Any circuit that converts an audio signal into a control voltage, which may be used to regulate the operating parameters of a signal processing device.
"We just wanted to send a great big thank-you for the GREAT job you did on our transfer. My husband loved this particular Christmas album and it is not available in any other format. The job you did transferring it to a CD is amazing. The sound clarity is more than we could have hoped for in a transfer. We will sure look to you for other jobs in ..."