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The navtec GmbH has developed a full digital signal generation and signal processing board for an existing guiding system for the blind to replace the conventional analog solution. The system was realised using a 40-MHz DSP.  All the  technical specifications by  DIN and RilSA are met.

A guiding system for the blind basically contains sound sources of a defined level. Two different signals are used:
 

A pilot signal, which sounds like a crack, and which is repeated at a low rate, helps the blind person to find the traffic light. This pilot signal is also active during the red light phase.  A clearance signal, a sinus or multi-tone burst, signals the green light for pedestrians to the blind person. The clearance signal then guides the blind person across the street. The clearance signal was designed according to psycho-acoustic characteristics of the human hearing. It stands out very well against the usual environmental noise and supports the ability to determine the direction of a sound.

Pilotton = pilot signal; Freigabe-Signal = clearance signal Fußweg = pedestrians sidewalk; Straße = Street

The generated sounds are dynamically adapted to the environmental noise and are always 0...5 dB above the registered environmental noise level.
 

The inside of a traffic light for the blind. One can clearly identify the DSP, the major component.  Other highly integrated circuits, which are used, are a dual CODEC and a PLD. Due to high integration and digitalisation of the functions only very few analog components are left.

These are the advantages of the system compared to the conventional analog solution:  Full digital signal generation with Taylor series in the DSP. Both the pilot tone and the clearance tone are generated with only one processor on one board. There are no tuning elements, such as trimmers or potentiometers. High stability of frequency in the entire temperature range. The system can also be progammed during operation with the help of switches or via an infrared interface (IrDa). The recorded signal of the environmental noise is digitally filtered in order to rule out any influence by opposite signal sources. The digital filters also select the typical traffic noise spectrum.  The processor takes over all control tasks at the same time. This includes extensive security and surveillance procedures.

 

To get an impression of the sound of a traffic light for the blind, click on one of the following signals. There has to be a sound card installed on your PC to play the WAV-files. The sound files are about 3 seconds or 24kByte in length. They were recorded with a sample rate of 8kHz and 8Bit resolution.    pilot signal Tacker   clearance signal FT mixed signal under the traffic light FT+Tacker (- 3 dB) clearance signal FTm mixed signal under the traffic light FTm+Tacker (- 3 dB)

The FT signal is a pure sinus burst.The FTm signal is a multi-tone burst. The type of signal can be programmed when the system is installed or programmed via IrDa remote control.

Exterior of a traffic light for the blind,  intended for installation on a mast. (The housing was not made by navtec GmbH)

In Germany the technical requirements for a guiding system for the blind are mostly laid down by

• DIN 32981,
• DIN VDE 0832 and
• RilSA (regulations for visual signal systems)

Extensive safety measures for self-supervision must be implemented to ensure that the system will not under any circumstances generate clearance signals or confusing signals outside the (visual) green light phase. That is also the reason why it switches off in case of low power.