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Off-line meeting
-- Digital filter experiment --



This photo shows an experiment.
Using a PC based digital channel divider, my newly designed transmission line loudspeaker system was driven by a couple of TUBE amplifiers ;)


Another person attended here but he went back home before taking a picture regrettably.

By using active crossovers and multi-amplifier, I feel the sound becomes clearer than LC filter, in particular with very steep slopes.
Tested filter slope were 180dB/oct to 2kB/oct.(Be care please, not a kilo byte, but kilo Bell* !! )

On the other hand, LC filter becomes more glossy or lustrous.

If we try to use the driver of metal diaphragm, hard dome or horn, I think the difference will be bigger..
Please find below figures, these are the characteristics of FIR filters we tested.



200[B/oct] filter. (Please notice, not deci Bell but Bell * !)


Zoom up nearby cut off frequency of above filter


Filter characteristic is 2[kB/oct]. (I am not sure "kilo Bell" is a valid unit or not;)


Zoom up nearby cut off frequency of above filter


All filters design and the digital system constructed by Prof. Raraki.
FIR filters are designed with the window function method, so of cause they are linear phase.

The sound source was PC. That is, a real time processing of MS-Windows(R), reading the musical CD by the same method of the CDDA ripping, and the data was sent to the signal processing part directly, then using a signal processing unit of the CPU (like as MMX or SSE).

The employed PC is not powerful in comparison with the contemporary standard machine, but it showed enough performance for our purpose.

We tested the 4th order IIR filter (butterworth function) also, but I felt that the sound of sharp cut-off FIR filter is clearer than gentle curve.


The tested drivers are paper cone and soft dome, so the difference seems to be smaller than hard material driver which has resonance peak. However, if I choose lower crossover frequency, the difference may become large.

This time, we compared with LC filter and digital filter, but I am interested in the comparison with analogue filtered multi-amp (especially the elliptic filter), or if compared with IIR digital elliptic filter, otherwise an analogue elliptic filter with using FDNR · · · never-ending stories ;)

For some unknown reason, every one listened to the "bad sound" CD. :-D

* 1/10 of the "Bell" is [dB] (deci-Bell, or decibel), and a thousand times of Bell can be kB [kilo Bell]
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15th/Sep./2004 Last update
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