|MLA Citation:||Bloomfield, Louis A. "Question 274"|
How Everything Works 17 Jul 2018. 17 Jul 2018 <http://howeverythingworks.org/print1.php?QNum=274>.
The term "BIAS" refers to a technique used to assist magnetic recording of weak audio signals. The magnetic particles on a tape's surface do not magnetize easily and need help when quiet sounds are being recorded. To provide that assistance, the tape recorder superimposes a strong, high-frequency "bias" signal on top of the weak audio signal. This inaudible bias signal allows the weak audio signal to influence the tape's magnetization. The characteristics of the bias signal must be adjusted to match the tape type.
The term "HX-Pro" probably refers to a variable bias techniques that prevents the bias signal from saturating the tape's magnetization and wasting some of the tape's dynamic range. (I'm less certain of this observation—please tell me if I am wrong and I'll fix this remark).
The term "Dolby Noise Reduction" refers to a collection of techniques for reducing high frequency noise on magnetic tapes. The higher the sound frequency, the smaller the patches of tape surface that are used to record each cycle of the sound. Since the recording occurs by magnetizing individual particles that are almost a micron long, the cycles of a high frequency sound do not use very many of the particles. A few miss-magnetized particles in each cycle can produce noticeable noise in the reproduced sound. To counter this noise, Dolby boost the volume of high frequency sounds during recording and then reduces their volume back to normal during playback. The noise caused by the particles is also reduced in volume and is less noticeable as a result. The different Dolby techniques refer to different filtering protocols, with C being an improvement over B, which was itself an improvement over A.
The term "20-bit LAMBDA Super-Linear converter" probably refers to a high performance Digital-to-Analog Converter (DAC). When a compact disc is played back, the audio signal must be converted from a stream of numbers into a smoothly varying electric current, which is then amplified and sent to a speaker. Turning each number into a current requires a DAC. The more carefully this DAC is built, the more perfectly the current passing through the speaker will represent the numbers on the disc and the recorded sound information. While most DACs work with only 16 bits, the one you mention provides 4 more bits of precision. However, the compact disc contains only 16 bits of sound information, so the 4 added bits must be created by some numerical analysis on the part of the compact disc player. This sort of signal processing may lead to reduction in noise during playback, but I wouldn't expect most people to be able to hear any difference.