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' / 'Bass/Treble' / 'Custom'. 'Bass/Treble' means
   the bass/treble gain controls are used, while 'custom' means the 
   coefficients have been set through the hwdep device (see below).
Note: bass/treble controls are not yet implemented

Filters
--------
Note: Setting a filter's coeffs automatically turns it off, it needs to
be turned on explicitly.

The De-emphasis filter can only be controlled on the machine driver level.
For example for the n900 (rx51.c) it is used as highpass filter for
speaker protection. See tlv320aic3x.h, aic3x_deemph_set_* for details.

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 
libasound. A short example follows, note that the data struct must be
passed *EXACTLY* as shown. Remember to link against libasound:
gcc myapp.c -lasound -o myapp

-------------------------------EXAMPLE----------------------------------
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <alsa/asoundlib.h>
#include <alsa/hwdep.h>

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

int main() {
    int ret;
    snd_hwdep_t *hwdep;
    struct iirfilter_data data = {
	    .N0 = 27619, .N1 = -27034, .N2 = 26461, .D1 = 32131, .D2 = -31506,
        .N3 = 27619, .N4 = -27034, .N5 = 26461, .D4 = 32131, .D5 = -31506
    };

/* Initialize */
    ret = snd_hwdep_open(&hwdep, "hw:0,0", SND_HWDEP_OPEN_DUPLEX);
    if(ret != 0) exit(1);

/* To write: */
    snd_hwdep_write(hwdep, (void*)&data, sizeof(data));

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

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