SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R26
System description
Page 73 of 217
Two different sample-based sample rate converters are used: an interpolator, required to convert the
sample-based voice-band processing sampling rate of 8 kHz to the analog audio front-end output rate of
47.6 kHz; a decimator, required to convert the circular buffer sampling rate of 47.6 kHz to the I2Sx TX or the
uplink path sample rate of 8 kHz.
The following speech codecs are supported by SARA-G340 / SARA-G350 firmware on the DSP:
GSM Half Rate (TCH/HS)
GSM Full Rate (TCH/FS)
GSM Enhanced Full Rate (TCH/EFR)
3GPP Adaptive Multi Rate (AMR) (TCH/AFS+TCH/AHS)
o
In AMR Full Rate (AFS) the Active CODEC Set is selected from an overall set of 8 data rates:
12.2 – 10.2 – 7.95 – 7.40 – 6.70 – 5.90 – 5.15 – 4.75 kbit/s
o
In AMR Half Rate (AHS) the overall set comprises 6 different data rates:
7.95 – 7.40 – 6.70 – 5.90 – 5.15 – 4.75 kbit/s
1.10.3.2
SARA-U2 modules audio processing system
The voiceband processing on the SARA-U2 modules is implemented in the DSP core inside the baseband chipset.
The external digital audio devices can be interfaced directly to the DSP digital processing part via the I
2
S digital
interface. With exception of the speech encoder/decoder, audio processing can be controlled by AT commands.
The audio processing is implemented within the different blocks of the voiceband processing system:
Sample-based Voice-band Processing (single sample processed at 16 kHz for Wide Band AMR codec or
8 kHz for all other speech codecs)
Frame-based Voice-band Processing (frames of 320 samples for Wide Band AMR codec or 160 samples for
all other speech codecs are processed every 20 ms)
These blocks are connected by buffers (circular buffer and voiceband sample buffer) and sample rate converters
(for 8 / 16 to 47.6 kHz conversion).
The voiceband audio processing implemented in the DSP core of SARA-U2 modules is summarized in Figure 33
and Figure 34.
Switch
I2S_RXD
UBF
2/6
UBF
1/5
Hands-
free
To
Radio TX
Uplink
Digital Gain
I2S_TXD
Switch
Scal_Rec
Digital Gain
Tone
Generator
I2Sx RX
UBF
4/8
UBF
3/7
DBF
3/7
DBF
4/8
DBF
1/5
DBF
2/6
Legend:
UBF= Uplink Biquad Filter
DBF = Downlink Biquad Filter
I2Sx TX
Sidetone
From
Radio RX
Speech level
PCM
Player
Mix_Afe
Figure 33: SARA-U2 “00” and “x3” product versions audio processing system block diagram