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RP2350 + LTE-M1 feasibility analysis

1. Input data specification

Data Channels Sample Rate Bit Depth Raw Data Rate
Audio (2 stereo mics) 4ch (2 mic x 2ch) 96 kHz 24-bit 1,152 KB/s
Sensors (vibration, magnetic, temp, humidity) 4ch 100 Hz f32 ~1.6 KB/s
Motor signals (brake 20ch RPM, current, voltage) 22ch ~100 Hz f32 ~8.8 KB/s
Total raw data rate ~1,162.4 KB/s

2. RP2350 resource analysis

Resource Specification Notes
CPU Dual Cortex-M33 @ 150 MHz ~300 MIPS total
SRAM 520 KB Includes stack, heap, static buffers
ADC 12-bit, 500 ksps (shared) NOT sufficient for 96kHz/24-bit audio
I2S PIO-based CAN handle 96kHz/24-bit with external ADC
SPI Up to 62.5 MHz SD card, external ADC connection
UART 2ch LTE-M1 modem AT commands
GPIO 30 pins Sensor, motor signal inputs
WDT Hardware Watchdog Timer Long-term stability
Flash External QSPI (typically 2-16 MB) Firmware + log storage

Key constraint: Built-in ADC is 12-bit/500ksps -- cannot capture 96kHz/24-bit audio. An external I2S ADC (e.g., PCM1808, CS5343) is required.

3. Codec feasibility

Codec MIPS/ch 4ch Total RP2350 Feasible? Compression Quality
Opus (music) ~80 320 NO (exceeds CPU) 10:1+ Excellent
Opus (voice) ~30 120 Marginal (1 full core) 10:1+ Good
IMA-ADPCM ~1 4 YES 4:1 Good
u-law/A-law <1 <4 YES 2:1 Fair
Raw PCM 0 0 YES 1:1 Perfect
FLAC ~30 120 Marginal 2:1 Lossless
XAP ~20 80 YES 10:1 Excellent
XAP (DSP-accelerated) ~14 56 YES 10:1 Excellent

Conclusion: XAP is the recommended codec on RP2350, offering 10:1 compression at excellent quality. With DSP-accelerated encoding (CMSIS-DSP optimized MDCT and FIR via the Cortex-M33 DSP extension), XAP 4ch @96 kHz uses ~56 MIPS -- well within the RP2350's 300 MIPS budget. IMA-ADPCM remains a viable fallback for extreme simplicity.

XAP and XMBP are patent-pending technologies of Xylolabs Inc.

DSP acceleration: The RP2350's Cortex-M33 includes single-cycle 32x32 MAC, dual 16-bit SIMD (SMLAD), and saturating arithmetic. These reduce XAP encoding cost by ~30% vs pure C. Enable via Rust SDK features = ["cmsis-dsp"] or C SDK XYLOLABS_USE_CMSIS_DSP=1. See docs/CODEC-ANALYSIS.md Section 6 and Performance Profile for detailed benchmarks.

4. Bandwidth analysis

LTE-M1 maximum bandwidth: 375 kbps = ~47 KB/s (theoretical, actual ~30-40 KB/s)

Data Raw Rate With ADPCM Fits LTE-M1?
4ch audio 96kHz/24-bit 1,152 KB/s 288 KB/s (4:1) NO
4ch audio 96kHz/16-bit 768 KB/s 192 KB/s NO
4ch audio 48kHz/16-bit 384 KB/s 96 KB/s NO
4ch audio 16kHz/16-bit 128 KB/s 32 KB/s YES
4ch audio 8kHz/16-bit 64 KB/s 16 KB/s YES (margin)
Sensors 100Hz x 4ch ~1.6 KB/s ~1.6 KB/s YES
Motor 100Hz x 22ch ~8.8 KB/s ~8.8 KB/s YES

5. Critical finding

4-channel 96kHz audio CANNOT be streamed in real-time over LTE-M1 using traditional compression methods alone.

