
skippy
New member
Igohman: Suggestions on how to evaluate your system without an investment? The gent who runs http://www.pcavtech.com/ appears to use SpectraLAB, a software package that aparently runs under Windows on the machine under test. Check out some of his card test data- it's downright beautiful. I don't know anything about this software at all, other than the fact that it appears to the casual nerd like me to be a very useful tool- and it looks like you can download a freebie test version from the vendor:
http://www.pmgrp.com/prod01.htm
So, if you want to make some serious measurements on your rig, looks like that might be worth a try: "free" is a quality that excuses many faults! That'd probably be more satisfying than trying to hack together differential noise measurements with just your ears- although that is always the final bottom line: can *you* hear it?
I don't have any analog tape machines to bias up any more, but if I did, I'd probably be all over that package muy pronto. It'd be sweet to see the distortion levels and frequency response changes with bias adjustment. Not "strain to hear them", but _see_ them, and know you had it right... Shoot. I knew these dadgum digipooter machines were good for _something_.
Time-domain anomalies: they come in two major flavors. The first is the phase anomaly issue you touch on: as you approach the "wall" frequency, you see more and more phase shift with respect to the unfiltered signal, and sensitive listeners can hear that. After all, these aren't little teeny baby phase shifts: the shift approaches 180deg at the deepest null (since the way you make a null in a filter is to add the input signal back to itself, but 180deg out of phase).
The second is probably best called "time/energy smearing", or just ringing for short. If you put an impulse, or a step, into a filter that has a section with a resonant peak, that section will store the excess HF energy present in the transient and release it over time: PIIiinnggg... Just like a bell, or plucking a ruler that's hanging over the edge of a desk. The excess HF energy doesn't go away, it winds up the resonance and comes back at you later, converted to the frequency of the resonant peak. The higher the Q, the more energy that gets stored up, and the longer the ring as it is gradually released. If you think about it, this is not a phase issue as such: it's really an energy-conversion thing. That's where your ringing on the tops of square waves comes from, since brickwalls by definition require at least a few high-Q filter sections.
You know, it is just downright *bizarre* to be talking about this stuff at all, let alone to be doing it as if I really have any direct knowledge of what's going on today. My knowledge of this field is pretty dated, and I haven't really been active in digital audio at all since about 1986. It was kind of amusing, looking at the comp.dsp FAQ, and seeing them say that the first place to start looking for stuff is Crochiere and Rabiner's "Multirate Digital Signal Processing". I thought "Shit! I _own_ that book!". And there it is, covered with dust on the shelf. Bought it in '83 when it came out, read the parts I needed then, and then put away and didn't touch it again (except to pack it to move, a few times...) until last night. It still has the paper dust cover, even, and it's a good thing: the layer on top was thick enough to bog down a Jeep. I'm a freakin' lousy student.
I've got a lot of catching up to do to understand all the stuff that's going on these days, especially on the high end...
http://www.pmgrp.com/prod01.htm
So, if you want to make some serious measurements on your rig, looks like that might be worth a try: "free" is a quality that excuses many faults! That'd probably be more satisfying than trying to hack together differential noise measurements with just your ears- although that is always the final bottom line: can *you* hear it?
I don't have any analog tape machines to bias up any more, but if I did, I'd probably be all over that package muy pronto. It'd be sweet to see the distortion levels and frequency response changes with bias adjustment. Not "strain to hear them", but _see_ them, and know you had it right... Shoot. I knew these dadgum digipooter machines were good for _something_.
Time-domain anomalies: they come in two major flavors. The first is the phase anomaly issue you touch on: as you approach the "wall" frequency, you see more and more phase shift with respect to the unfiltered signal, and sensitive listeners can hear that. After all, these aren't little teeny baby phase shifts: the shift approaches 180deg at the deepest null (since the way you make a null in a filter is to add the input signal back to itself, but 180deg out of phase).
The second is probably best called "time/energy smearing", or just ringing for short. If you put an impulse, or a step, into a filter that has a section with a resonant peak, that section will store the excess HF energy present in the transient and release it over time: PIIiinnggg... Just like a bell, or plucking a ruler that's hanging over the edge of a desk. The excess HF energy doesn't go away, it winds up the resonance and comes back at you later, converted to the frequency of the resonant peak. The higher the Q, the more energy that gets stored up, and the longer the ring as it is gradually released. If you think about it, this is not a phase issue as such: it's really an energy-conversion thing. That's where your ringing on the tops of square waves comes from, since brickwalls by definition require at least a few high-Q filter sections.
You know, it is just downright *bizarre* to be talking about this stuff at all, let alone to be doing it as if I really have any direct knowledge of what's going on today. My knowledge of this field is pretty dated, and I haven't really been active in digital audio at all since about 1986. It was kind of amusing, looking at the comp.dsp FAQ, and seeing them say that the first place to start looking for stuff is Crochiere and Rabiner's "Multirate Digital Signal Processing". I thought "Shit! I _own_ that book!". And there it is, covered with dust on the shelf. Bought it in '83 when it came out, read the parts I needed then, and then put away and didn't touch it again (except to pack it to move, a few times...) until last night. It still has the paper dust cover, even, and it's a good thing: the layer on top was thick enough to bog down a Jeep. I'm a freakin' lousy student.
I've got a lot of catching up to do to understand all the stuff that's going on these days, especially on the high end...