Show your square wave...

the thin end of the wedge
VinylScience
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Show your square wave...

Post by VinylScience » 04 Jan 2019 14:29

Previously I posted my DIY SUT based on the Lundahl 1636 transformer. Now I recorded a square wave, it looks really nice! No ringing, no resistors, 1:20 ratio, fed straight into the RME UFX mic input @ 50dB gain. The signal is from the new Ortofon test record, duty cycle 3:7 and a Denon DL103.
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SUT Square Wave.png
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VinylScience
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Re: Show your square wave...

Post by VinylScience » 14 Dec 2019 15:03

Here is a nicer imager, same as above, but 1:5, recorded straight into SMAART (96kHz), nice and straight, a perfect square wave.
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Screenshot 2019-12-14 at 15.41.56.png
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wolfie62
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Re: Show your square wave...

Post by wolfie62 » 16 Dec 2019 14:53

Those very rounded corners indicate a high frequency roll-off and an overdamped response. Not at all a perfect square wave. Your SUT does seem to mimic the input closely.

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Re: Show your square wave...

Post by VinylScience » 16 Dec 2019 15:46

Correct - that is more precise! Thank you. I guess we will not get more "square-wave" from a record, as cutting lathes generally implement a high-frequency roll-off?

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Re: Show your square wave...

Post by VinylScience » 16 Dec 2019 16:28

Good point actually. I suppose in the end the square wave of the Ortofon Test Record is not so square? Has anyone looked at that?

billshurv
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Re: Show your square wave...

Post by billshurv » 16 Dec 2019 17:04

Not sure if I have seen tests from the ortofon, I'll have a search. As the square wave is actually cut as a triangle on the disk you can get a pretty good square wave out.

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Re: Show your square wave...

Post by billshurv » 16 Dec 2019 17:10

Here is the sort of thing I would expect to see. Tiny overshoot then fairly flat. Not a true square wave still, but not bad for the technology.
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vinyl square wave.jpg
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wolfie62
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Re: Show your square wave...

Post by wolfie62 » 16 Dec 2019 17:18

That’s a nicely reproduced square wave. Nice verticals up and down. Virtually no overshoot.

How can a triangular groove wave produce a square wave stylus response? Where would the top shelf of the square wave come from?

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Re: Show your square wave...

Post by billshurv » 16 Dec 2019 17:21

it's due to RIAA pre-emphasis and cartridges being velocity sensitive not position sensitive.

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Re: Show your square wave...

Post by tep392 » 17 Dec 2019 20:40

So basically, the output of the cartridge is the first derivative of the waveform cut into the groove, and the derivative of a triangle wave is a square wave.

billshurv
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Re: Show your square wave...

Post by billshurv » 17 Dec 2019 23:09

To a first order yes. It's slightly complicated by the way RIAA is implemented, but broadly correct. As the needle flies across the record its going at constant speed, so this is the flat top/bottom of the square wave. As it changes direction at the 'points' of the triangle, pulling huge g forces that is the rise and fall of the square wave.

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Re: Show your square wave...

Post by chiz » 18 Dec 2019 00:19

billshurv wrote:
16 Dec 2019 17:10
Here is the sort of thing I would expect to see. Tiny overshoot then fairly flat. Not a true square wave still, but not bad for the technology.
Bill, do you know which test record was used to produce the image you posted?
I recorded the Ortofon square wave and it looks like this:
Ortofon_square.JPG
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I then tried the CBS STR-112 and it looks like a triangle wave:
CBS STR-112 square.JPG
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These are both post RIAA eq.

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Re: Show your square wave...

Post by billshurv » 18 Dec 2019 08:56

The one I posted was STR-112, which is recorded without RIAA. The Ortofon is recorded with RIAA. I hadn't spotted in the first post that the ortofon was not a flat recording.

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Re: Show your square wave...

Post by VinylScience » 18 Dec 2019 10:39

In chapter 25.6.4 in The New Audio Cyclopedia edited by G Ballou, 2nd Edition from 1991, I found a statement that the square wave test is subject to question. Apparently, already back then the records were not always all that great and the square wave was not as square but subject to recording artefacts.

In fact I noticed that the pink noise of different test records vary from record to record in balance and spectrum several dBs. Perhaps it also depends on the particular pressing...

Concerning the reproduction of a square wave by a cartridge, can't we consider the square wave as a superposition of sine-waves?

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Re: Show your square wave...

Post by billshurv » 18 Dec 2019 10:45

Pink noise is very questionable as has been found in some comparisons. I have the CH precision test record that I intend to experiment with, but spot frequencies or a sweep are best for FR testing. I would love to find time to calibrate up all the common test records, but right now can't.

Square waves are a superposition, but the square wave test is a particularly vicious one and useful for checking that everything is working as it should.

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