SD DVB-C Set-top box

Customer

SemiDevices is a Russian OEM/ODM electronics manufacturer.

Objective

The objective is to develop a digital set-top box receiver for DVB-Ñ SD signal. The STB should be based on the STMicroelectroniñs STi5197 processor. Set-top box enclosure should be implemented in the minimal form factor. The finished device targets the low-end market.

The engineering team had to perform the following tasks under the project:

• Selection and cost optimization of an elements base
• Calculation of the product’s cost in mass production
• Development of a circuit diagram
• PCB design layout and routing
• PCB enclosure design
• 3D enclosure design modeling
• PCB prototyping and debug
• Software development

Solution

The STMicroelectronics STi5197 processor was used as a key STB component. It features video/audio codecs and a demodulator on a single chip, which gives a competitive edge in the cost and size of the final product.

Along with processors, STMicroelectronics manufactures a number of additional chips and active STB components, covering virtually the whole range of digital TV components, which considerably cuts the total cost of the device.

ST provides full technical support and maintenance at the product development stage. The company also provides efficient debugged solutions for software implementation.

Basic Platform Design Requirements

The STB should receive DVB-C signals in the operating frequency range of 47-863 MHz, decode MPEG-2 video streams and send decoded video to a TV receiver. Video output format: SD (720x576), composite and component. The device is connected to the TV receiver through a SCART connector.

The STB should be fitted with a standby button, a smart card reader and an infrared receiver to receive remote control commands. Also, it should support a CAS (Conditional Access System) to deliver pay channels.

The device should have the following features:

• Electronic program guide (EPG)
• Switching between TV and radio channel lists
• Channel editing
• Favorite channels
• Multilingual user support
• Over-the-air software update option (OTA)

Figure 1. Overall layout of the device

Circuit Design

Figure 2. Structural layout of the device

The STi5197 by STMicroelectronics was chosen as the CPU for the project. It features hardware capabilities for MPEG-2 video and audio decoding as well as an integrated DVB-C demodulator. Linux, Windows CE and the ST40 OS21-compatible core is responsible for implementing the product’s user functions. The chip includes a DAC for analog audio and video. The STV6432 external chip is used for analog video and audio filtration and amplification. The STB is remotely controlled through an IR port.

While selecting a tuner for the project, two types of devices were considered: a traditional CAN tuner (a finished device in a metal screen) and a silicon tuner (basically an IC in which all processing is done digitally). The choice was made according to the following criteria: overall dimensions, solution cost, driver support and signal reception parameters. Finally, MXL203 - a silicon tuner from Maxlinear, was selected for the project. This solution hasa much smaller size andhigh quality signalreception.

A DDR chip, 32 MB total, was used as program and data memory. The STB was booted from an external SPI.

To reduce the cost, the PCB was implemented with two layers and single-sided mounting. The board is made ​​in the minimal form factor. DDR routing was based on signal integrity and calculations of the required wave impedances.

Software

The STB software is based on OS21 and STAPI, a low-level programming interface supplied by STMicroelectronics. OS21 was selected due to low demands to processor performance and size of RAM and flash memory.

Major software modules:

• A tuner control module (bandwidth scanning, switching between channels, stream reception)
• An MPEG-2 stream player
• A graphic interface (channel selection, scanning, favorite channels)
• A remote control support module
• A smart card module
• Service utilities (software update, etc.)

Enclosure Design

A number of enclosure design options and styles have been considered to meet the customer’s needs. In accordance with the selected option, an in-depth study of the product’s enclosure design was conducted. The finished design was implemented by the designer as a 3D model.

In the process of development, the engineers worked out a solution to the problem of ensuring the required thermal conditions while maintaining a compact size and an attractive design. Thus, a ventilation system was studied and designed. Thermal modeling of the device was performed in special CAD systems, using a PCB model and an enclosure model.

Another development feature is creation of a design with a minimal number of parts and fasteners. The problem was successfully solved - the finished enclosure consists of only two parts, while the PCB and the enclosure are mounted with four screws. This enclosure design provides maximum ease of assembly, which further reduces the production cost.

Advantages

• Use of chips by one manufacturer, which makes technical support easier
• Minimal number of components
• Two-part enclosure, which ensures a low cost and simple assembly
• Low production cost

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