1.8 Ensure that videophones provide accurate reproduction of text and sign language

Screen size and image quality must be of a sufficient standard to support lip-reading, sign language interpretation and textual communication.


Videophones can be particularly useful for those with hearing impairments, who can communicate using lip reading, sign language and text. However, if the display quality is not good enough to clearly distinguish lip movements, gestures and text characters, the communication will not be possible.

Directions and Techniques

Ensure screen resolution is sufficient

The screen resolution should be sufficient to render sign language and enable accurate lip-reading. At least 176x144 pixels and preferably 352x288 or more pixels.

Ensure transmission capacity is sufficient

To support lip-reading and sign language the picture update rate should be high enough for vivid signing and smooth movement reproduction, including finger spelling. This will require at least 12 pictures per second and preferably 21 or more pictures per second. This is not currently possible using PSTN (Public Switch Telephone Network). At least dual channel ISDN is required, allowing 128 kbps, and preferably six channel ISDN, allowing 384 kbps.

Ensure a minimal picture delay

The delay between the actions of the person at one end and the display of those actions at the other end should be less than 0.8 seconds, and preferably less than 0.4 seconds. This allows for fluid turn taking in conversation.

How you could check for this:

There are no specific test methods recommended for this guideline.