Behind The Scenes (Feb. 1974)

by Bert Whyte

AT THIS point in time, anyone remotely connected with audio and recording knows that Dolby noise reduction does an efficient and reliable job of reducing tape hiss during the recording process. This fact is attested by the virtual standardization on Dolby A-type noise reduction in recording studios all over the world.

On the other hand, if I had a nickel for every time an audio engineer wished he had a device that would reduce or remove noise from recorded material without degrading the signal, I'd be rich! The quest for such a device has got to be one of the Holy Grails of audio. Through the years various ideas have been tried. For example, in certain specific cases of single-frequency noise, steep notch filtering can be effective. But in general, if a tape has hiss, hum, room rumble, or any extraneous noises, that is that ... the noise is there to stay. Or is it? Hold that wistful sigh, Mr. Engineer! There is a ray of sunshine in this otherwise gloomy prospect. It is called the Burwen Dynamic Noise Filter.

Mr. Burwen gave a detailed technical description of his device in the June 1972 issue of AUDIO. With the Burwen Dynamic Noise Filter, there is no compression/expansion, encode/ decode function as in the Dolby System, but it does utilize the noise masking characteristics of human hearing, a basic of the Dolby System.

For our purposes, the operation of the Dynamic Noise Filter is neatly capsulized by Mr. Burwen.... "The operation of the Dynamic Noise Filter is based on two principles: (a) the noise output of an electronic system is dependent upon the system bandwidth and (b) the human auditory system `masks out' noise in the presence of the desired signal at frequencies in the vicinity of the signal frequency when the signal-to-noise ratio is sufficiently high. The Dynamic Noise Filter can be described as an automatically variable bandpass filter whose bandwidth changes rapidly with each musical note and whose high and low frequency cutoffs are independently controlled by the spectral content of the input signal. Noise reduction is achieved by restricting the bandwidth at high and low frequencies when the signal level is very low (the minimum bandwidth is 800 Hz). At medium and high signal levels, it passes the full 20 Hz to 20 kHz bandwidth. As in the other noise reduction systems, the filter reduces noise only for low level signals; the noise is passed along with the high level signals. However, due to the characteristics of the ear, the noise that is present during each note is masked by the music." Burwen's own catalog sheet describes the Dynamic Noise Filter as reducing noise by attenuating the high and low frequencies when there is no music present. This is rather an oversimplification as we have seen that it also operates at very low signal levels. However, it is true that in a great deal of music, there are many little pauses and rests ... cessation of sound between notes ... much more than we are consciously aware of hearing. In this circumstance, the filter works with maximum efficiency.

Since these music discontinuities are of extremely short duration, one might expect to hear the dynamic "working" of the filter. Burwen brings some psychoacoustics into play here by choosing attack and decay time constants for the high and low frequency sections of the bandwidth controller such that the action of the filter is inaudible. An attack time of 1 millisecond was found to be short enough to have no effect on musical transients.

Decay time was optimum at 50 milliseconds. A shorter attack time would have the undesirable effect of the filter being actuated by the "ticks" and "pops" of a disc recording. A shorter decay time would reduce the effective reverberation in the program material. The low frequency part of the bandwidth controller is about 14 dB less sensitive than the high frequency section because music tends to have more low frequency energy than high frequency energy.

The Dynamic Noise Filter sounded like an intriguing device, and if it worked as advertised, I had envisioned several applications where it would be invaluable. Dick Burwen was kind enough to furnish me with one of his Model 1000 units, and I found it a most useful and fascinating tool.

In a 1 3/4-in. high rack panel, up to four channels of the filter are available, with stereo channels ganged in pairs or independently. On the panel are controls for high and low frequency sensitivity, a high frequency cutoff, and a switch that permits listening to program material with the filters operating, or in the wide band mode in which the unit acts as a unity gain amplifier. The quality of the components in the Dynamic Noise Filter and the general construction is of the highest order, really of the caliber of aerospace equipment. In normal-use, the low frequency sensitivity is set so that for example ... rumble ... just begins to operate the low frequency filter. The high frequency sensitivity control is set so that hiss just begins to operate the high frequency filter. For poorer material, a limit can be placed on the upper cutoff frequency on the high frequency cutoff control.

