On 6/17/14 5:30 AM, Nigel Redmon wrote:
Well…yes, aliasing is the main issue that separates the digital world from
analog when it comes to amp modeling, but no, I don’t think it’s the main issue
in simulating a good amp :-)
There are a lot of details in simulating classic amps—the controls of the
passive filters interact, etc.
and if they stay linear, you oughta be able to lump some of these
passive filters, no? if some component of the passive filter goes
non-linear, you have to first decide if the nonlinearity is salient, and
if it is, then model it.
And you have to recreate the filtering before and after the “tube”—guitar
amps aren’t about flat frequency response.
isn't that par-for-the-course, Nigel?
And the cabinets are a huge part of the sound.
*that*, and the loudspeakers themselves, is the hardest part, no? it's
all three: 1. salient (so you can't ignore it), 2. non-linear, and 3.
non-memoryless.
Then there’s the approach that’s more like sampling (Kemper Profiling Amp), a
totally different direction—does it sound better?
i don't understand how they're a totally different direction. i haven't
seen nor heard these amps, i am merely reading the reference manual, but
it appears they are doing the "Power Sagging", "Tube Shape", and "Tube
Bias" emulation that, at least appears ostensibly the same direction as
other modeling efforts.
Anyway, just keep in mind that the particular classic amps don’t sound “better”
simply because they are analog. They sound better because over the decades
they’ve been around, they survived—because they do sound good. There are plenty
of awful sounding analog guitar amps (and compressors, and preamps, and…) that
didn’t last because they didn’t sound particularly good. Then, the modeling amp
has the disadvantage that they are usually employed to recreate a classic amp
exactly. So the best they can do is break even in sound, then win in
versatility. And an AC-30 or Matchless preset on a modeler that doesn’t sound
exactly like the amp it models loses automatically—even if it sounds better—
because it failed to hit the target. (And it doesn’t helped that amps of the
same model don’t necessarily sound the same. At Line 6, we would borrow a
coveted amp—one that belonged to a major artist and was highly regarded, for
instance, or one that was rented out for sessions because it was known to sound
awesome.)
what did you guys do with the amps when you borrowed/rented them? was
your analysis jig just input/output, or did you put a few high-impedance
taps inside at strategic places and record those signals simultaneously?
On Tue, Jun 17, 2014 at 6:58 PM, Nigel Redmon <earle...@earlevel.com>
wrote:
On Jun 16, 2014, at 7:51 PM, robert bristow-johnson<
r...@audioimagination.com> wrote:
one thing that is hard to replicate is a sample rate that is infinity
(which is how i understand continuous-time signals to be). but i don't
think you should need to have such a high sample rate. one thing we know
is that for *polynomial curves* (which are mathematical abstractions and
maybe have nothing to do with tube curves), that for a bandwidth of B in
the input and a polynomial curve of order N, the highest generated
frequency is N*B so the sample rate should be at least (N+1)*B to prevent
any of these generated images from aliasing down to below the original B.
if you can prevent that, you can filter out any of the aliased components
and downsample to a sample rate sufficient for B (which is at least 2*B).
This really goes out the window when you’re modeling amps, though. The
order of the polynomial is too high to implement practically (that is, you
won’t end up utilizing the oversampling rate necessary to follow it),
this is a curious statement *outside* of the case of hard clipping.
oversample by 4x and you can do a 7th-order polynomial curve and later
eliminate all of the aliasing. oversample by 8x and it's 15th-order.
do *no* oversampling and you can still make use of the fact that there's
not a lot above 5 kHz in a guitar and amp (so 48 kHz is sorta
oversampled to begin with). you can fit a quite curvy curve with a
7th-order polynomial.
so
you still be dealing with aliasing. Modern high gain amps have huge gain
*after* saturation. In practical terms, you round into it (with a
polynomial, for instance), then just hard clip from there on out, and there
goes your polynomial (it can be replaced by an approximation that's very
high order, but what’s the point).
yes, we splice a constant function against a curve. if at the splice as
many possible derivatives are zero as possible, that splice appears
pretty seamless. this is why i had earlier (on this list) been plugging
these curves:
x
f(x) = C * integral{ (1 - u^2)^M du }
0
(C gets adjusted so that f(1) = 1 and f(-1) = -1.)
you can splice that to flat values at +/- 1 and the nature of the
function will not change appreciably from the polynomial in the region
of the splice.
anyway, the whole point is to give the guys with golden ears no cause to
complain about hearing aliases. same with emulating sawtooths and
hard-sync synthesis.
Anyway, you pay your money, you make your choices. Obviously some really
good musicians making really interesting music use modeling amps. They
don’t have to be better than tubes, in order to be a win, just good enough
to be worth all the benefits. If you’re a session music, you can bring in
the truck with all of the kinds of amps that might be called on, or you can
bring a modeling amp, for instance. And going direct into the PA or your
recoding equipment…etc. I’m not going to make judgments on what people
should like, so I’ll leave it at that.
One happy thing about the aliasing is that, given a decent level of
oversampling, it won’t be bad at lower overdrive levels. At the higher the
overdrive levels, the harder it is to hear aliasing through all that
harmonic distortion you’re generating. So it could be worse...
i really agree with this, Nigel. with *some* oversampling (but
theoretically not sufficient oversampling), you can get away with a lot
(like hard limits or whatever stuff goes on inside a transformer with
core loss). i would not say that you have to oversample to a
ridiculously high degree just because there is a hard-limit saturation
in there or that your tube model is not a polynomial approximation (but
i wonder why you wouldn't try to fit the grid-to-plate tube curve to a
finite-order polynomial).
--
r b-j r...@audioimagination.com
"Imagination is more important than knowledge."
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