Browse Prior Art Database

IMPROVEMENT OF SIGNAL TO NOISE RATIO IN A VOICE COMMUNICATION PATH

IP.com Disclosure Number: IPCOM000007476D
Original Publication Date: 1995-Jul-01
Included in the Prior Art Database: 2002-Mar-29
Document File: 1 page(s) / 69K

Publishing Venue

Motorola

Related People

Stefan Lichterfeld: AUTHOR [+2]

Abstract

This idea relates to an audio processing circuit that enables voice communication systems to improve actual S/N performance.

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MO7VROLA Technical Developments

IMPROVEMENT OF SIGNAL TO NOISE RATIO IN A VOICE COMMUNICATION PATH

by Stefan Lichterfeld and Ernst Matthias Koenig

  This idea relates to an audio processing circuit that enables voice communication systems to improve actual S/N performance.

  Using a radio communication path the S/N ratio is limited by the modulator and demodulator stages to a finite value. With a low frequency signalling scheme or subaudible data modulation, this ratio rapidly decreases. Modulator and demodulator dis- tortion in today's standard transceiver designs is lim- ited so that small amounts of the subaudible modu- lation harmonics fall into the audio voice band. This is specified from 300Hz to 3kHz, is bandlimited by filters and will be affected by the harmonics and in-band noise.

  With standard analog techniques it is not possi- ble to achieve the same audio quality as with cost and bandwidth intensive digital techniques.

  To allow the audio response to meet the require- ments, the demodulated audio signal is processed as described in Figure 1.

  The unwanted frequency range below and above 1kHz has to be suppressed by a voltage controlled highpass-and/or lowpass filter to narrow the noise bandwidth dependent on the signal level amplitude. In comparison to standard wideband companders

where expensive compressors and expanders are nec- essary in transmit and receive stages, here a signal- level dependent audio filter expander with at least 1OdB expansion factor...