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Old 10th April 2013, 09:25 AM   #861
ChrisPa is offline ChrisPa  United Kingdom
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Quote:
Originally Posted by kazam View Post
He doesn't like pre-echo but of course not all FIRs have that problem...

FIR doesn't have to be linear phase.
Linkwitz links to the Grimm white paper. I believe the underlying reasoning is here
http://www.grimmaudio.com/whitepapers/speakers.pdf

In particular look at the comment at the end of section 2.2 where Bruno Putzeys says:
"The audibility of pre-ringing of >20 kHz filters in digital audio has always been a matter of some debate. Audibility of pre-ringing at 2 kHz is not."
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Old 10th April 2013, 12:01 PM   #862
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Quote:
Originally Posted by ChrisPa View Post
Linkwitz links to the Grimm white paper. I believe the underlying reasoning is here
http://www.grimmaudio.com/whitepapers/speakers.pdf

In particular look at the comment at the end of section 2.2 where Bruno Putzeys says:
"The audibility of pre-ringing of >20 kHz filters in digital audio has always been a matter of some debate. Audibility of pre-ringing at 2 kHz is not."
Bruno's a smart guy, and he may have a point about pre-ringing at 2 kHz (although correcting a diffraction blip in any manner except proper cabinet design is a fool's errand).

However, the frequency response dip pointed out in Bruno's section 2.2 is not a filter artifact, but instead a product of the 30 degree vertical measurement showing the mids working against each other, and indeed the blue frequency response line shows a first cancellation right where one would expect it for an off-axis path length difference. The trains have wrecked, all right, but the same would happen with an LR4 filter. The summed impulse response shows a hump at that point because that's the tweeter output without the mids's output counteracting it. Note the very last impulse response showing extensive pre-ringing would be objectionable even though the steeper filters, reducing crossover overlap, would have a narrower dip in the frequency response. Pre-ringing at audible frequencies sounds quite unnatural, best described as a "scratchiness" around the range where the oscillation occurs. On the other hand, how many people listen to their speakers at 30 degrees off the vertical axis? Room contributions to the listening position will have sufficient delays that the Haas precedence effect might make this anomaly a moot point.

The argument fails when the wavelength is considerably larger than driver separation, for example an WMW arrangement crossed over at 100 or 200 Hz. Cabinet diffraction, apart from baffle step (broadband anyway so its impulse response is docile), is not an issue here, nor is driver dispersion, so off-axis response would be about as flat as on-axis. At that point a reasonable complementary FIR is your friend, because it both keeps mid frequencies away from nasty high-Q woofer resonances and limits mid excursion by keeping bass energy out of mid drivers.

Moreover, FIR design methods such as windowed sinc show better pre-ringing characteristics than the omnipresent Parks-McClellan, which really shouldn't be used for crossovers. I'll confess to a preference for distinct crossover and correction filters for that reason, since specifying a crossover frequency response characteristic mixed in with correction could produce the dubious results Bruno points out. This doesn't mean FIR crossovers are to be shunned, but using them properly means looking at the time domain response first then checking to make sure the frequency response is what you want.

Last edited by DSP_Geek; 10th April 2013 at 12:05 PM.
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Old 10th April 2013, 01:16 PM   #863
jcga is offline jcga  France
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I would recommend to anyone trying to design a crossover (DSP or not) to read at least the enlightening paper from Mr Floyd Toole here

http://www.harman.com/EN-US/OurCompa...ndRoomsPt2.pdf

Jean Claude
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Old 10th April 2013, 04:10 PM   #864
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Originally Posted by jcga View Post
I would recommend to anyone trying to design a crossover (DSP or not) to read at least the enlightening paper from Mr Floyd Toole here

http://www.harman.com/EN-US/OurCompa...ndRoomsPt2.pdf

Jean Claude
Dr Toole, actually. Reading his stuff is good for the soul.

Note particularly his treatment of resonances -- drivers before they're crossed over are always minimum-phase systems, so correcting anomalies can be done with judicious parametric EQ. He's also not a fan of fixing diffraction problems with EQ since that will mess up the room response.
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Old 10th April 2013, 05:21 PM   #865
chaparK is offline chaparK  Luxembourg
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I'm looking at the 2-channel analogue expansion.

There are 3 options:

1. Cheapest is no analogue volume. Volume is handled by the DACs but output level is fixed to some value.

2. More expensive: we fit an analogue volume chip like CS3310 or PGA2311. Problem is that max output level will be less than on the 8 main outs due to lower admissible rails (+/- 5 V vs +/- 8.5 V).

3. We fit an expensive PGA2310. This will blow the bill however.

What do you think (Steve)?
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Old 10th April 2013, 07:36 PM   #866
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Originally Posted by chaparK View Post
I'm looking at the 2-channel analogue expansion.

There are 3 options:

1. Cheapest is no analogue volume. Volume is handled by the DACs but output level is fixed to some value.

2. More expensive: we fit an analogue volume chip like CS3310 or PGA2311. Problem is that max output level will be less than on the 8 main outs due to lower admissible rails (+/- 5 V vs +/- 8.5 V).

3. We fit an expensive PGA2310. This will blow the bill however.

What do you think (Steve)?
Thanks Nick,
I certainly only need digi volume control just as for the standard 8 outputs - no different handling at all.
Personally I can't see why you would need a separate control for them.
Do need all the same output settings and controls as the others have of course. Looks like that is fine as there are 2 more blanks there in the existing control panel begging to be used
I see them as just two more channels to make 5 way work perfectly.

Nice and simple

Last edited by Speedysteve7; 10th April 2013 at 07:39 PM.
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Old 10th April 2013, 08:27 PM   #867
chaparK is offline chaparK  Luxembourg
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Quote:
Originally Posted by Speedysteve7 View Post
Thanks Nick,
I certainly only need digi volume control just as for the standard 8 outputs - no different handling at all.
Personally I can't see why you would need a separate control for them.
Do need all the same output settings and controls as the others have of course. Looks like that is fine as there are 2 more blanks there in the existing control panel begging to be used
I see them as just two more channels to make 5 way work perfectly.

Nice and simple
Najda actually has analogue volume on the 8 outputs. Analogue volume is managed by the CS3318 chip which has these +/-8.5 V rails that I mentioned. (So it's analogue volume digitally controlled).

Keeping the same type of volume is indeed the ideal way to make it - but it's also the most expensive one (i.e. that's point 3 in my previous post).
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Old 10th April 2013, 09:19 PM   #868
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Or resample the output to 2x the original frequency, control the volume in the digital domain, and add dithering above the audio band before the signal hits the DACs.
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Old 10th April 2013, 09:52 PM   #869
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DSP_Geek,

I understand the 2x resample but why add dither?
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Old 10th April 2013, 11:16 PM   #870
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Originally Posted by Carl_Huff View Post
DSP_Geek,

I understand the 2x resample but why add dither?
To avoid quantisation when the volume is turned down. You'd be surprised at how much quantisation can be covered by proper dither [1], and 2x resampling means it can be spread across an entire octave above the audible range.

[1] Just for kicks & grins, I bit reduced a test file to 6 bits; it was ghastly without dither, but with dither it sounded pretty darned good.

Last edited by DSP_Geek; 10th April 2013 at 11:26 PM.
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