On the subject of "ringing"

[John_Atkinson]

On 1/24/2018 at 5:32 PM, fas42 said:

Does pre ringing matter? No.

 

As its frequency is at Nyquist, which is above almost everyone's HF cutoff frequency, you would think not. But when I discussed this with Karlheinz Brandenburg at an AES convention several years ago, he said that basically even if you can't hear the "ringing" as a tone, your brain could well be aware that something has happened when it starts and marks it as an acoustic "event." Then, when the peak is subsequently reached, that is marked as a spurious second "event," leading to confusion.

 

On the subject of the time-symmetrical ringing of linear-phase digital filters, I assume Robert Watts would disagree; see my comment on his "million-tap" filter at https://www.stereophile.com/content/chords-million-tap-digital-filter

 

Incidentally, I have recently been examining the time-behavior of A/D converter antialiasing filters and have found just one which captures a band-limited impulse without any ringing before or after: the Listen filter on Ayre's QA-9 converter, designed by Charley Hansen and Ariel Brown.

 

John Atkinson

Editor, Stereophile

[John_Atkinson]

13 minutes ago, mansr said:

Could you describe your "band-limited impulse" a bit more? How is it generated? What does the waveform look like?

 

In brief I created a "digital black " file sampled at 38kHz and drew the shape of the diagnostic waveform I needed with BIAS Peak's pencil tool. I then modified it  until I got the desired spectral content. To create an analog signal to feed to the ADCs under test, I decoded the signal with a DAC capable of handing 384k PCM data without downsampling. As the ADCs to be tested were all set to 96kHz sampling, the DAC's own ringing at Nyquist would be an octave above the ADC's output passband and would be rejected. Apologies but people will have to wait for a forthcoming article in Stereophile for more detail.

 

John Atkinson

Editor, Stereophile

[John_Atkinson]

47 minutes ago, buonassi said:

But isn't this ADC you mention achieving less ringing at the expense of high frequency loss?  I thought the ayer "listen" filters (at least in their DACs) were a super slow rolloff starting at about 15K?

 

The high-frequency rolloff of the Ayre QA-9's Listen filter seems pretty benign. You can find my measurements at https://www.stereophile.com/content/ayre-acoustics-qa-9-usb-ad-converter-measurements

 

Below is fig.2  on that page, showing the frequency response at –1dBFS with the Listen filter,  analyzed in the digital domain, and data sampled at 192kHz (left channel blue, right red), 96kHz (left green, right gray), 48kHz (left cyan, right magenta)  The vertical scale is 1dB/div. The response at 20kHz is down by just a fraction of a dB at all 3 sample rates, though the rolloff is slow, which will potentially allow some aliasing at single Fs rates with some music.

 

John Atkinson

Editor, Stereophile

 

[John_Atkinson]

33 minutes ago, adamdea said:

Why not just use a real thing like a castanet?

 

Because you have to capture the sound of the castanets to digital using an A/D converter whose antialiasing filter's behavior is unknown. You can't solve an single equation that has two variables.

 

John Atkinson

Editor, Stereophile

[John_Atkinson]

35 minutes ago, mansr said:

Something is off here. A 48 kHz sampled signal by definition has no content above 24 kHz. What is that graph actually showing?

 

Look more closely at the graph using the color coding for the traces I supplied in my earlier posting.

 

John Atkinson

Editor, Stereophile

[John_Atkinson]

1 minute ago, Shadders said:

The issue is ringing, does it exist and what causes it. The castanets example shows an audio file and sound causes ringing, so it exists. The isolated sine wave causes it too. What is the cause ?

 

You need to remember how a digital filter operates on a discrete-time-sampled datastream. The "ringing" is actually the filter's coefficient map plotted against time. That's why I use a single-sample-high signal in my published tests of digital products, to reveal the filter's coefficients and thus what type of filter it is: linear or minimum phase, fast or slow rollioff, etc.

 

John Atkinson

Editor, Stereophile

 

[John_Atkinson]

On 2/11/2018 at 1:18 PM, John_Atkinson said:

I created a "digital black " file sampled at 38kHz

 

384kHz. Apologies for the typo.

 

John Atkinson

Editor, Stereophile