# Project Specification — USB Measurement Audio Interface **Status:** Draft architecture review **Project:** High-end 2-channel USB measurement audio interface for Smaart/REW workflows **Primary goal:** Production-intent schematic architecture covering smart UI, audio signal path, digital infrastructure, clocks, isolation, power, routing, protection, and bring-up-critical decisions. --- ## 1. Project Overview This project is a compact high-end USB Audio Class 2.0 measurement interface intended for acoustic/electrical measurement workflows in tools such as Smaart and REW. The requested design combines: - XMOS USB audio core - Precision ADC/DAC audio path - Digitally controlled microphone gain - +48 V phantom power - Internal loopback/reference channel - Smart rotary encoder UI with RGB LED ring - Galvanic isolation between noisy USB/digital and precision audio domains - Low-noise multi-rail power architecture The current project stage is **architecture definition**, not schematic-capture complete. Before production schematic capture, the power-mode, clock-master/isolation, and calibration/loopback decisions must be finalized. --- ## 2. Intended Use Target use cases: - Acoustic measurement with calibrated microphones - Transfer-function and impulse-response measurements - REW/Smaart audio I/O interface - Field and lab measurement use - USB-connected computer host Expected user workflow: 1. Connect interface by USB-C. 2. Connect measurement microphone to female XLR input. 3. Enable phantom power only when required. 4. Use encoder to switch modes, adjust gain/volume, and toggle phantom. 5. Use LED ring for gain/VU/status indication. 6. Use internal loopback/reference path for timing/reference measurements. --- ## 3. What the Device Should Do - Enumerate as a USB Audio Class 2.0 device. - Support professional measurement sample rates, expected at minimum 44.1/48/88.2/96/176.4/192 kHz. - Provide one precision microphone input path into ADC channel 1. - Provide one internal loopback/reference path into ADC channel 2. - Provide balanced XLR output from DAC path. - Provide controlled microphone gain through a digital PGA. - Provide +48 V phantom power with default-off behavior and a yellow indicator LED. - Provide rotary encoder UI with push-button short/long press behavior. - Provide a 12-RGB-LED ring for level/status indication. - Protect USB, XLR, audio, and MCU/control interfaces against ESD, hot-plug, and fault conditions. - Maintain low-noise, low-jitter audio performance appropriate for measurement use. --- ## 4. Main Features | Feature | Architecture Direction | |---|---| | USB audio | XMOS XU208, USB Audio Class 2.0 | | Firmware storage | QSPI flash, e.g. Winbond 2 MB class | | Audio clocks | Dual ultra-low-jitter oscillators: 22.5792 MHz and 24.576 MHz | | Isolation | Isolated digital/audio boundary for I2S/control; exact channel count to be finalized | | MCU/UI | STM32 or RP2040 with SWD/debug access | | Encoder | Alps EC11 rotary encoder with push switch | | LEDs | 12x WS2812B RGB ring, current-limited/filtered rail required | | Metering | Prefer digital metering from ADC samples; analog envelope detector optional only for UI | | Mic preamp | THAT1580 balanced mic preamp | | Gain control | THAT5171 digitally programmable gain stage | | DAC | ESS ES9038Q2M | | DAC analog stage | OPA1612 I/V + reconstruction/filter stage | | Output driver | THAT1606 balanced line driver to male XLR | | ADC | PCM4222 or AK5572, final part TBD | | Phantom | +48 V boost/regulator, matched 6.81 kΩ feed resistors, fault/ramp/protection | | Power input | USB-C with ESD protection; USB-C PD strongly recommended | | Analog rails | ±15 V switching preregulators plus low-noise post-regulation/filtering | | Digital rails | 3.3 V, 1.8 V, 1.0 V as required by XMOS/MCU/converters | --- ## 5. System Architecture ```mermaid flowchart TD USBC[USB-C connector\nESD + CC/PD + inrush] --> XMOS[XMOS XU208\nUAC2 audio engine] XMOS --> FLASH[QSPI flash\nfirmware] XMOS --> ISO[Digital isolation boundary\nI2S / control / status] MCU[STM32 or RP2040\nUI controller] --> ENC[EC11 encoder\nshort/long press] MCU --> LEDRING[12x WS2812B RGB LEDs] MCU --> PHCTRL[Phantom enable / fault sense] MCU --> GAINCTRL[PGA control] MCU --> DACCTRL[DAC register control] OSC[22.5792 + 24.576 MHz\nultra-low-jitter oscillators] --> CLKBUF[Low-jitter mux/buffer] CLKBUF --> ADC[PCM4222 or AK5572 ADC] CLKBUF --> DAC[ES9038Q2M DAC] CLKBUF -. feedback clocking .