

Apart from picture elements, a CVS also contains blanking pulses. These pulses are used to make the retrace lines invisible. Retrace is nothing but the period when the horizontal scanning beam goes back to the left side of the raster (frame) and the vertical scanning goes to the top.

The adding of blanking pulse is done by increasing the video signal’s amplitude slightly above the peak black level. This means that they are added in blank level. Whenever a retrace is done, black spots are produced on screen. The blanking pulses are added at blank level so that no spots are observed on screen as blank level is darker than black level.

CVS contains horizontal as well as vertical blanking pulse to make the respective retrace invisible. The repetition rate for horizontal blank pulse, that is the period after which the next blank pulse will occur, is same as that of horizontal frequency that is 15625 Hz and the time period (T_P) = 12 µ sec. Similarly, the repetition rate for vertical blank pulse is same as that of vertical scanning frequency that is 50 Hz and the time period (T_P) = 160 µ sec.

To elaborate the answer, you'll can add the description of Peak White level, black level, blank level, etc.

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Composite Video Signal (CVS) - Pedestal height

Composite literally means ‘made up of various elements’. CVS has various elements like picture levels, sync pulses, etc. 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

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

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 is explained below is the first one.

Click the image to view it in full size.

A video signal is nothing but sequence of images stitched together. A CVS contain many video signals which means a CVS has different brightness levels depending upon the scene.

As video signals have various images, it is obvious that they will have various colour components. So in a video signal, the maximum whitness to be handled is known as Peak White level. This level is situated at 12.5% of the maximum value of the signal. For example if a signal having 100 volts amplitude is transmitted, then the peak white level for that signal will be at 12.5% of 100 = 12.5 volts.

Just like the white level, a CVS has black and blank level also. Black level is at 72% and blank level is at 75% of the maximum value of the signal. The synchronizing pulses, pulses that are used for keeping the horizontal and vertical oscillator in sync, are added at the blank level. The black level is used for separating the blank level from the picture information.

The difference between black level and blank level is known as pedestal and the difference between pedestal and DC value of the signal is known as the pedestal height. DC value of a signal is nothing but the average value of that signal.

To further elaborate the answer, you'll can add the details of horizontal and vertical sync pulses by stating their time duration, trace time, etc.

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Go digital, go green!

 It's a harsh fact but half of our college life is wasted in writing assignments and experiments.

Few weeks back, I had a visit to RGIT for promotional purpose. I felt really nice seeing a patch of green land, full of Mangrove trees, right in front of that college. But then, reality dawned upon me. How long will those trees survive there? Maximum 2 years. Read below to find out why.

In this era, when the world is going digital, we are still busy writing our assignments and experiments on paper! Just imagine guys, from first week of the college, we start writing experiments.

On an average, there are 8 experiments in each subject and there are 5-6 such subjects. Assuming 6 subjects, there will be 48 experiments to be written in all. If each experiment is of 5 pages minimum, then each student consumes 48 x 5 = 240 pages each semester!

If there are 5 departments in a college, each with 60 students in FE, SE, TE and BE, then total number of students will be 240 for each department and 240 x 5 = 1200 students for a college. Each of those 1200 students will consume 240 pages for every semester. This means, in one semester, 1200 x 240 = 2, 88, 000 pages will be utilized by one college.

After doing little research work, I came to know that one tree can produce 8333 pages. So this means, each college chops down 288000/8333 = 34.56 trees in each semester! What’s the use of this writing? In the end, all these pages are dumped in some storage room. I’m sure that this paper wastage is done in every college, be it VJTI or DJ or any other college.

Isn't this a total wastage of our natural resource? Instead of doing this, why can't we just type our assignments and send it to the respected teacher?

Every college has computer labs with MS Office embedded in it. Then why can't we utilize them? It’ll not only end up saving our time but will also reduce the effort we put in writing. For better understanding, I’ll cover the pros and cons of this advancement.

Pros:

1.     Biggest achievement will be saving of trees! Our earth has already suffered a lot. Man has exploited earth right since he started ruling this planet. If we don't act now, it'll be too late to handle.

2.     From student's point of view, we won't have to worry about losing our files anymore because they will be permanently stored on servers.

3.     Thirdly and most importantly, our basic knowledge about using MS Office will increase. I really feel sorry when I see that even after coming to engineering there are many people who still don't know how to set alignment of a page.

4.     From teacher’s point of view, it’ll be a huge relief for them. There won’t be any need for maintaining a separate register to keep track of those students who have submitted their assignments and those who haven’t. Neither they’ll have to sit for hours in front of their desks while checking the assignments nor will they have to worry about carrying a red pen always :P

Cons:

While discussing this with my friends, they said that by going digital, cases of copying assignments will increase. I just have one question for those who say this, ‘Aren't we already copying the assignments and experiments from last year's file?’

The reason for this article is that I want to create awareness among students like us that there are few things that need to be changed in our education system.

In the end I'd just say that if you'll agree with me, then please share this article. Share it on FB, Twitter and Google +. Share it everywhere and anywhere where people, especially students, can come across it and think over it.

Remember M. K. Gandhi has said “Be the change you want to see in this world”. If you are not ready to take the initiative yourself then don't expect anybody else to take it either! :)

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