[aprssig] SSTV camera from Argent

[Stephen H. Smith]

On 5/25/2011 2:32 PM, Andrew Rich wrote:
> Scott,
>
> What is the relationship between the number of lines in the video and the 
> SSTV image ?
>
> - Andrew -
>

Standard North American NTSC analog broadcast video is (or rather was before 
the digital changeover  a couple of years ago)  525 scanning lines 
top-to-bottom sent 30 times/second in two interlaced fields.   The odd lines 
are painted on the receive CRT, then the vertical scan is reset and the even 
lines drawn in.  This makes the actual vertical scanning rate (field rate) 60 
Hz, which in the early days of television was chosen to allow TVs to lock the 
vertical scanning rate to the frequency of US AC power lines.  It also reduced 
flicker in the rather dim early CRTs.   Since the system is analog, there is no 
defined grid of pixels -- just a fixed number of continuous horizontal lines.

The detail on each horizontal line is a function of the bandwidth of the the 
video amplifiers, and whether the red, green and blue components are carried 
over separate conductors (as in a CCTV monitor or RGB computer display) or 
encoded into a composite signal passed over a single conductor or over-the-air 
broadcast. In separate-path RGB systems, the bandwidth of  the video (which 
determines the amount of fine detail; i.e. "resolution") can be 4-5 MHz.

The color information in an NTSC composite signal, ready for over-the-air 
broadcast, is contained in a pair of phase-modulated subcarriers in quadrature 
at 3.58 Mz.  This forces the baseband video bandwidth to be limited to about 3 
Mhz to avoid interference with the color subcarriers (which were "shoe-horned" 
into the existing monochrome broadcast format over a decade and a half after 
black & white broadcasting began).

The equivalent values for the PAL system used in most 50-Hz-power countries is 
625 lines  25 frames/sec  50 fields/sec, yielding almost the same HORIZONTAL 
scanning rate per second.

Due to the overscanning of images on classic CRTs and vertical blanking 
intervals, only about 480 lines of the 525 NTSC lines are actually visible on 
most TVs. (Assuming the odd/even interlaced scanning is set correctly so the 
even lines land in-between the odd ones instead of on top of them, as they did 
on many cheap TVs.)   Approximating a grid of square pixels on the 4:3 aspect 
ratio analog screen, you got the equivalent of approximately 1.33 x 480 or 
about 640 equivalent pixels horizontally.

(This is where the 640x480 resolution of VGA computer displays comes from. At 
the time IBM introduced full color displays to PCs in the early 1980s, the only 
mass-produced reasonably-priced color displays were TV sets. The VGA standard 
was designed around the resolution limitations of what were basically TV sets 
with no tuners and component RGB input!)

The most common approach to scan conversion for SSTV was to grab a single 
fast-scan field of 240 lines (i.e. not a full interlaced frame of 480 lines) 
with a fast A-D converter write it into RAM, then throw out  every other 
horizontal pixel to restore the 3:4 aspect ratio of the original image, 
resulting in the QVGA 320x240 image format characteristic of SSTV at the time. 
    You then clocked out the data over 30-90 seconds to generate the SSTV 
version of the image.

When SSTV evolved in the late 1980s/early 1990s to being done with smoke, 
mirrors and software on PCs (instead of with a dedicated hardware scan 
converter tied to the line & frame rates of NTSC hardware),  additional SSTV 
resolutions appeared.  Since the source image is now an already-digitized grid 
of pixels in a JPG or TIFF file produced by a scanner or digicam, it's 
relatively easy to have any arbitrary horizontal/vertical pixel count.  Modes 
that do full 640x480 VGA and 800 x 600 SVGA are now common.


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Stephen H. Smith    wa8lmf (at) aol.com
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