Appendix F - Visual Signaling for the Hearing Impaired

The (O)ptions menu, (T)one submenu provides the signaling generator option G that uses screen blinking to send the Morse code. This form of signaling is used by the U.S. Navy for signaling between ships when other forms of communications is not available. This option has been added to Morse Academy because of several requests from hearing impaired persons who wish to learn the code.

There are many problems in using the PC screen to signal CW. To reduce the visual noise appearing on the screen care has to be taken to change the screen contents during times when the electron beam is not updating the screen image. The Morse Academy driver has been written to perform the screen update in the minimum number of CPU cycles possible while the beam is returning from the bottom to the top (HRI time). This restricts the maximum signaling rate to the screen refresh rate. For a 60 Hz non- interlaced screen, refresh occurs approximately every 16 milliseconds. At 10 WPM, where the sending time for a dot is 120 milliseconds, this means that an error of 15% can occur for one element. As the speed increases the error does also. High resolution non-interlaced monitors will perform better than interlaced types. LCD's used in laptop or notebook computers are very slow and are not suitable for visual signaling.

The eye can not easily handle signaling rates as fast as those as the ear. This fact combined with the larger error due to 16 ms timing increments make copying visual signaling difficult at high speeds.

More accurate visual signaling is possible using external hardware the PC's internal sound generator, e.g. Morse Academy's external tone generator, or the parallel or RS232 port's. (See appendix C and E for more information on methods of signaling).

Morse Academy was written to use the screen as a display of information while the student listened to the audio practice. Adapting it to display the same information and use the screen for signaling involved considerable modification. Hopefully any errors introduced as a result are minor. Still some sessions proved too difficult to modify and had to be dropped when using visual signaling, e.g. the (E)ndurance session. Depending on the response to this new mode of training, future developments may extend visual signaling to support all Morse Academy functionality.

User comments on the visual signaling option would be appreciated.

By Joe Speroni