zoom control works poorly

[BeerMan]

I can't get the zoom slider bars to work right. I'm using DCart32 v3.12 and Win98se. I start with a 5 sec wave file, then zoom in on about a 0.3 second window. At these extreme zooms, the amplitude seems to change. When I try to magnify the left/right channel waveforms using the slider controls, things don't work real good. The left slider has little effect, while the right slider causes the lower waveform to sink to the bottom of the source window almost immediately (at about 1/5 of slider travel) while the upper waveform drops out of sight at about 1/2 slider travel.

[Rick]

Re: zoom control works poorly

BeerMan
When you zoom in, at some point you change from viewing the peaks of the waveform to viewing the actual waveform. This may be what you are seeing with the amplitude changing. The peak waveform is symetrical to accentuate the peaks (Makes it easier to find clicks), so sometimes when you zoom in the peak amplitude will seem to change to only a positive or negative peak.

The left slider is the offset slider and has a limited range on DCart32. The Right slider is a gain slider and has a much larger range. If there is a DC offset in you signal, it may act as you have described. That is not normally what you should see.

[BeerMan]

Re: zoom control works poorly

Thanks for the reply Rick. How might I go about checking for a DC offset in my signal? I'm using a Technics turntable, going thru a Terratec Phono preamp, into a Soundblaster card.

[Doug]

Re: zoom control works poorly

I would have thought it quite unlikely that your phono-preamp has a large DC offset on its output. Often it should be around +/- 0.002% from memory - the value will depend on the method of amplification used in the preamp...I would think it would be a 0 DC offset method as it is quite common.
Anyway, to test the amount of offset, you could try shorting the input of your phono-preamp (the inner socket of the RCA connection) to the ground connection of the RCA socket with a very short piece of wire. Then record a short section using DC-Art with the phono preamp's input(s) shorted. If you dont want to go to that extent, then just leave the record player in its off condition, with RCA cords connected to the phono-preamp and do the same thing - record a short passage of nothing and look closely at the signal (it will be a little more noisy if the turntable is connected).
Both methods will pick up a little hum, but when you zoom into the peak file/audio signal, the noise should be symmetrical around the zero line in the display windows...

If it is (or is very close) - no DC offset problem
If it isn't - then your phono-preamp has problems

This would be a simple quick check that I would use to test for DC offset using DC-Art, but Craig/Rick may have a more accurate method.

[Craig Maier]

Re: zoom control works poorly

Dougs method is fine for sorting out DC offset sources in pre-amplifiers. However, the inputs of a sound card also have a pair of op amps, one on each input. If either of these are faulty, they can introduce a DC offset as well indicating a defective sound card.

[BeerMan]

Re: zoom control works poorly

I think you're on to something. It behaves just like there is a slight negative offset. I'm attaching a screenshot showing how far down the traces are after only about 20% slider travel.
Next question is can I use DCart to correct this?

[Craig Maier]

Re: zoom control works poorly

Run the signals through the High Pass Filter set to 6 dB/Octave at 20 Hz. That will fix the offset. However, it is still indicative of a hardware problem in your system. I would try to find the source of this problem and fix that for the best results.

[Doug]

Re: zoom control works poorly

Now that we have definitely established that there is a DC-offset problem, let's try and find out if it's your sound card -
Instead of shorting the inputs to the phono preamp, short the inputs to the sound card...If the same happens, then as Craig mentioned...your sound card is faulty [img]/ubbthreads/images/graemlins/frown.gif[/img]
However, if there is no longer a problem, then it is your phono-preamp that is faulty...

If the phono-preamp is getting on the older side, then it may be that the output capacitors that block DC voltages have failed (there is one capacitor for each channel). If this is the case, then the problem can be easily fixed using a simple capacitor-resistor junction on the phono-preamp output to restore it to its original working condition.

Let us know if it's the sound card or phono preamp, if it is the later - it should be a "quick fix" with minimal fuss (I hope!)