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Using Audacity to do NOS droop correction

Posted 29th April 2012 at 03:36 AM by abraxalito

If you have a NOS DAC (like a Metrum) and are curious to hear how it sounds with the zero-order hold corrected, here's a fairly simple way to try out my 3-tap filter using Audacity.

Load up the file you'd like to process and make two duplicates (select original and use Ctrl-D) - we'll call them A and B. We need to apply delay and gain to A and B which we do with Audacity's time-shift button (in the group of 6 tool buttons to the right of the transport buttons) and the 'Effects - Amplify' feature. A is shifted one sample to the right, and B, two samples right. I've shown how this looks with a mono chirp signal in the first image.

The gain is input into Audacity in dB so 0.15 becomes -16.5dB and 0.026 is -31.7dB. These are the gains for A and B, respectively. A needs to be inverted too - use 'Effects - Invert'. The second screen grab shows how it looks after these operations (I hid the device toolbar to give more space for waveforms).

Having processed the two duplicates then selecting all 3 tracks followed by 'Tracks - Mix and Render' gives the result in the 3rd pic. The sweep (called 'chirp' in Audacity) here is 1kHz - 20kHz, linear steps.
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