-> XMOS XLRIN[Female XLR input] --> EMIIN[Pin-1/chassis strategy\nESD + EMI/RFI + phantom protection] EMIIN --> PRE[THAT1580 mic preamp] PRE --> PGA[THAT5171 digital gain] PGA --> ADCCH1[ADC Ch1: mic] DAC --> IV[OPA1612 I/V + filter] IV --> LINE[THAT1606 balanced line driver] LINE --> XLROUT[Male XLR output\nESD + EMI + pin-1 strategy] IV --> LOOPBUF[Buffered/attenuated\npreferably switchable loopback] LOOPBUF --> ADCCH2[ADC Ch2: loopback/reference] ADCCH1 --> ADC ADCCH2 --> ADC ``` --- ## 6. Hardware Subsystems ### 6.1 USB / XMOS Digital Audio Core Requested core: - XMOS XU208 USB controller - USB Audio Class 2.0 firmware - QSPI flash for boot image/firmware - USB-C input with TVS ESD protection Required design items: - USB 2.0 high-speed differential pair routing at 90 Ω differential impedance. - USB ESD diode array close to connector. - USB-C CC pull-downs or USB-C PD controller, depending on power strategy. - Inrush limiting and power sequencing. - XMOS boot strapping per datasheet/reference design. - QSPI flash voltage compatibility and boot mode strapping. - XMOS firmware architecture supporting selected ADC/DAC clock topology and async feedback. Open decision: - Whether XMOS and converters share the same ground/clock domain, or whether the converters live in an isolated audio domain. ### 6.2 Audio Clocking Required clocks: - 22.5792 MHz for 44.1 kHz sample-rate family - 24.576 MHz for 48 kHz sample-rate family Production recommendation: - Place ultra-low-jitter oscillators in the **audio converter domain**, physically close to ADC/DAC. - Use a low-additive-jitter clock mux/buffer. - Keep ADC and DAC synchronous to the same selected oscillator. - Avoid passing converter MCLK through general-purpose digital isolators. - Use XMOS asynchronous USB feedback derived from the actual audio clock domain. Clocking truth table to finalize: | Sample-rate family | Oscillator | Clock master | ADC mode | DAC mode | XMOS mode | Feedback source | |---|---:|---|---|---|---|---| | 44.1/88.2/176.4 kHz | 22.5792 MHz | TBD, prefer audio domain | TBD | TBD | Async UAC2 | Audio-domain clock | | 48/96/192 kHz | 24.576 MHz | TBD, prefer audio domain | TBD | TBD | Async UAC2 | Audio-domain clock | ### 6.3 Digital Isolation Requested part: - Silicon Labs Si8640 for galvanic isolation on I2S/I2C lines Architecture correction: - A single Si8640 is likely insufficient for all required I2S + I2C + reset/mute/status lines. - I2C usually requires a dedicated bidirectional/open-drain isolator, not a generic unidirectional digital isolator. - I2S directionality and channel count must be explicitly mapped. - MCLK should not cross the isolation barrier if measurement-grade jitter is required. Likely isolated signals: | Signal | Direction | Notes | |---|---|---| | BCLK | Clock master → receiver(s) | Count direction after final clock topology | | LRCLK/FSYNC | Clock master → receiver(s) | Sample frame sync | | DAC SDATA | XMOS/audio bridge → DAC | Depends on topology | | ADC SDATA | ADC → XMOS/audio bridge | Depends on topology | | Control bus | Controller → converters/PGA | Prefer SPI or isolated I2C with proper isolator | | Reset/mute/fault | Mixed | Count all required lines | ### 6.4 Smart UI / MCU Candidate MCU: - STM32 or RP2040 Recommendation: - Prefer STM32 if analog ADC quality, low-power behavior, mature SWD/debug, and industrial control peripherals matter. - RP2040 is acceptable for UI/LED/encoder tasks but has a noisier digital profile and lacks on-chip flash. Required circuitry: - SWD header or test pads. - Reset/boot access. - Encoder A/B/push inputs with debounce and ESD protection. - WS2812B data output with series resistor. - LED-ring 5 V rail filtering, bulk capacitance, and hardware current limiting or enforced brightness limit. - MCU ADC input protection if analog envelope detector is retained. Control ownership decision: | Function | Recommended Owner | |---|---| | USB audio streaming | XMOS | | Sample rate state | XMOS primary, MCU informed | | Local encoder UI | MCU | | LED display | MCU | | Phantom enable | MCU, with hardware default-off safety | | PGA gain control | One clear master only: MCU or XMOS, not both without arbitration | | DAC volume/mute | Prefer XMOS-controlled audio state or MCU with explicit handshaking | | Calibration data | TBD: protected flash area or external EEPROM | ### 6.5 Metering / Envelope Detector Requested circuit: - TL072 + 1N4148 analog peak detector tapping mic preamp output and feeding MCU analog pin Production recommendation: - Prefer digital metering from ADC samples, sent to MCU over a control/status link. - If analog envelope is retained, treat it only as a UI indicator, not measurement data. - TL072 + 1N4148 simple detector is not precision and requires level shifting/protection for 3.3 V MCU ADC. Analog envelope circuit requirements if retained: - Buffer tap so the detector cannot load the mic/ADC path. - Precision rectifier or active peak detector rather than simple diode if low-level accuracy matters. - Defined attack/release constants. - ADC input RC filter. - Series resistor and clamps to MCU ADC rails. - Safe behavior if analog rails are powered while MCU rail is off. ### 6.6 Microphone Input Path Requested chain: ```text Female XLR → EMI/RFI/protection → THAT1580 → THAT5171 → ADC Ch1 ``` Required details: - XLR pin-1/chassis bond at connector. - ESD protection on pins 2/3. - Symmetrical RF filtering to preserve CMRR. - Phantom power injection through precision matched 6.81 kΩ resistors. - Phantom fault/hot-plug protection. - Input impedance target to be defined. - Optional line/pad mode should be considered if measuring high-level sources. - Gain range and gain step behavior to be defined. - PGA default safe gain at power-up. Recommended target specs to define: | Spec | Draft Target / TBD | |---|---| | Mic input impedance | TBD | | EIN | TBD | | Max mic input without pad | TBD | | Max line/reference input | TBD | | Gain range | TBD | | Gain step size | TBD | | CMRR | TBD | | Phantom voltage | +48 V nominal | | Phantom feed resistors | 6.81 kΩ matched | ### 6.7 ADC Path Candidate ADCs: - TI PCM4222 - AKM AK5572 Selection criteria: - Availability/lifecycle. - Supported sample rates and serial audio formats. - Dynamic range and THD+N. - Input topology and required analog driver/filter. - Clock ratios compatible with 22.5792/24.576 MHz oscillator plan. - Package/assembly complexity. - Digital interface compatibility with XMOS/isolation topology. Channel allocation: | ADC Channel | Signal | |---|---| | Ch1 | Mic preamp/PGA output | | Ch2 | Internal loopback/reference input | Recommendation: - Make ADC Ch2 loopback path buffered, attenuated, and preferably switchable. - Consider making Ch2 selectable between internal loopback and external balanced reference input if Smaart/REW two-input use is important. ### 6.8 DAC / Output Path Requested chain: ```text ES9038Q2M → OPA1612 I/V + filter → THAT1606 → Male XLR ``` Required details: - DAC supply filtering and register-control plan. - Low-jitter clock input and deterministic sample-rate switching. - I/V feedback network designed from DAC current/full-scale requirements. - Reconstruction/filter topology selected for flat measurement bandwidth and phase behavior. - Output muting during power-up, USB enumeration, sample-rate switching, and power-down. - Output ESD/EMI protection. - Ground-lift/pin-1 strategy defined. - Maximum output level and load drive target defined. Recommended target specs to define: | Spec | Draft Target / TBD | |---|---| | Max output level | TBD, e.g. +18/+20/+24 dBu | | Load | TBD, 600 Ω and/or 10 kΩ | | Output impedance | TBD | | THD+N | TBD | | Dynamic range | TBD | | Frequency response | TBD | | Output mute | Required | ### 6.9 Loopback / Reference Path Requested: - Hardwired split from OPA1612 directly to ADC channel 2 Production correction: - Do not hardwire directly from sensitive I/V output without buffering/attenuation. - Add a precision attenuator and low-distortion buffer before ADC Ch2. - Prefer switchable loopback to avoid permanent loading/coupling. - Define whether loopback pickoff is before