Pay-radio receiver rocks

You probably already pay a cable or satellite TV bill, even though free programming is out there for the taking. Now, in growing numbers, consumers are also buying their broadcast audio, despite the clutter of freely available AM and FM channels on their radio dials. The two primary providers of satellite pay-radio systems, Sirius and XM, hope that all of us spendthrifts keep it up, and they're responding to market growth with increasingly portable receiver designs to help listeners tune in-both inside and outside the car.

The Delphi Roady2 satellite radio receiver for XM was released in July 2004. Compared with the previous-generation Delphi SkyFI receiver (search www.eetimes.com for article ID =18308857), the Roady2 features improved integration in both the radio receiver and digital-processing aspects of the design. As you might expect, analog technology mans the front lines of pulling in signals from satellites (or, potentially, terrestrial repeaters) to bring an array of music, talk, news and other programming to listeners anywhere in the mainland United States and surrounding marine areas.

In contrast to previous designs, the receiver in the Roady2 is part of the main (and only) circuit board. A commercially available monolithic satellite digital audio radio service (SDARS) receiver chip, Maxim's MAX2140, replaces the highly discrete receiver solution, housed in a separate cast-metal enclosure, that was used with the first generation.

With a voltage-controlled oscillator on-chip, the BiCMOS-based Maxim device requires only a separate IF SAW filter and reference oscillator-supplied by RFMonolithics and KSS, respectively. Clearly, Maxim smelled a burgeoning opportunity with SDARS to shrink a previously burdensome radio module to a single chip.

Output from the satellite receiver is fed to an STMicroelectronics STA850 source and channel decoder chip, also a step forward in integration from the previous two-chip implementation of the receiver's digital signal processing. An Atmel ATMEGA128L-8MI microprocessor works with the STA850 to implement system control and interfacing, and a special-purpose encryption controller from ST (the ST19AF08) handles user authentication. Anyone can receive the SDARS signals, but only subscribers get to unlock the data stream and access the music. Several memory components round out the digital design, including a Micron 16-Mbyte SDRAM (the MT48LC16M8A2) for DSP working memory and a small, 32-kbyte SRAM for use by the Atmel device.

Received, decoded and unlocked audio in raw digital form is passed to a two-channel D/A converter from AKM Semi (the AK4384) to bring the signal back into the analog world. From there, a National Semiconductor audio power amp (the LM4880) completes the audio output food chain.

For in-car listening, audio can be sent to the auto's stereo via a cassette adapter. Alternatively, in a feature added with the Roady2, a Rohm wireless FM audio link (the BH1417F) can be used to pipe modulated audio through to an unused FM channel-ironic, when you think about it.

The "last few feet" of connectivity represents an unusual challenge for SDARS technology in aftermarket situations, since digital audio's high fidelity is squandered to some extent by cassette adapters or FM modulation. The lack of a direct line-level connection to the car radio, combined with an irksome tangle of wires, means audio purists and neat freaks may find the portable SDARS solution wanting compared with factory-installed car receivers.

Imperfections aside, SDARS hardware is experiencing integration-driven improvements in cost, size and power. With a bill of materials estimated to be in the $50 range for the Roady2, one can expect more users to embrace the pay-to-listen technology. In fact, the hardware cost becomes a secondary consideration for many in light of the $10-plus monthly service fees. Whether built-in or portable, SDARS technology is the next thing to try and tap into your wallet every month for delivering rock, talk and everything in between.