Videoconference Quality
Note: these comments refer to Standard Definition videoconferencing.
It is useful to consider some very basic information on how standard definition VC systems operate which will help in understanding the limitations of video quality over a videoconference link.
There are two measures of quality in an image:
- "Frame Rate" the number of images transmitted per second which results in smooth motion with a high frame rate or jerky motion with a low frame rate
- "Resolution" the number of pixels in the image resulting in a sharp or blurry image.
Given the restrictions in the overall bandwidth available within a videoconference link it is not possible to have High Frame Rate AND High Resolution. The norm is for sources likely to have significant motion: Cameras and VCR/DVD players to use High Frame Rate/Low Resolution and for sources likely to have less motion content: PCs and Document Cameras to use Low Frame Rate/High Resolution hence optimising the image quality.
| Image Source | Resolution | Frame Rate |
| Camera or VCR/DVD (Comp Video or S-Video I/P) | 352 x 288 | 15-30 |
| PC or Doc Camera (VGA I/P) | 1024 x 768 | ~5 |
It is also important to emphasise that a VC link transmits changes in the image rather than the entire image to optimise the use of the available bandwidth. Hence if the image contains a static talking participant only a small part of the image is changing and the overall image quality will be acceptable, however if the participant is walking around the scene and a large percentage of the image is changing then the overall quality of the image will be degraded as the system struggles to transmit the large volume of image information to the other sites. In such a case the image will become “blocky” and “blurry” until the movement declines and the image builds up to full resolution.
The degree of blockyness and blurryness depends on the overall bandwidth of the VC link, increasing the call bandwidth will increase the image quality for the same amount of movement.
The following results are taken from a VTAS product evaluation where a football sequence with significant motion was rated across a number of connection speeds. It is evident that even at 768Kbit/s the overall effect of blurring is slightly annoying and blockyness is perceptible. At 384Kbit/s blurring is annoying, blockyness is slightly annoying and jerkyness is perceptible.
| Subjective Impairments | 384kbit/s | 768kbit/s | 3Mbit/s |
| BLK | 3 | 2 | 1 |
| BLR | 4 | 3 | 2 |
| JRK | 2 | 1 | 1 |
When compared with the results below for a seated person talking the artefacts are reduced to perceptible but not annoying even at 384kbit/s.
| Subjective Impairments | 384kbit/s | 768kbit/s | 3Mbit/s |
| BLK | 2 | 1 | 1 |
| BLR | 2 | 2 | 1 |
| JRK | 1 | 1 | 1 |
| Block distortion | BLK |
| Blurring (reduced sharpness) | BLR |
| Jerkiness (distortion of smooth motion) | JRK |
| Scale of impairments: | |
| Imperceptible | 1 |
| Perceptible | 2 |
| Slightly annoying | 3 |
| Annoying | 4 |
| Very annoying | 5 |
Hence a key consideration in VC quality particularly at bandwidths below 1-2Mbit/s is to reduce the overall movement in the image, the quality of a seated participant will always be better than that of a standing participant who moves around the image. Static images will resolve better than those with continual movement.
Where PC images are sent at the higher resolution the restricted bandwidth has similar effects, where a large area of the image changes the overall effect is a blurring of the image as the frame builds up to full resolution, where the entire image changes for example in a PowerPoint slide change it may take a number of seconds for the image to build up to full resolution.
Jim Sheach
11/2/2008