What does mean 'bias' ? For instance in DAS digital analos sound device or I have seen it in a lot of other plugs... I remember I saw it on magnetic tape before cd writer exists. I don't find any satisfactory answer.
Thanks !
Bias ?
Re: Bias ?
A boost in current to improve the quality of recording to tape.
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jksuperstar
- Posts: 1638
- Joined: Mon Nov 15, 2010 12:57 pm
Re: Bias ?
Tube BIAS tends to mean "DC Bias"...a shift toward positive or negative.
In tube amps, if the amp was class A, then 1 tube is driving the full voltage spectrum (- to +), and the Bias sets the center point. In this case, pushing the bias toward positive gives a more asymmetrical waveshape....meaning more distorted when overdriven on the positive side, while less driven on the negative side. That creates various types of distortion from "warmth" through fuzz to fully overdriven. Adding gain in an asymmetrical vs symmetrical way, in terms of synthesis, is like creating a saw wave vs. a square wave. They have different combinations of odd vs. even harmonics, and sound different.
In Class B, the positive is driven by 1 tube, while the negative is driven by another tube. In this case, setting the Bias really means tuning the 2 tubes to become a matched pair, with minimal overlap in performing their duties. If not matched, you can get anything from clicks to loss of power to prematurely blown tubes.
There is some other forms of Bias, matching current as well as voltage (depending on the circuit). But in this case, most discrete emulations are trying to recreate the Class A situation, and so it more or less controls the symmetrical vs. asymmetrical nature of the distortion.
In tube amps, if the amp was class A, then 1 tube is driving the full voltage spectrum (- to +), and the Bias sets the center point. In this case, pushing the bias toward positive gives a more asymmetrical waveshape....meaning more distorted when overdriven on the positive side, while less driven on the negative side. That creates various types of distortion from "warmth" through fuzz to fully overdriven. Adding gain in an asymmetrical vs symmetrical way, in terms of synthesis, is like creating a saw wave vs. a square wave. They have different combinations of odd vs. even harmonics, and sound different.
In Class B, the positive is driven by 1 tube, while the negative is driven by another tube. In this case, setting the Bias really means tuning the 2 tubes to become a matched pair, with minimal overlap in performing their duties. If not matched, you can get anything from clicks to loss of power to prematurely blown tubes.
There is some other forms of Bias, matching current as well as voltage (depending on the circuit). But in this case, most discrete emulations are trying to recreate the Class A situation, and so it more or less controls the symmetrical vs. asymmetrical nature of the distortion.
Re: Bias ?
!! I didn't know this at all !!
Adding dc or better, high frequency ac current to the signal for magnetic tape ! Very interresting. Does it have any link with dithering ? I guess it has to do with hysteresis magnetism when the field change direction, without adding curent, the signal would be ugly as Jk explains...
I imagine the tape players had some feature for reading the signal properly ?
Thanks a lot for the info !
Adding dc or better, high frequency ac current to the signal for magnetic tape ! Very interresting. Does it have any link with dithering ? I guess it has to do with hysteresis magnetism when the field change direction, without adding curent, the signal would be ugly as Jk explains...
I imagine the tape players had some feature for reading the signal properly ?
Thanks a lot for the info ! -
hubird
Re: Bias ?
Can't be, it's about a 'physical' process of magnetizing tape coating, not a digital calculation process.JoPo wrote:!! I didn't know this at all !!
Adding dc or better, high frequency ac current to the signal for magnetic tape ! Very interresting. Does it have any link with dithering ?
But the link to dithering is interesting Jopo. Wiki says:
…[O]ne of the earliest [applications] of dither came in World War II. Airplane bombers used mechanical computers to perform navigation and bomb trajectory calculations. Curiously, these computers (boxes filled with hundreds of gears and cogs) performed more accurately when flying on board the aircraft, and less well on ground. Engineers realized that the vibration from the aircraft reduced the error from sticky moving parts. Instead of moving in short jerks, they moved more continuously. Small vibrating motors were built into the computers, and their vibration was called dither from the Middle English verb "didderen," meaning "to tremble." Today, when you tap a mechanical meter to increase its accuracy, you are applying dither, and modern dictionaries define dither as a highly nervous, confused, or agitated state. In minute quantities, dither successfully makes a digitization system a little more analog in the good sense of the word.
So, 'trembling' the tape with current, to make it more 'accurate', why not
Like the 1 bit black-and-white (or 2 color) comic paper technic of points division on a surface.
Interesting reads in the links by the way
Re: Bias ?
Ditherind is older than digital processing ! 
In hydraulic proportionnal valva, there is dithering to control it as accuracy as possible. As there is always small high frequency vibration, the drawer can move as soon as a coil push or pull it without any jolt. I "guess" bias is about the same function : as the magnetic field is always moving (because of ac curent ; curent = magnetism), the most little signal will be recorded on the tape without distorsion.
A bulldozer can take an egg without braking it !
I didn't know it was invented for bombing !
In hydraulic proportionnal valva, there is dithering to control it as accuracy as possible. As there is always small high frequency vibration, the drawer can move as soon as a coil push or pull it without any jolt. I "guess" bias is about the same function : as the magnetic field is always moving (because of ac curent ; curent = magnetism), the most little signal will be recorded on the tape without distorsion.
A bulldozer can take an egg without braking it !
I didn't know it was invented for bombing !
Re: Bias ?
actually it is similar to dither in a way. yes, playback has an opposite filter so that you don't hear the process. this is why high-end studio tape decks are "aligned" every session or so, to maintain optimal function. since tape is mechanical, tape and heads get just a little more worn out every pass.
Re: Bias ?
There is no better day than when I learn something about science or technology. And now, I know what I'm doing when I turn a 'bias' knob in a magnetic tape simulator plugin.
Merci !
Merci !