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German Translation

Traffic Light Guiding System for the Blind

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:
  1. 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. 
  2. 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.
Speaker Output 220 V at green 220 V
Preset Volumes Codec Relais
IrDa Communication DSP / PLD Transformer

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
clearance signal
mixed signal under the traffic light
FT+Tacker (- 3 dB)
clearance signal
mixed signal under the traffic light
FTm+Tacker (- 3 dB)
Außenansicht der Blindenampel
 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)

Technical Data for Guiding Systems for the Blind in Germany

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

This is a short summary:
installation height
2,10 ... 2,50 m
operation temperature
-25 o ... 75 o C
increase of the level above environmental noise, adjustable
0 ... 5 dB
mininum level at 1m distance from signal source
30 dBa 
maximum level at 1 m distance from signal source 
90 dBa
measurement of the generated signal level and switch-off
> 90 dB
gradient for increase of level in case of raised environmental noise
40 ... 120 dB/s
gradient for increase of level in case of lower environmental noise
max. 10 dB/s
repeat rate of pilot signal
1,1 ... 1,3 Hz 
audibility range of the pilot signal around the traffic light
4 ... 5 m 
repeat rate of clearance signal
1,8 ... 2,2 Hz
keying ratio
base frequency of the clearance signals (FT and FTm)
880 Hz, -3 dB
2nd harmonic of the clearance signal (FTm only)
2640 Hz, -0 dB
3nd harmonic of the clearance signal (FTmonly)
3520 Hz, -0 dB
frequency error 
< 50 Hz
frequency error above temperature
< 1%
measurement of generated frequency and switch-off
> 1%
operational power range
10 V
24 V
40 V
230 V
(-10 ...+6%)
measurement of operational power and switch-off in case it falls below required level
<10 % und >6%
type of housing
min. IP 55

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.

  © navtec GmbH 1997