atomicbob
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Trust the numbers? Maybe not so much.
Another Grim Fairytale on why I hate numerical specifications for audio components, especially THD+N and THD.
This is a brief, visually oriented tutorial.
Prerequisite reading:
Tutorial - Interpreting THD and THD+N specifications and graphs
https://www.head-fi.org/threads/tutorial-–-interpreting-thd-and-thd-n-graphs.885876/
Please read the above and familiarize yourself with the various information available on the graphs that will be presented below. I don’t wish to resort to eight-by-ten color glossy photographs with circles and arrows and a paragraph on the back of each one explaining what each one is (kudos to those who know this reference.)
Here are THD+N measurements for six hypothetical amplifiers, left and right channels:
Each amplifier measures very close to 0.01%. Given these measurements all six amps should be identical and sound the same, correct? No! While the THD+N numbers are almost identical, each one was derived from a different set of conditions occurring in the respective amplifiers. The resulting numbers are an oversimplification of the complex spectrums from which they were computed.
In reality unit variance from the production line is larger than those measured for our six hypothetical simulated amplifiers. The purpose here is to demonstrate how the same measurement number may be achieved in a variety of ways.
In each spectrum graph that follows, the second highest spike from the 1 KHz stimulus will dominate the measurements except the last one. Please note the dBu y-axis, which is held constant between graphs.
BS-1
Our first hypothetical amplifier has a 60 Hz mains spike dominating the THD+N measurement. Note that THD and harmonics are quite low. Easily observed in the spectrum above.
BS-2
The second amplifier has a 2nd harmonic dominating the THD+N and THD measurements. No AC mains noise to be seen.
BS-3
Our third amplifier THD+N and THD measurements are similar to the 2nd amplifier but now are dominated by a 3rd harmonic.
BS-4
The BS-4 distortion is dominated with a 4th harmonic for both THD+N and THD measurements.
BS-5
Our 5th amplifier has a 180 Hz mains noise harmonic dominating the THD+N measurement.
BS-6
The last of our hypothetical amplifiers has residual broadband noise dominating the THD+N measurement and contributing to a slightly higher than expected THD though there are no specific harmonics clearly responsible.
From the preceding it is obvious numeric representations of complex spectrum information is a tremendous oversimplification. These amplifiers each have a specific sonic signature differing from all others. They will sound different to those with trained listening skills. Yet their respective THD+N measured nearly identical.
In a similar fashion, the BS-2, BS-3 and BS-4 all measured close to 0.01% THD. Yet each one had a different harmonic dominating the THD measurement. Based on empirical listening evidence, the BS-2 might be described as warm sounding, the BS-3 as bright and the BS-4 having something about the sound that is fatiguing, possibly sour sounding. Please be aware these descriptions are highly subjective. However, the measurements are not.
Some general observations. The BS-1 has a lone 60 Hz mains component contributing to the THD+N, which is likely not to be noticed by most listeners in average ambient conditions. The BS-5 has a 180 Hz mains harmonic which is more likely to be heard due to increased sensitivity of the Human Auditory System (see Fletcher-Munson curves) at this frequency. As described in the previous paragraph the BS-2, 3 and 4 are all THD dominated with their respective subjective traits. Finally the BS-6 has broadband noise which will be readily apparent as hiss, especially to IEM listeners.
Hopefully this tutorial will raise awareness of how some numeric representations are just short of worthless given the drastic differences presented above resulting in almost identical computed results.
Major take-aways from this tutorial:
1. don't use a single numeric metric as a goodness indicator when comparing audio components
2. numbers typically represent oversimplifications of more complex data observed in spectrum graphs
3. learn what purpose a measurement serves, methods used to perform, and how to interpret results
For a great audio / visual by Avermetrics on this topic look at this video:
Further reading:
Fundamentals of Modern Audio Measurements, Richard C. Cabot
http://www.aes.org/e-lib/browse.cfm?elib=7080
*edit* - corrected video link, added take-away message
Another Grim Fairytale on why I hate numerical specifications for audio components, especially THD+N and THD.
