Why modern analog cameras still using TVL(Television-Line) as a spec? - Page 1
Forum section: electronics / projects

author: zajusako
author_rank: Newbie
timestamp: February 15, 2019, 05:20:33 pm
content: Hi all.I understand this value is from old line-scan cameras/TVs, so why modern CCD/CMOS based analog CCTV cameras still using TVL? And why don't that value equal the vertical pixel count?Thank you.

author: tooki
author_rank: Super Contributor
timestamp: February 17, 2019, 05:50:25 pm
content: "Lines of resolution" is the granularity in thehorizontal.For example. NTSC is 525 scan lines, of which historically around 485 are actually visible (due to overscan), which is why we chose 480 in digital standards for SD.The number of scan lines is fixed, as it is a fundamental characteristic of the analog signal format. So when we look at a stated spec like a standard VHS VCR having about 300 lines of resolution, it means that in each scan line, the VCR can resolve down to about 300 lines. But the key thing to remember is that it's analog, so there are no pixels. It's a continuous value within each line. Whether this signal originated in a vidicron tube or a CCD, the analog signal is still this analog signal.

author: dmills
author_rank: Super Contributor
timestamp: February 17, 2019, 09:38:33 pm
content: TV Lines is also a very useful measure when you look at a whole camera system, in particular with a modern HD or 4k chain, the glass on the front can be a significant limiting factor (particularly as you crank the iris open in low light conditions).Just because my sensor can manage 3840 pixels or whatever does NOT mean that I can resolve that by the time the prism block & lens have done their thing.Regards, Dan.

author: tuzakeyiwe
author_rank: Newbie
timestamp: April 25, 2019, 10:01:18 am
content: Quote from: dmills on February 17, 2019, 09:38:33 pmCCTV camera bodies also have that spec, and this is a mystery for me for a long time...

author: videobruce
author_rank: Frequent Contributor
timestamp: April 25, 2019, 12:29:53 pm
content: The Chinese CCTV industry appears to use the resolution of the actual sensor, not what the final video output actually is. Which is why thesegrossly inflatednumbers (typical marketing lies) come from. Mostly noticeable with old school analog cameras calming up to 800 lines when the NTSC limit is 480 (more or less as stated).One look at these so-called hi-res cameras on a high res display (1080p or even a 720) clearly shows that is a lie.

author: tooki
author_rank: Super Contributor
timestamp: April 25, 2019, 04:30:24 pm
content: Quote from: videobruce on April 25, 2019, 12:29:53 pmThat’s not what lines in analog video specs mean.As you can imagine, the number of scan lines is fixed (525 in NTSC, 625 in PAL), with a fair number at the edges lost to overscan (and some mandatory blank lines, etc), resulting in the ~480 visible lines. We are still talking aboutscan lines, that is, the lines going from left to right, aka theverticalresolution.On the other hand, asanalogstandards,there is no fixed horizontal resolutionin NTSC/PAL/SECAM, since it’s a continuous analog signal for each line. So when we say that S-VHS gets around 400 lines, laserdisc 450, broadcast 350ish, and regular VHS only about 250, we are talking about thehorizontalresolution. And this is why DVD looked so good: it does 720 lines. 800 is eminently possible, though it requires exceptionally good cabling and signal processing. Only broadcast monitors could do anything close to that.As for how it’s calculated, it means that when recording a test chart, you can make out that many alternating black/white lines (hence the starburst test patterns).

author: videobruce
author_rank: Frequent Contributor
timestamp: April 26, 2019, 07:05:57 am
content: But with a max of 480, resolution numbers above that (up to and over 700L) as these no-name companies advertise are basically meaningless.

author: helius
author_rank: Super Contributor
timestamp: April 26, 2019, 07:36:37 am
content: 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.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 26, 2019, 08:00:44 am
content: Resolution lines are nothing to do with the scanning lines, except in analog TV where thevertical resolutionis limited by the number ofscanning lineswhich are available for viewing.Resolution lines were used in photography many years prior to TV.Telecines & "Flying Spot Scanners" (FFS) were a common video source for displaying resolution charts in TV.In this case, the charts were supplied in the form of slides.You can use a resolution slide with a plain old optical projector, as I did years ago with the one from the FFS at my old work.I had purchased, at great expense, a "Kodak Carousel" slide projector, & wanted to check it against my old unit.It was astounding!The Kodak showed the resolution lines, both vertical & horizontal, out to "near as dammit" the limit of the original photographic process used to make the slide.(My old projector horrible in comparison)I then fired up the FFS.It was very good for a TV picture, but it looked crappy compared to the direct optical image.

author: helius
author_rank: Super Contributor
timestamp: April 26, 2019, 12:03:07 pm
content: Quote from: vk6zgo on April 26, 2019, 08:00:44 amThis 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_chartThis is an empirical test, but does not fully characterize the actual physical process that limits resolution. For that you need to find theMTFwhich 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.

author: tooki
author_rank: Super Contributor
timestamp: April 26, 2019, 01:48:18 pm
content: Quote from: helius on April 26, 2019, 07:36:37 amThanks 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!

author: vk6zgo
author_rank: Super Contributor
timestamp: April 27, 2019, 03:53:11 am
content: Quote from: helius on April 26, 2019, 12:03:07 pmThe 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, resolutioncouldbe expressed in MHzThis 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.