  • Opus at 4ch x 64kbps = 32KB/s would fit bandwidth, but RP2350 cannot encode Opus (~320 MIPS needed)
  • IMA-ADPCM 4:1 yields 288 KB/s -- 6x over LTE-M1 capacity (47 KB/s)
  • Downsampled to 16kHz: 4ch x 16kHz x 4-bit ADPCM = 32 KB/s -- fits

UPDATE (Section 12): XAP codec resolves this constraint. XAP at 80 kbps/ch achieves 10:1 compression with only ~56 MIPS (DSP-accelerated) on RP2350, yielding 4ch @96kHz = 40 KB/s — within LTE-M1 budget. See Section 12 for the revised recommendation.

Option A: Downsampled real-time streaming

(Recommended for continuous monitoring)

Architecture Overview

Option A: Downsampled Real-Time Streaming

  • Capture at 96kHz on external I2S ADC
  • Downsample to 16kHz on RP2350 (simple FIR filter, ~5 MIPS/ch, ~20 MIPS total)
  • Compress with IMA-ADPCM 4:1 (~1 MIPS/ch, ~4 MIPS total)
  • Audio: 4ch x 16kHz x 4-bit = 32 KB/s
  • Sensors + motor: XMBP protocol = ~10 KB/s
  • Total: ~42 KB/s -- fits LTE-M1 budget

Option B: Periodic high-res recording

(Recommended for quality)

Option B: Periodic High-Res Recording

  • Record to SD card at full 96kHz/24-bit (1.15 MB/s -- SD easily handles)
  • After recording window (10 sec = 11.5 MB), compress with ADPCM to 2.9 MB
  • Upload compressed file over LTE-M1 (~62 seconds per 10-sec recording)
  • 10-sec window every ~70 seconds -- 14% duty cycle
  • Or: 10-sec window every 5 minutes -- very relaxed

Option C: Hybrid (best of both)

(Best recommendation)

Option C: Hybrid Architecture

  • Core 0: Continuous low-rate monitoring (16kHz ADPCM stream)
  • Core 1: Sensor/motor data collection + periodic high-res burst (96kHz to SD card, upload later)
  • Server receives both streams, correlates by timestamp

7. Long-term stability (year+ operation)

Item Implementation Notes
Watchdog RP2350 hardware WDT, reset on hang Periodic feed in main loop
Memory Static allocation only, no heap fragmentation No malloc/free
Network Exponential backoff reconnect, session auto-resume Modem reset after max retries
Power PSM/eDRX for idle periods, voltage monitoring Safe shutdown on low voltage
Logging Error counts to flash, periodic health report Health report to server
OTA updates Firmware update via server command A/B partition recommended
Temperature RP2350 built-in temp sensor monitoring Reduce clock on overheat

8. Time synchronization

Time Synchronization

  • Server assigns authoritative timestamp on session creation
  • Pico uses monotonic counter (microseconds since boot)
  • Session start time = server timestamp at session creation
  • All samples relative to session start (drift < 1ms/hour with RP2350 crystal)

9. 10-second window alignment

10-Second Window Alignment

  • Server aligns received data to 10-second boundaries
  • Audio and sensor data in same window are correlated
  • Window ID = floor(server_timestamp / 10)
  • No overlap (0%)

10. Memory budget

RP2350 SRAM: 520 KB total

Purpose Size Notes
I2S DMA double buffer (4ch) ~32 KB 96kHz x 24-bit x 4ch x 2 buf
Downsample FIR filter ~4 KB Filter coefficients + state
ADPCM encoding state <1 KB Per-channel state variables
XMBP packet buffer ~8 KB Sensor + motor batch
HTTP transmit buffer ~4 KB AT commands + headers
Sensor collection buffer ~12 KB 26ch x 100Hz x 500ms batch
Stack (2 cores) ~16 KB 8 KB per core
SD card buffer (Option B/C) ~8 KB FatFS + read/write buffer
Misc (session, URL, stats) ~4 KB
Total ~89 KB 17% of 520 KB used

11. CPU budget

RP2350: 2 x Cortex-M33 @ 150 MHz = ~300 MIPS total

Based on Option A/C (continuous streaming):

