chore(radio): remove 20% deadband for low volume

[3djc]

This improves settings sound level by special function or sound settings by source. It reduces the dead band before any sound can be played (previously volume was set a 0 until at least -80%)

This also simplifies the mechanic when hardware needs a fixed volume limitation

Summary by CodeRabbit

• Bug Fixes

  • More immediate speaker volume updates by replacing hysteresis with a deadzone-based input filter.
  • Removed internal volume scaling so headphone volume matches the user-set level.
  • Lowered minimum volume threshold for improved low-volume audio fidelity.

• New Features

  • Added trace logging for volume changes to aid diagnostics.

• Chores

  • Standardized per-target maximum volume to 20 for affected transmitters.

[pfeerick]

@coderabbitai review please

[coderabbitai]

@pfeerick Sure, I'll review the changes in this PR now!

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 0b3583a2-2f8f-4f98-8336-d3e10da769d6

📥 Commits

Reviewing files that changed from the base of the PR and between e38147b and ea083b5.

📒 Files selected for processing (3)

• radio/src/hal/audio_driver.h
• radio/src/targets/tx15/hal.h
• radio/src/targets/tx16smk3/hal.h

✅ Files skipped from review due to trivial changes (1)

• radio/src/hal/audio_driver.h

🚧 Files skipped from review as they are similar to previous changes (2)

• radio/src/targets/tx15/hal.h
• radio/src/targets/tx16smk3/hal.h

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📝 Walkthrough

Walkthrough

Refactors speaker volume input from hysteresis to a deadzone-based mapping, adds per-target VOLUME_LEVEL_MAX overrides, lowers TAS2505 VOLUME_MIN_DB with added trace logging, and has the board driver pass unscaled volume to the TAS2505 driver.

Changes

Audio Volume Control Refactor

Layer / File(s)	Summary
Volume Configuration & Constants radio/src/hal/audio_driver.h, radio/src/targets/tx15/hal.h, radio/src/targets/tx16smk3/hal.h	Added comment about undefining/redefining VOLUME_LEVEL_MAX per target; tx15 and tx16smk3 now #undef and #define VOLUME_LEVEL_MAX 20. I2S DMA macros reformatted.
Core Volume Input Processing radio/src/edgetx.cpp	calcVolumeValue() replaced hysteresis/stateful update with a deadzone-based computation using VOLUME_SOURCE_DEADZONE; removed requiredSpeakerVolumeRawLast.
TAS2505 Driver Volume Control radio/src/drivers/tas2505.cpp	VOLUME_MIN_DB changed from 60 to 45; added TRACE logging in tas2505_set_volume() for volume and headphone_mode.
Audio Board Driver Integration radio/src/boards/rm-h750/audio_driver.cpp	Removed unused timers_driver.h include; audioSetVolume() now passes the unscaled volume directly to tas2505_set_volume() (headphone detection unchanged).

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Poem

🐇 I hopped along the audio stream,
Found deadzones where the signals dream,
The driver hums with quieter dB,
Boards pass volumes true and free,
A tiny rabbit claps—hooray, sound gleams! 🎶

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: removing a 20% deadband for low volume, which directly matches the core modification in calcVolumeValue() and related volume handling updates across multiple files.
Description check	✅ Passed	The description adequately explains the changes: it specifies the improvement to sound level behavior, clarifies the deadband reduction from -80% to 0%, and mentions simplification of hardware volume limitation mechanics. While it doesn't follow the template structure exactly, it provides sufficient context for understanding the PR intent.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch 3djc/improve-audio-volume

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