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Old April 25th, 2007, 01:19 AM   #1
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Mic comparison = audio lesson

For comparison, I recorded a short clip with an ATR-35s using my HV20 while at the same time recording with a Tram TR-50BPS using a Zoom H4.

To my surprise I didn't hear a whole lotta different.

But looking at their timeline profiles in FCP, the ATR-35s appears somewhat unbalanced with the top part (above the 'line') stunted while the bottom stretches further toward the bottom.

The Tram is nicely balanced.

Wondering if it was the HV20, I checked out other mics I'd tested on it, a DM-50, Rode VideoMic and Panasonic Shan MC-1. For the most part, they looked balanced in FCP timelines. So it didn't appear to be the HV20's doing.

Could someone explain exactly what I'm looking at in the timeline, is it just levels, and what does the stunting indicate?

Does the profile show bottom and high end levels recorded, with the high-quality Tram nicely balanced and the lesser-quality ATR-35s having strong bottom characteristics with the highs less strong. Or am I wrong?

Maybe my ears ain't so great, since listening back with and w/o headphones there was very very little difference I could hear.

Finally, when I recorded the ATR-35s with the Zoom H4 the profile seemed better balanced. Hmm. (And dang.)

Any comments or info would be appreciated.

best,
gwasshopper

ps On a side note, I've noticed that occasionally the Zoom H4 gets a buzz--I'm using the 4 track mode, with 2 tracks linked for stereo--actually kinda pisses me off since you don't know when it does unless you're watching the meter/listening.
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Old April 25th, 2007, 04:43 AM   #2
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Just guessing at what you're seeing here, but ...

The 'line' across the middle is the zero point of the waveform. Above the line represents positive voltages and below the line negative. If you were looking at a simple sine wave tracing, you'd see the wave start at zero and go up to positive level 'X' then down across the line to the equal but opposite value '-X' and then back up to zero again, making up one complete cycle and the frequency is the number of times per second it does this. The distance it goes on either side of the zero line represent the level while the number of times per second it crosses the line represents the tone, higher tones crossing more often. A simple wave is normally symmetrical around the zero point, going the same distance above and below the zero line. However if there's a DC voltage also applied to the signal, the zero point is shifted by an equivalent amount - add a -1 volt DC component and the zero point gets offset by that amount and the waveform will no longer be symmetrical - the voltages add together and the signal swings 1 volt farther in the negative direction than it does in the positive. Sounds like the recording you did on the HV20 has such a DC Offset applied to it somehow. If it doesn't show up in recordings made with the camera using other mics or using that mic with other recorders I expect it somehow crept in during the capture into the computer or when loaded into the editor. I don't use FCP so I don't know where to find it in the menus but if you check help for "DC Offset" you should be able to find the tool or setting that removes it.
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Old April 25th, 2007, 12:41 PM   #3
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Thank you very much for the reply and useful info! You could very well be correct. Unfortunately, I don't see a DC Offset filter in FCP, maybe I'll ask in the FCP forum.

What effect does the offset have on a clip that one wants to correct?

Thanks again!
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Old April 25th, 2007, 12:54 PM   #4
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Quote:
Originally Posted by Elmer Lang View Post
What effect does the offset have on a clip that one wants to correct?
Maybe I should have said, What effect does the offset create that necessitates correcting it
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Old April 25th, 2007, 02:42 PM   #5
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Quote:
Originally Posted by Elmer Lang View Post
Maybe I should have said, What effect does the offset create that necessitates correcting it
If severe it can produce low frequency distortion. It can also cause loss of dynamic range. And if you're mixing with other audio signals, it can cause them to interact in unpredictable ways. Luckily most editing software has a simple menu tool to remove it - I presume FCP would also. I think FCP calls it the "DC Notch Filter"
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