Tape saturation simulator

Hello folks,

this is a pretty simple but effective Tape Saturation simulation plug-in. If used correctly this plugins enhances apparent loudness of your track without adding a lot of distortion.

it’s based on the ‘Soft Clipping Limiter’ by Steve. but it uses a high shelf filter to shape the maximum possible output level on high frequencies on tape.
it’s really a first version and perhaps later some presets will be added. but for those I need to make measurements on real (hardware) tape machines.

the typical saturation onset threshold will be between 3 to 5dB. with a chosen limiter ratio of 2, the range between the treshold and the full level will be compressed (actually soft clipped) to half of the threshold. this means with a threshold selection of -3dB and full level input, the peak output will be -1.5dB.
harder limiting can be chosen by higher ratio. due to nonlinear calculations the ratio isn’t correct for all threshold settings. as always, experiment yourself.

the next section is about the frequency shaping of the soft clipper to simulate tape-like behaviour. the high frequency MOL (maximum output level) crossover selects the point where high frequency signals are limited earlier than low frequency signals. the recommended 4.5kHz should be fine for most cases, but sometimes you want to saturate at a lower or higher frequency. with the high freq MOL reduction you can choose how much more high freq signals are limited compared to low freq signals. as seen on a maxell XLII-S hi-fi tape, the difference can be up to 8dB, but on professional reel-to-reel machines the values are lower. as said before this still has to be checked.
an additional benefit of the frequency shaping is that some of the generated higher order harmonics get attenuated and therefore make the result less harsh.

finally there’s an automatic makeup-gain possible, off or on. the plug-in reserves a 0.2dB headroom for full scale input signals. if you process signals with ‘overs’ - higher than 1.0 in the waveform view - the result still can clip if high levels are present. in this case choose ‘amplify’ from effects menu to bring back to nominal full level.

as all analog tape machines have high pass filters, I added a first order one with 5Hz crossover. this is fixed because I didn’t want to add another control and should serve most cases. otherwise you can edit the code.

you can use this plugin with high input levels (above 1.0) but then perhaps distortion can get to an unacceptable level. as always, let your ears judge.

have fun with it.
Obsolete version
tapesat.ny (2.41 KB)
The latest version is now available on the Audacity wiki:

I’ve only had a brief look, but it looks promising.
I notice that sometimes the effect will introduce a bit of DC offset, and wonder if it might be better to have the high-pass filter at the end of the signal chain rather than at the beginning.

Normally I use this effect just slightly, but it can introduce a littlebit of DC for asymmetric source material when pushed harder.
So it make sense to have the highpass filter at the output rather than on the input. perhaps even on both, a real tape machine also has multiple highpass filters by using several electrolytic capacitors in the signal chain…

Ok, I added a 2Hz output highpass filter to remove any possible DC. see dl-link in first post.

Have you had chance yet to test against genuine tape saturation?
It’s been a long time since I did any serious recording on tape, but from memory the plug-in effect sounds a bit over-bright (though it could be specific to the equipment used and/or my memory being inaccurate).

I didn’t had the time yet for measurements. I do maintenance work in a studio with a 16 track studer a800, but we were busy with other stuff.

after some more tests with sine sweeps in audacity I’ve seen that there is a slight problem with plug-ins of this type. it’s aliasing. that’s a problem you don’t have on a real tape machine though. to circumvent aliasing in soft clippers will be very complicated and not worth the effort here. we would need at least the double sample rate and do a lot of serious filtering to suppress aliasing remnants. of course you would need to do this for a professional ‘high-end’ version.
unless you don’t push it too hard the plug-in still produces very good results without too much distortion. and the high frequency shaping helps alot.