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Line timing in digital video

Doug Kerr

Doug Kerr

Well-known member
I have always assumed that in analog video, the time represented by the last line of a field is later than the time represented by the first line of a field. (I've always considered myself quite knowledgeable with regard to analog video, and I'm embarrassed to not be completely sure of this!)

Further, I have assumed that the right end of a line represents a later time instant than the left end of the line (by about the line time).

In other words, I have assumed that the result of the raster scanning in that regard is just what we would think.

Now, if this is true in analog video, then what about in digital video (including the ATV standards). Is the same thing conceptually true there, or is each field a "snapshot" at a certain time instant, so that all lines of a field (and all pixels of a line) represent the same time instant?

(I realize that the effective "shutter speed" is not zero, so however it works, the imaging for a pixel averages the luminance over some time interval, perhaps an entire field time.)

Thanks for any help.

Best regards,

Doug
 
D

Daniel Browning

New member
Doug Kerr said:
Now, if this is true in analog video, then what about in digital video (including the ATV standards). Is the same thing conceptually true there, or is each field a "snapshot" at a certain time instant, so that all lines of a field (and all pixels of a line) represent the same time instant?
Click to expand...

The behavior of the video camera in this regard is determined by the presence (or lack thereof) of a electronic global shutter in the image sensor. Global shutters cause the exposure start and stop times to be so close that they are considered instantaneous (for some cameras, I know it's at least under a tenth of a millisecond). In contrast, a rolling shutter reads out each line sequentially, typically top-to-bottom and left-to-right (but that is of course a design choice), and the speed at which it can perform that is limited by the read-reset time of the pixel. In some cameras, it can be as slow as 20 milliseconds, which leads to a variety of motion artifacts such as skew, temporal aliasing, etc. Global shutters were common with CCD cameras, but less so with CMOS, as it generally requires an extra transistor.

As read-reset times drop with sensor improvements, even low-end CMOS video cameras will surpass the movement speed of the mechanical shutter in a cinema film camera, long considered to be sufficient for avoiding most rolling-shutter video artifacts.
 
Doug Kerr

Doug Kerr

Well-known member
Hi, Daniel,

Daniel Browning said:
The behavior of the video camera in this regard is determined by the presence (or lack thereof) of a electronic global shutter in the image sensor. Global shutters cause the exposure start and stop times to be so close that they are considered instantaneous (for some cameras, I know it's at least under a tenth of a millisecond). In contrast, a rolling shutter reads out each line sequentially, typically top-to-bottom and left-to-right (but that is of course a design choice), and the speed at which it can perform that is limited by the read-reset time of the pixel. In some cameras, it can be as slow as 20 milliseconds, which leads to a variety of motion artifacts such as skew, temporal aliasing, etc. Global shutters were common with CCD cameras, but less so with CMOS, as it generally requires an extra transistor.

As read-reset times drop with sensor improvements, even low-end CMOS video cameras will surpass the movement speed of the mechanical shutter in a cinema film camera, long considered to be sufficient for avoiding most rolling-shutter video artifacts.
Click to expand...

Much as I had suspected, but it's good to hear it from somebody who knows.

Yes, I have seen "rolling shutter" referred to, and assumed it referred to essentially this matter.

In any case, it sounds as if the situation in which the last line is read out essentially one frame time later than the first line is not likely to happen (as would be the case with a least certain types of analog video cameras); that is, the scene is "scanned" as fast as is practical. Do I have that correct?

Thanks so much.

By the way, I see you are a new member. Welcome aboard.

Best regards,

Doug
 
D

Daniel Browning

New member
Doug Kerr said:
In any case, it sounds as if the situation in which the last line is read out essentially one frame time later than the first line is not likely to happen (as would be the case with a least certain types of analog video cameras); that is, the scene is "scanned" as fast as is practical. Do I have that correct?
Click to expand...

Yes. Although I imagine that there could be (now or in the future) a camera with user-adjustable readout speed to allow use of the rolling shutter as a creative effect, in which case the behavior you described would be the lower limit of the adjustment parameter. For digital cinema cameras, I imagine such a parameter would often be used to match the rotary shutter of film cameras, where the difference from a global shutter in typical use is small, but detectable.

Doug Kerr said:
By the way, I see you are a new member. Welcome aboard.
Click to expand...

Thank you. My primary motivation for joining forums like this and sharing what I know with others is gratitude for the knowledge that others have shared with me. There is one source of knowledge in particular that stands out for its accuracy, depth, comprehensiveness, and clarity. I've referred to it many times over the span of years and am in debt to its author. It's called The Pumpkin.
 
Doug Kerr

Doug Kerr

Well-known member
Him Daniel,
Daniel Browning said:
Thank you. My primary motivation for joining forums like this and sharing what I know with others is gratitude for the knowledge that others have shared with me. There is one source of knowledge in particular that stands out for its accuracy, depth, comprehensiveness, and clarity. I've referred to it many times over the span of years and am in debt to its author. It's called The Pumpkin.
Click to expand...
Thank you so much.

Best regards,

Doug
 
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