Electronics > Projects, Designs, and Technical Stuff
Why modern analog cameras still using TVL(Television-Line) as a spec?
<< < (3/3)
tooki:

--- Quote from: helius on April 26, 2019, 07:36:37 am ---Tooki is on the money, except that the lines of resolution (LOR) is actually measured with respect to a horizontal distance equal to the vertical height. So it measures the horizontal resolution, but the result is scaled to the same units as the vertical scan lines. 480 lines of resolution is approximately square "pixels", although they are not addressed as actual pixels in an analog signal. Broadcast resolution is higher, with approximately 540 lines.
Broadcast quality digital VTRs like the D1 use non-square pixel frame formats to achieve this level of resolution. They have higher horizontal resolution than vertical, with a "lines of resolution" over 480.

--- End quote ---
Thanks for the added info! I had (naively?) assumed it measured across the whole video line.

I’ll agree that 800 lines within 3/4 of a video line is, um, optimistic. But in pure theory I suppose it’s possible!
vk6zgo:

--- Quote from: helius on April 26, 2019, 12:03:07 pm ---
--- Quote from: vk6zgo on April 26, 2019, 08:00:44 am ---Resolution lines were used in photography many years prior to TV.
--- End quote ---
This is also true, but that is a subtly different sense of the concept of "resolution". Resolution in photography (and printing, and microscopy) is in lpmm (line pairs per mm): How many pairs of light and dark lines per mm can be clearly seen. The military had an obvious interest in aerial reconnaissance photography, and developed the standard test target: https://en.wikipedia.org/wiki/1951_USAF_resolution_test_chart
This is an empirical test, but does not fully characterize the actual physical process that limits resolution. For that you need to find the MTF which is a spatial frequency response function.

By contrast (no pun intended), TV resolution lines is not a measurement of spatial frequency, but of signal fidelity. There are no mm here, as the video picture does not have a concrete size; instead it is an abstract object with dimensions in megahertz.

--- End quote ---

The only reason it may be expressed in MHz in TV use, is because the scanning line in an analog TV system takes, (for 625 & 525 line systems) approx 64 u/s to complete a scan.("spatial frequency" is a somewhat esoteric concept which has no real place in TV standards).

A sharp black to white transition (or vice versa) produces an electrical signal with a very fast rise time.
The frequency response of the Television system is the main determinant of how well that transient can be reproduced, hence, resolution could be expressed in MHz

This was not commonly done during my 30 plus years in Broadcast Television, where resolution, frequency spectra, & system bandwidth  were treated as obviously related, but separate things.

The horizontal resolution of the old 405 line system is not that much worse than that of the later systems, because the slower horizontal scan time produces lower bandwidth electrical signals that do not require as much system bandwidth.

Navigation
Message Index
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod