Author: John Rigg
This plugin gives a rough approximation of the low frequency distortion
produced by an audio transformer. It is intended to be used on the master bus
when mixing in a DAW like Ardour. It was made in order to get closer to the
sound of an analogue mix without the signal degradation caused by an extra
round trip through the DA-AD converters. Its effect is deliberately subtle and
may not be audible on undersized speakers or low quality headphones.
The plugin uses a low pass filter followed by a cubic function generating pure
third harmonic distortion. This is then mixed with the original signal. The LF
Distortion Bandwidth setting is the -3dB point of the low pass filter. The LPF
also serves as an anti-alias filter. Worst case alias product (with 0dBFS input
at just over fs/6, at maximum bandwidth setting) is below -115dBFS.
Third harmonic distortion is around 3% at 0dBFS at frequencies below the LPF
cutoff. Distortion decreases with the cube of the signal amplitude, so for
every 6dB reduction in signal level the distortion is reduced by 18dB. There's
around 1dB dynamic range compression at 0dBFS at low frequencies. LF makeup
gain of about 0.5dB is added. Input level needs to be fairly high (loud peaks
at -6dBFS or more) for the most noticeable results.
CPU load is very light. In Ardour 2 its DSP load is less than
0.3% on a 2.5GHz AMD64x2 system at 48kHz sample rate when used on the
At present this is the only plugin in the xfmr-plugins package.
It was developed for the author's own use and is released strictly
AS IS in the hope that others may find it useful.
There is ABSOLUTELY NO WARRANTY and NO SUPPORT.