Audio codec supports virtual speaker system

With the recent surge in consumer enthusiasm for DVD video, and with games-playing on the PC also garnering serious audio support, there is now a growing market for 6-channel 5.1 audio technology, and 6-channel codecs are making their appearance from design houses in Taiwan. The CMI9739, from C-Media of Taiwan, is one such codec, delivering full-duplex, 16-bit 6-channel audio, while maintaining full compliance with AC ’97 v2.2. The CMI9739-A, scheduled for launch this Q3, will comply with AC ’97 v2.3.

The CMI9739 codec delivers full-duplex 16-bit 6-channel audio at 96 kHz

The trick with a chip of this kind, which is destined to become mainstream as 96-kHz digital audio playback moves onto the PC, is differentiating it from the competition. With this codec that is achieved through use of a 3D engine that supports extended surround sound. Through the use of earphones, an end-user at a PC limited to a standard two-speaker stereo setup, can now adjust the position of five of the six speakers of 5.1 audio—Bass is excluded—in software. In other words, C-Media’s “Xear 3D” software creates five “virtual” speakers whose positions can be adjusted. This technique also eliminates the “sweet spot” of standard surround sound technology, which limits its effects to one spatial location.

On the hardware side, this codec offers full compatibility with southbridges from Intel and the three leading third-party logic designers, VIA Technologies, ALi, and SiS. An earphone buffer, PLL, and digital S/PDIF In and Out are all integrated on-chip. S/PDIF I/O is supported by an 18- to 20-bit DAC interface. The CMI9739-A will also integrate a line-out amplifier. The AA SNR is rated at 92 db and the DA SNR at 84 to 85 db, meeting the requirements of PC2002. This codec also meets the frequency response requirements of PC 2001 through the use of a 0.1 µF coupling capacitor. Packaging is 48-pin LQFP.

This codec offers support for the six speakers of 5.1 audio via the three standard I/O ports of mainstream PCI sound cards. The speakers port functions as Front Left and Front Right. Mic-In supplies Center and Bass (the LFE or low-frequency effects channel, akin to a sub-woofer). Line-In enables Rear Left and Rear Right. (Mic-In functions are maintained via the front panel.)

The 3D engine for extended surround sound has been licensed by C-Media from Sensaura of the UK, who supplied source code. The audio-spatial positionality is that of HRTF (head related transfer function) technology. C-Media has modified the Sensaura software, in particular adding a filter for support of C-Media’s proprietary “Xear” speaker shifter utility. The “speaker shifting” effect becomes available when the user dons a pair of “open” earphones, which fit loosely enough for reception from two speakers (front) in addition to sound from the phones. Xear 3D supports Microsoft DirectSound3D (DS3D), EAX, and I3DL2.


C-Media Electronics Inc
Fax (886) 2-8773-2211
www.cmedia.com.tw