Square Wave Testing of Audio Products (Video Tutorial)

[amirm]

The topic of using square waves to test audio products comes up quite frequently. Turns out that it is not a very useful signal for this application due to its nature and how our audio systems work. In this video I explain and demonstrate the signal processing behind square wave and reasons they can be problematic in audio measurements.

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As always, questions, comments, recommendations, etc. are welcome.

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[amirm]

You mentioned the bleed-through of digital information to the analogue circuits. What are the mechanisms that can cause this?

It could be many things. Shared power supplies without adequate filtering/decoupling can bleed this into DAC clock or reference voltage. It can even capacitively couple through the air (physical separation helps with this).

 

[amirm]

You can see example of such bleeding in this review I just did: https://www.audiosciencereview.com/forum/index.php?threads/periodic-audio-rhodium-dac-review.20981/

See how the series of pulses reduces in amplitude? There is a hidden square wave in jitter test running at 250 Hz. The USB receiver is seeing this and that activity is bleeding into the analog output of the DAC.

 

[trl]

A good writing back in '68: http://www.r-type.org/articles/art-125.htm.

A good explanation is also provided by AP too: https://www.ap.com/technical-library/square-waves/.

I always test my amplifiers with square waves when I purchase them. This tells me if there're oscillations or something wrong to the LPF or HPF filter. This also tells me if the amplifier is DC or AC coupled and, usually, if there's something wrong with the internal design, this usually can be represented by a scope fed with square waves signals. It feels much easier to me to find faults on amplifiers when fed with squares instead of sines, although I understand why this is an optional test and usually sinewave testing is more appropriate and easier to understand by the readers.

 

[pma]

Yes, stability, settling time, rise time, slew rate, slew rate limited circuit, small signal transient response, large signal transient response, complex load transient response - that’s the way professionals use square wave testing.

 

[amirm]

There is a difference between "testing" and "performance measurement." The former is regarding functionality, the latter is about fidelity.

 

[phoenixsong]

What about square wave measurements for headphones and iems?

 

[amirm]

I have no use for them. Tried them a bit but it had no value despite Tyll promoting them.

 

[pma]

What I desperately want to know is the large signal step response of ANY audio amplifier. Should it be preamp or poweramp. I want to see it is smooth, aperiodic, no overshoots, no ringing and no slew rate limitation (this is to be fixed by input RC). This tells me the thing is stable and not prone to AM demodulation (which happens if it is SR limited). In case of "normal" circuit, 0.35/Tr tells me the -3dB cut off. Tr is the rise time of the step reponse from 10% - 90%.

I am not sure if the popular view for the amateur audience, as shown, is helpful, I think it may be misleading to unqualified visitors.

My headphone amp square response. Please note high sampling rate and BW is needed even to test audio circuits. Please also note the output voltage swing.

 

[Frank Dernie]

Back in the late 1960s and 1970s when UK magazines always tested amplifiers using square waves there was always a substantial difference between amplifiers with the 40 Hz (chosen because it was below out 50Hz mains frequency).
The best amps all had better than average performance on this parameter, and a quick look at that graph always told whether the rest of the measurements would be good.
Mind you, most amps are good now.

 

[pma]

40 Hz pulse tilt told them about low frequency response and cut off. It also revealed possible clipping at lowest frequencies.

 

[restorer-john]

I use square waves pretty much as often I use sine waves.

I can see in an instant what is going on with tone controls, particularly stereo (2ch) and making sure both sets of tone controls and any filters or turnover frequency switches and components are all within tolerance, LF extension/issues, issues in the high end between channels, instabilities etc.

For repair/restoration work, square waves are my choice of function. An amplifier can be checked for all its basic functionality faster with a square wave source than anything else. Within perhaps 20 seconds I can zero in on problems with a square wave. Forget it with a sine.

Harman are/were a huge proponent of square waves and had been forever. Most of their advertisements through the 70s, 80s and into the 90s extolled the ultrawideband/linear phase of everything in their range using square waves. I could dig out the endless scope shots of square waves by all the manufacturers, some useful, most just about as deceptive as the Benchmark "crossover distortion" "scope" plot on their site.

