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Digital files for IM tests
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Old 9th April 2021, 12:05 PM   #1
hop333 is offline hop333  Wales
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Default Digital files for IM tests

Hi all

I'm not sure if this is the right place to ask, but I would like to either find or generate myself some test tones that can be used from CD or memory stick for challenging intermodulation distortion test tones that produce in-band residuals. So 44.1kHz 16-bit AIFF files would appear to be the most generally usable.

I am thinking about e.g. Bob Cordell's MIM tests (either the originals or shifted up to the top of the audio band as described here Bob Cordell Interview: Error Correction) or Dick Small's TDFD as described in his 1986 AES article.

They would be useful for a number of things, in particular allowing any digital playback device to become a sophisticated and repeatable test signal generator, but also to allow testing of the D/A itself, then of course through the line level and power amplifier chain from there.

If I was doing it myself, I would create the tests at different levels from maximum down in 10dB steps, with and without 1 LSB triangular dither.

If these files are already available somewhere then please point me there. Otherwise I will have to read up on the AIFF format and generate some myself from first principles. I would generate the signals in 64-bit double precision and then use Stochastic Rounding (essentially an unbiased method for getting from DP to 16-bit integer that is closely related to dither see e.g. [1904.11263] Stochastic rounding and reduced-precision fixed-point arithmetic for solving neural ordinary differential equations ). Both the SR and the dither signal would be generated from a top quality PRNG such as Marsaglia's KISS64.

Any thoughts or pointers welcomed - especially if I have missed something obvious!

M

Last edited by hop333; 9th April 2021 at 12:06 PM. Reason: typo
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Old 9th April 2021, 12:41 PM   #2
MarcelvdG is offline MarcelvdG  Netherlands
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If single precision and .wav would do, you could very easily create those files with the GoldWave audio editor using its expression evaluator: add a couple of sine waves and two random numbers for dithering. Maybe it also supports AIFF, never tried that.
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Old 9th April 2021, 01:00 PM   #3
hop333 is offline hop333  Wales
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Thanks @MarcelvdG I'll see if that is on my platform (Mac). I am pretty fussy about dither and the quality of PRNGs and conversion from floating-point to integer so I would have to have faith that they have considered all these issues carefully.

I only suggest AIFF because I would like to be able to put them on a test CD to use within CD players as well as a more general data files.
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Old 9th April 2021, 06:00 PM   #4
JohnPM is offline JohnPM  United Kingdom
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REW has a fairly flexible dual/triple/multitone generator that can save to WAV files.
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Old 9th April 2021, 06:47 PM   #5
hop333 is offline hop333  Wales
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Thanks @JohnPM. I have that on my Mac for doing loudspeaker and room measurements but I didn't realise that it had those capabilities too. I need to investigate it further but clearly it's a pretty powerful piece of kit, especially when you think that it's free to download.

If you put WAV files on a CD I assume that a normal CD player won't recognise them. Maybe load them into some software and export as AIFF?
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Old 9th April 2021, 07:17 PM   #6
hop333 is offline hop333  Wales
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Just checking my version of REW (5.19) which is a fresh download and it has the Dual Tone but not Triple Tone options.
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Old 9th April 2021, 07:27 PM   #7
hop333 is offline hop333  Wales
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Ah I see, it's in the beta of 5.20.
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Old 9th April 2021, 08:10 PM   #8
MarcelvdG is offline MarcelvdG  Netherlands
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With Windows computers, you can usually take 16 bit 44.1 kHz stereo .wav files and tell a CD-R writing program to put them on a CD-R and to make it an audio CD rather than a CD-ROM. Most regular CD-players can then read the CD-R with no problem. I never had to use AIFF to get a CD-R that plays on audio CD-players - in fact I never heard of AIFF before today.
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Old 9th April 2021, 08:49 PM   #9
phofman is offline phofman  Czech Republic
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IMO it's the Apple ecosystem, using apple proprietary formats. The burnt CD-DA medium will be identical for all OSes.
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Old Yesterday, 03:04 PM   #10
hop333 is offline hop333  Wales
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So creating the WAV files on REW, converting them to Apple AIFF (AIFF-C which I think is little-endian) in Audacity and then importing them into iTunes works and is painless. That way I have generated a bunch of test signals in a playlist and then burnt them to an audio CD which works perfectly. If anyone wants these files I can share them.

I have established that my Meridian 203 DAC has a cleaner noise floor and better harmonic structure from a variety of THD and IMD tests than the audio out of my re-capped Marantz CD94. So it would be great if it sounded better too but it really doesn't. The stereo depth is noticeably shallower on good recordings and there is a kind of 'grey veil' laying over the detail in a recording. I'm comparing with levels set very precisely and the DAC is taking either optical or electrical output from the CD94, so the same bitstream is being used. Perhaps there is something wrong with it, but it measures fine. Stereophile used to think it was a great DAC.

I'm not somebody who listens to cables and opamps because having run many ABX tests and other listening tests in carefully controlled environments whilst at Rogers and KEF and then as a consultant since (also less formally as a recording engineer), I have seen what people describe as 'night and day' differences disappear when they are made to listen double blind with levels set precisely. If one cannot reliably hear a difference then a perceived preference is irrelevant. This makes ABX tests a very powerful tool which should be used more often than they are (see e.g. Hopkins Research - ABX methods for measuring human perception of change). Unfortunately, the human motivations for profit and/or guru status are too powerful for people to forego. However, there is clearly some way to go finding measurements that correlate reliably with subtle aspects of sound quality.
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