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TAS5508APAGRG4 Datasheet(PDF) 35 Page - Texas Instruments |
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TAS5508APAGRG4 Datasheet(HTML) 35 Page - Texas Instruments |
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35 / 99 page  Introduction 27 SLES119 − September 2004 TAS5508A The audio input stream into the DRC must pass through DRC-dedicated programmable input mixers. These mixers are provided to scale the 32-bit input into the DRC to account for the positioning of the audio data in the 48-bit DAP word and the net gain or attenuation in signal level between the SAP input and the DRC. The selection of threshold values must take the gain (attenuation) of these mixers into account. The DRC implementation examples that follow illustrate the effect these mixers have on establishing the threshold settings. T2 establishes the boundary between the high-volume region and the mid-volume region. T1 establishes the boundary between the mid-volume region and the low-volume region. Both thresholds are set in logarithmic space, and which region is active for any given rms estimator output sample is determined by the logarithmic value of the sample. Threshold T2 serves as the fulcrum or pivot point in the DRC transfer function. O2 defines the boost (> 0 dB) or cut (< 0 dB) implemented by the DRC-derived gain coefficient for an rms input level of T2. If O2 = 0 dB, the value of the derived gain coefficient is 1.0 (0x00, 80, 00, 00 in 5.23 format). k2 is the slope of the DRC transfer function for rms input levels above T2 and k1 is the slope of the DRC transfer function for rms input levels below T2 (and above T1). The labeling of T2 as the fulcrum stems from the fact that there cannot be a discontinuity in the transfer function at T2. The user can, however, set the DRC parameters to realize a discontinuity in the transfer function at the boundary defined by T1. If no discontinuity is desired at T1, the value for the offset term O1 must obey the following equation. O1No Discontinuity + |T1 * T2| k1 ) O2 For ( |T1| w |T2| ) T1 and T2 are the threshold settings in dB, k1 is the slope for region 1, and O2 is the offset in dB at T2. If the user chooses to select a value of O1 that does not obey the above equation, a discontinuity at T1 is realized. Going down in volume from T2, the slope k1 remains in effect until the input level T1 is reached. If, at this input level, the offset of the transfer function curve from the 1:1 transfer curve does not equal O1, there is a discontinuity at this input level as the transfer function is snapped to the offset called for by O1. If no discontinuity is wanted, O1 and/or k1 must be adjusted so that the value of the transfer curve at the input level T1 is offset from the 1:1 transfer curve by the value O1. The examples that follow illustrate both continuous and discontinuous transfer curves at T1. Going down in volume from T1, starting at the offset level O1, the slope k0 defines the compression/expansion activity in the lower region of the DRC transfer curve. 1.11.2.1 Threshold Parameter Computation For thresholds, TdB = −6.0206TINPUT = −6.0206TSUB_ADDRESS_ENTRY If, for example, it is desired to set T1 = -64 dB, then the subaddressaddress entry required to set T1 to -64 dB is: T1SUB_ADDRESS_ENTRY + *64 *6.0206 + 10.63 T1 is entered as a 48-bit number in 25.23 format. Therefore: T1 = 10.63 = 0_1010.1010_0001_0100_0111_1010_111 = 0x00000550A3D7 in 25.23 format |
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