Recording - December 2015 - 64

drive the headphone amplifier into clipping. So if
you're monitoring with headphones while recording
and you hear clipping, your recording may actually
be fine. This is something that you should check out
when you get a new gadget.
Crossover distortion
Some amplifiers exhibit what's known as crossover
distortion. This type of distortion is a "glitch" in the
waveform right around zero voltage (the "zero crossing"), where the signal crosses between the positive
and negative halves of the cycle. It typically results
from imperfect biasing in "push-pull" amplifiers that
dedicate one tube or transistor to each half of the
cycle. It's most common in Class B tube amplifiers but
it can be present in solid-state integrated-circuit operational amplifiers as well; it's sometimes responsible
for "the IC sound" that manufacturers of products
using discrete circuitry denounce.
6

With power amplifiers, distortion is usually measured at near maximum power where harmonic
distortion is dominant. However, the crossover
glitch doesn't change with signal amplitude, so
crossover distortion may cause an amplifier to
sound "grainy" at low levels.
The two waveforms in Figure 3 show one cycle of
a 500 Hz sine wave, both having been passed
through signal paths that measure 5% THD. One path
has crossover distortion while the other has been overdriven to introduce clipping. Figure 4 shows the spectrum analysis of the distorted outputs, which illustrates
that the harmonic content of the two forms of distortion
is considerably different even though their THD measurements are identical. The clue is that the crossover
distortion created a large number of odd harmonics
with just a touch of 2nd harmonic, making it sound
quite a bit like a square wave (which is comprised of
all odd and no even harmonics). The clipped sine
wave has plenty of "good" even-order harmonics
along with the odd harmonics, making it sound sweeter than a square wave.
Since clipping occurs only when a peak exceeds
the available headroom, it only occurs at high levels.
Crossover distortion, however, occurs at every zero
crossing of the waveform regardless of its peak
amplitude, so when it's present, it's always present.




Time-based distortion
So far we've only been talking about distortion in the amplitude domain,
but there's distortion in the time domain as well. Echo, reverberation, wow
and flutter are all time-based distortion effects, but they're physical in nature.
Either they can be fixed with a screwdriver or a big enough hammer, or we
know there just isn't a darn thing we can do about them. Let's look at some
more interesting time-based forms of distortion in electronics.
Filters work by applying a phase shift to the incoming signal. Since phase
shift, time, and frequency are intimately related, passing a complex signal
(one that contains more than a single frequency) through a filter results in
harmonics being delayed by differing amounts so they no longer have their
original time relationship with the fundamental frequency. The technical
name for this is "group delay" but we usually just call it "phase shift."
Because each harmonic is delayed by a different amount of time, the audible result is usually described as "smearing the transients." Figure 5 is a
grossly exaggerated illustration of a spectrogram of a single tone with a few
harmonics in a vertical line as they're supposed to be, and scattered as a
result of group delay. Group delay is often specified for RF filters, but,
although DSP designers are aware of it and work to minimize it, you won't
find it on an audio spec.
Another type of time-based distortion is Transient Intermodulation Distortion
(TIM) or slew rate distortion. Real-world physical components require a little
time to respond to a fast-changing input, and the time response to a theoretically instantaneous rise or fall of the input voltage. Slew rate, specified in volts
per microsecond, is the measure of this response time (see Figure 6). Slew rate,
frequency, gain, and maximum output voltage are all interrelated, but what it
boils down to is that the steep rise and fall of a transient can become mushy if
the slew rate isn't fast enough to reproduce it accurately.
Slew rate distortion in audio gear was first observed with the introduction
of early solid-state operational amplifiers. While today's op-amp ICs can
easily handle the full audio frequency range without slew rate limiting, inadequate slew rate of the early generation op amps is one of the reasons why
we have the "consumer" or "semi-pro" -10 dBV operating level. Reducing
the maximum output voltage made it possible to get acceptable performance
with the components available inexpensively in the late 1970s.
You'll occasionally find slew rate on a spec sheet, but it's up to you to do
the math and decide if it will be a problem for your application. In practice,
slew rate distortion, particularly for small signal devices, is negligible, but it
may contribute to the increase in THD at high frequencies.
"Good" distortion
Sometimes we appreciate a bit of distortion, like what we get with tube
equipment and analog tape. Both vintage and new tube gear generally measure under 0.2% THD when operated within their normal range, but this
gear (or its modern-day digital simulation) is very often used for the
increased amount of THD when driven hard and causing the electronics to
saturate.
Saturation is a little different from clipping. Rather than coming on hard as
clipping does, once you get above the linear range, saturation increases
gradually with input level. This is due to the non-linear relationship between
input and output that occurs when the circuit begins to saturate. In Figure 7,
note the smoothly rounded peaks of the input waveform, and the squashed
down but still rounded peaks of the saturated waveform. Incidentally, this saturated signal clocks in at about 5% THD, numerically the same as our
clipped illustration, but different sounding (and often worth twice the price).
Transformers and analog tape saturate due to the non-linearity of magnetization with increasing current. Tubes saturate because there are only so many
electrons available to carry current from the cathode to the plate. A tube's
bias is adjusted for the widest linear range of output to input, but eventually
you'll run out of electrons that you can push across the vacuum. For a unity
gain amplifier, in the linear range, a change of 1 volt at the input results in a
change of 1 volt at the output. In the saturated range, that 1 volt input change
might result in the output changing only by 0.5 volts. Sound a bit like what a
compressor does? Well, that's why they call it "tube compression."
Designers of the best analog tape decks worked hard to minimize distortion, which typically runs around 1% at the nominal operating level.
However, recording engineers have long used the technique of pushing the
level sent to tape to the point where it saturates, in order to achieve "that
analog tape sound."



