Resolution
Understand video resolution measurement in VoIP calls — how CallMeter tracks resolution in megapixels, what resolution changes reveal about quality adaptation, and common causes of resolution drops.
Resolution measures the dimensions of the video being encoded or decoded, expressed in megapixels (MPx). For example, 640x480 is approximately 0.31 MPx, while 1920x1080 (Full HD) is approximately 2.07 MPx. CallMeter tracks resolution as a continuous metric so you can see exactly when and how the video quality adapts during a call.
Think of resolution as the sharpness of the picture. Higher resolution means more pixels, more detail, and sharper faces or text. Lower resolution means a blurrier image that uses less bandwidth.
How It Works
Video codecs can dynamically change the encoding resolution in response to conditions:
- Starting high — When bandwidth is plentiful, the encoder uses the maximum configured resolution
- Scaling down — When congestion is detected, the encoder reduces resolution to lower bandwidth demand
- Scaling back up — When conditions improve, the encoder increases resolution again
CallMeter converts the pixel dimensions (width x height) into megapixels for consistent comparison across different aspect ratios and provides this as a time-series metric.
Common video resolutions in VoIP:
| Resolution | Megapixels | Typical Use |
|---|---|---|
| 320x240 (QVGA) | 0.08 MPx | Low bandwidth, thumbnail video |
| 640x480 (VGA) | 0.31 MPx | Standard quality video call |
| 1280x720 (HD) | 0.92 MPx | HD video conferencing |
| 1920x1080 (Full HD) | 2.07 MPx | High-quality video |
Both directions
Resolution is tracked for both send and receive directions. The send resolution is controlled by your encoder. The receive resolution depends on what the remote encoder produces.
Why It Matters
Resolution is a primary quality indicator that users notice immediately. Text becomes unreadable, faces become blurry, and shared content becomes unusable when resolution drops too far.
For enterprise VoIP testing, monitoring resolution over time reveals:
- Bandwidth adaptation behavior — How aggressively the encoder reduces resolution under pressure
- Quality ceiling — The maximum resolution the infrastructure can sustain
- Asymmetric quality — One direction maintaining high resolution while the other drops
- Infrastructure bottlenecks — Load tests that show resolution dropping across all endpoints indicate a shared bottleneck
Common Causes of Resolution Drops
| Cause | Explanation |
|---|---|
| Network congestion | Encoder reducing resolution to fit within available bandwidth |
| REMB-driven adaptation | Remote receiver requesting lower bitrate via REMB |
| CPU constraints | Encoder unable to encode at high resolution in real time |
| Configuration limits | Maximum resolution restricted in encoder or SDP settings |
| Simulcast layer switching | In simulcast scenarios, the receiver may switch to a lower-resolution layer |
How to Fix It
- Check bandwidth — Compare resolution changes with Video Bitrate. If both drop together, bandwidth is the bottleneck.
- Increase available bandwidth — Ensure sufficient capacity for the target resolution. HD (720p) typically needs 1-2 Mbps, Full HD (1080p) needs 2-4 Mbps.
- Review QoS policies — Prioritize video traffic with DSCP marking to protect it from competing best-effort flows.
- Monitor CPU — If the encoder has sufficient bandwidth but still drops resolution, CPU may be the bottleneck. Reduce the target resolution to a sustainable level.
- Verify codec configuration — Ensure the encoder's maximum resolution is configured correctly and not artificially limited.
Related Metrics
- Video Bitrate — Resolution and bitrate are tightly coupled; both adapt to bandwidth
- Video FPS — Frame rate may adapt alongside or instead of resolution
- Video Freeze Events — Severe quality issues that resolution reduction failed to prevent
- Keyframe Interval — Keyframe size scales with resolution
Video FPS
Understand video frame rate measurement in VoIP calls — how CallMeter tracks FPS, what frame rate drops mean for visual quality, and common causes of choppy video.
Keyframe Interval
Understand keyframe interval measurement in video streams — how CallMeter tracks the time between consecutive keyframes, thresholds, and what the interval means for video recovery and bandwidth.