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KRAMER ELECTRONICS LTD.
Figure 2: The Digital “EYE” after accumulating Noise and Jitter
1.4
SDI Standards
“Uncompressed” digital video usually refers to “
Digital Component Video
” (ITU-R BT.601 standard). This is
based on sampling of component video. The sampling scheme stipulates one luminance (Y) sample for each
pair of color-difference (R-Y and B-Y) samples. Luminance sampling is done at 13.5MS/s (mega-samples per
second), and each color-difference is sampled at 6.75MS/s. This is also known as 4:2:2 (Y is sampled at 4f
sc
(more or less!), and the color differences at 2f
sc
).
The 8 or 10 bits of data are serialized to produce a single stream of bits (SMPTE-259 standard) at 270MS/s for
10 bits (10X(13.5+6.75+6.75) = 270). This is known as “
Serial Component Video
”, and is usually referred to
as
SDI
.
“
Serial Composite Video
” is (true) 4f
sc
sampling of composite video – PAL at ~177MS/s, and NTSC at
~143MS/s. This standard is hardly used, except in some older installations in the USA.
A standard launched about five years ago is “
Digital Widescreen
” – a digital version of PalPlus (16:9 or
Letterbox aspect ratio). This works at 360MS/s.
SDI
is continuously being upgraded and speeded up. Rates of 540MS/s and beyond 1GS/s are being tested to
be used in the near future for full digital HDTV.
1.5
Factors Affecting Quality of Results
There are many factors affecting the quality of results when signals are transmitted from a source to an acceptor:
Connection cables
- Low quality cables are susceptible to interference; they degrade signal quality due
to poor matching and cause elevated noise levels. They should therefore be of the best quality.
Sockets and connectors of the sources and acceptors
- So often ignored, they should be of highest
quality, since "Zero Ohm" connection resistance is the objective. Sockets and connectors also must
match the required impedance (75ohm in video). Cheap, low quality connectors tend to rust, thus causing
breaks in the signal path.
Amplifying circuitry
- Must have quality performance when the desired result is high linearity, low
distortion and low noise operation.
Distance between sources and acceptors
- Plays a major role in the final result. For long distances
(over 15 meters) between sources and acceptors, special measures should be taken in order to avoid cable
losses. These include using higher quality cables or adding line amplifiers.
Interference from neighboring electrical appliances
- These can have an adverse effect on signal
quality. Balanced audio lines are less prone to interference, but unbalanced audio should be installed far
from any mains power cables, electric motors, transmitters, etc. even when the cables are shielded.
