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In Reply to: Re: "passive" vs "active" posted by Presto on November 21, 2006 at 23:48:21:
*** Yes, I do see how the crossover and the driver "add up" (vectorally) to provide a net impedance as seen by the amp. ***This was the main point I ws trying to make, so I'm glad we both agree.
*** An amp in an active system only "sees" the impedance of the connected driver that are NOT in the stop-band (aka only the pass-band and crossover regions.) ***
I think you will find I have already aknowledged this earlier. To quote one of my earlier posts: "There is a counter argument that says filtering the amp used to drive the tweeter achieves better transparency."
I noticed you are mainly using the tweeter in your argument, and I seem to recall that research does suggest that filtering at the preamp primarily benefits the tweeter.
But a tweeter is very easy to amplify, typically only a few watts at the most. So the benefit of the approach that you suggest is minimal. Arguably filtering post amp is almost as good (but you do waste a lot of the power of the amp, admittedly).
Whereas it can be shown a woofer's impedance does change significantly with the addition of a filter post amp, and woofers require hundreds of watts to drive at high decibels (power required is inverse to frequency).
Anyway, I don't mean to say that there are no benefits to digital crossovers. And certainly you can potentially do digital crossovers, plus equalization, plus room correction more accurately than an analog only solution. I just don't like the "haze" that that imparts.
If you look at 32-bit floating point processing (which is what most software and DSPs do), it's not good enough to preserve 24-bit resolution. A 32-bit floating point number has 24-bit mantissa and 8-bit exponent. Numerical theory suggests you need approximately twice the resolution of your inputs when doing calculations in order to preserve the precision of the inputs. So 32-bit fp is barely adequate for 16-bit signals (the 24-bit mantissa has about 8 bit headroom for intermediate results), but clearly inadequate for 24-bit signals.
Ideally, you want to do all your calculations in 64 bit floating point (or simply use IEEE double precision which holds intermediate results to 80 bits), and then dither the final result to 24-bit fixed point before passing to the DAC.
I don't know any player or DSP that currently does this in real time. When that day comes, I'll be very keen to try it out. TI for example is starting to manufacture DSPs capable of 64-bit processing. And 64-bit VST and DirectX effects should hopefully be more prevalent when Vista is released. Sonar already has a 64-bit mixing engine, and I seem to recall Perfect Space is a 64-bit convolution reverb (but I could be wrong).
Follow Ups:
Christine:I was looking at Pristine Space - I had no idea it would be 64 bit capable - I will try and verify this. This would be phenomenal, since the Thuneau Allocator (according the website) now uses a 64 bit engine.
I got the SourceForge VST convolver working, but I think it's only 32 bit - I need to check this too.
What I really need these days is a really good prosumer or professional soundcard that has SOME ASIO mixing capabilities and goes beyound mapping only hardware I/O in ASIO. This will preclude the need to use ASIO4ALL and Virtual Audio Cable to stream data into the VST plugins via a kernel streaming output plugin from Foobar or Winamp.
The cards comes with a driver frontend called PatchMix.PatchMix allows dynamic assignment of ASIO inputs/outputs to hardware, and you can mix/route the signals, add effects etc. You can even do things like route an ASIO output to a Wave input etc.
The software works like a mixer so it should do what you are asking for. I can't remember if PatchMix is VST capable or not - would be cool if it was. Even if it doesn't, you can route an ASIO output to an ASIO input which an external VST host can then process and send back via another ASIO output back to PatchMix.
Christine:If Patchmix can handle VST plugins, this WOULD be awesome - eliminating the need for an external VST host.
I have looked long and hard at different cards - and the EMU 1820M is definately on my shortlist, along with others from Lynx, RME and Edirol.
Apparently, the 1820M uses some really good DACs in it as well. I can't remember if the Breakout box / dac module is PC bus powered or from an external source. The ability to power the DACs from a clean, regulated (or battery) source is a "must have" on my list.
ESI also makes some cards which use the "Directwire" patch software - hey didn't you mention this software some time ago? This is a neat "GUI" for those that want to "virtually connect" wires to create their PC's audio stream. Nothing wrong with an intuitive solution I guess...
Thanks again for the informative discussions.
No the 1820M is powered from the computer via a special cable (that patches into the computer's power supply directly), but it has an additional set of voltage regulators in the breakout box.The DACs are pretty good, Cirrus Logic 4398.
You might want to check out the 1616M PCI - which is the replacement for 1820M, which supports an external power source (I think).
The "directwire" facility on ESI cards is pretty primitive - it only allows routing, not processing.
Whereas Patchmix is a full mixer - you can do effects chains, multiple busses, mix and pan, etc.
Given that I don't do any processing (!), all this is wasted on me. But I realised from our discussion you may be very interested in it.
Another good thing is that the effects are run from the DSP on board the breakout box, so they don't take any CPU time :-)
Even if you don't want to use the on board effects, you can use an external processor, because all ASIO ins/outs are "virtual" on Patchmix and not tied to the hardware. Assume ASIO OUT 1/2 is the output from Foobar or Winamp. Within Patchmix, you can route this back out to ASIO IN 3/4, ASIO IN 5/6, ASIO IN 7/8. Now you can run three external programs that processes three separate stereo signals (for your digital crossover). For example, program 1 reads ASIO IN 3/4, processes, writes out on ASIO OUT 3/4. Ditto for 5/6 and 7/8
Now, back in Patchmix, you accept ASIO OUT 3/4 and route it the first physical DAC pair, accept ASIO OUT 5/6 and route it to the second DAC pair, etc.
You can set all this up so it's all connected and ready to go when you boot the PC.
COOOOOOOOOOOL!!! :D
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