User manual PHASE LINEAR AUTOCORRELATOR

[. . . ] Phase Linear Model 1000 Autocorrelator Noise Reduction System You have just purchased the most advanced and most versatile noise reduction and dynamic range recovery system available. Proper use of your Phase Linear 1000 will allow you to reproduce music with less noise and greater dynamic range than previously thought possible. We hope you will read this manual thoroughly in order to appreciate all the benefits of the 1000. Recording studios and record processing companies employ a number of "necessarily evil" techniques brought about by the state of the recording art. [. . . ] You should make a mental note of the final setting of this adjustment in the event that it is inadvertently moved THE DOLBY NOISE REDUCTION SYSTEM AND THE AUTOCORRELATOR The Autocorrelator differs from the Dolby system in that it is a single pass or open ended system which removes noise from the source without the necessity of the source material undergoing special encoding. The Dolby system is a noise preventing system designed to prevent additional noise from adding to the signal during tape recording. The Dolby system is a two pass or closed system which cannot remove noise once it has contaminated the signal, but prevents it from increasing during the tape recording process Both systems provide approximately 10dB of signal to noise improvement. It is possible to obtain a full 20dB increase in signal to noise by operating the Dolby system in tandem with the AUTOCORRELATOR. A tape recording that has been "Dolbyized" during recording may be "de-Dolbyized" by a Dolby decoder, then processed through the 1000. An interesting psychological phenomenon occurs when high frequency hiss is removed from the program source. It often seems that the absence of hiss is accompanied by a loss of high frequency program material, when in fact, no loss has occurred. At first, with certain kinds of music, the psychological suggestion that high frequency material has been removed along with the hiss is very difficult to overcome. With other kinds of music, it is very obvious that only hiss has been removed. However, after a brief exposure to noise-tree music, this psychological phenomenon seems to vanish as one becomes accustomed to a noise free background. Once acclimated to a noise free background, it becomes rather unbearable to listen to hisscontaminated music. Historically, it took several years before Ray Dolby was able to convince the world his system was able to reduce noise without affecting the high frequencies. This situation was caused by the aforementioned psychological effects. PEAK UNLIMITER/DOWNWARD EXPANDER Actual operation of the Peak Unlimiter circuit is relatively simple. Initially, set threshold control to the full counterclockwise position with the circuit engaged and source turned on. Gradually rotate the threshold control knob clockwise until the peak indicator lamp flashes on the peaks of the program material. The indicator lamp should NOT stay on all the time. No setting is necessary for the downward expand circuit since it is automatic CHARACTERISTICS OF THE EXPAND CIRCUIT The threshold adjustments will vary with the type of program material being played due to the different degrees of compression and gain riding used at the studio. You will find that the threshold setting for phono is different from either FM or tape sources. Generally the more compression used in recording and broadcasting, the more difficult it will be to make the indicator lamp flash on the peaks. It will want to conti nuously stay either on or off, which is not an optimum setting. This is because there are few if any peaks to expand due to excessive compression. [. . . ] The secondary circuits are located throughout the harmonic band, if above a level of -65dB, will cause each window associated with the appropriate harmonic to open. If energy appears in the harmonic band at a particular frequency, each window associated with each upper harmonic of that frequency will be opened. Notice that as the frequency increases, fewer and fewer windows are required to be opened. Notice also that for the autocorrelator to recover a harmonic "buried" beneath the noise, it's necessary that the harmonic have associated with it a lower frequency fundamental whose energy level is above the noise. [. . . ]