Forums

Removing White Noise

Started by Duane September 22, 2005
Jerry Avins wrote:

> Randy Yates wrote: > >> "Duane" <duanestorey@excite.com> writes: >> >> >>> Hey everyone, >>> [...] >>> I'd like to remove white/pink >>> noise from the signal somehow. >> >> >> >> Hey Duane, >> >> Yeah, I've got an algorithm that can do that. Let me finish up on this >> perpetual motion machine and I'll send it to you. > > > Poor Herman (I knew him*, Horatio) might take issue with you, as I > describe (with very broad brush strokes) yesterday. > > Jerry > _______________________________________ > * I politely chewed him out once. The push-pull output stage of his > power amplifier used the filament of the 12.6-volt low-level tube as its > load in order to minimize hum pickup. The usual 200 microfarad bypass > 'lytic was there. Before I knew any of this, I pulled the input tube to > test its Gm -- it was OK -- and replaced it. The filament blew out with > the surge from the cap, and it took me a long time to find that. (The > flash doesn't show up with a metal tube; 12SJ7.) After all, I knew the > tube was good because I had just tested it. Once I figured out it was > dead, I plugged in a new one and promptly blew that. A warning label on > the chassis would have been nice!
The filtering technique sounds like a clever idea -- if you wanted to be pedantic you could say it's an adaptive Wiener filter. Terminating the power amplifier load on the filament of the input tube sounds clever, too, in an egghead-physicist-surrounded-by-smoke sort of way. In fact, that's the sort of thing that I'd do, which is why my coworkers often won't let me design hardware without careful peer review. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Tim Wescott wrote:
> Jerry Avins wrote: >> ... The push-pull output stage of his >> power amplifier used the filament of the 12.6-volt low-level tube as >> its load in order to minimize hum pickup. The usual 200 microfarad
Check that: not load, but cathode resistor
>> bypass 'lytic was there. Before I knew any of this, I pulled the input >> tube to test its Gm -- it was OK -- and replaced it. The filament blew >> out with the surge from the cap, and it took me a long time to find >> that. (The flash doesn't show up with a metal tube; 12SJ7.) After all, >> I knew the tube was good because I had just tested it. Once I figured >> out it was dead, I plugged in a new one and promptly blew that. A >> warning label on the chassis would have been nice! > > > The filtering technique sounds like a clever idea -- if you wanted to be > pedantic you could say it's an adaptive Wiener filter. > > Terminating the power amplifier load on the filament of the input tube > sounds clever, too, in an egghead-physicist-surrounded-by-smoke sort of > way. In fact, that's the sort of thing that I'd do, which is why my > coworkers often won't let me design hardware without careful peer review.
My beef was no warning on the chassis -- there was one in the manual. It was a good design. Smartypants here didn't RTFM. Whose fault was that? Avoiding hum in tube power amplifiers with honest low-end performance was not trivial. We used a lot of techniques you probably wouldn't think of but whose effectiveness you'd recognize on sight. Herman later sent me printed labels to apply to any chassis that came to my shop. (And two replacement tubes. His idea of a joke.) Another case where "it's in the manual" isn't enough: There was a radar designed early in WW II when everything was hurry up and ship it. There were a lot of metal tubes in the design. One advantage of metal tubes is the shielding they provide. To make the shielding effective, a base pin is provided to ground them. A shield only needs to be an AC ground, so that pin was used as a convenient B+ tie point, killing two birds (and probably more radar techs) with one stone. I worked on one of those at school, and doubted the instructor when he warned us. I checked anyway. A meter showed that every tube exterior was hot by 400 V or more. Eddie Stinson, the aircraft designer, had good reason to say, "I can make it foolproof, but I can't make it damn fool proof". (The Voyager prototype was foolproof. Stinson himself was the damn fool.) All I ask is foolproof. Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
Fred Marshall wrote:
> "Duane" <duanestorey@excite.com> wrote in message > news:1127408349.860752.243720@g44g2000cwa.googlegroups.com... > >>Hey everyone, >> >>I have an audio application, and I'm trying to do some pre/post >>processing to improve the sound quality. I put in an adaptive filter >>to try and remove structured noise, but I'd like to remove white/pink >>noise from the signal somehow.. I've been looking at wavelet >>techniques, or whatever else I can find. >> >>Can anyone recommend a good way to reduce white noise from audio/ >> >>Also, in general, any suggestions on how to improve voice quality? > > > Duane, > > I've not done it .... but > I'd conceptually start with an Adaptive Line Enhancer (ALE) design that > removes white noise by passing the entire signal through an adaptive filter > that bandpasses the tones. I'd then ponder what changes would be needed for > an audio application. > > The success of an ALE depends on long term stability of the desired signal. > Since you don't have that situation exactly in audio then one might imagine > that the time constants would be shorter, the passbands wider, etc. Maybe > this hits a dead end and maybe not. > > I think it boils down to the method that Jerry mentioned - perhaps with a > different implementation approach. > > Cool Edit has / had? a filter to remove scratches and pops from phonograph > records - maybe that would work for you. I think it hurts the high > frequencies though... maybe not...
Scratches and pops are relatively easy to recognize and remove. they are so brief that filling in the gap left by removal is easy, and not a terrible task to ignore. People used to clean up tapes of old vinyl and shellac recordings by scratching out the pops with a needle after precisely locating them on the tape. The people who restore records now keep their methods secret, but only the details. The broad outlines are known. Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
"Jerry Avins" <jya@ieee.org> wrote in message 
news:k4qdnYCzvuSfuKneRVn-iQ@rcn.net...
> rhnlogic@yahoo.com wrote: >> Jerry Avins wrote: >> >>>Duane wrote: >>> >>>>Also, in general, any suggestions on how to improve voice quality? >>> >>>If the poor quality is caused by poor frequency response, equalizers can >>>help. (They can improve balance, but they can't restore missing >>>frequencies.) If that's not the case, singing lessons might be in order. >> >> >> Don't they make such things as rack-mount autotuners, which can re-tune >> a performers singing voice back on key? There are also vocoders, which >> can make someones voice sound like a robot, a distinct improvement for >> some "singers". I suppose an additive vocoder could even be used to >> restore some of the missing frequencies, if you have some external >> knowledge (a newer recording of a similar song by the same artist, >> for instance) of what might be missing on some particular vowel tone. > > You must be right. They frequently do that on TV shows.
Yes, that technology exists (and has for 5-10 years, IIRC). You can even generate background vocals in real-time based on a lead-vocal input which is pitch-shifted based on a MIDI or other indication of what the harmony note(s) should be. RB-J may have even worked on such products?