One area in which automotive electronics is changing rapidly is the automotive audio system. Along with the outwardly visible trends, such as the progression to radio/cassette/CD systems, there are internal technical changes. Among these is the transition from analog to digital signal processor-based audio signal processing. DSPs have already begun to show up in the in-dash radio/cassette/CD systems, handling signal processing tasks such as user equalization, sound-field effects processing and noise reduction for the cassette player. A less obvious emerging application for DSP-based audio-signal processing is in the front end of the automotive audio power amplifier.
The audio system in a typical mid- to high-end vehicle consists of an in-dash radio, or "head unit," and a separate audio power amplifier. The head unit typically includes an AM/FM tuner and a cassette tape player and/or CD player. The head unit's user interface provides the means to select the audio source, along with controls for adjusting the audio signal volume, pan (left/right balance), front/rear fader (front/rear balance) and equalization (generally a bass and treble control, or a graphic equalizer). The left-channel output feeds both front- and rear-channel amplifiers, as does the right-channel output. Thus, there are four analog outputs, even though the audio sources are only stereophonic (two channels). The front and rear channels generally carry the same stereo program, but at different signal levels as dictated by the front/rear fader control. The volume and pan controls also affect the signal levels on the four analog outputs.
The four analog outputs from the head unit are routed to the audio amplifier, which drives the loudspeakers located in the front and rear of the vehicle. In addition to power amplification, the audio amplifier generally includes some vehicle model-specific filtering or equalization. This equalization is used to compensate for vehicle-specific spectral characteristics, such as the type of speakers used, the location of the speakers and the spectral response of the passenger cabin itself. Inclusion of any vehicle-specific signal processing in the amplifier is advantageous because it isolates the head unit from the model-specific sound delivery system.
However, there is a major drawback. The amplifier must be calibrated, or tuned, to each vehicle model in which the design is to be used. Tuning is often accomplished by iteratively running spectral measurements and listening tests while changing the resistors and capacitors in the filter circuits on the amplifier circuit board. This procedure is time-consuming. And, the manufacturer must stock a separate amplifier for each vehicle model. Amplifiers are often built using the same circuit board with different bills of materials.
The four-channel amplifier can be used to drive four full-range speakers. Alternatively, passive crossover filters may be used to separate the low and high frequencies to drive a two-way speaker configuration. Another approach is to employ an active crossover in the amplifier, and drive the low- and high-frequency transducers-the woofer and tweeter-with separate power amplifiers. This approach can directly deliver all of the amplifier power to the transducers, where the passive-crossover approach will dissipate some of the amplifier output power in the passive-crossover components.
Digital signal processing can be used in place of analog circuitry for such functions as equalization and active-crossover filtering. In addition, DSP allows for the practical implementation of a number of features that would be impractical using analog circuitry - for example, adaptive signal-level-limiting functions or multichannel time delay for speaker alignment.
However, the strongest factors motivating the auto-amplifier industry to transition from analog to digital signal processing are related to manufacturability, rather than to new features or cost savings. The attractiveness of DSP implementation can be attributed to the ease of reconfiguring the amplifier without changing components.
For the amplifier maker, this can dramatically speed the tuning process, saving time and enabling the supplier to be more responsive to the customer: the automobile manufacturer. Reconfigurability of the DSP-based amplifier also allows the same amplifier to be used in different vehicle models, simplifying the automobile maker's inventory management. The amplifier can then be programmed for the particular vehicle model at installation, or configuration can even be accomplished automatically if the amplifier has an interface to one of the in-vehicle control buses-commonly, the J1850 bus for U.S. manufacturers or the CAN bus for European vehicles.
The components for a six-channel DSP-based amplifier include four analog-to-digital (A/D) conversion channels that convert the analog input signals into digital format, a DSP engine, typically 24-bit fixed-point for audio processing, with the associated program and data memories, and six channels of digital-to-analog (D/A) conversion.
This system also includes an interface to a serial control bus. The bus can be used to interactively reconfigure the amplifier's signal-processing functions during the vehicle model-specific tuning process. After the design has been tuned, the settings for a number of different vehicle models can be stored in the amplifier's internal memory, allowing the same amplifier to be easily reconfigured for any of the target vehicles. The configuration may be done manually prior to installation in the vehicle or, if the amplifier is connected to an in-vehicle control bus, done automatically within the vehicle.
The graphical user interface (GUI) of the Crystal Semiconductor CF4912CAR-01 firmware kit simplifies the DSP-based amplifier tuning process. Basically, the DSP code runs on the CRD4912 board and the GUI application runs on a personal computer under the Microsoft Windows 95 operating system. The process of tuning the CRD4912 board's DSP algorithms using the PC-based GUI is analogous to the amplifier tuning process described.
The software provides a graphical depiction of the signal flow through the signal-processing functions. Clicking with the mouse on any of the signal-processing blocks will bring up a more-detailed screen, which is used to configure the characteristics of that processing function. This reduces the tuning process from weeks to days.
One of the difficulties in DSP-based systems is maintaining good dynamic range. The problem is that the user volume, pan and fader controls scale the signal level in the head unit. This means that the signal level going into the amplifier may be tens of decibels below optimal for typical listening conditions. Operating in this scenario requires very good dynamic range in the amplifier's A/Ds and D/As.
A solution would be to convey the current user settings for volume, pan and fader from the head unit to the amplifier over a control-bus connection between the two units. The head unit could then be designed to output a full-scale signal level, regardless of the user settings. Once the fader is implemented in the amplifier, the audio signal path between the head unit and the amplifier is reduced from four channels to stereo.