1a. Removing hum from recordings (post processing)

Here is a technique I am using to remove hum from recordings with Cool Edit 96.  The result makes for awful listening, but does enhance the desired signal in the spectrograms. Here is the process and the settings I have been using.

• Capture the signal using Cool Edit 96.
• Process the signal using the DELAY tool set up as described below.
• Repeat.
• Process the signal using the NOISE REDUCTION tool set up as described below.
• Save the processed recording.
• Run through GRAM.

You may need to adjust the amplitude to get a good trace from GRAM.

DELAY

• 16.667 ms for 60Hz, 20 ms for 50Hz *
• ratio 50%
• invert selected

* Note that these values may need to be changed if the playback speed varies from the record speed on the tape deck.

NOISE REDUCTION

• Set the FFT to 16,384
• Select "Remove Noise"
• Set Precision to 9
• Set Smoothing amount to 1
• Set Transition width to 0 db
• Set number of statistical snapshots to 120
• Get noise profile from selection
• Set the Noise Reduction Level to 50%

Here is a signal through GRAM before this processing,

and after the two steps of delay processing,

and finally after noise reduction. The enhancement in S/N ratio is apparent. The last hop is barely visible in the first spectrogram, but fairly clear in the final.

A hardware humblanker is under development. The device should be usable in real time. The approach is somewhat different than the delay technique shown here. More to come...

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