PURPOSE: To prevent a reproduction sound from being masked owing to an noise and to improve a hearing sensation not only by varying a gain according to the level of the external noise and the level of an audio signal, but also by varying reproduction frequency characteristics.
CONSTITUTION: A voltage ΔVN corresponding to the increment ΔN of the noise level is supplied from a noise detecting part 1 to a level converter 19, which outputs 0.3ΔVN as its output voltage. Further, a voltage ΔVS corresponding to the level increment ΔS of the audio signal is supplied from a signal detecting part 9 to a level converter 20, which outputs 0.4ΔVS as its output voltage. Therefore, the output voltage VG of a subtracter 21 is 0.3ΔVN-0.4ΔVS. The output voltage VG is digitized by an A/D converter 22 and supplied to a decoder driver 23, so light emitting diodes 24 which are proportional in number to the output voltage VG illuminate. A switch circuit 25 consists of plural transistors (TR) which turn on and off according to the output level of the decoder driver 23, and frequency characteristics of a variable characteristic circuit 26 vary according to on-off states of the TRs. The variable characteristic circuit 26 is interposed in an audio signal line and the constant in a variable gain circuit 2 is varied to vary the frequency characteristics.
KOTAKA MAKOTO
SATOU TAKESHI
