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THD and IMD Distortion—Sidebar

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The term “distortion” is used pretty frequently in audio, but it’s not understood particularly well. There are different kinds of distortion, and some is more audibly bothersome than others.

By pure definition, if the output of an audio device differs in any way from the input, that difference is “distortion,” because theoretically, the device’s output should be a perfect mirror of the input signal. Nothing more, nothing less.

Well, unfortunately, this is the real world, and except for former Heavyweight Champion Rocky Marciano’s

49-0 record, nothing is perfect. Audio equipment produces distortion, and this distortion falls into two main categories—harmonic distortion and intermodulation distortion.

We’ll explain both kinds

(in simple terms, don’t worry!), and then you’ll know more than your friends.

Harmonic Distortion

This distortion is defined as unintended signal products generated by an audio product that are whole number multiples of the original signal. For example, if an audio device is asked to reproduce a 40Hz signal, and instead produces 40Hz and a small amount of 80Hz + 120Hz + 160Hz + 200Hz, the 80Hz + 120Hz + 160Hz + 200Hz product is called harmonic distortion. Small amounts of distortion close in multiples to the original signal (“lower-order harmonics,” like 80Hz) are barely audible; larger amounts of distortion in greater multiples far away from the original signal (“upper-order harmonics,” like 200Hz) are grossly objectionable to the human ear.

The sum total of all harmonic distortion products is expressed as a percentage of the original signal, or % Total Harmonic Distortion (% THD).

The human ear is most sensitive to midrange frequencies, so small amounts of distortion (less than 1%) in the 500-2000Hz range are clearly audible.

We’re pretty insensitive to harmonic distortion in the bass range, so even 5–10% THD in the 20–60Hz region is usually not objectionable, as long as the device (such as a subwoofer) is not exhibiting other forms of audible misbehavior as well (mechanical scraping, buzzing, port ‘chuffing’, etc.).

(See figure 1.)

 Distortion-FIgure1.jpg

Illustrations courtesy of the Joshua Cooper Company

Intermodulation Distortion

This is a bad one, because—unlike THD—it’s not harmonically-related to the original signal at all, so even very small amounts are audibly objectionable. IM distortion is when the distortion products occur at frequencies that are sums and differences of the original input signal. For example, if the input is 500 Hz and 2200 Hz, then the IM distortion products will occur at 1700 Hz and 2700 Hz. These frequencies have no musical relationship to the original frequencies at all, so the sound is very harsh and discordant.

(See figure 2.)

 Distortion-FIgure2.jpg

Illustrations courtesy of the Joshua Cooper Company

 

Now when you read those THD and IM distortion specifications, you’ll know what they mean.

A Note about THD + N vs IMD Distortion Testing

By. Gene DellaSala

Back in the early days, amplifier power over a wide bandwidth was lacking by today’s standards.  Output devices switched much slower and thus were subject to slew induced distortion (high distortion at high power due to decreased product bandwidth).  As a result IMD testing was often the best way to measure and characterize this behavior since it was a more revealing way of testing bandwidth limited devices at their upper frequency limits.  THD+N tests would be unrevealing in these cases since the generated harmonics would exceed the bandwidth of the Device Under Test (DUT).  A good example would be a DAC whose sampling frequency is 8kHz and you'd want to test how accurate it was right up to its usable bandwidth which in this case would be 4kHz (1/2 Nyquist) and clearly within the range of human hearing (20kHz).  When testing highly bandwidth limited systems containing A/D and/or D/A convertors, IMD testing (using DFD or twin-tone) is the ONLY way to measure non-linearity above about 50% of the system bandwidth.  THD+N measurements above 50% of the system bandwidth will simply reveal nothing about non-linearity because the harmonics will fall outside of the system bandwidth.  

Today we have much better (ie. wider bandwidth, faster) output transistors which allow audio amplifiers to have extremely wide bandwidth (>100kHz) octaves above the limits of human hearing.  Thus, THD+N testing can typically give us all of the relevant data we need if done properly.  This involves testing the amplifier full bandwidth (20Hz to 20kHz) at low and high power to determine consistency of performance.  If bandwidth is preserved and the distortion remains low (< 1% at rated power) than this is usually a good indication that the amplifier will also perform similarly well under IMD testing.  It’s also very useful to check amplifier distortion using a FFT analysis (1kHz fundamental) and observing the spectral content from the subsequent harmonics generated or any ill behavior caused by ground  loops or unwanted out of band noise.   

 

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Recent Forum Posts:

onthebus posts on May 12, 2022 23:04
davidacookie, post: 1547155, member: 98370
This article is fantastic.

I won’t joint the discussion about which receiver should be included or not as I only experienced some of those mentioned in Hi Fi shops and by the time, I could afford to buy any of them they had been superseded by later models.

The article and the forum discussion has jumped me right back to 1974.

In 1974 I was fifteen and still in school.

We had an old Philips Radiogram; I think we called it. A bit like a piece of furniture with a radio, record player and speakers built in, nice sound when listening to 45s or 78s but it wasn’t hifi.

