I have a question about the resolution of NBTV standard, in terms of 'pixels' as we are used to dealing with today.
Obviously the width is 32 scanlines, hence 32 pixels. The aspect ratio is standardised at 3:2, so this would suggest that the vertical resolution is 48 pixels (assuming square pixels).
Using the standards that I could find online, I tried to calculate the pixel resolution using the mechanics of the system...
The resolution, I expect, would be driven by the frequency of the input signal, and the ability of the light source to respond to that signal - and, of course, the 'hole' from the scanning device sweeping over this light source. I don't know about LED response, but I'd expect it is pretty rapid.
I see from the NBTV site the club standard is 2kHz - 10kHz signals. Given a 8kHz signal bandwidth, that would give 250 individual samples or pixels per scanline per second (8000Hz/32 scanlines). At 12.5 frames per second, that would be 20 pixels per scanline...?
On the other hand, using CD WAV files @44.1kHz that would suggest we could generate video to club frame rate (but not frequency bandwidth of course) at about 110 pixels per scanline.
The online test pattern images that I see appear for all the world to be drawn at a resolution of 32 x 60 (ref: http://www.nbtv.wyenet.co.uk/sound.htm)! I note the pixels in these images are drawn in a 4:3 aspect.
So, it seems to me that apparent resolution is somewhere between 32 across (defined) and vertically between 20 pixels (from the available bandwidth), or 48 pixels (using the published picture aspect ratio, assuming square pixels), or 60 pixels, from the test-pattern images, or 110 pixels (maximum using the 44.1kHz CD signal).
Commonsense tells me that it's actually 20 pixels vertical resolution, and these test patterns drawn at 32 x 60 are wishful thinking. Perhaps we get better apparent resolution due to persistance of vision, and the 'blurriness' introduced by the mechanical scanning merging pixels together, but by my calculations none of this can make it through the bandwidth pipe of 8kHz.
So, NBTV resolution at 12.5fps using 8kHz bandwidth is 32 x 20 pixels.
Can anyone correct my misunderstandings here?
Cheers
A