You may recall that last month I described that when I recorded Pablo Casals in Puerto Rico, the mics picked up the rumble of the air-conditioning ducts and also 60 cycle hum. Even though I was using 3 channel, half-inch. Ampex recorders (this was before Dolby of course), there was still a small residue of tape hiss, enough to be obtrusive and annoying. Running the tape through the Burwen filter, after some experimentation with various settings of the controls, I A/B flipped between filtered output and flat, and the transformation was truly impressive. I would say that I was getting no more than 2.5 to 3 dB reduction of tape hiss, but that was enough to remove the overlay of hiss. The rumble was considerably attenuated without much apparent effect on low frequency music content. However, the 60 cycle hum was a problem, but one that was manageable by resorting to a very steep notch filter. This notch filter would be used before running the tape through the Dynamic Noise Filter.

I tried the Burwen unit on a variety of tapes. Some were 15 ips masters or copies of masters, sans Dolby, and here too, they had that miserable little overlay of tape hiss which is so distracting. On this material, there was no low frequency noise problems, so the low frequency control was not employed. Judicious twiddling with the high frequency control again removed sufficient tape hiss to make the tapes significantly quieter, while having no or very minor attenuation of high frequencies. It is a fact that the Dynamic Noise Filter discriminates best between noise and music when the tape initially has a high signal-to-noise ratio. This was borne out by these experiments on my own tapes in this category, and also when I ran some of the better quality, least noisy commercially recorded music tapes through the filter. Of course, there is the natural tendency with this sort of device to go a bit overboard in the area of hiss attenuation. This was especially so when playing poorer material. A flip back and forte-on the FILTER/FLAT switch would SOON tell you that you had indeed removed the hiss . . . but that you also had succeeded in removing some high frequencies. Even so, in some cases, with manipulation of the high frequency cutoff control, and a bit less heavy hand on the high frequency sensitivity control, you could get an eminently listenable program, more enjoyable than suffering through the unfiltered tape. Another use of the Burwen filter is when you are making an improved tape copy with equalization. Burwen also claims that in recording at 7 1/2 or 15 ips, the un weighted tape noise reduction is typically 10 to 11 dB. Needless to say, the filter made considerable improvement in the playback of some of the earlier non-Dolby cassettes and 8 track cartridges. I should point out at this juncture that almost everything in life that is good, costs money, and the Burwen is no exception. It is, in fact, an admirable tool for the audio professional, who is probably better prepared to pay the $3500 for the filter than even the most earnest of audiophiles!

As you might expect, such a useful item as the Burwen Dynamic Noise Filter is finding much application in professional ranks, especially in FM broadcasting. The reason for this is that many stations are totally programmed for the special broadcast 8-track cartridges, and they suffer from S/N ratio problems, just the same as those used by consumers. One well-known New York FM station which uses cartridges say they get a 48 dB S/N ratio from these cartridges, a lot of which turns out to be 60 cycle hum and high frequency hiss. The chief engineer of the station says that with as many as six cartridge machines potted up at one time, the additive effects of the noise really can raise havoc with their "on-air" sound. He has been using the Burwen Filter and says that hum and hiss are minimized and they are broadcasting a much cleaner signal. He inserts the Dynamic Noise Filter in his broadcasting chain just before the stereo generator and, as a consequence, can get such added benefits as elimination of low frequency motor noise, transmitted as vibration to the microphone while a DJ is on the air, significant reduction of surface noise on older LP's and 45's, and automatic "cleaning up" of news feeds from such as bad phone lines and shortwave off-air pickup.

It was a pleasure to use the Burwen Dynamic Noise Filter. It performed well and would be an item that many an audiophile would love to have in his system, were it not for that cost. You know we are always looking for fancy professional devices to ultimately be revised and reworked and brought down in cost to consumer level. As precedent we have Dolby professional A-type noise reduction, and its offspring, Dolby B-type noise reduction. Who knows? Maybe Dick Burwen has something up his sleeve!

NOTE: In my column in the December issue of AUDIO, I described the new Revox A-700 tape recorder as the "successor" to the current A-77 deck. The A-700 is in fact a new generation, higher-priced addition to the A-77 line which will continue to be marketed.

(Audio magazine, Feb. 1974; Bert Whyte)

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