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HOLD FOR TESTING - fix(audio): reduce Windows USB audio latency for RX and CW sidetone (#3193) #3194

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HOLD FOR TESTING - fix(audio): reduce Windows USB audio latency for RX and CW sidetone (#3193) #3194

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15 changes: 15 additions & 0 deletions src/core/AudioEngine.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1043,6 +1043,15 @@ bool AudioEngine::startRxStream()
m_resampleTo48k = true;
m_audioSink = new QAudioSink(dev, fmt, this);
m_audioSink->setVolume(m_muted.load() ? 0.0f : m_rxVolume.load());
// Cap the WASAPI ring buffer to 100 ms. Class-compliant USB interfaces
// (Scarlett, Focusrite, etc.) advertise a 100–300 ms default ring that
// stacks on m_rxBufferCapMs and produces 500 ms+ speaker latency. (#3193)
{
constexpr int kWasapiTargetMs = 100;
const int bufBytes = (fmt.sampleRate() * fmt.channelCount()
* fmt.bytesPerSample() * kWasapiTargetMs) / 1000;
m_audioSink->setBufferSize(bufBytes);
}
m_audioDevice = m_audioSink->start();
if (!m_audioDevice) {
const QString firstError = audioErrorName(m_audioSink->error());
Expand All @@ -1054,6 +1063,12 @@ bool AudioEngine::startRxStream()
m_resampleTo48k = false;
m_audioSink = new QAudioSink(dev, fmt, this);
m_audioSink->setVolume(m_muted.load() ? 0.0f : m_rxVolume.load());
{
constexpr int kWasapiTargetMs = 100;
const int bufBytes = (fmt.sampleRate() * fmt.channelCount()
* fmt.bytesPerSample() * kWasapiTargetMs) / 1000;
m_audioSink->setBufferSize(bufBytes);
}
m_audioDevice = m_audioSink->start();
if (!m_audioDevice) {
const QString secondError = audioErrorName(m_audioSink->error());
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77 changes: 58 additions & 19 deletions src/core/CwSidetonePortAudioSink.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -30,45 +30,63 @@ PaDeviceIndex findPortAudioOutputDevice(const QAudioDevice& device)
return paNoDevice;
}

PaDeviceIndex partialMatch = paNoDevice;
QString partialMatchName;
bool partialMatchAmbiguous = false;
// Collect all partial-match candidates. On Windows a single physical
// device appears under multiple host APIs (MME, DirectSound, WASAPI);
// the partial-match list lets us prefer WASAPI instead of giving up. (#3193)
struct Candidate { PaDeviceIndex idx; QString rawName; PaHostApiTypeId apiType; };
QList<Candidate> partials;

for (PaDeviceIndex i = 0; i < count; ++i) {
const PaDeviceInfo* info = Pa_GetDeviceInfo(i);
if (!info || info->maxOutputChannels <= 0 || !info->name)
continue;

const QString rawName = QString::fromUtf8(info->name);
const QString candidate =
normalizedDeviceName(rawName);
const QString candidate = normalizedDeviceName(rawName);
if (candidate == target)
return i;

if (candidate.contains(target) || target.contains(candidate)) {
if (partialMatch == paNoDevice) {
partialMatch = i;
partialMatchName = rawName;
} else {
partialMatchAmbiguous = true;
}
PaHostApiTypeId apiType = paInDevelopment;
if (const PaHostApiInfo* api = Pa_GetHostApiInfo(info->hostApi))
apiType = api->type;
partials.append({i, rawName, apiType});
}
}

if (partialMatchAmbiguous) {
qCWarning(lcAudio) << "CwSidetonePortAudioSink: selected Qt output device"
<< device.description()
<< "matched multiple PortAudio outputs";
if (partials.isEmpty())
return paNoDevice;
}

if (partialMatch != paNoDevice) {
if (partials.size() == 1) {
qCWarning(lcAudio) << "CwSidetonePortAudioSink: selected Qt output device"
<< device.description()
<< "partially matched PortAudio output"
<< partialMatchName
<< partials[0].rawName
<< "by name substring";
return partials[0].idx;
}

#ifdef Q_OS_WIN
// Multiple matches — prefer WASAPI to avoid MME/DirectSound latency. (#3193)
QList<Candidate> wasapiCandidates;
for (const Candidate& c : partials) {
if (c.apiType == paWASAPI)
wasapiCandidates.append(c);
}
if (wasapiCandidates.size() == 1) {
qCInfo(lcAudio) << "CwSidetonePortAudioSink: selected Qt output device"
<< device.description()
<< "resolved to WASAPI output"
<< wasapiCandidates[0].rawName
<< "(preferred over" << partials.size() - 1 << "other host API(s))";
return wasapiCandidates[0].idx;
}
return partialMatch;
#endif

qCWarning(lcAudio) << "CwSidetonePortAudioSink: selected Qt output device"
<< device.description()
<< "matched multiple PortAudio outputs";
return paNoDevice;
}

PaDeviceIndex defaultPortAudioOutputDevice()
Expand All @@ -89,6 +107,27 @@ PaDeviceIndex defaultPortAudioOutputDevice()
}
}
}
#endif
#ifdef Q_OS_WIN
// Prefer WASAPI over MME/DirectSound/WDM-KS for lower CW timing jitter.
// Pa_GetDefaultOutputDevice() on Windows typically returns an MME device
// (first enumerated host API), which has 50–150 ms OS-level buffering.
// WASAPI shared mode runs at ~10 ms, eliminating the timing jitter that
// makes CW sidetone sound uneven on fast keying. (#3193)
if (devIdx == paNoDevice) {
const PaHostApiIndex apiCount = Pa_GetHostApiCount();
for (PaHostApiIndex i = 0; i < apiCount; ++i) {
const PaHostApiInfo* api = Pa_GetHostApiInfo(i);
if (!api || !api->name) continue;
if (qstrncmp(api->name, "Windows WASAPI", 14) == 0
&& api->defaultOutputDevice != paNoDevice) {
devIdx = api->defaultOutputDevice;
qCInfo(lcAudio) << "CwSidetonePortAudioSink: using WASAPI host API"
<< "(device" << devIdx << ")";
break;
}
}
}
#endif
if (devIdx == paNoDevice)
devIdx = Pa_GetDefaultOutputDevice();
Expand Down
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