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Distortion

cambridgecd116bit-60db

From 1986, the Cambridge CD1 with its Philips TDA1542 DAC produced a substantial 0.98% distortion at -60dB (summing 9 harmonics).


cambridgecd1thd-60db_hp3561

The Cambridge CD1 again, but measured by our Hewlett Packard HP3561A analyser. Summing 19 distortion harmonics produced by a -60dB test tone it gives a slightly higher value of 1.22%.. This instrument also shows the quantisation  noise.



WHAT IT TELLS US

Originally claimed to have vanishingly low distortion, in truth at low levels CD has very high distortion when inevitable quantisation noise is included. Worse, the distortion and noise products extend to 20kHz and are discordant, because they do not correlate well with the music. That’s because complex quantisation noise products are generated by music frequencies that do not coincide with discrete sub-multiples of the sampling frequency where error is at a minimum. Tests discs are structured to place test tones where quantisation distortion is low, to give a good measured result. Early discs from Japan did not do this and gave unpleasant looking results that Philips and Sony rushed to correct.

CD ‘distortion’ isn’t so much distortion as a nasty mush in modern players, with some harmonic distortion products thrown in. Early players suffered this heavily and had an obvious ‘buzzy’ quality. Modern players (and digital recordings) are greatly improved and our measurements show this.

nad-t557-cd-thd1

Discrete harmonic distortion from a NAD T775 Blu-ray player playing CD, at -60dB.


Distortion at high music levels on CD is always low; it’s the amount at -60dB that is the best single figure barometer of performance. Expect to see around 0.22% distortion at -60dB. Little lower is possible as this close to the noise floor of CD. There are still players and DACs (Digital-to-Analogue Convertors) that give worse figures.


cambrdacmagicthd2-16bit

Distortion from a Cambridge DacMagic measures a very low 0.18% at -60dB, consisting mainly of evenly structured modulation noise.


Distortion from CD is not audibly obvious any more, but you realise the subtle form it takes when listening to music recorded and replayed in 24bit code. Then the limitations of old 16bit PCM as used on CD becomes more obvious. It is slightly dirty, dark and opaque, and also short of fine, filigree detailing, making for a generalised outline of singers and instruments when compared to later, higher resolution codes, notably 24/96 (24 bit description of signal amplitude at a 96kHz sampling rate). This is becoming the new de facto standard for quality.


We measure distortion from CD, SACD and Blu-ray players, through their analogue outputs, and it tells us whether they meet good, current standards.


panasonicdmpbd4524bit-thd

Distortion from a Panasonic DMP-BD45  Blu-ray with 24bit audio.



Here are levels to expect from a top quality design -


distortion-table




rs-upl



HOW WE MEASURE IT

We measure distortion on CD Philips test disc SBC429 and SACD using Philips test disc 3122-783-00632. On DVD and Blu-ray players we use Rohde & Schwarz test discs from Burosch of Germany possessing suites of audio test signals using 24/96 and 24/192 PCM.


Distortion is recorded from 0dB down to -90dB using Rohde & Schwarz UPL and Hewlett Packard HP3561A  spectrum analysers. The UPL sums up to nine harmonics, when measuring total harmonic distortion (i.e. up to 10kHz with a 1kHz test tone). The HP3561A  measures up to 19 harmonics (i.e. up to 20kHz with a 1kHz test tone), giving a slightly higher distortion value as a result.



sacd-test-disc



 

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