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Old 12th October 2011, 04:01 PM   #511
AP2 is offline AP2  Italy
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Hi,
From Bruno: Even if the audibility of >20kHz content may be subject to discussion, the fact that <20kHz content is much more audible than >20kHz content is not controversial anywhere. I've never heard it claimed that above 20kHz is more important than below, even by the most seriously deluded woo-woos.

With respect, what kind of concept is this?
contains nothing. it is obvious that I have the first 20Khz.
You do not believe it makes sense to extend the bandwidth? or is it too complicated?
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Old 12th October 2011, 04:12 PM   #512
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Unless you take explicit measures to restrict bandwidth to 20kHz, it will be much greater as a matter of fact. UcD and Ncore have their corner frequency around 60kHz. There is no need for extra effort. It would only get complicated if you wanted it to stop at 20kHz...
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Last edited by Bruno Putzeys; 12th October 2011 at 04:15 PM.
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Old 12th October 2011, 04:24 PM   #513
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Quote:
Originally Posted by Bruno Putzeys View Post
The question therefore becomes: is there a frequency where we can safely say that the ear has thrown in the towel? Only listening tests (I mean good ones, not acoustic wine tasting sessions) can answer this. Since it remains a hot topic in some circles, the tests keep coming and the results are consistent: you can insert a 20kHz low-pass filter undetectably provided it is not too steep. Amazingly the sort of filter we use in AD/DA is still gentle enough. What matters is that the filter is free from pre-echos and other nonidealities (which most AD/DA filters aren't ) whilst remaining linear phase. On the other hand, a filter with a 100Hz transition band is very, very audible.
For that matter, transition bands are quite noticeable for lowpass filters in the audio band. The chief technologist at a previous job asked me to design and code up LPFs at 10 kHz with a 1 kHz transition, but they sounded raspy and unpleasant on music. I convinced him to try a 2 kHz transition, which sounded much less horrible. To forestall the inevitable question, the system ran well below clipping at all times.

Extremely steep anti-aliasing filters are the wrong way to go, IMO, precisely because of the pre-echo issue, but linear phase will always have some. On the other hand, one can design a minimum-phase FIR with a controlled rolloff which would show improved transient response at the cost of a bit more math.
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Old 12th October 2011, 04:31 PM   #514
AP2 is offline AP2  Italy
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Quote:
Originally Posted by Bruno Putzeys View Post
Unless you take explicit measures to restrict bandwidth to 20kHz, it will be much greater as a matter of fact. UcD and Ncore have their corner frequency around 60kHz. There is no need for extra effort. It would only get complicated if you wanted it to stop at 20kHz...
If I quit the band to 20Khz, the integration does not move, it stays out of the band. I do not see complicated.
I followed the speeches and I realized that you had set the frequency response to 20Khz.
well, then we will all be looking measures at 60kHz.
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Old 12th October 2011, 04:32 PM   #515
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@Francois Try not to confuse pre-ringing with pre-echo. Pre-ringing in filters above 20kHz is fairly innocuous (unless you go for fantastically sharp filters). Pre-echo always manifests itself over the whole bandwidth.
@AP2 The only thing that happens at 20kHz in Ncore or UcD is that that's the frequency where loop gain drops below the value at DC. You're being deliberately selective in your reading and I think from now on I'll be selective in what I bother replying to.
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Last edited by Bruno Putzeys; 12th October 2011 at 04:35 PM.
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Old 12th October 2011, 05:01 PM   #516
AP2 is offline AP2  Italy
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Quote:
Originally Posted by Bruno Putzeys View Post
@Francois Try not to confuse pre-ringing with pre-echo. Pre-ringing in filters above 20kHz is fairly innocuous (unless you go for fantastically sharp filters). Pre-echo always manifests itself over the whole bandwidth.
@AP2 The only thing that happens at 20kHz in Ncore or UcD is that that's the frequency where loop gain drops below the value at DC. You're being deliberately selective in your reading and I think from now on I'll be selective in what I bother replying to.
I did not ask you, as the band filter at 20Khz(what way have chosed). I do not want to know.
thanks for the "selective". certainly you have experience in dealing with people.
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Old 12th October 2011, 06:44 PM   #517
tengizk is offline tengizk  United States
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I might be missing something very obvious here, but isn't the amp performance well above 20k also very critical nowadays? With the modern noise shaping sources with more or less "gentle" output filters, the amp IMD products of out of band components will get folded into the audible range. So, unless there is a brick wall filter between the DAC and the amp or the amp IMD performance above 20k is spec-d, I don't see how one can ignore the elephant in the room.

