There are several factors that will determine which video card is the
best for your purposes. It will depend on the number and type of video
inputs, AD (Analog to Digital) conversion and system noise, frame rate,
video overlays and whether video capture is to be integrated with other
Grayscale and colour video capture cards are available. Grayscale
cards are usually 8 bit, but some are available for 12 bit conversion.
This means that the video intensity is sampled temporally, measured as
a voltage, then divided into 2^8 (2^12) or 256 (4096) discrete levels.
8-bit provides enough gray levels for most applications and approaches
the noise threshold in most video systems. Noise can be reduced in
this or any colour system by frame averaging.
Colour capture cards are available in 16, 24, 32 and more bit models.
They convert the individual red, green and blue video streams into
digital values separately, each stream being treated similarly to
grayscale digitization. 16-bit cards discretize RGB into 5, 5 and 6
bits, and so can record 65535 different colours. 24-bit cards provide
8 bits for each pixel for a total of up to 16.7 million colours. 24-
bit cards are also called Truecolour because most humans can distinguish
5-6 million colours. At 16.7 million, 24-bit colour can display more
different colours than anyone can perceive. Cards that provide 32 bits
or more of colour depth are usually Truecolour cards with overlay
capabilities. The overlay planes (8 bits in the case of 32 bit) can
be used to contain text or graphics overlays, or can store depth
information (z-buffer). In addition, extra video memory can be used
to double buffer the incoming digitized signal, up to doubling the
frame capture rate.
Video capture cards can often digitize different image sizes, though
the most common is 640x480. 640x480 is the maximum image size that is
meaningful for NTSC video signals. The 480 is derived from the number
of displayable scan lines in the NTSC standard (about 482 of the total
of 525 - the remainder are used for screen blanking, retrace and other
data). The 640 simply maintains the 4/3 aspect ratio of television
(that will change with HDTV) and does not mean that most video signals
provide horizontal resolution that high. Keep in mind that while the
horizontal resolution of a television signal is variable, the vertical
resolution is fixed to the number of scan lines in the video standard.
Horizontal resolution is typically described in terms of lines, which
is the maximum number of vertical lines that can be resolved.
A VCR provides ~250 lines, while S-video or laser disc provide about 400.
Many video cameras provide more, and because horizontal resolution is
dependent on the sampling device (i.e. CCD chip) and the bandwidth of
analog circuitry, it can theoretically be quite high. This means that
the capture card has to integrate along the horizontal scan lines to
determine pixel values. Capture cards are available that will
digitize larger images, but they require special-purpose video
equipment to be used to any advantage. For example, some video capture
cards will allow you to digitize a 1024x768 image from NTSC video. If
your video source is a VHS VCR, your resolution is effectively limited
to 250 (horizontally) by 480 (vertically). Digitizing at this higher
pixel resolution does not add any more information, since the source video
signal is the limiting factor. In fact, the interpolation which occurs
vertically will introduce artifacts that may result in a lower quality
image. Ensure that the resolutions you use maintain the 4/3 (horizontal/
vertical) aspect ratio.
Various types of input signals can be digitized including NTSC, PAL,
S-video and RGB. Some cards can handle all types, but most of the
less expensive ones can only understand NTSC. Boards that can
capture separate RGB signals can often be used to connect up to 3
grayscale video inputs.
Many video cards come with simple frame capture programs, but
if you are planning to integrate video capture with other operations
on the computer, like collecting data from an AD card, adding text
data as an overlay or changing video-in channels on-the-fly, you
will have to do some programming. In this case you will need good
programming libraries in a language you are familiar with for the
video card. Some companies include libraries with their cards, but
most charge extra. Most often libraries, when available, are for
C or BASIC, and sometimes Pascal.