Discussion on the design of feedback circuit of interactive set-top box

Digital terrestrial broadcasting ( DVB-T ) and cable television (DVB-C) set-top boxes are transitional products in the development of digital television. Their functional characteristics are crucial to the application of digital television technology, and the return function is essential for the development of two-way services. At present, there are different implementation methods for the return function of the set-top box. This article will explain the return circuit composed of different set-top box products and their peripheral components .

As the global digital TV program launch schedule approaches, the popularity of set-top boxes is also increasing. Currently, most set-top boxes on the market have a one-way receiving function, but as long as they are equipped with appropriate two-way circuits to enable these boxes to have a return function, they can become interactive products and can be used to process commands issued by users. Since there are many types of two-way circuits, this article will introduce the possible combinations based on the classification of set-top boxes.

DVB-T return circuit

At present, digital TV has been broadcasted in Taiwan. With the Athens Olympics, more and more users are buying terrestrial broadcast (DVB-T) set-top boxes, and some consumers are choosing to buy car set-top boxes. However, these two products can only watch one-way TV broadcasts. At the same time, China has begun to load MHP (Multimedia Home Platform) on the TV broadcast architecture as a development platform for interactive TV. From the perspective of the return path, it is a reasonable development trend for home set-top boxes to connect to modems and car set-top boxes to connect to GPRS modules.

modem

Most of the current set-top boxes are one-way receivers of digital TV broadcasts and do not have a return circuit. In the future, they can form a two-way interaction with the system head end by connecting a modem to receive multimedia content. Generally speaking, the interaction is just sending a few simple user commands back to the system end. The home set-top box can use the most popular telephone line and then connect a modem to form a return circuit.

The general STB chip can perform modem control functions by itself, and does not need to be connected to another microcontroller. ST's STB chip provides an external memory interface (EMI) bus, which can be connected to the modem DSP (such as Agere's DP2x series), and then connected to the codec of the telephone line (such as Agere's CSP1034), and then the return circuit can be completed (as shown in Figure 1). The modem driver has also been developed to support different modes. There are three main modem modes as shown in the table.

GPRS module

The car set-top box can also be designed as an interactive set-top box , but a mobile modem must be used. GPRS modules are currently easy to obtain and are only 4.3×2.9 cm in size. Although the transmission rate is only 9,600/14,400bps, it is sufficient for backhauling information. It is expected that after the price of GPRS modules continues to drop in the future, they can even be built into home set-top boxes. If each home set-top box has a built-in GPRS module, it will be convenient for system operators or TV stations to manage viewers and conduct two-way applications such as ratings surveys. The connection method only needs to use a serial port, and the hardware design is also quite easy (see Figure 2). However, manufacturers must develop drivers and application software by themselves. At present, some telecommunications system operators have noticed this business opportunity and invested in related software development.

DVB-C STB return circuit

Although the launch of digital TV in Taiwan has promoted the craze of terrestrial broadcasting set-top boxes, it cannot be ignored that the majority of viewers still use analog cable TV. Therefore, cable TV (DVB-C) set-top boxes will become an important transition product from analog to digital TV. In addition, digital cable TV is also the easiest to launch pay programs and on-demand information services, so the development of DVB-C towards two-way interaction will be an inevitable trend. In terms of the return path, the DVB-C set-top box can be connected to the telephone line by the PTSN modem, but the telephone line may not be distributed to each room along the cable cable, so if a cable modem can be used, it is the most efficient solution. Since 1999, system operators have begun to provide cable modems as return circuits for viewers.

There are several standards for current cable modems, including the IEEE802.14 protocol defined by the IEEE, the DAVIC protocol defined by the DAVIC (Digital Audio Video Committee), and the DOCSIS and Euro-DOCSIS protocols defined by the MCNS (Multimedia Cable Network System).

DOCSIS is the abbreviation of Data over Cable System Interface Specification, which is mainly used to standardize cable modems in the United States to ensure the speed and quality of two-way communication. This specification was originally a standard for personal computer users to use cable modems to connect to the Internet at high speed, but it can now be used as a return circuit standard for interactive DVB-C. Downlink data uses a 6MHz channel with 64-QAM or 256-QAM, ITU-J83 Annex B FEC. Uplink data uses QPSK or 16-QAM modulation in the VHF (5~42MHz) band to transmit IP interactive data.

