darTZeel NHB-108 model one User Manual

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darTZeel NHB-108 model one User Manual, version B 1.0 Page 22 of 35strumentation measurement, assuming this resultnecessarily leads to outstanding sound purity. Wetake this with some caution, since it recalls for usthe 70-80' Japanese philosophy, by which "per-formance" implies "quality". We have to be fair byacknowledging that such designs can be liked andeven loved by audiophiles. The world is big enoughfor multi musical tastes.The idea is very nice indeed, and we do admiresuch a philosophical approach, even if it is notours: the problem is that those designs are highlysophisticated – too much for our ears – involvinglots of added transistors, op amps, and higher thanreasonable NFB, leading finally to altered soundstructure.These electronics can be considered as very pureand detailed for the first listening hours or days,but once noticed, their sonic signature becomesharder and harder to bear.So if THD does not seem to be a determinant factorin the accuracy of the reproduced sound, we have tolook elsewhere.T5.1.2. Inter Modulation DistortionNow, let us listen to two flute players. The flute iswell known for its less rich than usual sound - har-monically speaking, of course! A kind of exceptionconfirming the rule.If each player produces a different, sustained note,what will we hear? One intermediary note, twodistinct notes, or more than this?In theory we should hear 2 distinctive notes. Intheory only? Well yes, because in practice our hear-ing is not perfect. Truly wonderful indeed, but lessthan perfect.Without entering into details, we can neverthelesssay that in fact we will not only hear 2 distinctivesounds, but also combinations of those primarynotes.We beg musicians and music lovers to forgive us,but for a while we need to replace notes by fre-quencies. Not very musical, but much easier for thepurposes of our explanation.If the first flute plays a 1,000 Hz tone, and the sec-ond one a 2,500 Hz tone, we will not only hearthose two discrete tones, but also the followingcombinations:2,500-1,000=1,500 Hz, also called the beating fre-quency, and also the mirroring part, say2,500+1,000=3,500 Hz. Fortunately, those combi-nations are of much less amplitude than the discretenotes themselves. But that’s not all! We will alsohear the harmonics of these combinations! So, fre-quencies of 1,500, 3,000, 4,500 Hz, but also 3,500,7,000 and 10,500 Hz will be perceived. Once again,their level will be very low compared to the 2 initialnotes played.It seems hardly believable, but it is true… In prac-tice, though, this is far less embarrassing than youmight think at first glance. As said above, the rela-tive level of those "ghosts" is much lower than thebasic notes. The result will be heard as if the fluteshad some tremolo, or vibrating behaviour, a veryeasily perceived phenomenon. And the combina-tion’s harmonics will add some "warming" factor,or on the contrary some "coldness" or "dryness" tothe perceived sound, depending on how they willcombine together.So you can see that lMD is even less easy to under-stand and quantify than THD. And how can weappreciate IMD, can we hear it, at what level does itbecome a nuisance?Well, the first thing we can admit is that it is notmusical instruments that generate inter modulationeffects, but our own hearing – brain included – thatis the cause. Some acousticians say than our inter-nal ear can "produce" inter modulation artefacts atlevels between 25 to 40%! Of course the brain thendoes some necessary "correction".However, this kind of "correction" cannot be com-pared with an electronic chain, say the microphoneand analysis system. So, we feel very uncomfort-able about advancing any IMD value from externalorigin for which our ear is sensitive. In practice itseems that IMD less than 2% (3 to 5% for THD) isconsidered as not disturbing, even not audible in amusical context, judging by the Single Ended Tri-ode aficionados or by all the psycho acousticalexperiments conducted over the past few decades.By way of conclusion, it appears that IMD, whileperhaps more disturbing than THD, remains practi-cally harmless, provided the levels are not outra-geously high.Okay. Now bring on the next suspect…T5.1.3. Temporal DistortionIn the ‘seventies, the famous Matti Otala high-lighted a new kind of distortion, not so easy tomeasure, but clearly audible, called Transient In-termodulation Distortion, TID (also called TIM, forTransient Inter Modulation).TID occurs when the negative feedback (NFB) loopis in a state of overflow, something that arises moreoften than you might think since the NFB correc-tion always applies after the phenomenon to becorrected appears. During these very short instants,the amplifier can produce more than 100% THDand/or IMD.At darTZeel, we prefer to call it Temporal Distor-tion, or TD, because this is obviously what it actu-