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18 Aug. 20, 2004; 6251-650-1AI Micronas 3. Audio Processing Mic Line Mic L Preamp A R A USB I 2 S (downstream) Mic-Bias I 2 S Synchronous Mode only in reduced feature set Q-Peak D D Mono Deemphasis off/50 µs/75 µs ADC Fig. 3–1: Signal flow in the audio processing unit (APU) mix USB mix I 2 S mix DSP-Loop Complementary High Pass Bass/ Treble/ AGC Loudness Q-Peak complete mix ADC+USBmix ADC input USB L/R I 2 S input complete mix ADC+USBmix ADC input USB L/R I 2 S input 5 4 0 1 2 3 5 4 0 1 2 3 Select Select EQ dashed blocks not available in reduced feature set Scale Scale Mono/ Stereo + Right Invert USB (upstream) I 2 S out Balance Vol. D D A A L R UAC 357xB ADVANCE INFORMATION
ADVANCE INFORMATION UAC 357xB The audio processing is realized by APU firmware. The audio building blocks can be split into USB-independent features such as parametric equalizer, I 2 S I/O, and blocks which belong to USB feature units, mixer units, and selection units defined in the USB Device Class Definition for Audio Devices. Output Level dBr −9 AGC off AGC on The USB feature unit provides basic manipulation of the incoming logical channels and can be controlled by the standard windows mixer tool. The parameters for the USB-independent features are predefined in the internal ROM, in an external EEPROM or a special host application which drives the IC. The UAC 357xB supports two logical channels (i.e. left and right). Multichannel or surround systems, however, can also be realized using more than one UAC 357xB, because phase or delay distortion is eliminated in the device by locking the audio processing to the USB frame rate. An overview of the architecture is given in Fig. 3–1 on page 18. −15 −21 −30 −24 −18 −12 −6 0 +6 dBr Input Level Fig. 3–2: Simplified AGC characteristics Table 3–1: AGC parameters Parameter Settings Default If the APU works with a 48 MHz clock it is necessary to select the reduced feature mode. The blocks, which are not available in reduced feature mode are shown with dashed lines in Fig. 3–1 on page 18. Decay time 8 seconds 4 seconds 2 seconds 20 ms 4 seconds 3.1. DSP Loop The DSP-Loop block symbolizes the option to route the audio signal to an external DSP and back into the UAC 357xB via I²S I/O. This allows to add more audio processing algorithms. 3.2. Automatic Gain Control The Automatic Gain Control (AGC) is one of the building blocks of the feature unit (USB Device Class Definition for Audio Devices 1.0, page 39). Different sound sources fairly often do not have the same volume level. The Automatic Gain Control solves this problem by equalizing the volume levels within a defined range. Below a threshold level the signals are not affected. The level-adjustment is performed with time constants in order to avoid short-time adjustments due to signal peaks. 3.3. Quasi-Peak Two quasi-peak detectors are provided: 1. after the ADCs. This allows the programming of an AGC in the microcontroller 1) or a VU-meter on the host side. 2. in the DAC channel. This can be used, e.g., for a VU-meter on the host side. The feature is based on using fast attack and slow decay time constants. 3.4. Bass Control The bass control provides gain or attenuation to frequency components below a corner frequency of 120 Hz. The bass control works identically on both channels in a range of −12 dB to +12 dB. 3.5. Treble Control The treble control provides gain or attenuation to frequency components above a corner frequency of 6 kHz. The treble control works identically on both channels in a range of −12 dB to +12 dB. 1) not supported by standard microcontroller firmware Micronas Aug. 20, 2004; 6251-650-1AI 19
Page 1 and 2: Hardware Documentation Advance Info
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