CEDAR Duo – declickle and auto dehiss
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Switch the processing off, and listen to the source material. Does the noise appear to be present
at all frequencies? If not, is it predominantly high-frequency (hiss) or low-frequency (rumble)?
If the former, turn the processing on again, and reduce the LF Bias. This will reduce the Level
below 5kHz, and reduce the amount of noise detected at these frequencies. As you do this, you
should hear the low+mid frequency signal content ‘open up’. This will allow you to increase the
Level without risk of damaging the desired signal at low+mid frequencies.
If the noise appears to be predominantly in the lower frequencies (as is often the case with
recordings obtained from discs) you should increase the LF Bias to tell the Duo that there is
more noise below 5kHz. This will increase noise suppression at low+mid frequencies without
increasing it at high frequencies, reducing the risk of high-frequency dullness when removing
large amounts of low-frequency noise.
Adjust the LF Bias to obtain a good balance between high- and low- frequency noise
attenuation.
There is a delay of a fraction of a second between adjusting the LF Bias and hearing the effect at
the output.
Attenuation Tutorial
With the Level and LF Bias set to suitable values, you can now adjust the Attenuation to
determine the amount of noise removed.
Decrease the amount of attenuation from -40.0 to 0.0, at which point you will hear that the
processed signal is identical to the unprocessed signal. This is because the Attenuation control is
limiting the amount of noise removal to 0dB - i.e. there is no effect.
Now increase the amount of Attenuation as defined by the material and your taste. You may
find that, if the Level is too high, you can only increase the Attenuation by a few dBs before the
signal starts to sound muffled. If the Level is too low, you can have more Attenuation, but with
reduced effect.
Iterating the Procedure
It is unlikely that the values of the three controls are optimised, so you should now attempt to
find a better value for the Level. Having done this you will probably wish to modify the LF Bias
and Attenuation values. Fine-tuning of these controls will lead to excellent noise removal with
few or no side-effects. However, the Duo is not a magic wand, and it may not be possible to
restore some badly degraded material beyond a certain point. Experience will enable you to
judge whether you have removed as much noise as possible without unacceptable
consequences.
Auto Mode (L-R channel mode selected)
Set the auto dehiss processor in AUTO mode, and ensure that the AUTO-OFF control is in the
AUTO position. Now set the Bias and LF Bias to zero, and the Attenuation to -40dB, and follow
the next three tutorials in sequence.
