Hi Uli, how well did you match the summing resistors and how much signal amplitude was left?
Note that with a 0.1dB (~1% tolerance resistors) level difference between the two signals, the difference would still have an amplitude about 40dB below the original signals. You can get to -60dB with resistors matched to 0.1%. (this is just a quick&dirty ballpark estimation) The difference test can be very useful in digital domain, where exact level matching comes for free. In the analog domain, however, it is very hard to achieve the necessary precision so you’re not chasing ghosts. Greets Christian > Am 21.07.2015 um 18:53 schrieb Uli Brueggemann <uli.brueggem...@gmail.com>: > > Theo, > > this reminds me on a simple test where I have never got a desired result. > Take a digital signal (a sine wave or your saw wave), send it thru a DAC. > For the second channel take the inverse wave. Add the DAC outputs e.g. by a > resistor network and try to get zero. > The digital signals add perfectly to zero but the analog signals behind the > DACs do not. > Now there is enough playground to modify the digital signal to get closer > to zero and this includes a research for all kind of the distortions you > have described. > > Uli > > 2015-07-21 17:50 GMT+02:00 Theo Verelst <theo...@theover.org>: > >> Hi DSPers, >> >> For a long time it has bothered me that there's a bit hidden way to deal >> with all kinds of signal "distortion", both in the analog domain as in the >> digital domain, that isn't necessarily clear. I think this is a real >> subject, and I'd prefer to have a good angle on it to essentially make >> clearer and more transparent recordings and DSP algorithms. >> >> Simply put, if you take a standard test signal or oscillator signal like a >> SAW wave (you know, linearly up to a fixed point, virtually zero time back >> to a given voltage, and then very linearly up again (obviously)), and you >> put that signal through a preamp or a digital processing device, you can >> get distortion because of transistors (or tubes) or errors in the AD/DA >> conversion, and you can get signal aberrations as a consequence of (linear) >> filtering, like coupling capacitors, DC offset control (similar function), >> limited frequency response, and general imperfections showing up as >> frequency dependent amplification errors stemming from linear filtering >> elements. >> >> To give an example, you could get exponential curves superimposed over >> your "pure" saw wave (forgetting for the moment the imperfections related >> to aliasing in case of digital signal processing) from non-linearities in >> the signal chain, such as small signal response curves in amplifier >> elements, or from filtering elements responding to the signal with normal >> (and linear) time responses based on the poles and zeros at stake. So the >> input of the amplifier amplifying a microphone could show some sort of >> transcendental non-linear distortion (standard for a lot of circuits). On >> the other hand, the high pass filter built in many amplifier stages (in >> terms of the coupling capacitor) and the limited frequency response of any >> analog circuit change the curves in the test saw wave a bit too, which >> translates to certain types of exponentials as well. (first year excercises >> for EEs). >> >> Now what's the point related to digital signal processing ? Well, there >> are a number of signal corrections, harmonic analysis and adaptation >> methods and even noise reduction methods that can work pretty ok in the >> digital domain that could benefit from analysis and tuning with standard >> test and synthesizer signals . So I'd prefer to work a bit on getting a >> very good quality saw wave, put that on a mixer and digitizer, and then use >> digital measurements to adjust whatever can be adjusted towards perfect >> mixing and pre-amplification (as far as there are errors than come to the >> attention in productions), but also, it would be interesting to then feed >> the "perfect" signal at the input of a AD convertor connected to the mixed >> signal through some form of digital correction audio streaming software >> such that we can compare the output of a also connected DAC to a mixer or >> electronic subtraction device to compare the Digital to Analog converters' >> output to the original signal (except for a phase shift) to as it were work >> on that sort of perfection! >> >> >> Theo V. >> -- >> dupswapdrop -- the music-dsp mailing list and website: >> subscription info, FAQ, source code archive, list archive, book reviews, >> dsp links >> http://music.columbia.edu/cmc/music-dsp >> http://music.columbia.edu/mailman/listinfo/music-dsp >> > -- > dupswapdrop -- the music-dsp mailing list and website: > subscription info, FAQ, source code archive, list archive, book reviews, dsp > links > http://music.columbia.edu/cmc/music-dsp > http://music.columbia.edu/mailman/listinfo/music-dsp -- dupswapdrop -- the music-dsp mailing list and website: subscription info, FAQ, source code archive, list archive, book reviews, dsp links http://music.columbia.edu/cmc/music-dsp http://music.columbia.edu/mailman/listinfo/music-dsp
