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Employing SoC for cost-effective 3D designs

Posted: 01 Oct 2011 ?? ?Print Version ?Bookmark and Share

Keywords:3D? SoC? active shutter?

Along with the rising adoption rates of 3D display technologies comes the challenge to design quality 3D glasses with attractive prices. Minimizing the physical size, developing true universal operation, and lowering power consumption have also become critical considerations for manufacturers vying for a piece of this market. This article will examine the 3D active shutter architectures employed today and contrast those with the next generation solutions now available.

Active shutter architecture
3D active shutter glasses operate by alternately driving a pair of liquid crystal lenses on and off. The switching of the lenses is synchronized with alternating "left-eye" and "right-eye" images generated from a 3D display. When the left-eye image is displayed, the glasses will open the left-eye lens and close the right-eye lens (and vice-versa). This synchronization happens at the refresh rate of your display (typically 120Hz) and your brain then combines the two images giving a perception of depth.

While the overall look of different 3D active shutter glasses may vary, the electrical architecture is very similar and typically broken into four distinct subsystems.

Synchronization of display
In order for the user to experience the 3D image, the display and the glasses must be perfectly synchronized as described above. To accomplish this, the display will transmit an infrared (IR) signal that contains synchronization information. This signal is detected by a photodiode in the glasses and then amplified and filtered to eliminate any ambient IR noise. Once complete, the signal is passed to the system controller for decode operations.

System control
The system controller is the heart of the glasses and interfaces all the subsystems together. It will take the amplified and filtered signal from the display and decode the information before passing it to the shutter control subsystem. The system controller will also interface to the battery management system to ensure power is supplied throughout the system. Finally, the system controller will typical interface to any external peripherals, such as control inputs and buttons or USB.

Shutter control
Once synchronization data has been decoded, the system controller will communicate with the shutter control system to operate the shutters in synchronization with the display. The shutter control system will typically boost the system voltage to match specifications of the liquid crystal shutters being used. The shutter voltage varies from vendor to vendor but is usually somewhere between 10 and 20V. This voltage is then supplied to the shutters, which are switched at a frequency that matches the refresh rate of the display.

Battery management
active shutter glasses require a battery to power the electrical components. The battery can either be a single-use coin cell battery or a rechargeable lithium based battery. Both systems require constant monitoring to ensure constant power output is being delivered to the system. In the case of rechargeable batteries, systems must be in place to monitor and control charging activities. This is specifically meant to safe guard against over voltage and over current which can cause damage to the device and the user in the case of a battery failure.

Discrete solutions
As mentioned, the general architecture for all 3D active shutter glasses is the same. For first generation active shutter designs, manufacturers have utilized discrete components for each subsystem of the overall architecture described above and shown in the block diagram in the figure. While this may have initially provided a quick time to market, this approach has three primary limitations that impact consumers.

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Figure: Here's a block diagram of 3D glasses solution employed nowadays.

The primary driver for manufacturers today is reducing overall cost, and a discrete solution tends to be the most expensive option. When you add up the necessary op amps, boost converters, switches, battery charging ICs, microcontrollers, and various passive components required to implement the design, the BOM costs quickly escalate. Handling, inventory, and assembly costs are also increased as the number of components increases, making this design methodology very expensive.

With a discrete component solution, the number of devices needed and real estate required to implement the design is significant. Even efficiently routed designs with proper noise isolation can require a significant amount of space on the PCB for routing traces of the numerous discrete components. Consumers are continually pushing for lighter weight glasses that have a sleeker design profile. Designs using discrete components struggle to deliver on both of the requirements.

Discrete component architectures offer far less flexibility in the overall design, making it difficult and expensive to create true universal operation. Instead, discrete component designs are targeted for a specific display or a specific model. While this may be effective for a quicker time to market, it reduces consumer options when buying glasses and locks them in to a specific brand specified by the display manufacturer.

Integrated solutions
While the first generation designs used discrete components, some vendors are now moving to a more integrated solution using an application specific integrated circuit (ASIC). An ASIC is ideal for 3D glasses because they can be specifically tailored to do the task of decoding the IR synchronization protocol, while also handling the battery charging and shutter control. This is accomplished by integrating the boost circuitry and switching FETs internally to the device. Additionally, ASICs can do these tasks efficiently with relatively low power consumption and with a limited number of required external components to implement the entire solution. Unfortunately, an ASIC based design provides a fixed solution that is unable to be modified as the device requirements change. ASICs are also expensive to design and provide limited configurability options once implemented in a design. If the design significantly changes, then the ASIC will no longer be an ideal solution. While the individual cost of an ASIC may be minimal, most ASIC manufacturers will require upfront non recurring engineering (NRE) fees that can reach cost levels of $1 million or greater.

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