So in 1974 my dad went and bought a Toshiba SA300 receiver, a Sony cassette recorder\player and a UK brand turntable, don’t remember the name.

OK, I know the Toshiba can’t compare to some of the models mentioned above but the sound from it was incredibly clear even at almost full volume.

It only put out 14 watts\ ch and had 0.8% THD according to the user manual.

Just to set the scene…I lived in Dublin, Ireland.

We didn’t have basements in most houses so we couldn’t put the system there and most Irish houses had fairly small rooms so putting a 50 watt \ ch system wasn’t practical and they were relatively expensive back then.

We couldn’t turn it up any further than two thirds volume as the neighbours would complain once or twice, third time they would phone the police.

Though many years later one of the neighbour’s sons told me they used to love listening to our music coming through the walls.

For about eight years all the crowd I hung around with would gather in our house and music played all day and night, sometimes into the next morning.

I moved out in 1984 and Dad gave the Toshiba SA300 to a relative.

The volume knob had started to give trouble, scratchy noise when you changed volume, and we couldn't get replacement pots. I think they were wire wound pots and were difficult to source.

My Dad replaced it with an early Sony home theatre system and always regretted it.

The Sony was fine for music but he always complained the Sony was bad for hearing dialogue on a TV programme and we could never seem to get it right.

He was sorry he gave away the Toshiba and later replaced the Sony home theatre system with a Pioneer 2.1 receiver. He had no problems hearing dialogue on the Pioneer.

In hindsight I think Toshiba were trying to create a Pioneer “clone” although at a much reduced power output and a lower price.

A few guys on YouTube have put up videos of Toshiba SA300 receivers and they seem quite impressed with them. See links below

54724
Toshiba SA-300 L Japan 1973 - YouTube

Toshiba SA-300 Mid 1970's Stereo AM/FM Receiver Demonstration - YouTube
Thanks again for the article, I now have a Denon system and really like it..but I would love the old Toshiba
Just picked up a pretty mint SA 300 today for free from an elderly neighbour, it’s
davidacookie, post: 1547155, member: 98370
This article is fantastic.

I won’t joint the discussion about which receiver should be included or not as I only experienced some of those mentioned in Hi Fi shops and by the time, I could afford to buy any of them they had been superseded by later models.

The article and the forum discussion has jumped me right back to 1974.

In 1974 I was fifteen and still in school.

We had an old Philips Radiogram; I think we called it. A bit like a piece of furniture with a radio, record player and speakers built in, nice sound when listening to 45s or 78s but it wasn’t hifi.

So in 1974 my dad went and bought a Toshiba SA300 receiver, a Sony cassette recorder\player and a UK brand turntable, don’t remember the name.

OK, I know the Toshiba can’t compare to some of the models mentioned above but the sound from it was incredibly clear even at almost full volume.

It only put out 14 watts\ ch and had 0.8% THD according to the user manual.

Just to set the scene…I lived in Dublin, Ireland.

We didn’t have basements in most houses so we couldn’t put the system there and most Irish houses had fairly small rooms so putting a 50 watt \ ch system wasn’t practical and they were relatively expensive back then.

We couldn’t turn it up any further than two thirds volume as the neighbours would complain once or twice, third time they would phone the police.

Though many years later one of the neighbour’s sons told me they used to love listening to our music coming through the walls.

For about eight years all the crowd I hung around with would gather in our house and music played all day and night, sometimes into the next morning.

I moved out in 1984 and Dad gave the Toshiba SA300 to a relative.

The volume knob had started to give trouble, scratchy noise when you changed volume, and we couldn't get replacement pots. I think they were wire wound pots and were difficult to source.

My Dad replaced it with an early Sony home theatre system and always regretted it.

The Sony was fine for music but he always complained the Sony was bad for hearing dialogue on a TV programme and we could never seem to get it right.

He was sorry he gave away the Toshiba and later replaced the Sony home theatre system with a Pioneer 2.1 receiver. He had no problems hearing dialogue on the Pioneer.

In hindsight I think Toshiba were trying to create a Pioneer “clone” although at a much reduced power output and a lower price.

A few guys on YouTube have put up videos of Toshiba SA300 receivers and they seem quite impressed with them. See links below

54724
Toshiba SA-300 L Japan 1973 - YouTube

Toshiba SA-300 Mid 1970's Stereo AM/FM Receiver Demonstration - YouTube
Thanks again for the article, I now have a Denon system and really like it..but I would love the old Toshiba
just picked up a pretty mint SA300 today from an elderly neighbour, it’s a black faced one though. Sounds sweet. Was yours silver or black?
davidacookie posts on March 20, 2022 18:17
This article is fantastic.

I won’t joint the discussion about which receiver should be included or not as I only experienced some of those mentioned in Hi Fi shops and by the time, I could afford to buy any of them they had been superseded by later models.

The article and the forum discussion has jumped me right back to 1974.

In 1974 I was fifteen and still in school.

We had an old Philips Radiogram; I think we called it. A bit like a piece of furniture with a radio, record player and speakers built in, nice sound when listening to 45s or 78s but it wasn’t hifi.

