On June 26th Texas Instruments announced the TAS3108, a new 8-channel audio processor targeting high fidelity home and car audio. The TAS3108's predecessor was a fixed-function filter engine. Customers would specify the filter properties and receive a customized fixed-function chip. With the TAS3108, TI has made the chip programmable and allowed customers the ability to implement their own filtering algorithms.
The TAS3108 contains a relatively simple fixed-point DSP core running at 135 MHz, coupled with an 8051 microprocessor that handles I/O and control operations. Audio data is input and output through serial ports with support for sampling frequencies ranging from 32-192 kHz and bit depths ranging from 16-32-bit. The DSP core, with one 28x48-bit multiplier (28 bits for filter coefficients and 48 bits for audio data), a 76-bit accumulator, and dither generator, is highly specialized for algorithmically simple filtering tasks such as FIR and IIR filters, mixing, equalization, and dynamic range compression. More complex tasks such as audio decoding are beyond the capabilities of the device. The high multiplier precision in the DSP core allows it to meet the precision requirements of high fidelity audio, which are typically met with floating-point DSPs.
Figure 1. TAS3108 block diagram
For home audio, the TAS3108 will be used primarily in flat panel televisions and HTIB systems, implementing such algorithms as Dolby Pro Logic and Audyssey's PrevEQ for room equalization. For automotive applications, TI provides an automotive-grade variant, the TAS3108IA, that is intended for use in in-dash head units, rear-seat entertainment systems, and external amplifiers.
One thing that sets the TAS3108 apart from TI's general-purpose DSPs is that it doesn't use the Code Composer Studio development environment. Instead, TI has provided two tools: a traditional development environment composed of separate IDEs for the DSP core and 8051 processor, and a graphical development environment with blocks for common filtering tasks such as FIR and IIR filters, volume control, and tone control.
While the TAS3108 is a very limited device, it provides a very cost effective solution for the applications it targets. At $4.91 apiece in 10k quantities, the TAS3108 is comparable in price to many more powerful fixed point DSPs, such as TI's own 300 MHz TMS320C5501 priced at $4.00. However, low-end fixed-point DSPs don't offer the precision needed for high-fidelity audio. Typically, the audio applications targeted by the TAS3108 require floating-point DSPs, most of which are considerably more expensive. The only floating-point DSP offered at a similar price point today is the $5 ADSP-21375 from Analog Devices. The ADSP-21375 is a viable alternative for the applications targeted by the TAS3108. With serial ports supporting up to eight channels of high-fidelity audio and a SIMD DSP core clocked at 266 MHz, the ADSP-21375 can handle the algorithmically simple filtering tasks the TAS3108 is designed for as well as more complex algorithms such as decoding. One advantage the TAS3108 might have for simpler filters is that its very wide data path will offer higher precision than floating-point DSPs.
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