How is the latency measured?
An on-screen timer Auvidea BigTimer (download) displays a counter in big letters in a window. This window is captured from the screen with an analog camera. The camera signal is digitized by the Winnov Videum capture card. The video stream is then compressed with a MPEG-4 software encoder. The compressed stream is packetized and send across 100 MBit Ethernet to the receiving PC. The video stream is decoded in software and displayed on the same display, which also shows the on-screen timer. A digital camera takes a snapshot of the screen, so that the delay may be determined.

The test setup includes one PC (sender) with digitizes and encodes the video, and a second PC (receiver), which receives and decodes the video stream. Both sides use the Auvidea RTPVideoSender to transmit and receive the video.

• Sender
• Processor: Athlon 64 3200+
• Codec: MPEG-4 Encoder
• Buffer: 0 sec
• Video: 720x576, 25 fps
• Mode: I frame only
• CPU load: 60%
• Receiver
• Processor: Pentium 4 2.4 GHz
• Codec: MPEG-4 Decoder
• Buffer: 0 sec
• Total latency: 90 ms (measured by the screenshot above)

How does the CPU load affect the latency?
For lowest latency the CPU should not be loaded more than 70%. Tests have shown, that at higher CPU loads the latency may go up to 200 to 300 ms under the same test conditions.

How do P frames affect the latency?
Tests have shown, that the use of P frames does not affect the latency. Example: if a key frame period of 10 frames is used, the encoder creates one I frame, then nine P frames, and then the next I frame. The latency stays below 100 ms, but the start up delay of the receiver increases, as the decoder has to wait for the first I frame, before video is decoded and displayed. If the key frame period is set to 100 and the video is encoded at 25 fps, the startup delay may be up to 4 seconds.

How do P frames affect the bandwidth?
At 720x576 (25 fps) the encoder uses 5 to 15 Mbit/s of network bandwidth in the I frame only mode depending on the amount of detail in the video. In this mode the amount of motion has no effect.
If the key frame period is set to 10, the bandwidth usage is reduced to 3 to 6 Mbit/s depending on the amount of detail and motion in the video. The size of P frames largely depends on the difference between two frames (the motion in the video).