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The Virtual Display Driver supports precise fractional refresh rates through a multiplier system, enabling accurate NTSC rates (59.94Hz, 29.97Hz, 23.976Hz) and other non-integer refresh rates.

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Refresh Rate Multiplier System

Floating-point refresh rates use a multiplier to represent fractional values accurately: 
Actual Refresh Rate = Nominal Rate × (Multiplier / 1000)

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Configuration Format

<MonitorMode>
  <Width>1920</Width>
  <Height>1080</Height>
  <RefreshRate>59.940</RefreshRate>
  <RefreshRateMultiplier>999</RefreshRateMultiplier>
  <NominalRefreshRate>60</NominalRefreshRate>
</MonitorMode>
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RefreshRate
double
Actual refresh rate in Hz (for display purposes)
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RefreshRateMultiplier
int
default:"1000"
Multiplier value (0-1000):
  • 1000 = Exact rate (no fractional component)
  • 999 = NTSC rate (×0.999)
  • Custom values for other fractional rates
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NominalRefreshRate
int
Base refresh rate before multiplier is applied

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Standard NTSC Rates

NTSC fractional rates use the 999/1000 multiplier:

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59.94 Hz (NTSC 60Hz)

<MonitorMode>
  <Width>1920</Width>
  <Height>1080</Height>
  <RefreshRate>59.940</RefreshRate>
  <RefreshRateMultiplier>999</RefreshRateMultiplier>
  <NominalRefreshRate>60</NominalRefreshRate>
</MonitorMode>
Calculation: 60 × (999/1000) = 59.94 Hz

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29.97 Hz (NTSC 30Hz)

<MonitorMode>
  <Width>3840</Width>
  <Height>2160</Height>
  <RefreshRate>29.970</RefreshRate>
  <RefreshRateMultiplier>999</RefreshRateMultiplier>
  <NominalRefreshRate>30</NominalRefreshRate>
</MonitorMode>
Calculation: 30 × (999/1000) = 29.97 Hz

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23.976 Hz (Film Rate)

<MonitorMode>
  <Width>4096</Width>
  <Height>2160</Height>
  <RefreshRate>23.976</RefreshRate>
  <RefreshRateMultiplier>999</RefreshRateMultiplier>
  <NominalRefreshRate>24</NominalRefreshRate>
</MonitorMode>
Calculation: 24 × (999/1000) = 23.976 Hz

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Custom Fractional Rates

For non-NTSC fractional rates, calculate custom multipliers:

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47.952 Hz (Double Film Rate)

<MonitorMode>
  <Width>1920</Width>
  <Height>1080</Height>
  <RefreshRate>47.952</RefreshRate>
  <RefreshRateMultiplier>999</RefreshRateMultiplier>
  <NominalRefreshRate>48</NominalRefreshRate>
</MonitorMode>

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119.88 Hz (NTSC 120Hz)

<MonitorMode>
  <Width>1920</Width>
  <Height>1080</Height>
  <RefreshRate>119.880</RefreshRate>
  <RefreshRateMultiplier>999</RefreshRateMultiplier>
  <NominalRefreshRate>120</NominalRefreshRate>
</MonitorMode>

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Internal V-Sync Calculation

The driver converts fractional rates to v-sync ratios using GCD (Greatest Common Divisor) reduction: 
// Driver.cpp:1445-1453
void float_to_vsync(float refresh_rate, int& num, int& den) {
    den = 10000;
    num = static_cast<int>(round(refresh_rate * den));
    
    int divisor = gcd(num, den);
    num /= divisor;
    den /= divisor;
}
Example: 59.94 Hz 
Input: 59.94 Hz
1. num = 59.94 × 10000 = 599400
2. den = 10000
3. GCD(599400, 10000) = 600
4. Reduced: num = 999, den = 16.67
5. Final ratio: 999:1000

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Complete Configuration Examples

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Multi-Rate 4K Configuration

<MonitorModes>
  <!-- Exact rates -->
  <MonitorMode>
    <Width>3840</Width>
    <Height>2160</Height>
    <RefreshRate>24.000</RefreshRate>
    <RefreshRateMultiplier>1000</RefreshRateMultiplier>
    <NominalRefreshRate>24</NominalRefreshRate>
  </MonitorMode>
  
  <!-- NTSC film rate -->
  <MonitorMode>
    <Width>3840</Width>
    <Height>2160</Height>
    <RefreshRate>23.976</RefreshRate>
    <RefreshRateMultiplier>999</RefreshRateMultiplier>
    <NominalRefreshRate>24</NominalRefreshRate>
  </MonitorMode>
  
  <!-- Exact 30Hz -->
  <MonitorMode>
    <Width>3840</Width>
    <Height>2160</Height>
    <RefreshRate>30.000</RefreshRate>
    <RefreshRateMultiplier>1000</RefreshRateMultiplier>
    <NominalRefreshRate>30</NominalRefreshRate>
  </MonitorMode>
  
  <!-- NTSC 30Hz -->
  <MonitorMode>
    <Width>3840</Width>
    <Height>2160</Height>
    <RefreshRate>29.970</RefreshRate>
    <RefreshRateMultiplier>999</RefreshRateMultiplier>
    <NominalRefreshRate>30</NominalRefreshRate>
  </MonitorMode>
  
  <!-- Exact 60Hz -->
  <MonitorMode>
    <Width>3840</Width>
    <Height>2160</Height>
    <RefreshRate>60.000</RefreshRate>
    <RefreshRateMultiplier>1000</RefreshRateMultiplier>
    <NominalRefreshRate>60</NominalRefreshRate>
  </MonitorMode>
  