Euro-DOCSIS is a standard derived for compatibility with the European 8MHz frequency channel. Downlink data follows the DVB-C standard ETS300 429/ITU-J83 Annex A FEC. The downlink data bit rate is 2MBps, and the uplink data uses QPSK or 16-QAM modulation. Its VHF band extends to 5~65MHz, with extra bandwidth to accommodate the return channel.

Since cable modems all have USB interfaces that can be connected to computers, so as long as the set-top box has a USB interface, it can also be connected to the cable modem. Here we divide the USB interface into several categories, from the perspective of technical difficulty, they are USB1.1 peripheral level, USB1.1 host, and USB2.0 host.

[page]

USB is used to replace the traditional RS232 serial interface and IEEE1284 parallel interface. It has been several years since its introduction in 1997 and is a mature technology for PC-related applications. The transfer rate of USB1.1 is 12Mb/sec, while USB2.0 is up to 480Mb/sec, and is divided into Host level, Hub and Peripheral.

For consumer electronics, it is not technically difficult to use them as a peripheral level of a personal computer. For example, digital cameras and MP3 players can be connected to computers and managed as storage devices through a file manager. The same is true for set-top boxes. You can add a serial interface to USB IC to connect to the computer, allowing the computer to retrieve the content received by the set-top box. If it is a set-top box with a hard disk, the computer can also manage it as an additional hard disk. However, since the file format (TS) stored in the hard disk of the set-top box still needs to be read and decoded by the set-top box, it is not a computer-readable MPEG format. At the same time, GB-level files are too large, and it is not practical to transfer them using USB1.1. Therefore, if the set-top box only has a USB1.1 peripheral, it is mostly used to diagnose built-in programs and debug, or to develop or update programs.

If the set-top box has the USB1.1 host function, it can issue commands to use the cable modem, whether external or built-in, to transmit signals or other digital data to achieve the return function, as shown in Figure 3. If the STB core chip does not have a USB1.1 host, a USB1.1 host chip must be added (Cypress or TransDimension provide such chips).

If the set-top box has the host function of USB2.0, it can not only be used to connect to a cable modem, but also as an interface for a hard disk, making it easier to design a DVR (digital video recorder). It can also be connected to a personal computer in a Host-Host manner, but the software stack of the driver and the hard disk file management program still need to be developed.

IP Box Internet Box Return Circuit

Chunghwa Telecom has recently launched MOD (Multimedia on Demand) multimedia on-demand information for ADSL users. The data compression format currently transmitted is MPEG2. As a result, the compression ratio is not high and the bandwidth is limited, so the services that can be provided are limited. In the future, when the system-side MPEG-4 encoding settings are completed and the MPEG-4 decoding chip is popularized, standard-screen and even high-resolution videos can be downloaded from the Internet for a fee. This will have a huge structural impact on the entire business model of DVD movie sales and rentals, and the annual demand for global IP Boxes will reach tens of millions of units.

IP Box can connect to ADSL by using only an Ethernet transceiver chip, as shown in Figure 4. ADSL can also be built into the IP Box. However, the TCP/IP software stack must be obtained from a contract with a relevant developer, or it must be developed by yourself.

DMA transfer circuit

DMA (data media adapter) and DMR (data media receiver) can be used together to receive Internet data, and can also be used as a transmission device for household consumer electronics, such as CD and DVD analog /digital signals. The wireless transmission between DMA and DMR is constructed by using a wireless network card or a built-in WiFi wireless network chipset. The 802.11b standard transmits data at a rate of 11Mbps on a 2.4GHz carrier. This frequency is sufficient to transmit video compression data with MPEG2 standard resolution, but it will not be enough when it is upgraded to high resolution in the future, and 802.11a/g must be used, as shown in Figure 5.

With the upcoming launch of digital television, the use of set-top boxes will become increasingly popular. When developing such products, only a few design changes can be made to make the set-top box interactive and more attractive, thereby accelerating its promotion in the market.