Converter box brings digital TV into analog realm

There's a revolution coming in broadcast television, and for those not yet equipped to handle the change, solutions are standing by. The long-discussed Federal Communications Commission (FCC) mandate to switch off analog television signal transmission across the country is now slated for Feb. 17, 2009. Digital TV broadcasting will replace remaining analog signals, a move already begun or implemented in many U.S. markets. People who use cable or satellite TV, regardless of their televisions' functionality, will not be affected by the conversion; those with antenna-based TVs that have only analog tuners, however, have been bracing themselves for the inevitable kick of their old TVs to the curb.

Not so fast. Technology comes to the rescue, in the form of a converter box that massages DTV signals back to the analog format needed for legacy television compatibility. Converter boxes from a variety of manufacturers have made their way to consumer electronics store shelves in recent months to help ease customers through the pending DTV changeover. Because the analog signal “go-dark” order was imposed by the FCC, there's even an offer for U.S. government help: To defray the cost of spectral repurposing, consumers can apply for $40 coupons from the Department of Commerce to be used toward the purchase of an approved DTV converter box.

We examined the LG Insignia NS-DXA1 converter box, a unit offered at Best Buy that is similar, if not identical, to the Zenith (an LG subsidiary) DTT900, which is sold at several other retailers. Currently, each box sells for $60, so those who get their coupons and paperwork straight end up paying about $20 out of pocket.

The Insignia box is relatively simple, consisting of a main board and a power board wrapped in a painted two-piece stamped-metal enclosure with a plastic front face. The front panel has just a few buttons, two for channel up/down select and a third for powering on and off. An infrared receiver for use with the included remote control also hides behind the front plastic fascia, along with an LED indicating the box's active status. The back of the enclosure has two coaxial cable connectors, one for the DTV antenna input and the other for analog RF output. These connectors are joined by three RCA jacks, two for left/right stereo audio and the third for line video.

The functional concept of the converter box is straightforward. RF-modulated DTV broadcast input (by way of “rabbit ears” or other antenna) comes in, and RF-modulated analog TV signals go out, feeding the non-DTV-compatible set. Don't confuse DTV with high-definition (HD) broadcasts. Although HD might be supported, DTV signals can be of similar resolution to conventional analog signals. (Of course, any HD programming will be lost on a standard- definition analog TV using the box output, but that's unavoidable.)

Inside, just a few components serve the primary task of going back to analog; all signal processing takes place on the main board, a two-metal-layer FR4 PCB whose simplicity helps keep costs down. A can tuner module from Sanyo, mounted to the main board, lies at the signal headwaters. The can assembly contains a TI SN761668 digital TV tuner for demodulation of the incoming RF DTV signal and a Toshiba TA1372FNG modulator that takes the analog signal back to the RF spectrum for television interface.

Between these two components (which are supported in-can by a number of discrete coils, filters and other parts) lies the MPEG decoder system on chip (SoC) (LGDT1111D) supplied by LG. The SoC is tied to 32 MB of DDR SDRAM memory (HY5DU561622FTP) from Hynix and 2-MB NOR flash memory (MX29LV160C) from Macronix. In addition, a 32-Kb x 8 serial EEPROM (24LC256) is supplied by Microchip for boot-up memory. A Cirrus Logic CS4344 stereo D/A converter is used to create line audio out to accompany line video from the rear panel.

In essence, the tuner brings in DTV and demodulates the signal to baseband, where the MPEG signal stream is decoded and converted to an analog signal equivalent. The resulting analog versions of the signal go to the rear RCA jacks while also forking back to the tuner can, where the signal is remodulated back onto an RF carrier for analog-only TV input.

The power board, which feeds all TV signal processing, is a single-metal-layer phenolic PCB whose main semicon- ductor content is the usual set of discrete devices for rectification and an Infineon switch-mode power supply controller (ICE3B0365J). The power board implements the DC-to-DC conversion needed to go from (rectified) wall- power input, with attention to achieving low standby current requirements.

As might be expected, the $60 retail price implies a low-cost box to build; our detailed cost analysis points to a hardware production cost of about $30 for the NS-DXA1. Combining a modest purchase price (made possible by a carefully whittled bill of materials) with a significant coupon offset from the government allows folks to hang on to their still-good-enough TVs through the impending transition.

With a reported government subsidy of $1.5 billion in play, sales are likely to be stimulated to the extent that legacy analog TVs remain in the mix. In the end, the February 2009 switch-off of analog broadcast television will free up spectrum that has huge intrinsic value. Claims of improved broadcast quality from DTV could indicate another upside. But for those not quite ready to take the plunge, an inexpensive option will still exist in the form of boxes such as the NS-DXA1. p

See related image: New digital-to-analog signal converters will ease broadcast customers through the impending transition to digital TV.

See related image

David Carey is president of Portelligent, a TechInsights company that produces teardown reports and related industry research on wireless, mobile and personal electronics.

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