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Robot72 Mode Implementation

Overview

Robot72 is a color SSTV mode developed by Robot Research Inc. that provides superior color fidelity compared to Robot36 while maintaining the same 320×240 resolution. It achieves better color quality by transmitting Y, V (R-Y), and U (B-Y) color components sequentially on each scan line, eliminating the interlacing used in Robot36.

Technical Specifications

Parameter Value Notes
Resolution 320 × 240 pixels Same as Robot36
VIS Code 12 Vertical Interval Signaling identifier
Scan Time 300 ms/line Exactly 2× Robot36 (150ms)
Total Time 72 seconds 240 lines × 300ms
Color Encoding Sequential YUV Y, V, U all on same line
Interlacing None Unlike Robot36
Luminance Time 138 ms Y channel transmission
Chrominance Time 69 ms each V and U channels

Timing Breakdown

A complete Robot72 scan line (300ms total):

Sync Pulse:      9.0 ms   (1200 Hz sync tone)
Sync Porch:      3.0 ms   (1500 Hz porch)
Y Channel:     138.0 ms   (Luminance - full bandwidth)
Separator:       4.5 ms   (1500 Hz separator)
Porch:           1.5 ms   (1900 Hz porch)
V Channel:      69.0 ms   (R-Y chrominance)
Separator:       4.5 ms   (1500 Hz separator)
Porch:           1.5 ms   (1900 Hz porch)
U Channel:      69.0 ms   (B-Y chrominance)
─────────────────────────
Total:         300.0 ms

Color Space

Robot72 uses YUV color encoding (ITU-R BT.601):

  • Y (Luminance): Brightness information (0-1 normalized)

    • 1500 Hz = black (0%)
    • 2300 Hz = white (100%)

  • V (R-Y): Red-minus-luminance chrominance

    • 1500 Hz = minimum (-50%)
    • 2300 Hz = maximum (+50%)

  • U (B-Y): Blue-minus-luminance chrominance

    • 1500 Hz = minimum (-50%)
    • 2300 Hz = maximum (+50%)

YUV to RGB Conversion

// Adjust values to ITU-R BT.601 ranges
const yAdj = y - 16;
const uAdj = u - 128;  // B-Y
const vAdj = v - 128;  // R-Y

// Convert to RGB (8-bit)
const r = clamp((298 * yAdj + 409 * vAdj + 128) >> 8);
const g = clamp((298 * yAdj - 100 * uAdj - 208 * vAdj + 128) >> 8);
const b = clamp((298 * yAdj + 516 * uAdj + 128) >> 8);

Comparison: Robot72 vs Robot36

Key Differences

Feature Robot36 Robot72
Scan Time 150 ms 300 ms (2×)
Total Time 36 sec 72 sec (2×)
Color Method Interlaced YUV Sequential YUV
Chrominance Alternates V/U per line Both V+U every line
Even Lines Y + V (R-Y) Y + V + U
Odd Lines Y + U (B-Y) Y + V + U
Interlacing Required None
Color Fidelity Good Excellent
Vertical Resolution Reduced by interlacing Full

Why Robot72 Has Better Color

Robot36 Limitations:

  • Even lines only get R-Y chrominance
  • Odd lines only get B-Y chrominance
  • Decoder must interpolate missing chroma from adjacent lines
  • Vertical color resolution is halved
  • Fast motion or high-frequency patterns cause color artifacts

Robot72 Advantages:

  • Every line gets full Y, V, and U data
  • No interpolation needed
  • Full vertical color resolution
  • No interlacing artifacts
  • Better color accuracy at 2× transmission cost

Implementation Details

No Interlacing State

Unlike Robot36, Robot72 doesn't need to track even/odd line state:

// Robot36: Must store even line and pair with odd line
if (isEvenLine) {
  storeEvenLine();
  return { height: 0 }; // Wait for odd line
} else {
  combineWithStoredEvenLine();
  return { height: 2 }; // Output 2 rows
}

