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Synchronous Digital Video Switch Disclosure Number: IPCOM000121708D
Original Publication Date: 1991-Sep-01
Included in the Prior Art Database: 2005-Apr-03
Document File: 6 page(s) / 209K

Publishing Venue


Related People

Farrell, RH: AUTHOR [+1]


This article describes a digital switch for use in a video subsystem on a personal computer (PC).

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 35% of the total text.

Synchronous Digital Video Switch

      This article describes a digital switch for use in a
video subsystem on a personal computer (PC).

      In a video subsystem on a PC, multiple connections between
inputs and outputs need to be made simultaneously. These connections
carry video information which requires high bandwidth at a relatively
constant rate.  The interconnection of several of these sources to
more than one output quickly utilizes the full bandwidth of a
standard 'flat' bus.  To expand bandwidth and connectivity, a digital
switch, as disclosed herein, replaces the standard flat bus. An NxN
switch permits N interconnections simultaneously. Conceptually the
interface for input and output devices is simpler also.  As opposed
to a complex time division multiplexed bus, the switch allows each
input to run on its own bus (input to the switch) and each output to
receive only data required by that device (output from the switch).

      The synchronous digital video switch of this disclosure is
shown in Fig. 1.  It receives and transmits byte-wide data on each
port.  Each device connected to the switch occupies an input port and
an output port of the switch.  A video source transmits its data to
the switch for distribution, but also has connection from the switch
(output port) for synchronization and interface control. Each video
output device receives data from the switch (output port) but also
has an input port connection to the switch for communication to other
devices, especially the video switch controller which handles the
switch control functions.  The data on any given input can be
switched to any number of outputs.  These connections are controlled
by entries in a control table.  The control table is continuously
stepped through to provide control entries to the switch. The control
entries describe the connections to be made by the switch for each
time period.  Shown in Fig. 1 is an entry from the table indicating
that output port 1 will be connected to the input port indicated by
entry C1, output port 2 connected by entry C2, etc.  This set of
connections is maintained for a short time period, and then the next
table entry is read, allowing different sources to be connected to
the video output devices.  This control table is used to generate
hardware windows for multiple devices by loading the entries to
compose the output screen desired.

      The size of the control table is governed by two
considerations.  The width of each entry, i.e., the number of bits
per access, is determined by the size of the switch. For a 4x4
switch, the design would call for 4 sub-entries (four output ports)
of two bits each (describing which of 4 inputs), or a byte per entry.
In the case of an 8x8 switch, there are 8 sub-entries of 3 bits each
or 24 bit entries in the table.

      The other dimension of the table determines the resolution of
the hardware windows.  A 1K depth would allow 32 horizontal break