This is a brief, visually oriented tutorial.
Prerequisite reading:
Tutorial - Interpreting THD and THD+N specifications and graphs
https://www.head-fi.org/threads/tutorial-–-interpreting-thd-and-thd-n-graphs.885876/
Please read the above and familiarize yourself with the various information available on the graphs that will be presented below. I don’t wish to resort to eight-by-ten color glossy photographs with circles and arrows and a paragraph on the back of each one explaining what each one is (kudos to those who know this reference.)
Here are THD+N measurements for six hypothetical amplifiers, left and right channels:
Each amplifier measures very close to 0.01%. Given these measurements all six amps should be identical and sound the same, correct? No! While the THD+N numbers are almost identical, each one was derived from a different set of conditions occurring in the respective amplifiers. The resulting numbers are an oversimplification of the complex spectrums from which they were computed.
In reality unit variance from the production line is larger than those measured for our six hypothetical simulated amplifiers. The purpose here is to demonstrate how the same measurement number may be achieved in a variety of ways.
In each spectrum graph that follows, the second highest spike from the 1 KHz stimulus will dominate the measurements except the last one. Please note the dBu y-axis, which is held constant between graphs.
BS-1
Our first hypothetical amplifier has a 60 Hz mains spike dominating the THD+N measurement. Note that THD and harmonics are quite low. Easily observed in the spectrum above.
BS-2
The second amplifier has a 2nd harmonic dominating the THD+N and THD measurements. No AC mains noise to be seen.
BS-3
Our third amplifier THD+N and THD measurements are similar to the 2nd amplifier but now are dominated by a 3rd harmonic.
BS-4
The BS-4 distortion is dominated with a 4th harmonic for both THD+N and THD measurements.
BS-5
Our 5th amplifier has a 180 Hz mains noise harmonic dominating the THD+N measurement.
BS-6
The last of our hypothetical amplifiers has residual broadband noise dominating the THD+N measurement and contributing to a slightly higher than expected THD though there are no specific harmonics clearly responsible.
From the preceding it is obvious numeric representations of complex spectrum information is a tremendous oversimplification. These amplifiers each have a specific sonic signature differing from all others. They will sound different to those with trained listening skills. Yet their respective THD+N measured nearly identical.
In a similar fashion, the BS-2, BS-3 and BS-4 all measured close to 0.01% THD. Yet each one had a different harmonic dominating the THD measurement. Based on empirical listening evidence, the BS-2 might be described as warm sounding, the BS-3 as bright and the BS-4 having something about the sound that is fatiguing, possibly sour sounding. Please be aware these descriptions are highly subjective. However, the measurements are not.
Some general observations. The BS-1 has a lone 60 Hz mains component contributing to the THD+N, which is likely not to be noticed by most listeners in average ambient conditions. The BS-5 has a 180 Hz mains harmonic which is more likely to be heard due to increased sensitivity of the Human Auditory System (see Fletcher-Munson curves) at this frequency. As described in the previous paragraph the BS-2, 3 and 4 are all THD dominated with their respective subjective traits. Finally the BS-6 has broadband noise which will be readily apparent as hiss, especially to IEM listeners.
Hopefully this tutorial will raise awareness of how some numeric representations are just short of worthless given the drastic differences presented above resulting in almost identical computed results.
Major take-aways from this tutorial:
1. don't use a single numeric metric as a goodness indicator when comparing audio components
2. numbers typically represent oversimplifications of more complex data observed in spectrum graphs
3. learn what purpose a measurement serves, methods used to perform, and how to interpret results
For a great audio / visual by Avermetrics on this topic look at this video:
Further reading:
Fundamentals of Modern Audio Measurements, Richard C. Cabot
http://www.aes.org/e-lib/browse.cfm?elib=7080
*edit* - corrected video link, added take-away message
Last edited: