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| LEDs screen display concept |
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Main features
General presentation
Features impact on the various
criteria of the screen
LEDs screen demonstrator |
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Main features |
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- 32 x 16 or 16 x 16 pixels display modules
- 3 LEDs per pixel (Red, Green, Blue)
- Screen size : 64 x 48 pixels to 384 x 288
pixels
- Resolution : to be chosen by the
manufacturer of the screen
- Serial differential transmission lines
< 20Mbits/s trough a single pair for
each column of display modules (no ribbon
cables)
- LEDs modulation with PWM programmable
resolution (between 8 and 14 bits per RVB
channel at display) , and refresh rate
(from 100 Hz to 6 400 Hz)
- Analog or digital video acquisition
- Resizing of the displayed picture
- TV output
- Ambient light measuring and setting of
the LEDs current (to optimize the
visualization quality and increase the
reliability)
- Management of gammas (television, data
processing, LEDs)
- Firmware and FPGA configuration download
from a PC
- Dispersion correction per module and per
colour
- Backup of the configuration parameters
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General presentation |
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MVD has developed a
LED screen display system.
This system allows to control the transmission of
video data, control commands and display
parameters which can be dynamically adapted
during the normal operation of the screen.
The principle is based on the use of programmable
components (Xilinx FPGA) and low cost LEDs
drivers. The development enables optimal
flexibility for different types of environments
(indoor, outdoor) and a great facility for
adjustments. Integration and maintenance are
simple.
Each pixel is made of 3 LEDs (Red, Green, Blue).
For a given number of pixels per line and
available lines the pitch between consecutive
pixels determines the size of the screen.
The source of the images can be a PC (VGA, DVI)
or any source compatible with video standards
(PAL, SECAM, NTSC, Y-C). The demonstrator
designed by MVD uses the smart camera Vision
Booster II as image acquisition and transmission
unit.
The maximum size of a screen is 384x288 pixels.
The screen contains a given number of elementary
modules, each of these modules having a
resolution of 32 lines of 16 pixels (512 pixels),
or 16 lines of 16 pixels (256 pixels).
The minimum size of a screen is 64 pixels x 48
lines. All other sizes are supported.
The 384 x 288 screen is organized in 12 columns
of 32 pixels. The acquisition board transfers the
data of each column via a differential serial
transmission line (2 wires) at a speed less than
20Mbits/sec. |
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Each LED board is associated with
a control board which extracts pixels to be
displayed from the data stream, and manages the
lighting of the LEDs. The control boards are
daisy chained via a differential serial link.
Led lighting time is controlled by a highly
parameterizable PWM, supporting a resolution
between 8 and 15 bits, that is between 256 and
32.768 levels of intensity per LED.
The LED lighting current is linearly adapted for
each colour independently, in order to adjust the
efficient current according to the ambient
luminosity. |
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Unit Block-diagram (power supply,
control board, LEDs board) : |
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Main functions managed by the
system :
- Video acquisition (analog or digital)
- Resizing of the displayed picture
- TV screen for image monitoring
- Ambient light measuring and adjustment of
the LED current
- Gammas management (control monitor, LEDs)
- Download of software, firmware and FPGAs
configuration from a PC
- Leds efficiency dispersion correction per
module (independently for each color)
- Backup of the configuration parameters in
a flash memory
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Features impact on the various
criteria of the screen |
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| Technical
elements / features |
Impact on
the various criteria of the screen
Image quality,
flexibility, reliability, production and
maintenance cost. |
Resolution
: auto configurable
(maximum 384 x 288 in our demonstrator)
The chaining method of the control and
display boards offers a feedback path to
the acquisition board. |
This makes
possible, amongst other things, to
determine the size of the screen, and to
check its correct operation. Easy
configuration and maintenance
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Pixel
number per display module :
- 32 pixels x 16 lines (512 pixels
ie 1536 LEDs)
- 16 pixels x 16 lines (256 pixels
ie 768 LEDs)
The control board FPGA can manage all
LEDs, whatever the number of pixels per
module (512 or 256 pixels)
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For screens
with large pixels, (more than 10 mm), it
is generally preferred to use 2 LEDs
boards to build a 32 x 16 module in order
to reduce the size of LED boards. In this
case, one control board manages a set of
32 x 16 pixels.
On the other hand, for applications using
small pixels, it is preferred to consider
LEDs boards of 32 x 16 based on compact
LED drivers (QFN packages) .In this case,
only one control board will manage the
whole module.