Table of Contents for the Digital Edition of Recording - December 2015

Contents
Recording - December 2015 - Intro
Recording - December 2015 - Cover1
Recording - December 2015 - Cover2
Recording - December 2015 - 1
Recording - December 2015 - 2
Recording - December 2015 - 3
Recording - December 2015 - 4
Recording - December 2015 - 5
Recording - December 2015 - Contents
Recording - December 2015 - 7
Recording - December 2015 - 8
Recording - December 2015 - 9
Recording - December 2015 - 10
Recording - December 2015 - 11
Recording - December 2015 - 12
Recording - December 2015 - 13
Recording - December 2015 - 14
Recording - December 2015 - 15
Recording - December 2015 - 16
Recording - December 2015 - 17
Recording - December 2015 - 18
Recording - December 2015 - 19
Recording - December 2015 - 20
Recording - December 2015 - 21
Recording - December 2015 - 22
Recording - December 2015 - 23
Recording - December 2015 - 24
Recording - December 2015 - 25
Recording - December 2015 - 26
Recording - December 2015 - 27
Recording - December 2015 - 28
Recording - December 2015 - 29
Recording - December 2015 - 30
Recording - December 2015 - 31
Recording - December 2015 - 32
Recording - December 2015 - 33
Recording - December 2015 - 34
Recording - December 2015 - 35
Recording - December 2015 - 36
Recording - December 2015 - 37
Recording - December 2015 - 38
Recording - December 2015 - 39
Recording - December 2015 - 40
Recording - December 2015 - 41
Recording - December 2015 - 42
Recording - December 2015 - 43
Recording - December 2015 - 44
Recording - December 2015 - 45
Recording - December 2015 - 46
Recording - December 2015 - 47
Recording - December 2015 - 48
Recording - December 2015 - 49
Recording - December 2015 - 50
Recording - December 2015 - 51
Recording - December 2015 - 52
Recording - December 2015 - 53
Recording - December 2015 - 54
Recording - December 2015 - 55
Recording - December 2015 - 56
Recording - December 2015 - 57
Recording - December 2015 - 58
Recording - December 2015 - 59
Recording - December 2015 - 60
Recording - December 2015 - 61
Recording - December 2015 - 62
Recording - December 2015 - 63
Recording - December 2015 - 64
Recording - December 2015 - 65
Recording - December 2015 - 66
Recording - December 2015 - 67
Recording - December 2015 - 68
Recording - December 2015 - 69
Recording - December 2015 - 70
Recording - December 2015 - 71
Recording - December 2015 - 72
Recording - December 2015 - Cover3
Recording - December 2015 - Cover4
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