VGA - image transmission standard

VGA (Video Graphics Array) is an analogue image transmission system using three RGB components. Thus, it is susceptible to interferences, in particular during transmission at slightly higher distances compared to its digital counterparts, e.g. DVI or HDMI. VGA was introduced in 1987 for use in IBM PC-compatible graphics cards.

The term VGA often refers to 640x480 pixels regardless of the colour depth. VGA graphics cards allowed to divide the screen into two individual parts and smoothly move the image left and right or up and down.

The first VGA cards included an undocumented function, i.e. the X mode increasing the capabilities and performance of the graphics card by utilizing all of the available memory at a cost of increased complexity.

Currently, analogue VGA resolutions reach up to 2560×1600 (WXGA) at 60 Hz. Due to a wide band and analogue transmission, VGA cables used for high resolution image transmission must be of a very high quality, i.e. should include ferrite beads on both sides to eliminate the return loss. They should also eliminate crosstalk between adjacent lines, usually observed as the ‘ghost images’.

VGA cables are prone to all kinds of interferences, since the RGB signals are transmitted via individual lines and undesired signals can be induced in the adjacent lines due to insufficient shielding. Thus, VGA cables are often quite short (up to 5 m).

Maximum VGA distance can be improved with extenders. e.g. TRVGA-300-P. The devices (active receiver and transmitter) transmit VGA signal via a twisted pair. TRVGA-300-P transmits data up to 300 m at 1024×768, 60 Hz or up to 100 m at 1920×1080 (Full HD), 80 Hz.

Fig. 1. TRVGA-300-P transmits VGA signal up to 300 m

VGA uses a standard D-Sub-based 15-pin DE-15 connector. VGA output (port) in the recorder and similar connectors are available in most laptops or graphics cards.

Fig. 2. DE-15 port at the back panel of the recorder

Since 1999, DE-15 connector has been gradually replaced with its digital counterparts, i.e. DVI (Digital Visual Interface) and HDMI, however, it is still used in some devices.