Sunday, 14 March 2010

Digital Video Resolution is different to digital photography resolution

Oh, how naive I was, when as a photographer I thought that image resolution of my camcorder in 4:3 and in 16:9 aspect ratios would be as simple as with digital photos; after all wasn't it just x pixels wide by y pixels high? Apparently not.

Digital video has taken over conventions from analogue video so that the whole industry could have a seamless transition, with interesting consequences.

My camcorder produces PAL compatible videos according to the 625/50i system. 625 refers to the number of horizontal lines in an analogue image. However the images are created by first scanning the odd numbered horizontal lines at 25 frames per second and then the even numbered ones at 25 frames per second, and interlacing them (25 +25 = 50 and i stands for interlacing - hence the 50i).

Of the 625 analogue TV lines, a number are not part of the image itself, 576 lines are used for the actual image. In a digital system this means the image recorded by my camcorder is 576 pixels high.

Finding the width is a bit more complicated.  In a conventional analogue TV system, the width of each line (and therefore the image) is defined by a squiggly wave that lasts 52 micro seconds. To translate this into pixels, you have to take discrete samples at regular intervals along the line. An industry standard is to sample 702 times; digitally, this is therefore equivalent to a width of 702 pixels. Actually, in real life, the sampling is often 704 or 720 pixels.
(Note, 52 micro seconds divided by 702 equals 74.1 nano seconds. In TV and radio physics, people like to express things in frequencies; we obtain the sampling frequency in Hertz by dividing 1 second by 74.1 nanoseconds = 13500000 Hz or 13.5 MHz - the way in which the industry standard is expressed)

Next comes a piece of strange news for a photographer - a pixel does not have to be a fixed square. in fact, with video, pixels are not square and can have a variety of different aspect ratios!

So, when I record a video in a conventional 4:3 aspect ratio, it is 704 x 576 pixels in size. However, when I record in a 16:9 aspect ratio, it is still 704 x 576 pixels in size, it's just that the video pixels are wider!

Even worse, if you use an on-screen ruler, you will find that none of the formats actually precisely fit the 4:3 or 16:9 image ratios!

So, when I record my video and display it on a screen, it should be 576 pixels high; however, the width of my video on screen will vary, depending on the  width of the 704 video pixels!

If you want to have more detailed information, see the references below!

Wikipedia on Camcorders
The 625/50 PAL Video Signal and TV Compatible Graphics Modes
A Quick Guide to Digital Video Resolution and Aspect Ratio Conversions