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DOSBox-X SoundBlaster Microphone Implementation Notes

Build Environment

Visual Studio 2022 Build Command

powershell.exe -NoProfile -ExecutionPolicy Bypass -Command "& 'C:\Program Files\Microsoft Visual Studio\2022\Community\Common7\Tools\Launch-VsDevShell.ps1' -Arch x86; Set-Location C:\temp\dosx\dosbox-x; msbuild vs\dosbox-x.sln /t:dosbox-x /p:Configuration=Debug /p:Platform=Win32 /p:PlatformToolset=v143 /m"

Key Build Notes

  • Use /p:PlatformToolset=v143 for VS 2022 (v142 is VS 2019)
  • Close DOSBox-X before rebuilding (executable gets locked)
  • If Microsoft.Cpp.Default.props not found, install C++ Desktop workload via: 
    winget install Microsoft.VisualStudio.2022.BuildTools --override "--add Microsoft.VisualStudio.Workload.VCTools --includeRecommended"

Testing

Test Configuration (dosbox-x-clean.conf)

[sblaster]
sbtype=sb16
sbbase=220
irq=7
dma=1
hdma=5
sbmixer=true
recording source=microphone

Test Programs

  1. SBREC.COM - DMA-based recording (DSP commands 0x24, 0x2C, etc.)

    • Works well with MIC_GenerateInput()

  2. Creative Parrot - Direct ADC recording (DSP command 0x20)

    • Polls DSP for individual samples
    • Requires MIC_GetDirectADCSample()
    • Location: C:\temp\dosx\dos_test\SBPRO\PARROT\

SoundBlaster Recording Modes

DMA Recording (High-level)

  • DSP commands: 0x24, 0x2C, 0x98, 0x99, etc.
  • Uses DMA transfers for bulk sample data
  • Called via gen_input() in sblaster.cpp
  • Sample rate specified by program

Direct ADC Recording (Low-level)

  • DSP command: 0x20
  • Polls for one sample at a time
  • Used by Creative Parrot and similar programs
  • Polling rate varies: 2700-6400 Hz (average ~4500 Hz)

Key Technical Findings

Polling Rate Measurement

From debug logs, Creative Parrot polls at:

  • Range: 2700 - 6400 Hz
  • Average: ~4500 Hz
  • Varies significantly during recording
  • Drops to 71-878 Hz during playback

Sample Rate Selection

  • Wrong approach: Match sample rate to polling rate dynamically

    • Causes audio artifacts from constantly changing resample rate

  • Wrong approach: Fixed high rate (22050 Hz) with 1:1 poll mapping

    • Causes slow motion (5x slower) and large delay

  • Correct approach: Time-based sample selection at average polling rate

    • Use real elapsed time to determine sample position
    • Generate at ~4500 Hz (measured average)
    • Returns sample based on wall-clock time, not poll count

Time-Based Sample Generation

// Calculate which sample corresponds to current real time
double elapsedSec = (now.QuadPart - startTime.QuadPart) / perfFreq.QuadPart;
size_t timeSample = (size_t)(elapsedSec * SAMPLE_RATE);
return buffer[timeSample % BUFFER_SIZE];

Benefits:

  • Correct playback speed regardless of polling variations
  • If Parrot polls faster: some samples duplicated (minor)
  • If Parrot polls slower: some samples skipped (minor)
  • Overall timing preserved

Windows WASAPI Audio Capture

Key Components

  • IMMDeviceEnumerator - Enumerate audio devices
  • IMMDevice - Audio device handle
  • IAudioClient - Audio stream configuration
  • IAudioCaptureClient - Capture buffer access

Capture Format

  • Windows typically provides: 48kHz, 32-bit float, stereo
  • Must resample to DOS rates: 4-22 kHz
  • Must convert to 8-bit unsigned or 16-bit signed

Threading Model

  • Capture thread polls every 10ms
  • Writes to circular buffer (256KB, ~1.3 sec at 48kHz)
  • Main thread reads and resamples on demand

Common Issues

  1. NOMINMAX - Define before windows.h to avoid min/max macro conflicts
  2. KSDATAFORMAT_SUBTYPE_IEEE_FLOAT - May need local GUID definition
  3. COM initialization - Call CoInitializeEx() in each thread

Files Modified

src/hardware/mic_input_win32.cpp

  • WASAPI microphone capture implementation
  • MIC_GenerateInput() for DMA recording
  • MIC_GetDirectADCSample() for Direct ADC

src/hardware/mic_input_win32.h

  • Class declarations and global function prototypes

src/hardware/sblaster.cpp

  • Added REC_MICROPHONE enum value
  • Modified gen_input() to call MIC_GenerateInput()
  • Modified DSP command 0x20 handler for Direct ADC

src/dosbox.cpp

  • Added "microphone" to sb_recording_sources array

vs/dosbox-x.vcxproj

  • Added mic_input_win32.cpp/h to project

Debugging Tips

Add Logging

#include "logging.h"
LOG_MSG("MIC: poll rate: %.1f Hz", measuredRate);

Check Log File

C:\temp\dosx\dosbox-x\bin\Win32\Debug\dosbox-x.log

Monitor DSP Commands

Look for command 0x20 in logs to confirm Direct ADC is being used.

Performance Considerations

  • Pre-buffer 100ms of audio to minimize latency
  • Generate samples ahead to prevent underruns
  • Use 2048-4096 sample chunks for efficient resampling
  • Ring buffer size: 32KB (~7 sec at 4500 Hz)