The VT1724 audio controller works in pair with a high-performance eight-channel audio-codec WM8770IFT from Wolfson. This chip supports all external analog ins and outs of the card (there is a twofold operational amplifier of the 4580 series before each analog output). Here are some basic parameters of WM8770IFT:
- 106dB SNR (“A” weighted @ 48kHz) DAC;
- 102dB SNR (“A” weighted @ 48kHz) ADC;
- Sample frequency on the DAC path: 8kHz – 192kHz;
- Sample frequency on the ACD path: 8kHz – 96kHz;
- Eight stereophonic input ADCs with analog amplification control in a range from +19dB to –12dB with 1dB increment.
8-channel WM8770IFT audio codec
with analogue volume control
WM8770IFT audio codec block diagram
Four stereo 24bit DACs are used with digital interpolation filters that perform sampling with a reserve of sampling frequency (so-called oversampling). They support digital audio input with 16-32bit capacity and 8-192kHz sampling frequency. Every digital channel allows independent volume control; the set of input multiplexers allows sampling from three external analog inputs.
The codec’s audio interface supports I2S and DSP data format. It is controlled via a three-wire serial interface, including channels selection, volume control, elimination of high-frequency constituents (de-emphasis) and power supply control.
The use of such a powerful codec as WM8770IFT in a SOHO-level audio card indicates that the requirements to modern home multimedia devices have grown up. Interestingly, these chips are also used in hi-fi auto radio-recorders and audio-visual systems.
The world of digital electronics stands on three foundations: microprocessors, memory and logic. Microprocessors execute program instructions to perform a variety of tasks, memory chips store various information, while logic devices serve as an interface between these two components. Aureon 7.1 Space uses a XC9536XL chip from Xilinx as such complex programmable logic device. This device is equipped with Fast Flash memory controlled by an integrated programmable controller. The micro program is flashed into the chip at the manufacturing stage, so very close to this chip there is a special 10-pin connector marked “System Port” on the back side of the PCB.
Just a few years ago such a chip cost about $150 and worked at up to 40MHz frequencies. Now technological advances have reduced the prime cost to $10, while the frequencies have grown to 400MHz. A curious fact: CPLD-chips were often used in military onboard computers, but now you can find them in cell phones, navigation systems and audio cards called Aureon.
One more codec, SigmaTel STAC9744T, supports the internal AUX analog audio input. This codec series is out of production now (“Not available for new designs”, to be exact, which is the same thing, actually).
The flowchart of the codec is shown above. At the hardware level STAC9744T supports the following sampling frequencies: 48kHz, 44.1kHz, 22.05kHz, 16kHz, 11.025kHz and 8kHz. Other frequencies are available through the audio driver. The digital interface of the codec communicates with the AC’97 codec via AC-Link. STAC9744 is rather an old solution and was mostly intended for dual-channel audio support, so in Aureon it fulfills some secondary fucntions. Interestingly, this codec was used in some audio cards to reproduce Dolby Digital streams. Several such codecs would make up a multi-channel solution, sometimes a four-channel codec (9704/07/21/23) was used among them.
Besides the above-described chips, Aureon has some simple logic chips. They are Philips 74HC4052D and Philips 74HC125D. The first one is a high-speed doubled four-channel analog multiplexer/demultiplexer, the latter – a chain of four logical “AND” elements. In fact, the 74th logic series is the most “long-living” generation of semiconductors and has been produced since late 80-s (these chips have survived a few PC generations).