  <!-- NTSC 60Hz -->
  <MonitorMode>
    <Width>3840</Width>
    <Height>2160</Height>
    <RefreshRate>59.940</RefreshRate>
    <RefreshRateMultiplier>999</RefreshRateMultiplier>
    <NominalRefreshRate>60</NominalRefreshRate>
  </MonitorMode>
</MonitorModes>

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High Refresh Gaming Setup

<MonitorModes>
  <!-- 144Hz exact -->
  <MonitorMode>
    <Width>2560</Width>
    <Height>1440</Height>
    <RefreshRate>144.000</RefreshRate>
    <RefreshRateMultiplier>1000</RefreshRateMultiplier>
    <NominalRefreshRate>144</NominalRefreshRate>
  </MonitorMode>
  
  <!-- 165Hz exact -->
  <MonitorMode>
    <Width>2560</Width>
    <Height>1440</Height>
    <RefreshRate>165.000</RefreshRate>
    <RefreshRateMultiplier>1000</RefreshRateMultiplier>
    <NominalRefreshRate>165</NominalRefreshRate>
  </MonitorMode>
  
  <!-- 240Hz exact -->
  <MonitorMode>
    <Width>1920</Width>
    <Height>1080</Height>
    <RefreshRate>240.000</RefreshRate>
    <RefreshRateMultiplier>1000</RefreshRateMultiplier>
    <NominalRefreshRate>240</NominalRefreshRate>
  </MonitorMode>
</MonitorModes>

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EDID Profile Integration

EDID profiles automatically include fractional rates with correct multipliers: 
<!-- From monitor_profile.xml -->
<MonitorMode>
  <Width>720</Width>
  <Height>480</Height>
  <RefreshRate>59.940</RefreshRate>
  <RefreshRateMultiplier>999</RefreshRateMultiplier>
  <NominalRefreshRate>60</NominalRefreshRate>
</MonitorMode>
The EDID parser (edid_parser.txt:80-87) automatically calculates multipliers: 
int calculate_z_multiplier(double actual_refresh, int nominal_refresh) {
    // For standard NTSC fractional rates (59.94, 29.97, 23.976)
    if (std::abs(actual_refresh - nominal_refresh * 1000.0/1001.0) < 0.01) {
        return 999; // NTSC factor
    }
    return 1000; // Exact refresh rates
}

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Filtering Fractional Rates

When using auto resolutions, you can exclude fractional rates: 
<auto_resolutions>
  <enabled>true</enabled>
  <edid_mode_filtering>
    <exclude_fractional_rates>true</exclude_fractional_rates>
  </edid_mode_filtering>
</auto_resolutions>
This filters out all modes where RefreshRateMultiplier ≠ 1000.

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Global Refresh Rate Application

Apply specific refresh rates globally to all resolutions: 
<global>
  <g_refresh_rate>60</g_refresh_rate>
  <g_refresh_rate>120</g_refresh_rate>
  <g_refresh_rate>144</g_refresh_rate>
</global>
Global refresh rates always use exact rates (multiplier = 1000). For fractional rates, define them explicitly in individual resolution entries.

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Refresh Rate Limits

Constrain refresh rates when using EDID or auto-resolution features: 
<auto_resolutions>
  <edid_mode_filtering>
    <min_refresh_rate>24</min_refresh_rate>
    <max_refresh_rate>240</max_refresh_rate>
  </edid_mode_filtering>
</auto_resolutions>
Filtering applies to the NominalRefreshRate value, so both 60Hz and 59.94Hz would be included if the range includes 60.

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Common Refresh Rates Reference

RateTypeMultiplierNominalRefreshRate
23.976 HzFilm (NTSC)9992423.976
24.000 HzFilm (Exact)10002424.000
25.000 HzPAL10002525.000
29.970 HzNTSC9993029.970
30.000 HzExact10003030.000
47.952 HzFilm 2×9994847.952
50.000 HzPAL10005050.000
59.940 HzNTSC9996059.940
60.000 HzExact10006060.000
119.880 HzNTSC 2×999120119.880
120.000 HzExact1000120120.000
144.000 HzGaming1000144144.000
165.000 HzGaming1000165165.000
240.000 HzHigh-refresh1000240240.000

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VIC Mode Support

The EDID parser includes VIC (Video Identification Code) lookup tables for standard fractional rates (edid_parser.txt:436-481): 
static const std::map<uint8_t, VICMode> vic_table = {
    {2, {720, 480, 60, true}},   // 720p NTSC (59.94Hz)
    {4, {1280, 720, 60, true}},  // 720p NTSC
    {5, {1920, 1080, 60, true}}, // 1080p NTSC
    {32, {1920, 1080, 24, true}}, // 1080p Film
    // ...
};
The true flag indicates fractional rate support (multiplier = 999).

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Troubleshooting

Refresh rate not showing correctly:
  • Verify RefreshRateMultiplier is set correctly (999 for NTSC, 1000 for exact)
  • Check that NominalRefreshRate matches the base rate
  • Ensure RefreshRate calculation is accurate
Application doesn’t detect fractional rate:
  • Some applications only detect nominal rates
  • Provide both exact and fractional versions of the same rate
Performance issues with fractional rates:
  • Fractional rates require more precise timing
  • Use exact rates (multiplier=1000) for better performance if fractional accuracy isn’t needed

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