Audio Cleaning filters?

Quu · Post by **Quu** » Tue Jul 29, 2003 1:03 pm

I pretty much have a good handle on how to clean up video footage... if I am given an old VHS tape of a music video... the video portion is fixable...

but I know next to nothing about audio cleaning? given a wav file... how hard is it to clean...

yea... that's vague... but examples of audio damage would be
Compression (someone used mp3 source... i need to re compress it to AC-3 or Layer-II... is there a pre filter to clean up the audio slightly... like doing a msmooth on video before compressing it?)
over driven (someone doubled the audio volume when recornding to tape)
under driven... the audio uses 20% of the avaible range... (can it be increased to 95%... with out a sinple volume adjustment... to help with quality?)
reduced quality... (someone stored the audio as 22 khrz and/or 8 bit)

I am ultimatly looking for a nice setup of standardized steps that I can take older audio and store it in a 192-320 kbps AC-3 file

zalas · Post by **zalas** » Tue Jul 29, 2003 5:31 pm

A lot of the problems you stated cannot be cleaned very well.

(I'm not gonna use the quote command because it's too big)

> Compression (someone used mp3 source... i need to re compress it to AC-3 or Layer-II... is there a pre filter to clean up the audio slightly... like doing a msmooth on video before compressing it?)
I don't think audio compression introduces "blocks." Hence you can't actually smooth it out. Most audio compression (DCT based ones) give watery sounds, and I haven't found something that would clean that...

>over driven (someone doubled the audio volume when recornding to tape)
Best you can do is low pass filter it. Overdriven means you've lost a lot of the original signal... (Think of a video clipped in brightness, you can't *really* do much about it)

>under driven... the audio uses 20% of the avaible range... (can it be increased to 95%... with out a sinple volume adjustment... to help with quality?)
I think volume adjustment is your best bet (try adjusting it by an integer multiple, as long as it's a linear coding scheme (not a logarithmic AC3 type one))

>reduced quality... (someone stored the audio as 22 khrz and/or 8 bit)
Nothing much you can do there, I think. Granted, you can "hi-pass" the audio or something, but that just doesn't work. It's like sharpening an image. You might be thinking you're getting quality back, but unless you're using a smart sharpen and you know stuff about the blur filter, it's really hard to get quality back. You might be able to get some results by slightly boosting the higher end of the spectrum... As for 8bit, you can try low pass filtering it to remove some of the quantization noise (same idea in video is like a nice color image being mapped to 256 colors or a 7/6/6 palette)

madmallard · Post by **madmallard** » Tue Jul 29, 2003 7:49 pm

soundforge full version should be able to do a lot of things to audio.

um, if you're trying to kill hiss, you should be able to record a sample of the hiss you're trying to kill from the tape itself, and i think Soundforge should be able to apply the sample as a type of filter.

for over driven signal, there isn't much to be done. You can try to give it a new celiing by reducing the volume, increasing the resolution, then applying some type of DSP just to get it to break the distortion line. I've only had nominal results doing this. a low pass filter probably wont make much of a difference because its seldom the bass that distorts, and even if it is, you cant tell as easily when bass distorts as you can when hi's do.

um. . .there's a number of things for reduced audio quality depending on what it sounds like now.

if its sounds muddy, resample to 44.1khz or higher, run a low pass filter, use interpolation filter in soundforge to fill in the new resolution gaps, then eq gain up the midrange (4k~8khz, you'll have to play with it.)

if it sounds tinny or artificial because of low rez, resample to high rez and eq gain down anything above 17khz, and fatten up the low-mids (800hz~2khz)

zalas · Post by **zalas** » Wed Jul 30, 2003 3:02 pm

sixstop wrote:for over driven signal, there isn't much to be done. You can try to give it a new celiing by reducing the volume, increasing the resolution, then applying some type of DSP just to get it to break the distortion line. I've only had nominal results doing this. a low pass filter probably wont make much of a difference because its seldom the bass that distorts, and even if it is, you cant tell as easily when bass distorts as you can when hi's do.

Well, when you overdrive a signal, you technically get harmonics of the original signal. For example, at the extreme, I overdrive a sine wave, and I get back a square wave, which is just the sine wave and all its odd harmonics. Hence the recommendation for low pass filtering. Typically, when you get distortion due to overdrive, you're gonna get a lot of harmonics, and since they're at least double the frequency of the original signal, you can get rid of a lot of the harmonics by cutting out the higher half (granted this also takes out a good amount of your signal).

As for hearing, I tend to hear bass + midrange distortions more. Bass distortion creates buzz at extremes. The really high pitched distortions just make the audio sound not as clean.

madmallard · Post by **madmallard** » Wed Jul 30, 2003 5:28 pm

but the 'overdrive' he's talking about is signal clipping. i just said 'overdrive' again for familiarity. Its not the same principal as overdriving a current.

when the signal clips, particularly from being gained up too much on a digital file, or just an analog signal clip, i've have yet to see any example where the bass is distorting except in bad DJ mixes.

zalas · Post by **zalas** » Tue Aug 05, 2003 12:42 pm

Technically, clipping is equivalent whether you do it digitally or analog(err)ly. All it is is that the top of the waveform gets chopped and replaced with a horizontal line across the top. There are some audio differences between harsh straight clipping and smoother clipping (found more in audio equipment since you don't get exact clipping in transistors and even less exact clipping in tubes)

The reason you don't hear much bass distorting would be that the original signal did not have as much bass in it. However, if you take a bassy signal (like DJ mixes, since dance beats concentrate on bass beats), you will hear the bass distortion at lower clipping levels, and it'll be more prominent than in a non-bassy signal.

Quu · Post by **Quu** » Tue Aug 05, 2003 1:45 pm

so basically there is not a magic cocktail I can use to fix old VHS audio recordnings....

crap...

there is a magic cocktail for video... was hoping to automate the audio also

madmallard · Post by **madmallard** » Tue Aug 05, 2003 5:39 pm

the problem is too variable.

you can sample one frame of flawed video and usually get to the probs across the board.

you can't sample one. . . .well, sample of music and do the same thing because its too miniscule for the brain to process, and the problem dynamic has changed already by the next sample.

A truly imperfect process compared to video. . . .

post-it · Post by **post-it** » Tue Aug 05, 2003 5:48 pm

used to be able to do this ^^ the files were called .snd and .rol/.mod
today we use .wav and .mid
( I know I've got that stored on CD-R somewhere - but I'd need a catalog program to find it T_T )
can't even recall its name, but what you are discribing is exactly what this software did ^^

now, if there were a way to wav2snd and snd2wav then I might just have to look that program-up again ^^

zalas · Post by **zalas** » Tue Aug 05, 2003 7:19 pm

What exactly are you talking about?

And I don't see why there wouldn't be snd2wav converters if snd is just a
sampled format. I remmeber a TON of converters back in the day when
there were a billion formats floating around.