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On 5/25/2011 2:32 PM, Andrew Rich wrote:
<blockquote cite="mid:B882F9DDFD2449D0B1285DFDF4AA06C3@MACBOOKPRO"
type="cite">Scott,
<br>
<br>
What is the relationship between the number of lines in the video
and the SSTV image ?
<br>
<br>
- Andrew -
<br>
<br>
</blockquote>
<br>
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. <br>
<br>
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. <br>
<br>
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). <br>
<br>
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. <br>
<br>
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. <br>
<br>
(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!)<br>
<br>
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. <br>
<br>
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. <br>
<br>
<br>
<hr size="2" width="100%"><br>
--<br>
<br>
Stephen H. Smith wa8lmf (at) aol.com <br>
EchoLink Node: WA8LMF or 14400 [Think bottom of the 2M
band]<br>
Skype: WA8LMF<br>
Home Page: <a class="moz-txt-link-freetext" href="http://wa8lmf.net">http://wa8lmf.net</a><br>
<br>
===== Vista & Win7 Install Issues for UI-View and Precision
Mapping =====<br>
<a class="moz-txt-link-freetext" href="http://wa8lmf.net/aprs/UIview_Notes.htm#VistaWin7">http://wa8lmf.net/aprs/UIview_Notes.htm#VistaWin7</a><br>
<br>
*** HF APRS over PSK63 ***<br>
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<br>
"APRS 101" Explanation of APRS Path Selection & Digipeating <br>
<a class="moz-txt-link-freetext" href="http://wa8lmf.net/DigiPaths">http://wa8lmf.net/DigiPaths</a> <br>
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