wjm (12/13/82)
As I mentioned in an earlier article, there are 3 types of tape noise reduction in common use: Dolby B (10 dB of noise reduction) Dolby C (20 dB) dbx (>30 dB) Briefly they work as follows: Both Dolby systems are "pre-emphasis/de-emphasis" systems. They boost the signal level above a certain frequency by a predetermined amount (which depends upon signal level and frequency) when recording and then cut it by the same amount on playback. The effect is that the boost and cut cancel each other out and the same signal comes out as went in. However, the tape hiss was not boosted and is reduced by the cut in the playback process. Since the Dolby system is level sensitive, tape decks are provided with adjustments and test tones to calibrate it. The difference between Dolby B and Dolby C is the amount of boost/cut that is applied to a given input signal. Dolby B tried to keep some compatability where undecoded Dolby material could be listened to (albeit with a treble boost) and this limited the amount of noise reduction it provided while Dolby C is designed to provide more noise reduction without compatability. dbx is a "compander" system. In the dbx system, the dynamic range of the input signal is compressed 2:1 when recording (the loud passages are made softer and the soft passages louder). On playback, the process is reversed, and the dynamic range is expanded back to that of the original signal. At the same time, the noise (which is usually softer than the signal) is expanded to a softer level (into virtual oblivion). There are two major advantages to dbx over the Dolby systems: it is not level sensitive and it provides more noise reduction. However one cannot listen to dbx material without decoding. I'll repeat my recommendations from the earlier item. Since dbx is the most effective noise reduction system, it is the one to get for home use. Many new decks have it built in or one can add dbx's outboard processor (a 222 for 2 head decks or a 224 to allow simultaneous monitoring from a 3 head deck). I expect it to be available soon in car decks (if it isn't already). For compatability for use with "Walkmen" and pre-recorded tapes the deck should also have Dolby B. Generally, most mass-market pre-recorded tapes use Dolby B and the "audiophile" ones give you your choice of noise reduction systems (I'll take dbx, thank you.). Bill Mitchell BTL Whippany harpo!wjm