or after the THAT1606 line driver. Loopback options: | Option | Pros | Cons | |---|---|---| | Pre-line-driver loopback | Easier level control, low noise | Does not measure final line-driver output | | Post-line-driver differential loopback | Measures full output path | Needs precision differential attenuation/protection | | Selectable internal/external Ch2 | Best measurement flexibility | More circuitry and firmware states | Recommended default: - Buffered/attenuated internal loopback, preferably switchable. - Consider external reference input if product must behave as a true two-input measurement interface. ### 6.10 Phantom Power Requested: - 5 V to ~+50 V boost - LT3042 LDO for exactly +48 V - MCU-controlled MOSFET switching - DC-blocking caps and matched 6.8 kΩ resistors - Yellow indicator LED Production requirements: - Verify selected regulator supports required input/output voltage and dissipation. - Boost to sufficient headroom for regulation after ripple/filter losses. - Use precision matched 6.81 kΩ phantom feed resistors. - Provide soft-start/ramp to prevent pops. - Provide per-channel current/fault limiting. - Provide discharge path when phantom is disabled. - Provide input protection for hot-plug and shorts. - Default phantom OFF at boot/reset/firmware fault. - Yellow LED should indicate commanded/valid phantom state, not just MCU GPIO state. Critical note: - The exact high-voltage regulator topology must be verified from datasheets. LT3042 suitability at +48 V must not be assumed without checking voltage rating and application constraints. ### 6.11 Power Architecture Requested rails: - USB-C input - 3.3 V digital - 1.8 V digital - 1.0 V core - ±15 V analog - +48 V phantom Production recommendation: - USB-C PD is strongly recommended. Plain USB 2.0/USB-C default 5 V current may be insufficient for ±15 V rails, phantom, RGB LEDs, XMOS, converters, and line output headroom. - If strict USB bus power is required, implement power-mode detection and disable/limit phantom, LED brightness, and max output level when power is insufficient. Preliminary power-tree architecture: ```mermaid flowchart TD USBC[USB-C Input] --> ESD[TVS ESD + EMI] ESD --> PWRMODE[CC/PD current detection\nor USB-C PD controller] PWRMODE --> INRUSH[Inrush / load switch / protection] INRUSH --> BUCK33[3.3 V buck or high-efficiency regulator] BUCK33 --> MCU[MCU + UI logic] BUCK33 --> XMOSIO[XMOS I/O + flash + isolators] BUCK33 --> LED5CTRL[LED rail control] INRUSH --> CORE[1.0 V XMOS core regulator] INRUSH --> DIG18[1.8 V digital/converter rail] INRUSH --> ANAPRE[Analog boost/inverting preregulator] ANAPRE --> P15[+15 V analog filtered/LDO] ANAPRE --> N15[-15 V analog filtered/LDO] INRUSH --> PHBOOST[+55 to +60 V phantom boost] PHBOOST --> PHFILTER[Filter / regulator / current limit] PHFILTER --> PH48[+48 V phantom injection] ``` Power budget must be completed before selecting fuses, load switches, regulators, inductors, and USB power strategy. Known high-current risks: | Load | Risk | |---|---| | 12x WS2812B | Up to ~720 mA at 5 V full white if not current-limited | | +48 V phantom | Significant boost input current, especially if supporting high-current microphones | | ±15 V rails | Boost/inverting converter current can be substantial from 5 V input | | Balanced line output | Output current depends on max dBu and load impedance | | XMOS + converters | Multiple digital rails and clocking loads | ### 6.12 Grounding / Shielding / Ground Lift Ground domains to define: - USB shield - USB/digital ground - XMOS ground - isolated audio digital ground, if used - analog ground - chassis/shield - XLR pin 1 - phantom return - UI/LED return Production guidance: - Do not split ground casually under high-speed or precision analog circuits. - Use a continuous reference plane inside each domain. - If isolation is used, the isolation boundary is a deliberate barrier with controlled crossing points. - XLR pin 1 should bond to chassis/shield at the connector where possible. - Ground lift should not disconnect internal signal reference or break ESD/phantom return behavior. - Ground lift should be implemented as a shield/chassis/audio-ground network decision, not as a simple