Distortion is any change to the original signal by a system. In sound reproduction, distortion is considered a flaw since it reduces the accuracy of reproduction by generating frequencies that were not included in the original content. This colors the sound and could make the music sounds impure, harsh, or muddy. It is also worth noting that distortion differs from noise by being dependent/related to the original signal, whereas noise is a random external/unrelated signal added to the original signal.
We perform harmonic and intermodulation distortion tests for TVs at 80 dB SPL and at Max SPL (i.e. the TV is set to maximum volume).
It is important to have a TV that produces low amounts of harmonic and intermodulation distortion when a clean and pure sound reproduction is desired. For example, in critical listening application or in situations where the sound reproduction is excepted to be full-spectrum and at loud volumes, like when watching an action movie in a large room, since TVs generally produce more distortion under heavier loads.
However, since moderate - and even in some cases - high amounts of harmonic and intermodulation distortion is not very audible to humans, most TVs should be considered good-enough in this regard. Except for extreme cases, their THD and IMD performance shouldn't be a deciding factor. Audible levels of distortion tend to deteriorate the sound reproduction by making it muddy, colored, or harsh.
Harmonic distortion is an overtone that is a whole number multiple of a fundamental frequency. In other words, it is an unintended frequency which is an integer multiple of an intended frequency fed to the system. Total harmonic distortion (THD) is the ratio of the sum of the powers (RMS amplitude) of all the harmonics to the power (RMS amplitude) of the fundamental frequency. Inharmonic distortion differs from harmonic distortion by being an overtone that is not an integer multiple of the fundamental frequency. For example, if the original signal is a 100Hz sine wave, a system with second-order harmonic distortion would output a 200Hz tone in addition to the original 100Hz signal, and a system with inharmonic distortion would output a non-integer multiple, like 273Hz, in addition to the input.
The test signal for our THD measurements is the same as our frequency response test signal. It is a 16-bit/48KHz 20-second sine wave swept at -6dB FS (RMS) between 10Hz and 22KHz. The TV is placed on a table, as close as possible to the back wall, and is connected to the test PC through HDMI. The measurements are recorded using a calibrated Dayton Audio EMM-6 microphone, placed at the optimum viewing distance for each TV's size, and is connected to a Focusrite Scarlett 2i2 audio interface. The signal level is calibrated post-compensation (i.e. after being flattened by applying the target response) using a pink noise limited between 500Hz and 2KHz. The resulting sound pressure level is measured and calibrated with a GalaxyAudio CM-140 SPL meter, which is set to C-weighting and Slow.
The measurements are performed at two different intensity levels; 80dB SPL and Max SPL. Due to the sample-rate of 48KHz and the test signal being limited to 22KHz, the THD results are capped at 10KHz, since harmonics of higher frequencies would fall outside of the test bandwidth.
The THD (total harmonic distortion) of the 80dB SPL pass is calculated by our test software (Room EQ Wizard), as a percentage of the fundamental frequency's power and is exported for further calculations. Since speakers tend to produce more distortion in lower frequencies and human hearing is less sensitive to low-frequency harmonic distortion, a perceptual weighting filter is applied to the THD calculations which gives as much as 20x less weight to the lower frequencies compared to the higher frequencies. The final THD value is derived by calculating the variance of the weighted THD response.
The THD (total harmonic distortion) of the Max SPL pass is measured the same way as the 80dB SPL, but at the maximum volume setting of the television.
Intermodulation distortion (IMD) differs from THD by being produced when the device is excited by two frequencies rather than one as in THD. The resulting distortion frequencies will occur at the sum and difference frequencies of the original frequencies,, and at sums and differences of multiples of those frequencies. For example, if a 100Hz and a 105Hz signal are fed to a system, the intermodulation distortions will occur at 110Hz and 95Hz (and their multiples).
Our IMD test is performed with the same procedure as the THD test but with two sets of test signals. The first is a DIN (250Hz & 8KHz) dual tone with a 4:1 ratio at -9dB FS (RMS), and the second is a CCIF (19KHz & 20KHz) dual tone with a 1:1 ratio at -12dB FS (RMS).
The intermodulation distortion results for DIN and CCIF tones at 80dB SPL are calculated by REW (Room EQ Wizard) as a percentage of the fundamental frequency power and are then averaged to get the final IMD @ 80 value.
The intermodulation distortion at Max SPL are measured in the same way as the 80dB SPL, but at the maximum volume setting of the television.
Although THD and IMD have been a staple of audio measurements and specifications, studies have shown that there is only a moderate correlation between the THD/IMD response of a device and its perceived audio fidelity1, 2. Depending on the ratios between the different harmonics produced by a device, auditory masking, and other psychoacoustic effects, it is possible for a TV with high levels of THD/IMD to produce very little audible effects. It is also possible for a TV with audible levels of distortion (or compression and pumping) to have a typical THD/IMD performance. This could also be due to the fact that harmonic distortion is measured with a single tone sweep, and IMD with a dual tone, neither of which put the TVs under as much load as a full-spectrum, bass-heavy piece of music or movie. Additionally, some TVs are able to perform very well in THD at Max SPL simply due to being volume-limited, so they won't get loud enough to put the TV under heavy load and produce high distortion.
Other methods for measuring distortion such as multi-tone distortion and non-coherent distortion have been proposed and studied, and among these, non-coherent distortion has significantly outperformed THD. There have also been new methods developed for interpreting THD, such as the GedLee metric, which incorporate auditory masking and other psychoacoustic phenomena into their calculations. We have plans for adding such measurements to our distortion tests in the future.