If I was into making youtube videos like Amir, I'd do one on the usefulness of square waves in my context (repair/construct/restore etc) But he's the man and I don't want to upstage him

 

[pma]

Back in the late 1960s and 1970s when UK magazines always tested amplifiers using square waves there was always a substantial difference between amplifiers with the 40 Hz (chosen because it was below out 50Hz mains frequency).

40Hz square response for you (headphone amp)

 

[Aerith Gainsborough]

Wee, this brings me back to my math lessons in Uni.

Those were fun times.

 

[Blumlein 88]

I use square waves pretty much as often I use sine waves.

I can see in an instant what is going on with tone controls, particularly stereo (2ch) and making sure both sets of tone controls and any filters or turnover frequency switches and components are all within tolerance, LF extension/issues, issues in the high end between channels, instabilities etc.

For repair/restoration work, square waves are my choice of function. An amplifier can be checked for all its basic functionality faster with a square wave source than anything else. Within perhaps 20 seconds I can zero in on problems with a square wave. Forget it with a sine.

Harman are/were a huge proponent of square waves and had been forever. Most of their advertisements through the 70s, 80s and into the 90s extolled the ultrawideband/linear phase of everything in their range using square waves. I could dig out the endless scope shots of square waves by all the manufacturers, some useful, most just about as deceptive as the Benchmark "crossover distortion" "scope" plot on their site.

If I was into making youtube videos like Amir, I'd do one on the usefulness of square waves in my context (repair/construct/restore etc) But he's the man and I don't want to upstage him

Please do a video. Square waves or any other subject. Heck you can do a nice one with a cell phone and some planning/forethought. I bet if you corroborate with Amir he'd host it on his you tube channel.

 

[Mnyb]

I use square waves pretty much as often I use sine waves.

I can see in an instant what is going on with tone controls, particularly stereo (2ch) and making sure both sets of tone controls and any filters or turnover frequency switches and components are all within tolerance, LF extension/issues, issues in the high end between channels, instabilities etc.

For repair/restoration work, square waves are my choice of function. An amplifier can be checked for all its basic functionality faster with a square wave source than anything else. Within perhaps 20 seconds I can zero in on problems with a square wave. Forget it with a sine.

Harman are/were a huge proponent of square waves and had been forever. Most of their advertisements through the 70s, 80s and into the 90s extolled the ultrawideband/linear phase of everything in their range using square waves. I could dig out the endless scope shots of square waves by all the manufacturers, some useful, most just about as deceptive as the Benchmark "crossover distortion" "scope" plot on their site.

If I was into making youtube videos like Amir, I'd do one on the usefulness of square waves in my context (repair/construct/restore etc) But he's the man and I don't want to upstage him

I think you said it "context" that's it

Its actually also in the video itself something about the amplifier designers may find it useful for some purposes.

 

[KSTR]

[rant]I think the mass production of videos here is getting out of hand. I don't ever watch technical videos, nor does anybody I know. It just doesn't work. Writing good articles is much better for understanding, both for understanding basics as well as details, for a lot of well-known reasons.[/rant]

 

[pma]

[rant]I think the mass production of videos here is getting out of hand. I don't ever watch technical videos, nor does anybody I know. It just doesn't work. Writing good articles is much better for understanding, both for understanding basics as well as details, for a lot of well-known reasons.[/rant]

Agreed. Seems that the purpose is not technical, but rather to rise attraction and number of visits to the forum. A kind of marketing . And it even does not make sense to argue with the content, because of the target set of the readers.

 

[KSTR]

I would agree videos are ok for very entry-level basic things, the 1x1. Especially if the addressed clientele would have problems collecting the relevant starter level literature themselves (which today, thanks to the internet, is really comfortable and easy).

 

[Mnyb]

ON that topic . What would a starter library for the Audio nerd be

Build a bookshelf for me with 10 books .

Dr Tooles book on speakers i know off but have not ordered yet . 9 to go ..