The upper right-hand plot shows the log-spectrum of the output waveform. The noise floor for the system can be seen along the bottom of the plot. The THD figure is not really affected by the presence of noise, having a value similar to that shown in Figure 1. IMD is measured by using the sum of two or more sinusoids as an input signal. Typically, the frequencies combined in the input are not harmonically related.
With multiple frequencies present at the input, system nonlinearities produce distortion products at sums and differences of multiples of the input frequencies. Thus, rather than being harmonically related, IMD components are separated by the lower of the two input frequencies.
This is convenient, because it allows for examination of many distortion components in a narrow bandwidth. With THD techniques, it is necessary to use low frequency input signals to create multiple distortion components in a narrow band. If the system being measured does not support low frequencies, THD measurements then become difficult.
Total Harmonic Distortion. What is THD and how is it measured? Figure 1: Block diagram for flanger and phaser. Figure 2: Transfer function for flanger's delay element. Since individual harmonic amplitudes are measured, the manufacturer must state the test signal frequency , its level , and the gain conditions set on the tested unit, as well as the number of harmonics measured.
And more different yet, if one manufacturer measures two harmonics while another measures five. For all signal processing equipment, except mic preamps , the preferred gain setting is unity. For mic pre amps, the standard practice is to use maximum gain. Too often THD is spec'd only at 1 kHz, or worst, with no mention of frequency at all, and nothing about level or gain settings, let alone harmonic count.
Similar to the THD test above, except instead of measuring individual harmonics this tests measures everything added to the input signal. This is a wonderful test since everything that comes out of the unit that isn't the pure test signal is measured and included -- harmonics, hum, noise, RFI, buzz Distortion analyzers make this measurement by removing the fundamental using a deep and narrow notch filter and measuring what's left using a bandwidth filter typically 22 kHz, 30 kHz or 80 kHz.
The remainder contains harmonics as well as random noise and other artifacts. Weighting filters are rarely used. When they are used, too often it is to hide pronounced AC mains hum artifacts. The preferred value is a 20 kHz or 22 kHz measurement bandwidth, and "flat," i. One argument goes: it makes no sense to measure THD at 20 kHz if your measurement bandwidth doesn't include the harmonics. Valid point, and one supported by the IEC, which says that THD should not be tested any higher than 6 kHz, if measuring five harmonics using a 30 kHz bandwidth, or 10 kHz, if only measuring the first three harmonics.
Another argument states that since most people can't even hear the fundamental at 20 kHz, let alone the second harmonic, there is no need to measure anything beyond 20 kHz. Fair enough. However, the case is made that using an 80 kHz bandwidth is crucial, not because of 20 kHz harmonics, but because it reveals other artifacts that can indicate high frequency problems. A more meaningful test than THD, intermodulation distortion gives a measure of distortion products not harmonically related to the pure signal.
This is important since these artifacts make music sound harsh and unpleasant. Intermodulation distortion testing was first adopted in the U. The test signal is a low frequency 60 Hz and a non-harmonically related high frequency 7 kHz tone, summed together in a amplitude ratio.
Vibration and Sound
This signal is applied to the unit, and the output signal is examined for modulation of the upper frequency by the low frequency tone. As with harmonic distortion measurement, this is done with a spectrum analyzer or a dedicated intermodulation distortion analyzer. The modulation components of the upper signal appear as sidebands spaced at multiples of the lower frequency tone. The amplitudes of the sidebands are rms summed and expressed as a percentage of the upper frequency level.
SMPTE specifies this test use 60 Hz and 7 kHz combined in a 12 dB ratio and that the peak value of the signal be stated along with the results.
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However, measuring the peak value is difficult. This tests for non-harmonic nonlinearities, using two equal amplitude, closely spaced, high frequency tones, and looking for beat frequencies between them. Use of beat frequencies for distortion detection dates back to work first documented in Germany in , but was not considered a standard until , when the CCIF International Telephonic Consultative Committee recommend the test. A mistake compounded by the many correct audio references to the CCI R weighting filter. The common test signal is a pair of equal amplitude tones spaced 1 kHz apart.
Nonlinearity in the unit causes intermodulation products between the two signals. These are found by subtracting the two tones to find the first location at 1 kHz, then subtracting the second tone from twice the first tone, and then turning around and subtracting the first tone from twice the second, and so on. Usually only the first two or three components are measured, but for the oft-seen case of 19 kHz and 20 kHz, only the 1 kHz component is measured.
Many variations exist for this test. Therefore, the manufacturer needs to clearly spell out the two frequencies used, and their level. The ratio is understood to be This specification indirectly tells you how noisy a unit is. This figure is used to calculate a ratio between it and a fixed output reference signal, with the result expressed in dB. No input signal is used, however the input is not left open, or unterminated.
The usual practice is to leave the unit connected to the signal generator with its low output impedance set for zero volts.
Total Harmonic Distortion
Alternatively, a resistor equal to the expected driving impedance is connected between the inputs. The magnitude of the output noise is measured using an rms-detecting voltmeter. Noise voltage is a function of bandwidth -- wider the bandwidth, the greater the noise. This is an inescapable physical fact. Thus, a bandwidth is selected for the measuring voltmeter. If this is not done, the noise voltage measures extremely high, but does not correlate well with what is heard.
The most common bandwidth seen is 22 kHz the extra 2 kHz allows the bandwidth-limiting filter to take affect without reducing the response at 20 kHz. This is called a "flat" measurement, since all frequencies are measured equally. Alternatively, noise filters, or weighting filters, are used when measuring noise. This filter is preferred because it shapes the measured noise in a way that relates well with what's heard. Pro audio equipment often lists an A-weighted noise spec -- not because it correlates well with our hearing -- but because it can "hide" nasty hum components that make for bad noise specs.
Always wonder if a manufacturer is hiding something when you see A-weighting specs. While noise filters are entirely appropriate and even desired when measuring other types of noise, it is an abuse to use them to disguise equipment hum problems. A-weighting rolls off the low-end, thus reducing the most annoying 2 nd and 3 rd line harmonics by about 20 dB and 12 dB respectively.
Sometimes A-weighting can "improve" a noise spec by 10 dB. Fletcher-Munson curves document equal loudness of single tones. Their curve tells us nothing of the ear's astonishing ability to sync in and lock onto repetitive tones -- like hum components -- even when these tones lie beneath the noise floor. This is what A-weighting can hide. This follows the semiconductor industry's practice of spec'ing delta-sigma data converters A-weighted. They do this because they use clever noise shaping tricks to create bit converters with acceptable noise behavior.
All these tricks squeeze the noise out of the audio bandwidth and push it up into the higher inaudible frequencies. The noise may be inaudible, but it is still measurable and can give misleading results unless limited.