In both cases, the model and the
configuration of FPGA are identical.Flexibility,
cost reduction
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Number
of bits number per RGB component
(display):
Programmable between 8 and 15
bits/component (between 24 and 45
bits/pixel).
Selection is done through a command sent
by the acquisition board to PWM control
modules
Display refresh rate :
Between 100 Hz (14-bit modulation) and
6.400 Hz (8-bit modulation) according to
the number of bits chosen for the PWM
modulation. |
Allow to
select the PWM modulation. For example
both following configuration can be
selected :
- 12 bits/component (36 bits/pixel)
with a refresh rate of 400 Hz
- 14 bits/component (42 bits/pixel)
with a refresh rate of 100 Hz
When a LED screen has to be filmed by
a TV camera - sporting events for
instance - a high refresh rate is
preferred to reduce aliasing effects.
In other cases, a higher number of bits
per pixel will allow a better image
restitution with a lower refresh rate.
(100 Hz with a 14 bits per component
modulation).
Flexibility
of use : the same screen supports various
types of applications
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Number
of bits number per RGB component
(acquisition) :
8 bits per color (24 bits per pixel).
The image transmission rate between
acquisition board and display system is
depending on the acquisition input
standard. |
Market
standard
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Gamma
correction table :
dynamically modifiable |
The
acquisition board can dynamically
transmit to PWM control modules a new
gamma correction table without stopping
the operation of the screen. Dynamic
adaptation to the display conditions,
flexibility
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Lighting
of the LEDs :
Current-controlled with specialized, low
cost circuits (drivers) proposed by
several manufacturers.
The method of images transmission and
lighting management of the LEDs has been
especially designed to benefit of the low
cost of both last generation Xilinx FPGA
(Spartan3), and constant current LED
drivers (actually current is
adjustable)These LEDs drivers are mature
products and are proposed by several
manufacturers, electrically, mechanically
and pin to pin compatible. Only a minor
adaptation of the bitstream is necessary
to pass from a type of component to
another.
Many supply sources. |
Reduction of
production costs
Supplier-independent
In addition, since the current of LEDs
is regulated by specialized drivers said
"constant current" (in fact
dynamically adjustable), serial resistors
become completely useless.
Reduction
of complexity and cost of the boards
Moreover, the assembly of
the screen is simplified : since this LED
current is independent of the supply
voltage, the 3 channel Red, Green and
Blue can be powered by the same power
supply (typically comprised between 4,5
and 5 volts).
Cost
Reduction and assembly simplification
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LEDs
lighting intensity (current) :
Dynamically programmable, according to
external conditions.
Linear adjustment between 3 and 20 mA or
more if necessary.
Examples :
- Operation during the night :
Current adjusted to 3 mA
- Operation outdoor - regular
luminosity : Current adjusted to
approximately 8 or 10 mA
- Operation outdoor - sunny
luminosity : Current adjusted up
to 20 mA
Most of screens of the market use the
same lighting LED current, whatever the
ambient luminosity.
Indoor screens operate with a current of
approximately 5 mA , whereas outdoor
screens operate with a current of
approximately 20 mA.
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It should be
noted that an outdoor screen must provide
a "maximum" luminosity in
daylight even in full sun, whereas this
same screen must considerably reduce its
luminosity in half-light or during the
night.
Current systems work with constant
current (20 mA for example for an outdoor
screen). To reduce the luminosity they
must decrease the display resolution by
reprogramming the PWM parameters.
The restitution of images is affected,
particularly the dark parts.
On the other hand, an indoor screen
working with constant current (typically
4 or 5 mA) cannot work outdoor because of
its weak luminosity.
These difficulties lead many
manufacturers to develop different
electronics for indoor and outdoor
screens.
In order to solve this problem, our
equipment allows to permanently control
the current provided by the LEDs drivers,
so as to use the optimum current
according to the day/night light
conditions and intermediate situations,
so that the number of levels per channel
RVB - is unchanged. Flexibility,
optimization of the display quality and
improvement of the reliability.Flexibility,
optimization of the display quality and
improvement of the reliability
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Links
between acquisition board and display
modules :
Transmission of the video data to display
as well as commands and auxiliary data is
done by only one differential line
working at low speed (less than 20 MHz).
The cable contains also a ground wire and
a wire use to protect the system when
cables are not correctly connected to
control and display modules. (total : 4
wires).
The chaining between consecutive boards
of the same column is ensured by a
differential connection of the same type
(twisted pair of approximately 30 cm for
a screen based on 16 mm pixels).
The sending of orders and auxiliary data
does not stop displaying.
