900TVL Cameras? Good? Bad?

900TVL appears to be a marketing trick, like 700TVL and 800TVL before it. My understanding is that these work with conventional DVRs while magically increasing video quality even though the rest of the infrastructure (e.g., the analog encoder) does not support those higher TVLs.

HD SDI is something totally different, using digital signals rather than analog, requiring a completely different encoder/DVR. HD SDI has many weaknesses but unlike 900TVL actually offers HD resolution.

Btw, if you want to look at analog alternatives, there is also 960H, which fits in between traditional analog and HD SDI.

Well if these are on conventional DVR and NTSC standard then...

From Wiki

NTSC color encoding is used with the System M television signal, which consists of 29.97 interlaced frames of video per second. Each frame is composed of two fields, each consisting of 262.5 scan lines, for a total of 525 scan lines. 483 scan lines make up the visible raster. The remainder (the vertical blanking interval) are used for synchronization and vertical retrace. This blanking interval was originally designed to simply blank the receiver's CRT to allow for the simple analog circuits and slow vertical retrace of early TV receivers. However, some of these lines may now contain other data such as closed captioning and vertical interval timecode (VITC). In the complete raster (disregarding half lines due to interlacing) the even-numbered scan lines (every other line that would be even if counted in the video signal, e.g. {2,4,6,...,524}) are drawn in the first field, and the odd-numbered (every other line that would be odd if counted in the video signal, e.g. {1,3,5,...,525}) are drawn in the second field, to yield a flicker-free image at the field refresh frequency of approximately 59.94 Hertz (actually 60 Hz/1.001). For comparison, 576i systems such as PAL-B/G and SECAM use 625 lines (576 visible), and so have a higher vertical resolution, but a lower temporal resolution of 25 frames or 50 fields per second.

Of course this does not address the compression of video at all.

Most of these analogue camera's with 720+TVL use 1.3MP sensors (All the re-branded sensors come from a Korean manufacturer).

From what I have seen the performance is the best I have seen for standard conditions in analogue world. The prices for these cameras are also very good.

On all the variations they have effective pixels of 1280(H)×960(V), some people consider this 720TVL others 800TVL. I haven’t seen the 900TVL line yet….

Although I only tested one 960H camera (Vitek) with a 960H encoder, I didn't see any difference in resolution between it and a good 600TVL camera through a 4SIF or D1 encoder. Considering the limitations of analog, I wasn't surprised. Pixel counts mean little with analog once you reach approximately 500TVL.

And despite D1's supposed limitation of 720 horizontal resolution, it cannot capture the full resolution of even 470-480TVL analog cameras. That was proven in a series of encoder tests we performed last year. We tested Avigilon, Axis, Bosch, Sony, TKH, Verint, IndigoVision, Pelco Endura and Dallmeier encoders and not one of them was able to provide encoded resolution anywhere near what we obtain from 480TVL to 600+TVL analog cameras fed directly to Orion and ViewZ 19RTC monitors via a Pelco 9780 matrix. Not even close!

900TVL appears to be a marketing trick, like 700TVL and 800TVL before it. My understanding is that these work with conventional DVRs while magically increasing video quality even though the rest of the infrastructure (e.g., the analog encoder) does not support those higher TVLs.

My thoughts exactly. The weakest link theory.