TAS3001CPWR datasheet(25/56 Pages) TI

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4−1

4 Digital Audio Processor

4.1

Input Mixer Control

The TAS3001 is capable of mixing two channels of serial audio data. The mixer permits each input to scaled (−

∞ to

+18 dB) independently and then a stereo summation is performed. One important function of this circuit is to scale

the input signals down to compensate for gains in the equalization settings and the bass and treble controls. This

prevents the system from exceeding the maximum digital signal output.

The mixer operation is controlled by loading values into the MIXER1 (07h) and MIXER2 (08h) control registers. The

mixer control values are in 4.20 format—4 bits for the integer and 20 bits for the fraction. The formula for converting

a 4.N number to dB is: dB = 20 log(X), where X is a positive 4.N number. To mute a mixer channel, 0s are loaded into

its respective mixer control register.

In order to transmit mixer control values over I2C, it is necessary to separate each value into three bytes. The first

nibble of byte 2 is the integer; the second nibble of byte 2 and bytes 1 and 0 are the fraction.

Table A−7 contains dB converted into 4.20 numbers for the range –70 dB to 18 dB, although any positive 4.20 number

can be used.

The mixer operation is updated instantly in response to a control register change. This may cause audible artifacts

when changing mixer settings outside of fast load mode.

4.2

Biquad Block

The biquad block consists of six digital biquad filters per channel organized in a cascade structure as shown in

Figure 4−1. Each of these biquad filters has five downloadable 24-bit (4.20) coefficients. Each stereo channel has

independent coefficients.

Biquad 1

Biquad 2

Biquad N

Figure 4−1. Cascaded Biquad Filters

4.2.1

Filter Coefficients

The filter coefficients for the TAS3001 are downloaded through the I2C port and loaded into the biquad memory space.

Digital audio data coming into the device is processed by the biquad filters and then output from the device, usually

to an external DAC. Any biquad filter may be downloaded and processed by the TAS3001. The biquad structure that

is used for the parametric equalization filters is:

H(z)

0 )

Z–1

) b

Z–2

) a

Z–1

) a

Z–2

The coefficients for these filters are quantized and represented in 4.20 format—4 bits for the integer part and 20 bits

for the fractional part. Each biquad uses five coefficients to define the operation.

4.3

Volume Control Functions

4.3.1

Soft Volume Update

The TAS3001 implements a Texas Instruments proprietary soft volume update. This update allows a smooth and

pleasant-sounding change from one volume level to another over the entire range of volume (18 dB to mute).

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