The TLV320AIC3X is a powerful four channel low power audio codec family.
More information is available at:
    http://focus.ti.com/docs/prod/folders/print/tlv320aic34.html
Datasheet:
    http://www.ti.com/lit/gpn/tlv320aic34

The codec driver leverages the codecs effects through alsa controls and a 
hwdep device for controlling the hardware fourth-order IIR filter block.

There's an alsa control, "3D Control - Depth" for depth simulation.
The rest of the controls are for the IIR filter:

1- A control for setting the bass/treble gain, which sets the filter's
   coefficients to certain precalculated values.
2- A control for 'off' / 'on' / 'external control'. 'On' means the 
   bass/treble gain is used, while 'external control' means the 
   coefficients have been set through the hwdep device (see below).



The IIR Filter consists of 2 cascaded biquads. The formula is:
 /                                \  /                                \
|    (N0 + 2*N1*z^-1 + N2*z^-2)    ||    (N3 + 2*N4*z^-1 + N5*z^-2)    |
|  ------------------------------  ||  ------------------------------  |
 \ (32768 - 2*D1*z^-1 - D2*z^-2)  /  \ (32768 - 2*D4*z^-1 - D5*z^-2)  /

The filter can be controlled through an alsa hwdep device, via 
libalsa. A short example follows, note that the data struct must be
passed *EXACTLY* as shown:

#include <stdint.h>
#include <alsa/hwdep.h>

struct _iirfilter_data {
    int16_t N0, N1, N2, D1, D2;
    int16_t N3, N4, N5, D4, D5;
} iirfilter_data;

/* Initialize */
snd_hwdep *hwdep;

snd_hwdep_open(&hwdep, "IIR Filter", "w");

/* To write: */
iirfilter_data = {
	.N0 = 27619, .N1 = -27034, .N2 = 26461, .D1 = 32131, .D2 = -31506,
    .N3 = 27619, .N4 = -27034, .N5 = 26461, .D4 = 32131, .D5 = -31506
};

snd_hwdep_write(hwdep, (void*)iirfilter_data, sizeof(iirfilter_data));

/* To read: */
snd_hwdep_read(hwdep, (void*)iirfilter_data, sizeof(iirfilter_data));

/* To enable/disable filtering: */
int arg = 1; /* 1 = enable, 0 = disable */
snd_hwdep_ioctl(hwdep, 1, &arg);