TAS3103 datasheet(71/146 Pages) TI

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3−11

The direct form I structure provides a separate delay element and mixer (gain coefficient) for each node in the biquad

filter. Each mixer output is a signed 76-bit product of a signed 48-bit data sample (25.23 format number) and a signed

28-bit coefficient (5.23 format number). A 76-bit ALU in the TAS3103 allows the 76-bit resolution to be retained when

summing the mixer outputs (filter products). This is an important factor as it removes the need to carefully tailor the

order of addition for each filter implementation to minimize the effects of finite-precision arithmetic. Intermediate

overflows are allowed while summing the biquad terms to further minimize the effects of finite-precision arithmetic

(see the Digital Audio Processor – DAP – Arithmetic Unit section for more discussion on intermediate overflow).

3.5

Bass and Treble Processing

The TAS3103 has three fully independent bass and treble adjustment blocks—one for each of the three monaural

channels. Adjustments in bass and treble are accomplished by selecting a bass filter set and a treble filter set, and

then selecting a shelf filter within each filter set. The filter set selected, of which there are five sets for treble and five

sets for bass to select from, determines the frequency at which the bass and treble adjustments take effect. The shelf

filters determine the gain to be applied to the bass and treble components of the incoming audio. All selections are

independent of one another—any bass filter set can be combined with any treble filter set and any shelf filter can be

selected in a given filter set.

Figure 3−12 shows the bass and treble selections available, their I2C subaddresses, and the data fields in each

subaddress used to make the selections. Each bass filter set has 150 low pass shelving filters to choose from, with

the shelves ranging from a cut (attenuation) of 18 dB to a boost (gain) of 18 dB. A shelf selection of 0 dB effectively

removes bass processing. All 150 filters in a given filter set have the same 3-dB frequency, as measured from the

shelf. The only difference in the 150 filters in a given bass filter set is the gain of the shelf.

Each treble filter set has 150 high-pass shelving filters to choose from, with the shelves again ranging from a cut

(attenuation) of 18 dB to a boost (gain) of 18 dB. A shelf selection of 0 dB effectively removes treble processing. All

150 filters in a given filter set have the same 3-dB frequency, as measured from the shelf. The only difference in the

150 filters in a given treble filter set is the gain of the shelf.

Commands to adjust the bass and treble levels within a given filter set (by commanding the selection of different

shelving filters) results in a soft adjustment to the newly commanded levels. The filters are labeled 1 through 150,

with filter #1 implementing the maximum cut (18 dB) and filter #150 implementing the maximum boost (18 dB). If a

command is received to change a shelf setting, the transition is made by stepping through each filter, one at a time,

until the shelf filter commanded is reached. A soft transition is achieved by residing at each step (or shelf filter) for

a period determined by the programmable parameter TBLC (bit field 7:0 of subaddress 0xF1). The time period set

by TBLC is TBLC/FS (where FS is the audio sample rate). For an audio sample rate of 48 kHz and a TBLC setting

of 64, the dwell time on each shelf filter is approximately 1.33 ms. The transition time then depends on the number

of shelves separating the current and commanded shelf values. For 48-kHz audio and a TBLC setting of 64, a

maximum transition time of approximately 198 ms is required to transition from shelf #1 to shelf #150 and a minimum

time of approximately 1.33 ms is required to transition from shelf x to shelf x + 1.

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