SARA-G3 series - System Integration Manual
UBX-13000995 - R06
Objective Specification
System description
Page 70 of 218
1.10.3
Voice-band processing system
1.10.3.1
Audio processing system overview
The voice-band processing on the SARA-G350 modules is implemented in the DSP core inside the
baseband chipset. The analog audio front-end of the chipset is connected to the digital system through 16
bit ADC converters in the uplink path, and through 16 bit DAC converters in the downlink path. External
digital audio devices can directly be interfaced to the DSP digital processing part via the I
2
S digital
interface. The analog amplifiers are skipped in this case.
The voice-band processing system can be split up into three different blocks:
Sample-based Voice-band Processing (single sample processed at 8 kHz, every 125 µs)
Frame-based Voice-band Processing (frames of 160 samples are processed every 20 ms)
MIDI synthesizer running at 47.6 kHz
These three blocks are connected by buffers and sample rate converters (for 8 to 47.6 kHz conversion)
I2S_RXD
Sw itch
M IC
Uplink
Analog Gain
Uplink
Filter 2
Uplink
Filter 1
To
Radio TX
Uplink
Digital Gain
Dow nlink
Filter 1
Dow nlink
Filter 2
M IDI
Player
SPK
Sw itch
I2Sx TX
I2S_TXD
Scal_Rec
Digital Gain
SPK
Analog Gain
Gain_Out
Digital Gain
From
Radio RX
Speech
Level
I2Sx RX
Sample Based Processing
Frame Based Processing
C
ir
cu
lar
B
u
ff
er
Sidetone
Digital Gain
DAC
ADC
Tone
Generator
AM R
Player
Hands-Free
V
o
ic
e
b
a
n
d
S
a
m
p
le
B
u
ff
e
r
Figure 22: SARA-G350 modules voice-band processing system block diagram
The sample-based voice-band processing main task is to transfer the voice-band samples from either
analog audio front-end uplink path or I2Sx RX path to the Voice-band Sample Buffer and from the
Voice-band Sample Buffer to the analog audio front-end downlink path and/or I2Sx TX path. While doing
this the samples are scaled by digital gains and processed by digital filters both in the uplink and
downlink direction and the sidetone is generated mixing scaled uplink samples to the downlink samples
(refer to the
u-blox AT Commands Manual [2], +UUBF, +UDBF, +UMGC, +USGC, +USTN commands).
The frame-based voice-band processing implements the Hands-Free algorithm. This consists of the Echo
Canceller, the Automatic Gain Control and the Noise Suppressor. Hands-Free algorithm acts on the uplink
signal only. Algorithms are configurable with AT commands (refer to the
u-blox AT Commands Manual
[2], +UHFP command). The frame-based voice-band processing also implements an AMR player. The
speech uplink path final block before radio transmission is the speech encoder. Symmetrically, on downlink
path, the starting block is the speech decoder which extracts speech signal from the radio receiver.