Last edited by tengizk; 12th October 2011 at 06:51 PM.
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Old 12th October 2011, 06:57 PM   #518
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You're not missing something. It is important that HF noise doesn't get demodulated. This is one reason why it's not a good idea to try to increase bandwidth too far beyond the power bandwidth.

In the case of linear amplifiers the problem is particularly tricky because demodulation happens outside the feedback loop (at the input stage) so the feedback loop is powerless against any in-band products that turn up.

In an amp like Ncore on the other hand (and many other class D amps) the only place where you can get demodulation is inside the feedback loop, which comes down like a ton of bricks on any audio-band products that might have otherwise resulted. So when I say that what matters is performance <20kHz I do mean: including any IMD products that might land there. So I should be more specific: performance for output signals up to about 20kHz (and automatically a goodish bit beyond that) but for input signals however high in frequency they come. So for instance, if harmonic distortion for a 50kHz signal is 0.1%, I don't care. If the same 50kHz signal causes in-band spuriae that is a problem. Thanks to control theory I can get good in-band audio performance without having to optimize supersonic performance. I have an old SACD player that puts out way more HF noise than a modern DAC. I can drive the amp flat out without any audible demodulation.

I think the way in which I might be confusing people is that it sounds like I think amps can have horrid performance above 20kHz. That's definitely not what I'm saying. What I'm saying is that I won't bend over backwards to get a 200kHz bandwidth if that adversely affects performance below 20kHz. Neither will I bend over backwards to make THD at 50kHz better if that results in worse performance at audible frequencies. But make no mistake about it: this amp is more than ordinarily capable at supersonic frequencies. Just not unnecessarily so.
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Last edited by Bruno Putzeys; 12th October 2011 at 07:19 PM.
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Old 12th October 2011, 08:22 PM   #519
AP2 is offline AP2  Italy
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I state that I'm talking to Bruno, because he is an engineer, and not becouse Hypex.
Your justification for limiting the bandwidth is absurd.
the problem of aliasing filter, if it is multi-pole, or if DAC is very good, or if the output buffer is guessed, let those who develop the DAC Professional.
it is obvious that if you want to 50Khz bandwidth and get real high-end performance.
IMD-thd-Linearity Low-latency-perfect clips and 104-db SNR that is really complex.
This can be a project that has value.
NuForce has a very wide band, amp has no noise (sounds dumb).
DXA have 60KHz-1dB, IMD perfect and noise-104dB.
if you present the measures extended to 60kHz or 50Khz, and known issues of class D are all resolved, then you have an amplifier of prestige.
But it is useless to say that more than 20Khz, because the noise is demodulated within the loop. then? this is just not easy to solve. are ten other things. Rest test of listen.. as others.
Just when extend the band and all performances remain good is goal!
Or because you are "Bruno", then I can not dare to ask? in Italy does not work type of name. works that demonstrate. (from Prof. up to the scientist).
Regards
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Old 12th October 2011, 08:31 PM   #520
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Quote:
Originally Posted by Bruno Putzeys View Post
@Francois Try not to confuse pre-ringing with pre-echo. Pre-ringing in filters above 20kHz is fairly innocuous (unless you go for fantastically sharp filters). Pre-echo always manifests itself over the whole bandwidth.
I see. I haven't run across this pre-echo so what came to mind was the pre-ringing familiar to anyone who's worked with linear phase FIRs. How does pre-echo manifest itself in a system? It implies a fair amount of stored state, so that's of some interest to me.
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