Recording - December 2015 - 63
3a
3b
4a
its effect depends on what's coming out in addition to what we put
in, which may not be simply harmonics of a single test frequency.
A better way than expressing distortion simply as a number
is to show a spectrum plot of the output for a given test frequency. You'll rarely see this on a manufacturer's spec sheet,
but it's starting to show up in some product reviews.
Figure 1 shows the output spectrum of an audio interface that
I reviewed recently. The large spike near the center of the graph
is the 500 Hz test frequency, and the other three most significant spikes are its first three harmonics. The largest harmonics
are in the neighborhood of 100 dB below the fundamental
(about 0.001 %), and other distortion products are so low as to
be insignificant. The spec sheet for this device lists THD+N as
-96 dB measured at 1 kHz, -1 dBFS over a 20 kHz bandwidth,
so this specification passes the Pinocchio test-remember
Pinocchio, whose nose grew longer every time he told a lie? A
proper harmonic distortion will state %THD+N, the signal level,
gain, and bandwidth.
Intermodulation distortion
Harmonic distortion consists solely of frequencies that are
harmonics (multiples) of the source. Since most of those harmonics are already present in music and speech, small amounts
of harmonic distortion don't color the sound very much.
Intermodulation distortion (IM or IMD), however, is nastier. IM
creates new frequencies in the output that are not harmonically
related to the input-there's nothing musical or pleasant-sounding about IM. It's usually perceived as buzziness or mushiness.
IM is measured by applying a test signal to a device that consists of two different tones mixed together. Due to non-linearity in
the device, frequencies that are the sum and difference of the
two test frequencies are created. Since harmonic distortion can
cause the test frequencies to generate harmonics of themselves,
we can end up with a family of new frequencies including sums
and differences of the more than just the two test frequencies.
Like THD, IM is expressed as a percentage by measuring what
remains after the test frequencies are filtered out.
There are a few standard test methods. Audio amplifiers are
typically measured using the SMPTE method: 60 Hz and 7 kHz
tones mixed with the 7 kHz tone 12 dB lower than the 60 Hz
tone. The ITU-R method is more commonly used for measuring
A/D and D/A converters, which is where IM is likely to be present in modern studio gear. This method uses two tones of equal
amplitude spaced 1 kHz apart, typically 19 and 20 kHz for 48
kHz sample rate or a bit lower for a 44.1 kHz system.
In the IMD plot shown in Figure 2, the test frequencies are 18
and 19 kHz, both at -6 dBFS. Spikes immediately above and
below them (19 and 17 kHz) are the test frequencies plus and
minus their difference. What's more interesting are the spikes at
1 kHz and 2 kHz, the difference and 2nd harmonic of the difference between the two test frequencies. These represent the
unpleasant sound of IM. Generally the output is lowpass filtered
above the lowest few distortion products, so the high frequency
sum and difference is ignored. A proper IM specification will
look like: Less than 0.02%, 18/19 kHz, -6 dBFS, ITU-R.
4b
Loudspeakers are the most notorious IM sources because a
certain amount of mixing is inevitable in the mechanical motion
of the speaker cone. However, good electronics should have IM
on the order of a few hundredths of a percent.
Clipping
Clipping is a major (and usually avoidable) source of harmonic distortion. Clipping can sometimes be used creatively if it makes
what you're recording sound better. Most input devices such as
mic preamps, recorders, and computer audio interfaces have an
indicator that tells you when you're at or near the clipping level.
The specification that covers this is maximum input level, which we
addressed in the previous article in this series (July 2015).
In reviewing handheld recorders and interfaces in the past several years, I've observed a sneaky form of clipping that won't appear
in the spec sheet, and that's with the device's analog output stage.
If the line output level is fixed, as it typically is with an interface,
you're OK, but if there's an output volume control as there nearly
always is for a headphone output, if you have a recording near full
scale, turning the headphone volume up beyond a certain point will
5
Frequency
chop off high frequencies within the audible bandwidth, it also
cuts low frequencies, reducing the contribution of power supply
hum to the THD+N measurement.
THD can also vary with frequency, particularly for large signal
devices. A good spec sheet for a power amplifier will include a
plot of THD vs. frequency for both high and low signal levels.
THD of most of today's solid state small signal gear tends to be
in the ballpark of a few hundredths of a percent, with power amplifiers getting up to maybe a tenth of a percent when pushed near
their limit. Back in the bad old days, less than 1% THD was considered high fidelity, but with the advent of lower-distortion devices,
we've learned that the amount of distortion it takes in order to hear
Time
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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