The interpreter terminals were selected using a quality evaluation method developed in a research project by the Swedish Association of the Deaf. The method extracts quality factors in a way that is related to sign language so that they can be enumerated and compared. Experience shows that the quality factor where the products have most trouble fulfilling the needs is regarding the picture update rate with signing in the picture. Often this figure is much lower than the figure used in product brochures usually indicating the highest possible picture rate achieved in a completely still view. It is observed that real usability starts at 12 pictures per second and increases up to 25 pictures per second when signing.
Lower values give jerky motion reproduction and loss of language contents.
The same resolution as used in consumer video recording is found sufficient but important, reproducing 352 points horizontally. It has been found possible to use a lower resolution, reproducing a coarser image with only 176 points horizontally. That resolution introduces some uncertainty and irritation to the users, and is not normally used in the Swedish service. One important factor when video communication is used for conversation is the delay of the picture from the camera to the screen of the other terminal. Today, with current ISDN terminals the value is usually around 0.7 seconds.