I give up. . .how do I eliminate hiss in post? - Page 2 at DVinfo.net
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Old May 23rd, 2003, 12:16 PM   #16
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yes it does, but that would then allow a steady stream of your distorted sound as opposed to a fluctuation which is easier to manage...

even with hiss, if u run it thru a stereo enhancer, you can probably make the hiss sound like part of the environment...

If you compress your sound, it might help a bit, but compresion will also raise the lower end of the freq field and give you an inaudible low rumble(you wont hear it as such, but if u look a your speakers you will see them jump)

in the end, i suggest you experiment to find what works for you.
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Old May 23rd, 2003, 10:43 PM   #17
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<<<-- Originally posted by Peter Jefferson : compression is used to confine teh freq distribution range of certain frequencies..
... compresion is "tightening " up the freq fluctuations of the signal.. ... compression it wont do much for noise reduction thou...
... [a compressed signal'll...] make it harder to rectify later on as the freq are bought closer together, and it'll end up being a lot more work to segregate them again if you wnat to do so in teh future. -->>>

Forgive me for being picky, but words have meanings and people can be confused if terms are not used consistently. I believe you are correct but are using the term "frequency" when/where what is really meant is "gain" or "level".

That being said, compression does not work in the frequency domain, at least not unless one is using a sidechain and filtering or eq with a higher-end compressor in order to do frequency-dependent compression. But that's an advanced recording and pro sound technique and I believe is not what you are referring to.

Instead, compression works in the gain domain. The idea behind compression is to try and limit the overall dynamic range of a signal.

A compressor is basically a bandpass (in the frequency domain) amplifier that samples the gain level of the input signal and the rate of change (attack, first derivative of) the input signal. The output (gain) of the amplifier is modulated as a function of those two control parameters in such a way that the dynamic range of the output is lower than the dynamic range of the input.

A compressor can help keep noise out of a signal during recording by allowing one to run a signal "hotter" through the signal chain without running into headroom problems and clipping, since the dynamic range in the compressed signal is lower. In other words, since the gain level of the highest-gain transients and passages is lowered, the overall signal gain can be lifter higher above potential noise and the overall noise floor.

All that being said, you are correct that in general, compressing a noisy recorded signal in general will not help to ameliorate the noise, either in absolute terms or as a matter of perception.

As to recommending threshold, attack, release, hard/soft knee, and (in the case of a compressor/limiter) the limit etc, settings, it is best to either experiment, perhaps starting with manufacturer-recommended settings as a starting point.

BTW, by all accounts a really sweet analog stereo compressor is the RNC (Really Nice Compressor.)
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Old May 24th, 2003, 12:01 AM   #18
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Hi Lance,
the reason i kept it in very lamens terms was due to the fact that i didnt want to confuse the situation by goin on about compression and its variable uses.

in this case, compression was considered to lower noise.
I totally agree with your comments re: compression works on a gain level, however compression does indeed lower or lift the raw freq (as well as gain) even without a bandbapss filter or EQ or shelf, depending on how its used of course.
When i referred to "lift the freq", i should have worded it , "increased the gain of the frequency", so thats my mistake..

That said, in this case we're dealing with post production compression, and my suggestion was to not compress the noise as it may be detrimental to future EQ and noise reduciton repairs.

thats the beauty of Audio these days, u can pretty much do anything with anything...

In this case for noise reduction, i was focussing on compression used for that noise reduction purpose.
I hope i didnt confuse anyone, i jsut didnt want to go too far into the depths of compression as i dont think it would help in this case.

If you are after a nice analogue style software solution, TRacks 24 is absolutely incredible...
Its a standale eq, compressor and limiter modelled on ye ol analogue workmanshit...
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Old June 18th, 2003, 03:34 PM   #19
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A notch filter is useful to take out small groups of frequencies.
So, they're best used on hums (60 Hz from electrical group loop,
camera noise, fan noise) which are generally low-mid single
frequencies that are often acompanied by harmonics. When
there are a few harmonics or inharmonics, the sound is called a buzz. Hiss is generally spread across all frequencies, but is
most troublesome in the mid and high frequency regions. For
such wide-band signal, use a low pass filter or a high frequency shelving filter with the filter frequency at 5000 Hz or as low as you can get it without significantly dulling your material.
Most audio editors only offer 2nd order (2 pole, -12 dB/octave)
filters, so if you can manage to serialize two or more, you can
get a steeper rolloff and preserve the brightness of the
material you want to keep.

As for the source of hums, the 60 Hz group loop can often
be eliminated by plugging all of your eletrical devices into
the same outlet and eliminating extension cords. Often,
the 60 Hz hum has 120 Hz and other harmonics, so you need
to notch them out, too. . My Sony VX-2000 seems to add
a large 15217 Hz spike. While you often can't hear this,
these inaudible signals will mess up your level meters and
other signal processing like noise gating. You can identify
these hums by taking a respresentative sample of your noise and
using SoundForge, CoolEdit, etc. as a spectrum analyzer.
Increase the number of bins to the max. Then, sweep the
frequency a parametric equalizer with max boost, high-Q or smallest bandwidth. When you hear a single frequency
boosted, that's one your your hums. Then, decrease the
gain on that eq until you don't hear the hum. Use the minimum
amount of reduction because these filters will also attenuate
neighboring frequencies.

Most of the hiss is probably from your microphone preamps and
A/D circuitry, either on the camcorder or in your mixer.
Consider turning off or unplugging all extra audio inputs.
Position your microphone close to your source so you don't have
to amplify weak signals.

As for post production, the easiest is to use a low pass or high shelf filter on hiss and several parametric filters on hums.
I've tried the Sonic Foundry noise reduction. It's pretty good,
for any reasonable amount of attenuation, you will be left with
a dull soundtrack with robotic artifacts. A noise gate will
kill the hiss most apparent when your talent isn't speaking.

As for the the fellow with the high shelf/300 Hz, issue, were you
able to set the gain of the high shelf ? You should be able to
get at least -20 dB above the cutoff frequency. Stack two or more
if possible. I get a major reduction even with one if I set the gain to -20 dB and the frequency to 5000 Hz.

As for compression, it will amplify your ambient noise. So, use a noise gate/reduction wired *before* the compressor.
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Old June 18th, 2003, 04:55 PM   #20
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Ow, my brain hurts. Does Vegas 4 have a low pass filter? As you said, hiss is spread across most frequencies, and using the track eq effect, if I do a high shelf, I have to cut everything from 20,000 hz down to around 640 hz to eliminate most (still not all!) of the hiss, and of course then it sounds like crap.
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Old June 19th, 2003, 01:47 PM   #21
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Regarding the Vegas Video Track EQ, you made the right choice in using the High Shelf. Your goal is to assemble a filter with a steeper rolloff by using 2-4 high shelf filters all set to the same frequency. The It's a pain to adjust 4 filters, but you'll be able to save more of your material. Start by setting each of the four filters to: 5000 Hz frequency, gain = -Inf, rolloff= -12 dB.
Then, toggle them on one by one to hear the effect. Then,
drop the frequency in 250 and 500 decrements, band by band, auditioning each time.
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