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Author Topic: Probably a dumb question (DC offset)  (Read 2438 times)

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Offline heathen

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Probably a dumb question (DC offset)
« on: July 14, 2017, 11:40:04 PM »
In the attached image, is the part circled in red DC offset?
Mics: Core Sound TetraMic | AT4031s | Countryman B3 | CA-14 omnis | AT853 cards | AKG 460/ck61 | Studio Projects CS5
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Offline bombdiggity

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Re: Probably a dumb question (DC offset)
« Reply #1 on: July 17, 2017, 02:33:48 AM »
DC offset deals with the entire segment not a tiny segment of it. 

"DC offset is a mean amplitude displacement from zero. In Audacity it can be seen as an offset of the recorded waveform away from the center zero point. DC offset is a potential source of clicks, distortion and loss of audio volume."
 
DC offset - Audacity Development Manual
manual.audacityteam.org/man/dc_offset.html

There's a dramatic illustration of what it could look like there ^. 
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Audio:
Schoeps MK4V
Nak CM-100/CM-300 w/ CP-1's or CP-4's
SP-CMC-25
>
Oade C mod R-44  OR
Tinybox > Sony PCM-M10 (formerly Roland R-05) 
Video: Varied, with various outboard mics depending on the situation

Offline Gutbucket

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Re: Probably a dumb question (DC offset)
« Reply #2 on: July 17, 2017, 10:22:43 AM »
That image shows a plot of voltage over time, with the change in voltage representing pressure variations. See how the area you've circled undulates above and below the zero voltage line?  The center of undulation indicates the average voltage value.  If that center value was constantly above or below the zero voltage line over the entire length of the file, it would indicate the presence of DC offset.  A recording system intentionally sensitive to zero-hertz pressure variations acts as a barometer.  Since audio recording systems are designed to measure pressure variations only in the audible range, with a sensitivity that intentionally drops off to nothing before reaching zero hertz, significant DC offset in an audio recording system indicates something may be incorrect in the signal chain.  Or that Jimmy Cliff is in town.

« Last Edit: July 17, 2017, 02:01:15 PM by Gutbucket »
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Offline bombdiggity

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Re: Probably a dumb question (DC offset)
« Reply #3 on: July 17, 2017, 12:27:53 PM »
Great breakdown (as usual) ^. 

I've noticed that certain pre's have a defined DC offset.  For example my Tinybox runs at .03% every time and always has.  My Oade modded R-44 always exactly at 0%, running the same mics (the only other difference in the chain being the cables).  I can't say I hear anything amiss with the sound of the Tinybox though I always correct it in editing. 

Old analog sources (like cassettes) transferred to digital tend to show much larger DC offsets.  Sometimes different offsets in each channel.  Cassette recordings almost always have an offset. 

Some instruments will appear to have much greater energy above the zero voltage line than below (mostly brass ones) and it can look to the eye like the recording would have to have a large offset but when you check the stats it doesn't. 

It is easy to get a read on it just by pulling the stats on a file in your editor and easy to correct.  See the other recent thread about correcting in Audition. 

Gear:
Audio:
Schoeps MK4V
Nak CM-100/CM-300 w/ CP-1's or CP-4's
SP-CMC-25
>
Oade C mod R-44  OR
Tinybox > Sony PCM-M10 (formerly Roland R-05) 
Video: Varied, with various outboard mics depending on the situation

Offline heathen

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Re: Probably a dumb question (DC offset)
« Reply #4 on: July 17, 2017, 12:43:51 PM »
Thanks for the info guys.  Bottom line: is the above anything to worry about?
Mics: Core Sound TetraMic | AT4031s | Countryman B3 | CA-14 omnis | AT853 cards | AKG 460/ck61 | Studio Projects CS5
Decks: Zoom F4 | Tascam DR-70D | Roland R-05

Offline Moke

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Re: Probably a dumb question (DC offset)
« Reply #5 on: July 17, 2017, 02:05:29 PM »
That image shows a plot of voltage over time, with the change in voltage representing pressure variations. See how the area you've circled undulates above and below the zero voltage line?  The center of undulation indicates the average voltage value.  If that center value was constantly above or below the zero voltage line over the entire length of the file, it would indicate the presence of DC offset.  A recording system intentionally sensitive to zero-hertz pressure variations acts as a barometer.  Since audio recording systems are designed to measure pressure variations only in the audible range, with a sensitivity that intentionally drops off to nothing before reaching zero hertz, significant DC offset in an audio recording system indicates something may be incorrect in the signal chain.  Or that Jimmy Cliff is in town.



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Offline perks

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Re: Probably a dumb question (DC offset)
« Reply #6 on: July 17, 2017, 02:21:40 PM »
I'm a bit dense but is this due to the pressure (SPL) is going to drop on you?
Mics: Schoeps MK5, Schoeps MK41, AT853u (C,SC,H)
Preamps/converters: Schoeps VMS52UB (x2), Nbox (x2), E.A.A. PSP-2 (x2) Grace Lunatec V2 (for sale), Sound Devices MP-2 (for sale), DPA MMA6000, Naiant Tinybox v1.5, Apogee Mini-Me, Benchmark AD2k+
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Offline Gutbucket

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Re: Probably a dumb question (DC offset)
« Reply #7 on: July 17, 2017, 02:40:01 PM »
Cuban missile crisis as alternate metaphor- When your ports get blocked the pressure gonna rise!

heathen, no offset worries in the file you posted the image from.  And minor amounts of offset like those mentioned above with regards to various preamps and recorders and sources which produce somewhat asymmetrical waveforms such as horns are likewise not a problem.  Like Bombiggity suggests, if you suspect an issue, the easiest way to check is to take a look at the file/waveform stats.
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Offline F.O.Bean

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Re: Probably a dumb question (DC offset)
« Reply #8 on: August 05, 2017, 12:13:40 AM »
I often forget to do the waveform analysis when I'm using WL6 or SF10, but its certainly helpful at times! I have honestly NEVER had to apply DC Offset that I'm aware of? I do remember doing it a few times back in the day like 10-15 years ago when I first started using WL4, just because it was something to try, and I never heard an audible difference between running WL DC Offset or not! But that doesn't mean that it's not useful! Ive only recorded to cassette/microcassette a few times back in the mid-90's, but I quickly moved up to a Sony TCD-D8 DAT shortly after that, and I've never noticed needing to do a DC Offset since being in the digital format of the DAT>HDD>CF Card>SD Card era's! So according to yinz guys in this thread, I should be OK not doing it for the most part, since it seems to affect analog recordings more than digital!

But I'm glad that I opened and read this thread, because I want to start doing a little bit more of the waveform analysis more than I was using it! Thanks for all of the info too :)
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