[LAU] Re: Tape emulation - is it worth the effort ?

Take what I say with a large grain of salt as I am both a software engineer
and electrical engineer with a large strictly hardware only studio with 152
inputs, 16 effect sends  and 16 subgroups.

I own a real tape delay - largely similar to a Roland Space echo with
similar looks and dimensions - that anyone can buy new today: the EchoFix
ef-x2 for $1999 that includes a reverb tank and digital chorus, while their
ef-x3 with an analog chorus is $2699.

When talking about the tape bias levels, the EchoFix is around 50hz and is
annoying enough that I high pass filter it out. EQ is one of the areas the
cheap clones haven't conquered yet imho and I use API 550a/550b mostly so I
am unclear if digital EQ is going to help in emulation.

The EchoFix prices have been dropping and imho it is only a matter of time
before there is competition at the lower end by Berhinger etc - who by the
way kind of proves my point as their kt-1176 compressor has dropped from
$499 to $150 today.

My point here is that with a plethora of proprietary tape plugins that also
emulate the Ampex recorders that I used the real ones first hand while in
audio engineering school in the early 90's, as well as new hardware tape
delays on the market, I wonder if it's a little late now.

In all that I do applaud your efforts as it is a difficult software
problem, in my mind close to impossible when hearing the proprietary
efforts being so far off.

On Wed, Oct 16, 2024 at 3:24 AM Fons Adriaensen <[email protected]> wrote:

> Hello all,
>
> The purpose of this post is to gather some opinions on the
> use of 'tape distortion' plugins.
>
> The way an analog magnetic tape recorder distorts the signal
> is quite complicated. The non-linear response is mainly due
> to the hysteresis of the magnetisation process, and how this
> interacts with the HF bias added to the recorded signal.
>
> Simplifying things a bit, as the tape moves past the recording
> head and away from it, it goes through several cycles of an
> hysteresis loop with decreasing amplitude, and it finally
> converges to some value more or less proportional to the signal.
> The bias signal itself does not remain on the tape, or at most
> at a very low level.
>
> The same process but with just a very high level HF bias is
> used to erase the tape.
>
> Simulating this accurately is quite complicated, and very CPU
> intensive. The sample rate must be at least twice the bias
> frequency (around 100 kHz for 'pro' equipment) and for each
> sample you need to evaluate the hysteresis loop up to a hundred
> times (depending on tape speed) to simulate the convergence.
>
> Over the past few months I've written the code to do this,
> with the aim to present a useful 'tape emulation' plugin
> at the next LAC. It uses a somewhat simplified form of the
> Preisach algorithm for the hysteresis.
> <http://kokkinizita.linuxaudio.org/linuxaudio/downloads/preisach-model.pdf
> >
>
> But the first results are somewhat sobering.
>
> Have a look at
> <http://kokkinizita.linuxaudio.org/linuxaudio/downloads/tapesaturation.png
> >
>
> This shows the tranfer function (recorded magnetic signal as
> a function of the input) for three values of the HF bias.
>
> The blue line is what you get without bias. It result in a
> very smooth saturation for high level signals, but a low
> amplitude signal will be severly distorted due to the flat
> region around zero.
>
> As bias level is increased, this effect is reduced and the
> low level gain increases up to some maximum. This is shown
> by the orange curve.
>
> The recommended way to set the correct bias level is to
> further increase it until the small signal gain is reduced
> by a few dB (typically 1 or 2 dB, depending on tape type).
> This is shown by the green curve.
>
> The central part of this is very linear (less than 1% THD).
> But now the smooth saturation is almost transformed into
> hard clipping. The sharp bend in the curve occurs when the
> signal amplitude is higher than the bias amplitude.
>
> The only effect that remains in a complete simulation is
> the result of the EQ applied to the signal to be recorded.
> Higher frequencies have a 'self erasure' effect (which is
> nicely reproduced by the simulation) and so need to be
> amplified. The net result is that they will saturate at
> a lower input level.
>
> How much EQ is required depends mainly on tape speed, higher
> speeds need less. At the most popular 'pro' speed (381 mm/s)
> this would be something around 10 dB at 10 kHz. At the higher
> (762 mm/s) speed typically used for 'master tapes' it's just
> a few dB.
>
> So what does effectively remain of the 'smooth saturation
> and compression' that is claimed to give tape recording its
> magical 'warm' character ? Is it just a myth ?
>
> I also simulated the green curve directly without going
> through the complicated full simulation, and honestly,
> to me that sounds just the same. And unless you really
> use very high levels (much more than would actually be
> used) the net effect is marginal. Maybe the hard clipping
> can be useful when applied to individual tracks (e.g.
> drums or bass), but then there are much simpler ways
> to do this than 'tape emulation'.
>
> Comments invited !!
>
> _______________________________________________
> Linux-audio-user mailing list -- linux-audio-user-cunTk1MwBs/[email protected]
> To unsubscribe send an email to
> linux-audio-user-leave-cunTk1MwBs/[email protected]
>

_______________________________________________
Linux-audio-user mailing list -- linux-audio-user-cunTk1MwBs/[email protected]
To unsubscribe send an email to linux-audio-user-leave-cunTk1MwBs/[email protected]