Compander noise reduction method and apparatus

A method of encoding an information signal for transmission over an amplitude-limited transmission channel, comprising the steps of:

• sensing the information signal level,
• generating a gain control signal representative thereof,
• delaying the information signal until the gain control signal has been substantially generated,
• encoding the delayed information signal together with the gain control signal,
• isolating the gain control portion of the encoded signal, and controlling said encoding with said isolated signal to compress the level of the encoded signals with respect to the original signal levels.

The method of recording an audio signal on a film medium, comprising the steps of:

• generating a gain control signal, said gain control signal having an amplitude which represents the amplitude of the audio signal, and a rise time corresponding to a subaudible frequency,
• delaying the audio signal until the gain control signal has been substantially generated,
• forming a composite signal by adding together the audio and gain control signals,
• amplifying the composite signal,
• controlling the amplification gain with the gain control signal to compress the amplified composite signal level, and
• optically recording the amplified composite signal on a film medium.

Encoding circuit apparatus for use in a compander system, comprising:

• a variable gain amplifier means, the output of which is adapted to be applied to a transmission channel,
• circuit means adapted to receive an input information signal, and in response thereto to generate a gain control signal which is shifted in frequency from the information signal,
• said means for generating a gain control signal including a level sensing circuit adapted to produce an output representative of the peak values of the input information signal, said level sensing circuit having a predetermined response time, and further including a delay circuit connected to delay the information signal until the output of the level sensing circuit has been substantially fully generated,
• means for presenting said delayed information signal and said gain control signal as a composite input signal to said amplifier means, and
• an amplifier control circuit responsive to the amplified gain control portion of the output of said amplifier means for establishing an amplifier gain to compress the level of said composite signal.

Circuit means for conditioning an audio signal for optical recording on a film medium, comprising:

• (a) an input terminal for an electrical audio signal,
• (b) a level sensing circuit connected to the input terminal and adapted to produce an output the amplitude of which is representative of the peak values of the audio signal, said circuit being actuable over a predetermined response time,
• (c) a delay circuit means connected to delay the audio signal by a period substantially equal to the response time of the level sensing circuit,
• (d) an amplifier means having an input connected to receive the level sensing and delayed audio signals as a composite input, and
• (e) an amplifier feedback control circuit, comprising
• (1) a filter means connected to the amplifier output, said filter means being adapted to pass the amplified level sensing signal and block the amplified audio signal,
• (2) a multiplier circuit connected to multiply together the signal at the amplifier output and the signal passed by the filter means, and
• (3) means connecting the multiplier circuit output as a feedback control to the amplifier means.

A compander signal transmission system, comprising:

• a gain control circuit connected to receive an input information signal, and to generate a gain control signal which represents the information signal level and is shifted in frequency therefrom, said circuit being actuable over a predetermined response time,
• a delay circuit connected to receive the input information signal, and to delay said signal by a period substantially equal to the response time of said gain control circuit,
• an encoding circuit connected to receive the outputs of said gain control and delay circuits, and to encode the gain control signal together with the delayed information signal, said encoding circuit including means responsive to said gain control signal to compressively encode the information signal,
• means for applying the gain control and encoded information signals to a transmission channel,
• means for receiving said signals from the transmission channel, and
• a decoding circuit including means responsive to a received gain control signal to expansively decode a received encoded signal and thereby substantially reproduce the original input information signal.