The boards of the same column of 32
pixels X 16 lines can be chained on the
top or on the bottom side
Control informations and/or data that can
be transmitted at any time :
- Adjustment of the LEDS current
for each color (Red, Green and
Blue) according to the variations
of the external luminosity.
- Gamma correction curve
modification
- Lighting/extinction of the screen
- Modification of the bitstream
(configuration file of the FPGA)
used to completely reconfigure
the screen without manual
intervention (specific
application, evolutions...)
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- Low cost connections
- Screen assembly simplification
- EMI reduction
- Protection against bad
connections (Transmit and receive
cables inverted)
Control informations transmitted
concurrently with pixel stream .
Assembly simplification and optimization
Dynamic adaptation of the LEDs intensity
according to the external environment.
The same screen can adapt to ambient
light : night, indoor and outdoor
daylight(even in full sun).
LEDs are operating with optimal current
value, adjusted in real time.
Improvement of the lifetime of LEDs and
associated drivers.
Runtime adaptation to conditions of
acquisition and display.
Power management of power electronics
circuits
Easy
evolutions, facilitated maintenance
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Calibration
of the boards according to LEDs
sensitivity :
Adjustment and recording of compensation
parameters related toof dispersion of
luminosity between LEDs boards, for each
component Red, Green and Blue. |
Each control
board records a luminosity correction
coefficient per RVB component.
This makes possible to automatically
initialize the response of the display
modules according to the LED sensitivity
, which makes easier the manufacturing
and the maintenance (Module change).
This adjustment can be achieved manually
or, automatically from the intelligent
camera Vision Booster II designed by MVD.Reduction of
the maintenance and production costs
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LEDs screen demonstrator : |
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MVD has developed a
demonstrator which allows to check the complete
functionality of the theoretical concept.
This demonstrator uses the camera Vision Booster
II for video acquisition and transmission and
also for control, commands and auxiliary data.
In a professional application, it is necessary to
design a video acquisition board which supports
main video standards (PAL, SECAM, NTSC, Y-C, VGA,
DVI...). MVD can take care of this study. We have
a long experience in this type of design.
Among our previous designs
- Correlation unit for analogical video
reception thanks to 4 different antennas
(for TV reception in cars). This project
includes an intensive real time
processing of digital data.
- Device of video channels encryption and
decryption (multichannel PAL, SECAM,
NTSC)
- System of video monitoring including
MPEG2 compressors, and storage of the
video stream in CompactFlash memory.
- Vision Booster II: smart camera for
industrial control applications
- Different types of video games
- Other video applications, industrial
controls, telecommunications, ASIC
prototypes ...
Among our most prestigious customers :
- AIRBUS
- Alcatel CIT
- Alcatel Espace
- Alstom
- Areva
- Gemplus
- Hewlett Packard
- Indra
- Sagem
- SCM
- Thalès
Services offered by MVD:
MVD is a company of 20 people including a
majority of engineers, based in Plaisance du
Touch (near Toulouse - France), with branches in
Paris (training center), and South America.
Among our principal activities :
- Trainings : MVD is the official and
exclusive training center, of the
following electronic components and tools
manufacturers :
- Xilinx : we offer all types of
courses in connection with Xilinx
technology, including VHDL, PCI,
System on Chip - MicroBlaze and
PowerPC405,
MultiGigabitTranceivers, Digital
Signal Processing...)
- Freescale : courses on any
processor of this manufacturer
(PowerPC, ColdFire...)
- IBM : PowerPC courses
- AMCC : PowerPC courses
- ARM : ARM7, ARM9 and ARM11 cores
- WindRiver : VxWorks, Tornado,
Workbench
- Linux for embedded applications
- High end buses : PCI Express,
serial ATA, RapidIO, Gigabit
Ethernet, USB2...
- Design of FPGAs, electronic boards, and
expertise for assistance in designs :
- Design of FPGAs, electronic
boards (specifications, choice of
the components, schematics,
boards routing).
- Manufacturing of prototypes and
pre-production boards.
- Design of FPGA (official design
and training center of Xilinx
since 1989)
- Software development (low level
and/or application).
According to the needs of our customers, our
services can be partial or cover the totality of
the design as well as the delivery of functional
boards.
Regarding the LEDs screen control system,
services of MVD can go from the delivery of
documentation and VHDL files of the FPGAs
including technical support and transfer of
"Know How", to the development of the
complete system (only the electronic part)
including also the transfer of "Know
How".
According to the wishes of our customers the
services can be exclusive or not (i.e. a part of
the design can be re-used or not for other
customers).
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