connected to an amplifier (leading to the receiver or monitor) may present a capacitive and/or an inductive
load to the amplifier. This is the main cause for instability. The non-ideal behavior of a load or cable may
severely degrade the performance of the amplifier - it’s bandwidth, linearity, and stability - and in general it’s
ability to faithfully reproduce the signal.
The cables affect image resolution. Longer cables, due to non-ideal characteristics, cause high frequency
deterioration and hence image “smear” and loss of resolution. In computer graphics especially, this adverse
effect is very much accentuated. The amplifiers should cope then with an additional task - compensating for
cable losses up to the maximum useful operation distance. High-resolution graphics systems must use very
high quality cables for image transmission. The cables should be shielded - to eliminate externally induced
interference but the shield might increase the capacitance of the cable, and therefore, cause deterioration in the
image’s resolution and clarity. The standard cables can only be a few meters long. For longer distances, the
compound cable is broken into five individual coax cables, which are bulky and cumbersome for use. Even so,
the distance is limited to several tens of meters.
The non-ideal behavior of a cable may create other problems resulting from the failure to accurately match the
system’s required impedance. The result of this, especially at high frequencies, is “shadows” or “ghosts” on
the image, resulting from standing waves and electronic reflections running back and forth between transmitter
and receiver. Another aspect to consider is the sync. As those signals are logic signals, which are not treated as
analog signals, the receiver does not terminate the line, and therefore the line is not matched. A host of
problems might occur when the signals are sent over long, unterminated, unmatched cables. The result might
be image breakdown or distortion due to improper sync information. The amplifier that drives the analog
section of the graphics data should also be able to buffer, recover and send the sync information in such a way
that it would be received properly at the receiver end.
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 target. Sockets and connectors must also 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 end 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 of
over 15 meters (~2 to 3meters for VGA/XGA) 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.
