teensy4: add capture-side USB Audio Feature Unit (mute/volume) for recording devices

[JayShoe]

Summary

Adds a USB Audio Class 1.0 Feature Unit on the device-to-host (capture/recording) path so the host OS can control mute and volume on audio the Teensy sends. Previously the AudioControl descriptor only had a Feature Unit on the playback path (FU 0x31), so the host never exposed a recording-device volume slider — the Microphone level control in Windows Sound Settings had nothing to bind to.

Topology after the patch:

capture:   IT1 → FU 0x30 → OT2    ← new
playback:  IT3 → FU 0x31 → OT4    ← unchanged

Changes

usb_desc.c

• Insert a 10-byte FEATURE_UNIT descriptor block (ID 0x30: master mute + L/R volume) between Input Terminal 1 and Output Terminal 2 on the capture path
• Retarget OT2 bCSourceID from IT1 (0x01) to FU 0x30
• Bump AC header wTotalLength from 62 to 72
• Bump AUDIO_INTERFACE_DESC_SIZE to account for the new block
• Both high-speed (480 Mbps) and full-speed (12 Mbps) descriptor copies are updated

usb_audio.h

• Add static struct usb_audio_features_struct features to AudioOutputUSB, mirroring the existing AudioInputUSB::features
• Add volume() and mute() accessors to AudioOutputUSB
• Add mute() accessor to AudioInputUSB for parity (the data was already there — volume() reads features.mute internally but there was no standalone accessor)
• Add friend declarations for usb_audio_set_feature/usb_audio_get_feature to AudioOutputUSB

usb_audio.cpp

• Define AudioOutputUSB::features (initialized to same defaults as AudioInputUSB::features)
• Add usb_audio_features_for_entity() helper that routes by the Entity ID in the wIndex high byte:
  • FU 0x31 → AudioInputUSB::features (playback, unchanged behavior)
  • FU 0x30 → AudioOutputUSB::features (capture, new)
• Replace hardcoded AudioInputUSB::features references in usb_audio_set_feature() and usb_audio_get_feature() with the entity-routed pointer

Usage

// Playback volume (existing PJRC API — host speakers/output slider):
float playVol = usbIn.volume();   // 0.0–1.0, returns 0.0 when muted

// Capture volume (new — host recording/microphone slider):
float capVol  = usbOut.volume();  // 0.0–1.0, returns 0.0 when muted
bool  capMute = usbOut.mute();

// Or via the struct for separate volume/mute tracking:
int rawVol  = AudioOutputUSB::features.volume;  // 0 to 255
int rawMute = AudioOutputUSB::features.mute;    // 0 or 1

Notes

• No changes to usb.c — the existing word1 match in the control-endpoint dispatcher deliberately ignores wIndex, so SET_CUR requests to either Feature Unit already fall through into usb_audio_set_feature(), which now disambiguates by entity ID.
• The core does not auto-apply capture volume to the transmit path. The sketch decides how to use the value (same pattern as the existing playback-side AudioInputUSB::volume()).
• bcdDevice is not bumped (it's used for board identification by teensy_ports). Windows re-enumerates on descriptor-length change; if it caches stale topology, unplug/replug clears it.
• Tested on Teensy 4.1, Windows 11 — recording-device slider in Sound Settings correctly drives AudioOutputUSB::features.