Analog cameras, digital video and aspect ratios

[tuonela 0]

Hi all,

 

I have noticed that analog video signal is often digitized and stored to different CIF formats (e.g CIF, 2CIF or 4CIF). However, the pixel aspect ratio of CIF formats is not square (see e.g. Wikipedia and "Common Intermediate Format"). Does this mean that, for example, images exported from a DVR with CIF resolution should always be resized to square pixel format in order to maintain the correct aspect ratio?

 

Thank you for your help!

-Tuonela

[CollinR 0]

If you want a 1:1 pixel map on a square display, yes. However you will be hard pressed to find a square display, 4:3 (CCTV and SD TV), 16:9 and 16:10 are the most common aspect ratios but are all rectangular.

[tuonela 0]

Thank you for your reply. Maybe my question was a bit unclear. As non-native writer, it's sometimes difficult to find the most suitable words. I'll try to rephrase it.

 

I meant pixel aspect ratio, not display aspect ratio. I was talking about non-square vs. square pixels. For example, if I take an 4:3 analog source (e.g. VHS tape) and digitize it according to the ITU 601 standard (PAL version), I will get resolution of 720x576 pixels. Now the aspect ratio is actually 1.25, not the original 1.333... This is because the pixels according to ITU 601 are non-square, although in computer displays they are always square. When I view such frame on my computer display, it will have kind of a "stretched" look. Therefore, the frame needs to be scaled to 768x576 pixels in order to obtain the original aspect ratio.

 

Back to CIF resolution. The pixels in CIF formats are not square either (e.g Wikipedia "Common Intermediate Format" talks about this), and therefore the aspect ratio of a CIF frame is 352/288 = 1.2222..., not 1.3333... On a computer display, in order to get the aspect ratio of 1.333..., a CIF frame needs to be scaled to 384x288 resolution.

 

Now CCTV and DVRs. Many DVRs use CIF formats (e.g. CIF or 4CIF) when storing digital video from analog cameras. Now if I export 352x288 frame from a CIF video, and view it with my computer display, do I actually need to rescale it to 384x288 in order to get the absolutely correct aspect ratio, or is this taken care of somehow else.

 

Tuonela