windows: add LHDC V5/aptX sink and low-latency audio path

[Loukious]

Summary

Adds a Windows WinUSB A2DP sink demo with SBC, LHDC V5, and optional aptX Classic/aptX HD decoding, together with the PortAudio changes needed for stable low-latency playback at high sample rates.

Highlights

• LHDC V5: 44.1/48/96/192 kHz, 16/24-bit stereo
• aptX Classic + aptX HD: 44.1/48 kHz stereo
• aptX endpoints are advertised only when an external openaptx0.dll decoder backend is available; no aptX decoder code/binary is bundled
• fixes Android aptX Classic media handling (no RTP header when SCMS-T is not enabled)
• decodes aptX/aptX HD one codec block at a time
• handles the observed LHDC V5 192 kHz transport layout with two [tag, sequence] subpacket prefixes per 20 ms A2DP payload
• aptX keeps its 40/48 ms PCM target/high-water limiter
• LHDC does not discard decoded PCM for latency control; testing showed queue trimming can cause audible hitches, especially around packet-burst boundaries
• PortAudio staging scales with sample rate (128 @ 48 kHz, 256 @ 96 kHz, 512 @ 192 kHz) to preserve a similar callback period
• Windows WASAPI shared-mode auto-convert fallback for rates rejected by the active Windows mix format; no exclusive-mode device takeover
• monotonic PortAudio producer/consumer counters prevent stale staging buffers from growing latency indefinitely

Validation

Tested on Windows 11 with an Intel Bluetooth controller and an Android 16 Xiaomi onyx source:

• SBC playback
• aptX Classic playback and repeated aptX <-> aptX HD switching
• LHDC V5 playback at 44.1, 48, 96 and 192 kHz
• 44.1 kHz capture replay: 5,628 LHDC frames decoded, 0 bad frames
• 192 kHz capture: 6,574 A2DP packets / 26,296 LHDC frames; after stripping the two transport subpacket prefixes, decoder replay completed with 26,296 good frames and 0 bad frames
• sustained 192 kHz playback is clean with the parser fix and without destructive LHDC PCM trimming

The external aptX backend is intentionally not included because libopenaptx is GPLv3; the sink omits aptX endpoints when the DLL is unavailable.

[mringwal]

Hi. Thanks for your PR. It contains a number of independent changes. From a first glance: change to Intel chipset support, adds git submodules for LHDC v5 and PortAudio, support for LHDC v5, support for aptX, changes to PortAudio driver, some changes for Windows.

Let's see. So far, we don't use git submodules (to make it easier as well as keep closer control of them) and I'd rather not add additional audio codecs to the BTstack sources directly. Detecting libraries via PkgConfig is fine, though.

Could you split this into multiple single topic PRs for manual review & discussion? Btw. please base these on the 'develop' branch instead of 'master'.

I'm not sure what's a good way to support custom codecs like AAC, aptX, LDAC, LHCD.. without increasing the size of the a2dp_sink_demo too much.

We recently discussed if we should switch to "btstack_run_loop_execute_on_main_thread" to trigger refill by the application instead of using a timer. It looks like this would help you here, too.

Btw. what's your goal here? Did you just wanted to high-quality audio playback on your Windows laptop from your Android phone?

[Loukious]

Could you split this into multiple single topic PRs for manual review & discussion? Btw. please base these on the 'develop' branch instead of 'master'.

Sure thing.

Btw. what's your goal here? Did you just wanted to high-quality audio playback on your Windows laptop from your Android phone?

Yes exactly. I was also working on reverse engineering the codecs to bring them to AOSP and I didn't really have a headset that supports them for testing so I implemented them here (so I can test directly on my PC) and thought other people might find it useful.