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How to Clean Up Audio from a Phone Recording

January 20, 2005

Repair and restore your audio:

RX 7

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Description:

Recordings made off of a telephone line can be heavily distorted and difficult to understand. The De-clip module in iZotope’s RX software is designed to help repair analog and digital distortion by redrawing squared-off waveforms.

Sample:

In this phone recording of an interview segment, we can hear significant overmodulation and see the clipped waveforms that square off at 0 dB:

Standalone Workflow

  1. Open an audio file in the RX Audio Editor or send it using RX Connect.
  2. Select the distorted portions of the audio, and open the De-clip module.
  3. For this audio sample, we can start by selecting the preset “Subtle analog clipping.” Click Preview to hear the results.
  4. Adjust the Threshold sliders to the right of the Histogram or Spectrogram to tailor the settings to your audio, or click Suggest to have the De-clip module analyze your audio and set the threshold automatically. For this example, any Threshold setting above –1.0 will work fine, so we don’t need to adjust the Threshold slider.
  5. Once you’ve found the desired settings, click Process to repair your audio.
  6. Use the Dialogue De-noise plug-in to remove the remaining hiss from the recording. For more information on removing background hiss, follow the recommendations in the recipe for Repairing Noisy Vocal and Instrument Tracks.
  7. Phone lines are usually band limited to a range of 300 Hz to 4 kHz. There can be certain frequencies in this region that resonate because of this. Try using the Corrective EQ module to notch out these resonant frequencies and improve intelligibility

Plug-in Workflow

  1. Select the distorted portions of the audio and open the RX De-clip plug-in.
  2. Select a preset from the drop down menu, or set the threshold manually to identify where the De-clip algorithm should begin processing. For this example, the default threshold of –1.0 dB worked well.
  3. Click Preview to hear the results. Adjust the threshold and quality settings as necessary.
  4. Once you’ve found the desired settings, click the Process or Render button to complete the repair.
  5. Use the Dialogue De-noise plug-in to remove the remaining hiss from the recording. For more information on removing background hiss, follow the recommendations in the recipe for Repairing Noisy Vocal and Instrument Tracks.
  6. Phone lines are usually band limited to a range of 300 Hz to 4 kHz. There can be certain frequencies in this region that resonate because of this. Try using an EQ plug-in to notch out these resonant frequencies and improve intelligibility
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