Design Article
MP54 ups car audio's cool factor
2/6/2006 10:00 AM EST
While digital music has been bolstered by phenomenal progress in mobile media players, the one area where mobility and modernity in audio still fall short is the car. Compared with consumer gadgets, automakers move to a different beat on product schedules, given the complexity and longevity of their platforms. Typically, the result is outdated OEM audio in your ride.
How to bring it up to speed? Go aftermarket, of course.
Vendors serving the car-audio aftermarket have started to fill the digital audio void, producing devices like the MP3-disk-capable receiver from Blaupunkt analyzed here. Combining MP3 with a more DSP-based radio orientation, the German manufacturer's design for the Casablanca MP54 radio highlights the role of cheap signal processors and commodity-scale optical drives in moving car audio closer to the current standards for mobile music.
The Casablanca MP54 is one of many modernized car audio systems from Blaupunkt. Along with the requisite AM/ FM radio, the design is equipped with a CD player that can also play MP3 files written to CD-R. It's not a 10,000-song plug-in library, but it's progress.
Better audio means higher risk, so a detachable front panel on the design guards against theft. And just in case the "defacing" feature fails to deter thieves, the user receives 12 months of anti-theft insurance along with the one-year warranty.
The Casablanca MP54 contains a total of four circuit boards. The main board supports core radio electronics; a faceplate board implements the input controller and display driver functions. A third board is associated with the optical-drive electronics, and the fourth (not shown) provides system interconnect. The use of low-layer-count pc board technology throughout--including silver-paste-filled through-hole vias on the optical-drive board--speaks to the relaxed component density and tight cost pressures characteristic of car audio.
MP3 support may be the bigger story, but the radio side has also benefited from technical advances. Under the trade name DigiCeiver, the MP54 converts received signals to digital form in the early going, allowing DSP technology to take over closer to the antenna.
Conventional, analog-domain-received signals must still be pro- cessed, but only an input-RF amplifier, first-stage mixer and first-IF filter are left behind as analog vestiges of the traditional radio architecture. Here the core of the DigiCeiver design is implemented in a U4257BM AM/FM receiver IC from Atmel--a BiCMOS part that produces a 10.7-MHz intermediate-frequency output. Beyond the analog silicon, a quick inspection of the board layout shows a healthy dose of supporting passive components and filters whose functions remain cheapest off-chip.
The IF signal also represents the input for the Blaupunkt/Bosch Audio Subsystem + DSP (the SC540206), whose high-speed A/D conversion and downstream digital signal processing address demodulation, filtering and multiplex processing via digital manipulation rather than analog signal conversion. The approach is claimed to improve stereo sound, reduce unwanted interference and distortion, and allow better use of subcarriers such as the MP54's Radio Data System (RDS), which enables the display of such information as the artist and song title.
The digital-domain focus of signal processing is not to be confused with the digital audio broadcasting (DAB) available in some geographies. But the design tries to correct shortcomings of broadcast radio sooner, rather than later, by leveraging the ever-declining cost of sophisticated DSPs.
Turning to the MP3-capable optical drive, several ICs support the basics of operation. An AN22002A head amplifier from Matsushita, for example, boosts signals from the CD read head. The read head's output passes to a second Matsushita part, the MN6627482WA, where an additional DSP layer generates digital audio for output to the rest of the system by way of an NEC microcontroller (the µPD780076). A Matsushita AN8785SB driver addresses control of spindle motor and read head sled mechanisms.
Output from the optical drive subsystem enters an STMicroelectronics STA016A audio decoder, which can convert either standard CD or MP3 digital audio. The next conversion takes place in a stereo D/A from Philips (the UDA1330ATS). At this point, analog audio from either the receiver or CD drive is available for final amplification in the Toshiba TA8272H, which is capable of 43 watts of output audio power using a bridge-tied-load amp topology.
For orchestration of signal flow and system control, two additional microcontrollers are present, one on the main board (NEC's µPD70F3079) and one on the detachable faceplate display board (STMicroelectronics' ST72C334). The latter manages user input and signal sourcing for the Epson S1D15206 display controller/driver. The display itself is a custom dot-matrix/icon combination LCD with four white LEDs for backlighting.
The cost of goods sold for the product comes to less than $125, about a third of the suggested retail price. Of this total, electronics--circuit boards and their related component content--account for about 75 percent. The disk transport mechanism contributes 15 percent to the total, with the balance spread among the display panel, enclosures and other system mechanicals.
The MP54 highlights the growing influence of digital media formats on both product design and function in automotive audio. Of course, in the fast-paced world of digital audio, even this aftermarket design can quickly look a little dated. Without support for memory card- or hard-drive-based audio, the MP54 is still a step behind mobile-player alternatives, but it's a start.
By David Carey(dcarey@portelligent. com) , president of Portelligent (Austin, Texas), which produces teardown reports and related industry research on wireless, mobile and personal electronics.
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