CIRCUIT FOR CONCENTRATION AND CONCENTRATION EQUALITY OF A SPECTRUM

摘要

In order to reduce the effect of quantizing noise in digital transmission of an audio signal, a delta modulator (22) providing the digital signal is preceded by a circuit (16) providing adaptive pre-emphasis. In the decoder a complementary circuit follows the demodulator. The preprocessing circuit (16) includes a spectral analysis circuit (52) providing a control signal indicating the predominant frequency region of the audio signal. The control signal is subjected to bandwidth limitation and controls an adaptive pre-emphasis circuit (56) which, when quiescent provides high frequency boost above say 500 Hz. The control signal causes the corner frequency of the boost characteristic to slide upwardly so as to stay above the predominant frequency region. Moreover above about 2 kHz cut of the predominant frequency region itself is introduced. In order to simplify the decoder the control signal is converted to digital form by a converter (72) and transmitted to the decoder. Timings are such that this information is transmitted advanced relative to the main digital signal so as to compensate for slow response of the control signal in the decoder arising from bandwidth limitation after conversion to an analog control signal.