Noise reduction, audio repair, audio "freshening"

[swiego]

I wasn't sure whether to break this into several posts; I'll just dump out my questions all at once!

1. What are the best solutions for hiss removal of an audio file? Of these, which are effective / most effective with higher resolution audio files, i.e. 24 bit, 96kHz captures? And a related question, are there software filters that approximate the effect of Dolby B/C/S, dbx, etc? If I have content that was Dolby NR encoded somehow, should I allow the playback device to apply the NR before capture, even if it does so imperfectly? Or is it better to capture then apply NR digitally?

2. I have several captures where the audio tends to fall out. It as as though a certain area of the tape (physical spool location) got garbled, so there is a two second drop in amplitude, every ten seconds (give or take) for maybe a minute. That sort of thing. I've experienced the same with cassette. Any way to clean this up?

3. I have an audio cassette deck by Pioneer with a technology called FLEX - Frequency Level EXpander. It's a gimmick; the deck digitizes the audio off the tape, then passes it through a DSP. The DSP is a little more inteligent than a regular EQ; it actually attempts to determine what frequencies have been attenuated due to signal fade in the tape, then applys the EQ. This isn't a fixed boost, it's constantly adjusted in real-time during playback, and on very old tapes the effect is simply extraordinary. Old, faded tapes sound new again. So, are there any equivalents over on the software side? I have VCR tapes that sound much the same, and I'd love to be able to liven them up somehow with a similarly intelligent and adaptive EQ algorithm.

Thanks for the patience with my questions!

[SaSi]

1. Tape hiss removal, an almost impossible task in the analog audio days, is now much simpler by means of fast fourier transform filters.

I am doing all my audio editing, filtering and restoration work using CoolEdit PRO v.2 - now sold as Adobe Audition v.1.5

It has a noise reduction filter which samples a small (1-2 sec) portion of the source material that has no real content - just background tape noise - and generates a filter to remove that background noise. It works wonders, as the "filter" is dynamically adjusted to the particular hiss.

Some tapes have a loud and "juicy" hiss while others have different flavours...

With Cooledit/Audition, you can operate the filters at any sampling frequency/bit resolution you want, although even if you convert a 44kHz/16bit audio file to 96kHz/32bit and apply the filter, the operation will be more precice and downsampling it back to 44/16 will produce a better result (slightly). Of course, filtering is slower this way - and you have to wait for the up-sampling / downsampling parts to finish as well.

A Dolby B playback filter is easy to produce with CoolEdit - or any other audio editor. However Dolby C and S are not static filters and you cannot easily reverse engineer them with a simple filter. I would allow the cassette deck to apply the playback filters.

Additionally, correcting the playback head azimuth is important during playback. Even if the decks azimuth is perfectly aligned, a tape recorded on an imperfectly aligned deck plays back better with azimuth adjusted for this tape specifically.

2. If the problem with amplitude is only for a minute or so, you could easily spot the low amplitude sections and increase the amplitude by the required amount for each section. A little trial and error will result in something acceptable.

3. A parametric equalizer filter is usually sufficient to boost the frequency response profile of a recording. CoolEdit has a frequency response display - in real time - and by means of the parametric equalizer filter you can boost the parts of the audio spectrum needing this.

Or, you can try recording the VCR tapes audio on tape and then play back this cassette through the Pioneer deck. This will require additional effort to sync the audio with the video stream, though.

[Soopafresh]

2) You can run the audio through a Limiter or Compressor, which will keep the volume more level.

3) BBE Sonic Maximizer will do that. There is a hardware version and a software version. This is also refered to as an "Exciter".