bh | Becker Grand Prix 612 'Digital' Car Stereo |
The encroachment of digital technology and embedded processing into common consumer items has been mentioned in articles about the Heathkit AJ-1510 (1972) and
the Sherwood Micro/CPU-100 (1976) FM stereo tuners.
Here we have an example of the continuation of that process, the Becker Grand Prix 612 AM/FM/Cassette Car Stereo, dating from the early 1980s.
Less than 10 years after the AJ-1510, it was possible to jam a complete microprocessor-controlled stereo, with phase-locked-looped tuning for both AM and FM, into a car radio form factor. As an early example of this degree of miniaturisation the Grand Prix 612 was a high-end item in its day. It was the standard offering for a radio in Mercedes-Benz automobiles for several years in the early 1980s.
In a notable break from convention, the volume and tone controls are also under digital control, the design completely avoids the use of rotary controls in favor of pushbuttons. The volume and auto-scan tuning 'bars' push both up and down for direction. The bass and treble pushbuttons function in a cyclical manner, pushing one of the buttons increments the respective level till the maximum, subsequent pushes then decrement the level till the minimum, and so on. The tone can be reset to flat by pushing both tone pushbuttons simultaneously.
The digital technology now allowed a clock to be included in the radio. A timer is also included, allowing one to set a time at which the radio will automatically turn on.
There were apparently two versions of the 612, an earlier version with two flat pushbuttons each for the volume and auto-scan, and a later version with the toggle bars for these controls, as seen on the unit here. In the later version a noise suppression IC was added in the signal path between the demodulator and multiplex decoder, the AM front end was redesigned, the tape drive electronics changed, and other alterations made in the electronics.
Internally, the microcontroller used appears to be a Mostek MK3870, going from the pinout. The 3870 is a descendant of the Fairchild F8 family. The firmware is mask-programmed into the microcontroller. Communication between the microcontroller and devices - the front panel pushbuttons, LCD display, tuning PLL, and audio control D-A converters - is accomplished with a clocked serial bus. The bus is composed of primarily 3 lines: data, clock and latch. This may be seen as a precursor to such schemes as the I2C and SPI busses.
A schematic for the later version is available upon request. (See email contact).
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Unit Log
Becker Grand Prix 612 |
bhilpert 2015 Nov |