So in 1974 my dad went and bought a Toshiba SA300 receiver, a Sony cassette recorder\player and a UK brand turntable, don’t remember the name.

OK, I know the Toshiba can’t compare to some of the models mentioned above but the sound from it was incredibly clear even at almost full volume.

It only put out 14 watts\ ch and had 0.8% THD according to the user manual.

Just to set the scene…I lived in Dublin, Ireland.

We didn’t have basements in most houses so we couldn’t put the system there and most Irish houses had fairly small rooms so putting a 50 watt \ ch system wasn’t practical and they were relatively expensive back then.

We couldn’t turn it up any further than two thirds volume as the neighbours would complain once or twice, third time they would phone the police.

Though many years later one of the neighbour’s sons told me they used to love listening to our music coming through the walls.

For about eight years all the crowd I hung around with would gather in our house and music played all day and night, sometimes into the next morning.

I moved out in 1984 and Dad gave the Toshiba SA300 to a relative.

The volume knob had started to give trouble, scratchy noise when you changed volume, and we couldn't get replacement pots. I think they were wire wound pots and were difficult to source.

My Dad replaced it with an early Sony home theatre system and always regretted it.

The Sony was fine for music but he always complained the Sony was bad for hearing dialogue on a TV programme and we could never seem to get it right.

He was sorry he gave away the Toshiba and later replaced the Sony home theatre system with a Pioneer 2.1 receiver. He had no problems hearing dialogue on the Pioneer.

In hindsight I think Toshiba were trying to create a Pioneer “clone” although at a much reduced power output and a lower price.

A few guys on YouTube have put up videos of Toshiba SA300 receivers and they seem quite impressed with them. See links below

54724
Toshiba SA-300 L Japan 1973 - YouTube

Toshiba SA-300 Mid 1970's Stereo AM/FM Receiver Demonstration - YouTube
Thanks again for the article, I now have a Denon system and really like it..but I would love the old Toshiba
tonyE posts on August 24, 2019 18:22
MrBoat, post: 1335169, member: 80705
….

I bought a 100 watt Fisher rack system in the 80's and never looked back from there. Attached to that, was some of the most enjoyable listening ever, a whole lot of experience gained with things like EQ effects, and other ways to further enhance sound. Come 2019, that experience has proven invaluable, with finding the truth and fiction with marketers today, with somewhere north of 90% of available information being complete, and utter BS.
….

Gulp… from ‘83 to ’88 I sort of stopped listening to music, even though I had a very fine top of the line Sony Super Beta HiFi and a matching Laserdisk… I dumped the audio receiver and used an all Sony ES system driving ADS car amplifiers with a BIG 12VDC power supply into my ADS L810s. I guess the sound of CDs sucked. I used my Kenwood integrated as phono preamp for my then Dual/Grado turntable.

Sometime in ‘88 I got a used Linn LP12, Conrad Johnson PV9, Audio Research D70 MkII and a pair of Acoustic Energy AE1s. I do own some classic receivers, but I don’t drive them in a serious audio system.

Once you go to separates, it's very hard to go back to an all in one. Even though my receivers are gorgeous. Heck, my High End audio stuff looks boring, specially my all black, no switch, class A FET mono amps. I bought a Radio Shack APM-200 power analog meter thingie… in very good shape. My kids think it's a hoot that the cheapest thing in the stereo, cheaper than my cheapest interconnect, is the one thing we end up looking at most.

I did own an equalizer once… it just made things worse.
MrBoat posts on August 24, 2019 16:42
tonyE, post: 1335034, member: 89479
And let's not even discuss those solid state receiver monstrosities from the early 80s. Yuck.

In spite those being mid-fi at best, they managed to match or exceed the recording quality of most popular music of that era and before. What 75% of the planet was listening to. Those Japanese engineers knew what they were doing. The least of which was bring affordable, and better sound quality to the masses. I grew up with my parents systems with names like RCA, Zenith, etc., which was so horrid that the speakers came essentially blown before the first signal ever passed through them. House brand electronics from Sears, M. Ward, Kmart etc., were what was affordable before the Japanese figured out the next step up from that, which was up and coming audiophiles blessings, truth be told.

I bought a 100 watt Fisher rack system in the 80's and never looked back from there. Attached to that, was some of the most enjoyable listening ever, a whole lot of experience gained with things like EQ effects, and other ways to further enhance sound. Come 2019, that experience has proven invaluable, with finding the truth and fiction with marketers today, with somewhere north of 90% of available information being complete, and utter BS.

ETA: With many of those systems being quite functional nearly 40 years later, it tends to discount the notion of them being low quality.
tonyE posts on August 23, 2019 23:58
Marantz 2325, 4215 Sansui G7500, Kenwood separates, etc.. etc.. that, and lots more in the closet. Those were indeed the days. Yet, as beautiful as those receivers were, stuff like Nelson Pass and Audio Research were still ahead.

And let's not even discuss those solid state receiver monstrosities from the early 80s. Yuck.

BTW- I'm really surprised you didn't bring up the Sony VFET TA-5650 and 5450! Those were (are still) likely some of the best sounding amplifiers of all time. Just make sure to replace the kamikaze diodes.
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