At 10:07 AM 9/30/2014, Bruce Whisler wrote:
>I am working with an old tape recording that has several instances of
>distortion that sound like clipping. When I view the waveform in my DAW,
>I see two things that are puzzling:
>1. The waveform in the distorted areas is not at a higher amplitude
>than other undistorted sections.
>2. The waveform amplitude appears to be attenuated on the negative
>side of the waveform, but not on the positive side.
>The distorted sections usually last only about a second and do coincide
>with loud high notes from a trumpet soloist. The recordings are from live
>performances in the 1970s.
>Any thoughts on what I am dealing with?
The first thought that comes to my mind is slew rate limiting (aka
transient intermodulation distortion). For those who don't know what slew
rate limiting is, good explanation here:
It sounds like distortion, but reduces the waveform amplitude of high level
high frequency components compared to lower frequencies, so the distorted
waveform looks quite different from simple clipping. Some audio equipment
built in the 1970's used early IC op amps which had slew rate too slow for
Trumpet is a very asymmetrical waveform. When you look at undistorted
sections of the trumpet waveform, are the fastest-moving portions on the
negative side, or the positive side? If negative, that supports this
theory. If positive, this theory is almost certainly wrong.
> I have Izotope RX 3 Advanced, and have had little success in repairing
> this particular problem with the Declip, Decrackle, or Declick modules.
I've never tried to use digital tools to repair slew rate limiting, but it
doesn't surprise me that those tools won't do it.
-- John Chester