signal-ground switch. Ground-lift network to evaluate: - Direct chassis bond option - Lifted LF shield connection with RF capacitor retained - Audio ground to chassis through resistor/capacitor/antiparallel diode network as appropriate - Exact network depends on enclosure and regulatory/EMC strategy --- ## 7. Interfaces and Connections ### External Interfaces | Interface | Connector | Signals / Purpose | Protection | |---|---|---|---| | USB | USB-C receptacle | USB 2.0 HS data, 5 V/PD power | TVS, CC/PD, inrush, shield strategy | | Mic input | Female XLR | Balanced mic input + phantom | ESD, RF filter, pin-1/chassis, phantom protection | | Line output | Male XLR | Balanced output | ESD, RF filter, output fault tolerance, pin-1/chassis | | Encoder | Panel control | A/B/push | Debounce, ESD if exposed | | LEDs | 12 RGB LEDs | UI/VU/status | Filtered rail, current limiting | | Debug | SWD/test pads | MCU programming | ESD optional, accessible pads | | Production test | Test pads | Rails, clocks, reset, phantom, audio refs | To be added during schematic | ### Internal Digital Interfaces | Interface | Used For | Notes | |---|---|---| | USB 2.0 HS | Host to XMOS | Controlled impedance, short pair | | QSPI | XMOS firmware flash | Per XMOS reference design | | I2S/TDM | XMOS/audio bridge to ADC/DAC | Exact master/slave and isolation TBD | | I2C/SPI | DAC/PGA/control | Isolated/control ownership TBD | | UART/SPI/I2C | XMOS ↔ MCU coordination | Needed if MCU owns UI/control | | WS2812 data | MCU to LEDs | Series resistor, filtered 5 V rail | | SWD | MCU programming/debug | Test pads/header | --- ## 8. Power and Runtime Expectations This is not a battery product. Runtime is continuous while USB/external power is present. Power-source decision required: | Option | Status | Notes | |---|---|---| | USB-C 5 V only | Risky | May be insufficient with phantom, LEDs, ±15 V rails, and pro output level | | USB-C PD | Recommended | More realistic for production; negotiate 9/12/15 V power profiles | | External DC + USB data | Robust alternative | Lower risk for phantom and analog rails | | Hybrid | Possible | USB-only limited mode, full performance with PD/external power | Recommended behavior: - Detect available USB-C current or negotiate PD. - Keep phantom disabled until sufficient power is available. - Limit LED brightness in hardware/firmware. - Sequence rails to avoid pops and brownouts. - Mute output during rail transitions and sample-rate changes. --- ## 9. Power Tree and Power Budget A full datasheet-backed budget is required before component selection. The table below is a placeholder structure for the production budget. | Rail | Loads | Typical Current | Peak Current | Notes | |---|---|---:|---:|---| | USB input / negotiated input | All downstream converters | TBD | TBD | Use worst-case input voltage and converter efficiency | | 5 V LED/UI rail | WS2812B ring | firmware-limited TBD | up to ~720 mA if full-white | Hardware current limiting recommended | | 3.3 V | MCU, XMOS I/O, flash, isolators, logic | TBD | TBD | Separate noisy UI and quiet digital filtering may be needed | | 1.8 V | converter digital / XMOS as required | TBD | TBD | Datasheet required | | 1.0 V | XMOS core | TBD | TBD | Datasheet required | | +15 V | op amps, preamp/PGA, line driver | TBD | TBD | Post-filter/LDO after switching preregulator | | -15 V | op amps, preamp/PGA, line driver | TBD | TBD | Inverting converter plus filtering/LDO | | +48 V | phantom | TBD | TBD | Include worst-case microphone current and boost efficiency | Sizing rules: - Size regulators and inductors from peak current plus margin. - Reflect boost/buck loads back to the input rail using converter efficiency. - Verify LDO thermal dissipation. - Verify USB-C advertised/negotiated current before enabling high-power features. - Include inrush and startup sequencing. --- ## 10. Manufacturing and Assembly Expectations Production-intent assumptions: - SMT assembly. - Controlled-impedance PCB fabrication. - At least 4 layers; 6 layers recommended for compact low-noise/high-performance implementation. - Precision passive tolerances for gain, loopback, filters, phantom matching, and calibration-critical paths. - Factory