Task Core MIPS Notes
I2S DMA management 0 ~2 DMA hardware handles most
FIR downsample 4ch 0 ~20 96kHz to 16kHz, ~5 MIPS/ch
IMA-ADPCM encode 4ch 0 ~4 ~1 MIPS/ch
Sensor ADC read 26ch 1 ~5 100Hz, lightweight
XMBP encoding 1 ~2 Binary serialization
HTTP transmit (AT cmds) 1 ~10 Includes UART I/O wait
WDT, logging, misc 1 ~3
Core 0 total 0 ~26 17% of 150 MIPS
Core 1 total 1 ~20 13% of 150 MIPS

With DSP acceleration (Rust SDK features = ["cmsis-dsp"] or C SDK XYLOLABS_USE_CMSIS_DSP=1): The FIR downsample uses CMSIS-DSP arm_fir_q15 (dual 16-bit SIMD), reducing from ~20 MIPS to ~12 MIPS. ADPCM benefits from saturating arithmetic. Core 0 total drops to ~18 MIPS (12%).

With XAP instead of ADPCM (Option D -- direct 96 kHz streaming):

Task Core MIPS Notes
I2S DMA management 0 ~2 DMA hardware handles most
XAP encode 4ch @96 kHz (DSP) 0 ~56 ~14 MIPS/ch with CMSIS-DSP
Sensor ADC read 26ch 1 ~5 100Hz, lightweight
XMBP encoding 1 ~2 Binary serialization
HTTP transmit (AT cmds) 1 ~10 Includes UART I/O wait
WDT, logging, misc 1 ~3
Core 0 total 0 ~58 39% of 150 MIPS
Core 1 total 1 ~20 13% of 150 MIPS

XAP at 80 kbps/ch yields 4ch x 80 kbps = 40 KB/s -- fits LTE-M1 without downsampling. This eliminates the FIR downsample step entirely and preserves full 96 kHz bandwidth at 10:1 compression.

12. Conclusion and recommendations

  1. 96kHz/24-bit 4-channel audio real-time streaming over LTE-M1 is achievable with XAP.
  2. XAP at 80 kbps/ch: 4ch = 40 KB/s -- fits LTE-M1 budget
  3. DSP-accelerated XAP encoding uses 39% of one RP2350 core

  4. No downsampling needed -- full 96 kHz spectral content preserved

  5. Recommended solution: Option C (Hybrid) with XAP

  6. Continuous monitoring: XAP 4ch @96 kHz (40 KB/s) + sensor XMBP (10 KB/s) = 50 KB/s (exceeds LTE-M1 theoretical max of 47 KB/s — requires 64 kbps mode or sensor data reduction)
  7. Or: XAP 4ch @96 kHz (32 KB/s @64 kbps) + sensor XMBP (10 KB/s) = 42 KB/s (comfortable)
  8. Periodic high-res: 96kHz raw to SD card for lossless archival, batch upload

  9. Server correlates both streams

  10. Server-side changes needed:

  11. Integrate XAP decoder for XAP frame decoding (or use FFmpeg 7.1+ with XAP support)
  12. Add ADPCM WAV format reception and decoding support (fallback path)

  13. XAP → FLAC/WAV transcode pipeline for archival storage

  14. RP2350 resources are sufficient:

  15. Memory: 89 KB / 520 KB (17%), or ~121 KB with XAP encoder state (23%)
  16. CPU: 39% and 13% per core with DSP-accelerated XAP
  17. DSP extension provides 25-40% speedup for codec operations (single-cycle MAC, SIMD, saturating arithmetic)

  18. Long-term stability achievable (WDT, static allocation, PSM)

  19. DSP hardware acceleration is key:

  20. Cortex-M33 DSP extension reduces XAP encoding from ~80 MIPS to ~56 MIPS for 4ch @96 kHz
  21. CMSIS-DSP library provides optimized FFT, FIR, and vector routines

  22. See docs/CODEC-ANALYSIS.md Section 6 for per-platform DSP capabilities and benchmarks

  23. Implementation note: The Rust SDK is now the recommended implementation for RP2350 projects. The Embassy-based async runtime provides compile-time safety guarantees while matching C performance via zero-cost abstractions. See sdk/examples/ for ready-to-build RP2350 Rust examples.

  24. Performance validated via burn-in testing: E2E tests confirm XAP encoding at avg=9us/frame with 99.3% headroom on the RP2350's 10ms frame budget. Multi-device (10x concurrent) and stress (96kHz) scenarios pass with 0 failures.