PRELIMINARY DATA SHEET
UAC 3552A
2.2.1.4. HID Report Descriptor
2.2.2.1. Sample Rate Converter
The HID report descriptor defines the functionality of
the GPIO Pins. The basic information here are the
usage IDs for the key inputs. These IDs are stored in
the EEPROM and can therefore be modified if the
default configuration does not fit the application. The
UAC 3552A, however, only supports the default func-
tions: volume up/down, mute on/off and BassBoost on/
off. This means, that all nonstandard usage IDs will be
transmitted to the host on request and can be used
with vendor specific software, but only the default IDs
will work together with the operating system.
The purpose of the sample rate converter is first to
transform the block transferred audio data into a con-
tinuous data stream and second to convert all incom-
ing sample rates to a fixed 50-kHz sample rate. This
technique eliminates input data jitter. Furthermore, all
audio algorithms and the DAC run on a single sample
rate and no parameter switching is required on change
of audio sampling rate. Furthermore, all audio clocks,
such as sampling clock, noise-shaping clock, and
DAC-clock can be derived from a single free-running
12-MHz oscillator. This mechanism allows continuous
input sampling rates from 5 kHz up to 50 kHz.
2.2.2. Audio Processing Software
2.2.2.2. Automatic Gain Control
All audio processing is realized by DSP-software,
apart from volume control which is located in the ana-
log back-end. The audio building blocks split into USB-
independent features, like sample rate conversion and
oversampling filters and blocks which belong to the so
called USB feature unit, defined in the USB Device
Class Definition for Audio Devices. The feature unit
provides basic manipulation of the incoming logical
channels. The UAC 3552A supports two logical chan-
nels (i.e. left & right). Multichannel or surround sys-
tems, however, can also be realized using more than
one UAC 3552A, because phase or delay distortion is
eliminated by locking the audio processing to the USB
frame rate. An overview of the architecture is given in
Fig. 2–3.
The Automatic Gain Control (AGC) is one of the build-
ing blocks of the feature unit (USB Device Class Defini-
tion 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 theshold 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.
Feature Unit
Sample Rate
Converter
Bass/Treble
Bass Boost
Volume/
Balance
Over-
Sampling
AGC
Equalizer
Fig. 2–3: Audio processing
Micronas
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MICRONAS [ MICRONAS ]