test/calibration process required. - Test pads for all rails, reset, boot, clocks, phantom voltage, I2S/control debug, and selected audio test nodes. Recommended stackup: | Layer Count | Suitability | Notes | |---|---|---| | 4-layer | Minimum | L1 signal/components, L2 solid GND, L3 power, L4 signal | | 6-layer | Recommended | Better shielding and separation for clocks, USB, converters, analog, and power supplies | Routing zones: 1. USB/XMOS/high-speed digital 2. UI MCU/LED/encoder 3. Isolation boundary 4. Low-jitter clock island 5. ADC/DAC converter island 6. Mic preamp/PGA low-level analog 7. DAC I/V and line output 8. Switching power and phantom boost 9. XLR pin-1/chassis/protection area --- ## 11. Firmware-Relevant Hardware Requirements Firmware must support: - USB Audio Class 2.0 streaming. - Sample-rate family selection and oscillator/mux control. - Async feedback based on actual audio clock domain. - Encoder short press: mode toggle. - Encoder long press: phantom toggle, with safety interlocks. - LED ring mode/status/VU indication. - Gain setting control and gain-step smoothing/muting. - Phantom default-off and fault handling. - DAC mute/volume/register initialization. - Output mute during startup/shutdown/sample-rate changes. - Calibration constant storage and retrieval. - Host control interface, if required, for gain/phantom/status. Required safe defaults: | Condition | Required State | |---|---| | Power applied | Phantom off, output muted, LEDs low/off, safe gain | | MCU reset | Phantom off by hardware default | | XMOS not enumerated | Outputs muted or safe | | Sample-rate change | DAC/output muted, clocks switched cleanly | | Power brownout | Phantom disabled, outputs muted | --- ## 12. Physical Design Expectations TBD by enclosure/mechanical design. Important physical constraints: - XLR connectors should sit at board/enclosure edge with pin-1/chassis connection close to connector. - USB-C connector requires controlled impedance routing and robust mechanical anchoring. - Encoder and LED ring require panel alignment. - LED ring and MCU should be physically separated from mic preamp input and clock island. - Switching power and phantom boost inductors must be far from low-level mic input and ADC reference areas. - Thermal and creepage/clearance considerations apply to +48 V phantom rail. --- ## 13. Routing and Protection Requirements ### USB-C / USB 2.0 - Route D+/D- as 90 Ω differential pair. - Keep pair short, length-matched, and referenced to a continuous ground plane. - Place ESD protection close to USB connector. - Use USB-C CC resistors or PD controller as required. - Define USB shield termination to chassis/ground. ### Clocks - Keep oscillator-to-mux/buffer-to-converter traces short. - Avoid vias where practical. - Do not cross plane splits or isolation barriers with MCLK. - Keep away from switching nodes, WS2812 LEDs, USB PHY, and MCU clocks. ### I2S / Digital Audio - Explicitly map BCLK, LRCLK, data directions and isolator channels. - Use controlled return paths on each side of isolation. - Avoid unnecessary vias and long parallel runs near analog inputs. ### Analog Input - Maintain symmetry on XLR pins 2/3 and mic preamp differential input routing. - Place EMI/RFI network close to XLR connector. - Preserve CMRR with matched component placement and values. - Keep phantom/current paths away from low-level differential traces. ### DAC / Output - Keep DAC output and I/V feedback loops compact. - Separate reconstruction filter/line driver from switching supplies and LEDs. - Add output mute/protection and XLR ESD/EMI network. ### Phantom Power - Keep boost switching node compact and shielded from audio. - Filter before injection network. - Maintain appropriate clearance for high-voltage nodes. - Use matched layout for phantom feed resistors. --- ## 14. Important Design Decisions ### Accepted from user request - XMOS XU208 as USB audio controller. - QSPI flash for XMOS firmware. - Dual audio-rate oscillator families. - Smart UI with encoder and RGB LED ring. - THAT1580/THAT5171 input architecture. - ES9038Q2M/OPA1612/THAT1606 output architecture. - ADC Ch1 mic and Ch2 loopback concept. - USB-C input and protected multi-rail power tree. ### Recommended changes before schematic capture 1. **Use USB-C PD or external power** for full-performance operation; plain 5 V USB-only is high risk. 2. **Do not isolate converter MCLK** through Si8640-class isolators; keep low-jitter clocks local to converters. 