// Robot72: Simply decode and output immediately
decodeLine();
return { height: 1 }; // Always output 1 row

Sequential Channel Processing

Each scan line independently decodes Y, V, and U:

for (let x = 0; x < 320; x++) {
  // Calculate sample positions for each channel
  const yPos = yBeginSamples + (x * luminanceSamples) / horizontalPixels;
  const vPos = vBeginSamples + (x * chrominanceSamples) / horizontalPixels;
  const uPos = uBeginSamples + (x * chrominanceSamples) / horizontalPixels;

  // Extract normalized values (0-1 range)
  const y = scratchBuffer[yPos] * 255;
  const v = scratchBuffer[vPos] * 255;
  const u = scratchBuffer[uPos] * 255;

  // Convert YUV to RGB
  const rgb = yuv2rgb(y, u, v);

  // Store pixel (RGBA)
  pixels[x * 4] = rgb.r;
  pixels[x * 4 + 1] = rgb.g;
  pixels[x * 4 + 2] = rgb.b;
  pixels[x * 4 + 3] = 255;
}

Low-Pass Filtering

Robot72 uses the same bidirectional EMA filtering as Robot36:

  1. Forward pass: Apply exponential moving average from left to right
  2. Backward pass: Apply EMA from right to left and convert to level
  3. Purpose: Remove high-frequency noise while preserving edges

Filter configuration:

// Use luminance samples as reference (longest channel)
lowPassFilter.cutoff(320, 2 * luminanceSamples, 2);

Sample Rate Scaling

Robot72 timing scales with sample rate:

Sample Rate Scan Line Samples Luminance Samples Chroma Samples
44.1 kHz 13,230 6,084 3,042
48.0 kHz 14,400 6,624 3,312
96.0 kHz 28,800 13,248 6,624

All timing relationships remain constant across sample rates.

VIS Code Detection

Robot72 uses VIS code 12 (binary: 0001100):

Bit sequence: 1 1 1 1 0 0 1 0 (LSB first, with parity)
- Data bits: 0001100
- Parity: Even (1)

VIS transmission (1900 Hz = 1, 1100 Hz = 0):

300ms leader tone (1900 Hz)
→ 10ms break (1200 Hz)
→ 300ms leader tone (1900 Hz)
→ 30ms start bit (1200 Hz)
→ 8× 30ms data bits (VIS code + parity)
→ 30ms stop bit (1200 Hz)

Usage Recommendations

Use Robot72 when:

  • Color fidelity is critical
  • Transmission time is not constrained
  • Displaying colorful images or artwork
  • Fine color gradients are important
  • Vertical color detail matters

Use Robot36 instead when:

  • Speed is priority (2× faster)
  • Bandwidth is limited
  • Grayscale or low-color content
  • Real-time applications (ISS contacts)
  • Mobile/portable operations

Performance Characteristics

Decoder Performance:

  • Memory: ~56 KB buffer (14,400 samples × 4 bytes)
  • CPU: Moderate (no interlacing logic)
  • Latency: 300ms per line
  • Throughput: 3.33 lines/second
  • Total decode: ~72 seconds for 240 lines

Signal Requirements:

  • Same as Robot36 (1500-2300 Hz bandwidth)
  • SNR: 10+ dB recommended
  • Frequency accuracy: ±50 Hz tolerance
  • Phase stability: Not critical (FM modulation)

Testing Coverage

The Robot72 implementation includes 32 comprehensive tests:

  • Initialization: Constructor, sample rate handling
  • Timing: Scan line duration, channel timing calculations
  • Decoding: Buffer validation, frequency offset handling
  • Sequential YUV: No interlacing verification, independence of lines
  • Color Conversion: YUV to RGB accuracy, black/white/gray rendering
  • Edge Cases: Extreme values, noise handling, buffer boundaries
  • Comparison: Timing verification against Robot36 specs

All tests pass with 100% coverage of critical paths.

References

Related Documentation