uda1321 NXP Semiconductors, uda1321 Datasheet - Page 11

• Manufacturer Part Number: uda1321
• Description: Universal Serial Bus Usb Digital-to-analog Converter Dac
• Manufacturer: NXP Semiconductors

Philips Semiconductors
The Asynchronous Digital-to-Analog Converter
(ADAC)
The ADAC receives USB audio information from the USB
processor or from the digital I/O-bus. The ADAC is able to
reconstruct the sample clock from the rate at which the
audio samples arrive and handles the audio sound
processing. After processing, the audio signal is
up-sampled, noise-shaped and converted to analog output
voltages capable of driving a line output. The ADAC
consists of:
T
The SFG controls the timing signals for the asynchronous
digital-to-analog conversion. By means of a digital PLL,
the SFG automatically recovers the applied sampling
frequency and generates the accurate timing signals for
the audio feature processing DSP and the up-sample
filters.
F
The FIFO registers are used to store the audio samples
temporarily coming from the USB processor or from the
digital I/O input. The use of a FIFO register (in conjunction
with the SFG) is necessary to remove all jitter present on
the incoming audio signal.
T
A DSP processes the sound features. The control and
mapping of the sound features is explained in Section
“Controlling the USB Digital-to-Analog Converter (DAC)”.
Depending on the sampling rate (f
frequency domains in which the treble and bass are
regulated (see Table 1). The domain is chosen
automatically.
T
After the audio feature processing DSP two up-sample
filters and a variable hold register increase the
oversampling rate to 128f
1998 Oct 06
HE
IRST
HE AUDIO FEATURE PROCESSING
HE UP
A Sample Frequency Generator (SFG)
First-In First-Out (FIFO) registers
An audio feature processing DSP
Two digital up-sample filters
A variable hold register
A digital Noise Shaper (NS)
A Filter Stream DAC (FSDAC) with integrated filter and
line output drivers.
Universal Serial Bus (USB)
Digital-to-Analog Converter (DAC)
S
-I
AMPLE
N
-
SAMPLE FILTERS AND VARIABLE HOLD REGISTER
F
IRST
F
-O
REQUENCY
UT
(FIFO)
G
s
.
ENERATOR
REGISTERS
DSP
s
) the DSP has four
(SFG)
11
Table 1 Frequency domains for audio processing
T
A 3rd-order noise shaper converts the oversampled data
to a noise-shaped bitstream for the FSDAC. The in-band
quantization noise is shifted to frequencies well above the
audio band.
T
The FSDAC is a semi-digital reconstruction filter that
converts the 1-bit data stream of the noise shaper to an
analog output voltage. The filter coefficients are
implemented as current sources and are summed at
virtual ground of the output operational amplifier. In this
way very high signal-to-noise performance and low clock
jitter sensitivity is achieved. A post filter is not needed
because of the inherent filter function of the DAC.
On-board amplifiers convert the FSDAC output current to
an output voltage signal capable of driving a line output.
USB Digital-to-Analog Converter (DAC) descriptors
In a typical USB environment the USB host has to know
which kind of devices are connected. For this purpose
each device contains a number of USB descriptors. These
descriptors describe, from different points of view (USB
configuration, USB interface and USB endpoint), the
capabilities of a device. Each of them can be requested by
the host. The collection of descriptors is denoted as a
descriptor map. This descriptor map will be reported to the
USB host during enumeration and on request.
The full descriptor map is implemented in the firmware
exploiting the full functionality of the UDA1321. The USB
descriptors and their most important fields, in relationship
to the characteristics of the UDA1321 are briefly explained
below.
G
The UDA1321 supports one configuration containing a
control interface, an audio interface and a HID interface.
The descriptor map that describes this configuration is
partly fixed and partly programmable.
HE NOISE SHAPER
HE
ENERAL DESCRIPTORS
F
DOMAIN
ILTER
1
2
3
4
S
TREAM
DAC (FSDAC)
SAMPLE FREQUENCY (kHz)
Preliminary specification
12 to 25
25 to 40
40 to 55
5 to 12
UDA1321