3. **Re-count isolation channels**; one Si8640 is probably not enough. 4. **Use a proper I2C isolator** if I2C crosses domains, or change to SPI/unidirectional control isolation. 5. **Prefer digital metering** from ADC stream over TL072/1N4148 analog detector. 6. **Buffer/attenuate and preferably switch** the loopback path instead of hardwiring directly from OPA1612. 7. **Define ground-lift as shield/chassis strategy**, not signal-ground disconnection. 8. **Add output mute and phantom fault/ramp protection** as production requirements. 9. **Add calibration storage/process** for measurement accuracy. --- ## 15. Assumptions - The product is production-intent, not a one-off proof of concept. - The current “2-channel” requirement means one physical mic input plus one internal loopback ADC channel, unless revised. - USB audio transport is USB 2.0 high-speed over USB-C connector. - Sample rates up to at least 192 kHz are desired. - The device is not battery powered. - The enclosure may provide a chassis/shield reference; final grounding depends on mechanical design. - Exact ADC choice is not finalized. - Exact MCU choice is not finalized. - Exact power source mode is not finalized. --- ## 16. Open Questions Before Schematic Capture 1. Should the product be strictly USB bus-powered, USB-C PD-powered, externally powered, or hybrid? 2. If USB-C PD is allowed, which negotiated profiles are acceptable: 9 V, 12 V, 15 V, or 20 V? 3. Is phantom power required in all power modes? 4. Is this truly one physical input plus one internal loopback channel, or should there be two physical balanced inputs? 5. Should ADC Ch2 be selectable between internal loopback and external reference input? 6. What maximum output level is required: +18, +20, +22, or +24 dBu? 7. What minimum load must the output drive: 600 Ω or only modern high-impedance line inputs? 8. What are the target published specs for EIN, THD+N, dynamic range, CMRR, frequency response, and phase accuracy? 9. Should the design prioritize ultimate measurement accuracy over visual LED effects/current draw? 10. Which MCU should be used: STM32 or RP2040? 11. Who owns gain/phantom/DAC control: MCU, XMOS, or a coordinated split? 12. Where should calibration constants be stored? 13. What enclosure/chassis strategy is expected for XLR pin-1 and ground lift? 14. Is an analog envelope detector actually required, or can metering come from digital audio samples? 15. Which ADC should be selected after availability/performance review: PCM4222, AK5572, or another part? --- ## 17. Validation and Bring-Up Requirements Schematic validation must include: - Datasheet review for every IC before final wiring. - ERC/design review clean or justified. - Power budget and regulator thermal checks. - USB-C current/PD behavior verification. - Phantom voltage/current/fault tests. - Audio clock jitter/layout review. - ADC/DAC interface timing review. - Analog simulation or calculation for filters, loopback attenuation, and detector/protection paths. - Grounding/shielding review before PCB routing. Factory/bring-up test plan should include: - USB enumeration and current draw. - All rail voltages and ripple. - Oscillator frequency and sample-rate switching. - XMOS flash boot. - MCU SWD programming. - Encoder and LED ring test. - Phantom ramp, voltage, current limit, fault recovery. - Input gain calibration and noise test. - DAC output level/THD+N test. - Internal loopback gain/phase calibration. - Output mute/pop test. --- ## 18. Change Notes - Initial project specification created from the pasted high-end USB measurement audio interface architecture. - Added production-critical architecture corrections around USB-C power, clock isolation, I2C/I2S isolation channel count, loopback buffering, ground-lift strategy, phantom protection, and calibration. - Renamed project to **USB Measurement Audio Interface** and added project description/properties.