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 <fons@xxxxxxxxxxxxxx> 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 !!
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