Getting ELF Out of a Sound Card Using EMF Cancellation

On a sunny day (Mon, 20 Nov 2017 02:45:09 -0800 (PST)) it happened snipped-for-privacy@gmail.com wrote in :

If there is one thing that is still stuck with me after all those years it is: RF bias symmetry to reduce recording noise.

The professional audio recorders had an adjustment for RF bias symmetry, and often used a balanced oscillator (Studer). The low end sold as high end had just some one transistor oscillator with not exactly a symmetric waveform (Philips).

That is the difference between -40 dB and -70 dB noise in recordings.

LOL I once had a dictating recorder with DC bias, and a permanent magnet for erase :-) had variable speed too (no capstan IIRC). It is all possible....

Wow and flutter measurements... frequency response, part of the job. Wow and flutter on perfotape is a totally different experience, especially when it starts up :-)

I was talking about cassette tape players, not 1/4" tape or multitrack studio tape recorders. I agree with you. There were good 1/4" tape decks and recorders available in the 1960's and later. At the time, I had a small collection of Akai and Roberts 1/4" tape recorders that I liked. However, the cassette stuff was not much better than junk even when some companies put cassette drives into impressive looking packages.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

I had an Akai 1/4" deck that was pretty good, even though it was on the lower end of the line. Nakamichi had some pretty good cassette decks in the '70s. I had a Yamaha that wasn't too bad (couldn't afford a Nak). Metal tape was a big win (though $$).

Den mandag den 20. november 2017 kl. 19.16.58 UTC+1 skrev snipped-for-privacy@notreal.com:

S+S is amazing. After you play with it for a while, it becomes intuitive and you see it everywhere. A few basic concepts become instant simplifications of complex situations, like conservation of energy solves some complex problems instantly.

One consequence of the the forward and reverse Fourier transform is that some problems that are hard to think about in one domain become easy in the other.

We have a lot of fun with stuff like that.

S+S is often taught as a heap of equations. I was lucky to be doing electronics (like, designing modems) when I took the course, so I saw the concepts. The other guys were grunting math while I was having revelations.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Sure, it's good to be able to think in both domains. (You come across as a bit time-centric. :^) Maybe I can learn the difference between a Fourier transform and a Laplace. (I always just replace the s with an i*omega. (j*omega for EE's) But the 'real' Laplace transform has a real part in it... I never figured out what that was for.

The best way to get a grip on FFT is with a digital 'scope.

In grad school I dealt with Green's functions sometimes. I'd follow along in the math, but was always somewhat confused. Then later in life I learned that Green's functions are the impulse response of the system. Light dawns, and all was much clearer.

George H.

Did you miss the bit about rectifying to capture the beat envelope?

--
Regards, 
Martin Brown

Indeed I did. I got snared by your contorted phrasing.

Jeroen Belleman

You record a fixed tone on a spare flutter compensation track and subtract the demodulated compensation track from the others.

I did it with a cassette recorder using a four track head for three geophone (triaxial pack) signals before digital storage was realistic. Quite a few were made, battery powered and timer controlled, mostly for recording vibrations at various points around an explosive demolition site.

Cheers

--
Clive

On a sunny day (Tue, 21 Nov 2017 11:07:47 +0000) it happened Clive Arthur wrote in :

That is actually also how VHS color phase compensation works.

In record the horizontal sync frequency from the luminance is PLL multiplied to about 560 kHz, and added to the 4.43 (PAL) chroma to make what was it 4.99 MHz or so, The 4.43 MHz chrominance signal is then subtracted from the 4.99 MHz to create a 560 kHz chroma, and superimposed on the much higher (few MHz) FM luminance carrier, using it a RF bias.

In playback the now 560 chroma is easy to filter out with a lowpass. That 560 kHz is then subtracted from that 4.99 MHz that is now locked to the playback horizontal sync and changing with it, leaving 4.43 MHz time compensated chroma again.

The result is a few ns stability for the chroma, even with huge tape speed changes. Very old system from about 1976.

Subtract how? If you literally mean subtract, that will inject the comp tone into all the other signals.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Sorry, probably lost the context. This is FM tape recorder to allow very low frequencies to be recorded, so a constant tone demodulates as a constant voltage with added ripple corresponding to tape wow and flutter. You subtract this from the other demodulated signal channels with eg an op-amp.

Cheers

--
Clive

Linear subtraction of different frequencies? I can't see how that could work.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

I suspect you're thinking this is something complicated, it isn't.

Four track cassette tape.

Low frequency AC FM modulates a (say) 1kHz centre frequency carrier which excites one winding of the four track tape head. Three of these each to a separate track.

Fixed 1kHz modulates a fourth head winding. Wow and flutter (tape slowing and speeding due to being crap) mean that this signal will vary around it's recorded 1kHz when replayed. When demodulated there is a DC corresponding to 1kHz with a superimposed voltage signal corresponding to the wow and flutter.

The signal channels are demodulated to recover the original LF AC signals which also have the superimposed wow and flutter signal. So you subtract it using the demodulated 1kHz reference track.

Cheers

--
Clive

Not so fast. For both rigour's sake and as a courtesy, we probably ought to at least mention _Reference Data For Engineers_ (

), which Steve Wilson kindly posted a few days ago. "Table 1. Nomenclature of Frequency Bands" appears on page 1-3.

Thank you,

--
Don Kuenz, KB7RPU

How low are you trying to go? I was having problems with the analog out of my work PC (Dell 7020). Flat to 50Hz but like 30dB down at 20 Hz. 1KHz squa re wave looked so bad as to be unrecognizable as a square wave. Got a StarT ech USB audio interface. 10 Hz square wave