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Greg Berchin wrote:
On 29 May 2005 21:28:41 -0700, wrote:

Since those preprints are not free to download, can you tell me in what
way it is relevant to my question ?


Well, I figured that the title -- "Perfect Reconstruction Digital
Crossover Exhibiting Optimum Time Domain Transient Response in All
Bands" -- would be a good hint.


Hi Greg,

My apologies, I didn't realise that YOU were the author of the paper
you were refering to ;-) Somehow I didn't connect 2 and 2
together...thanks for your email, I'll give the article a good read.


In your original post, you said:

could you synthesize the same
summed electrical response (and therefore the on axis response) in
terms of amplitude and phase response if you were to lower the Q of the
high pass section, and raise the Q of the low pass section from their
standard 0.707, without changing anything else ?


[...]

Some classes of high pass and low pass filter have ringing at their
crossover frequency whose phase is opposite to their complementary
filter


[...]

It occurs to me that this cancellation of ringing only occurs on axis,


[...]

With the output from the low pass section unable to balance the output
from the high pass section in these off axis directions, the ringing
will not be canceled.


In the AES paper, I address exactly this problem. First, the lowpass
and highpass filters are perfectly complementary -- that is; their
responses sum to unity magnitude and linear phase (pure delay) at all
frequencies. Second, their responses are in-phase at all frequencies,
so there are no off-axis anomalies. Third, ringing is eliminated
because the filter upon which this system is based is Gaussian (or a
Bessel approximation to Gaussian).


I don't see how being in phase at all frequencies could eliminate
off-axis anomolies ? Surely going off axis vertically will introduce a
time delay that will cause problems no matter what type of filtering
you use. Or are you only refering to the effects of horizontal off axis
response where the drivers are still equidistant from the listener, but
the narrower dispersion of the larger driver causes an additional
change in the response that causes incomplete summing of the response.
(Which is what my original post was about)

(Note: I havn't read your article yet)


Has anyone tried something like this ? Any gaping holes in my idea ?


Basically, it lends itself only to digital signal processing, because
there are time delays inherent in the technique. Implementing pure
delay in analog electronics is difficult.


Yes I kind of suspected that at the very least it could only be
implemented using an active filter, or possibly even only a digital
filter.

Regards,
Simon