David E. Blackmer (January 11, 1927 – March 21, 2002) was an American audio electronics engineer, most famous as the inventor of the DBX noise reduction system and founder of dbx.
As well as audio noise reduction, Blackmer worked on extending the frequency response of audio electronics beyond the conventionally accepted audible range of 20 kHz. He also published research on the value of ultrasonic frequencies in sound reproduction, claiming that the time resolution of human hearing is 5 microseconds or better—which would correspond to a frequency of 200 kHz, requiring audio equipment ideally to have a flat response to that frequency.
Blackmer attended High Mowing School in Wilton, New Hampshire. He started in audio at Lafayette Radio in Boston in the 1940s and studied electronics in the U.S. Navy and at Harvard University and MIT. He later worked at Trans-Radio Recording Studio, Epsco, Hi-Con Eastern and Raytheon, where he designed telemetry systems for the Mercury space program. He founded dbx in 1971, selling it to BSR in 1979 and staying on with the company for several years. In the late 1980s he formed Earthworks, producing studio microphones, preamplifiers and studio reference monitors. He also founded Kintek (now Colortek) and Instrumentation Laboratory, as well as running the Cafe Pierrot restaurant in Wilton for a time.
Blackmer was a life member of the IEEE and a fellow of the Audio Engineering Society from 1976. He was also a great reader of science fiction. He had ten children.
A partial list of patents held by David Blackmer
- U.S. Patent 3,681,618, August 1, 1972: RMS Circuits with Bipolar Logarithmic Converter
- U.S. Patent 3,714,462, January 30, 1973: Multiplier Circuits
- U.S. Patent 4,403,199, September 6, 1983: Gain control systems
- U.S. Patent 6,091,829, July 18, 2000: Microphone apparatus
- U.S. Patent 6,526,149, February 25, 2003: System and method for reducing non linear electrical distortion in an electroacoustic device
dbx (noise reduction)
dbx is a family of noise reduction systems developed by the company of the same name. The most common implementations are dbx Type I and dbx Type II for analog tape recording and, less commonly, vinyl LPs. A separate implementation, known as dbx-TV, is part of the MTS system used to provide stereo sound to North American and certain other TV systems. The company, dbx, Inc., was also involved with Dynamic Noise Reduction (DNR) systems.
The original dbx Type I and Type II systems were based on so-called “linear decibel companding” – compressing the signal on recording and expanding it on playback. It was invented by David E. Blackmer of dbx, Inc. in 1971.
A miniature dbx Type II decoder on an integrated circuit was created in 1982 for use in portable and car audio, although only a few devices took advantage of it, such as certain Panasonic portable cassette players and Sanyo car stereos. dbx marketed the PPA-1 Silencer, a decoder that could be used with non-dbx players such as the Sony Walkman. A version of this chip also contained a Dolby B-compatible noise reduction decoder, described as dbx Type B noise reduction; this was possible after the Dolby patent (but not the trademark) had expired. Software implementations have been developed.
Magnetic tape consists of microscopic particles that can be magnetically charged to record signals. The size of the particles and the speed of the tape transport defines the maximum frequency that the media can record. For high fidelity recordings, reel-to-reel audio tape recording typically works at tape speeds of 15 or 7.5 inches-per-second (38 or 19 cm/s), but this requires a lot of tape for a given amount of recording. Lower fidelity recordings can be made at 3.75 or even 1.875 ips, which allows more recording time on a given tape, but at the cost of adding more high-frequency noise.
The cassette tape was designed for convenience, not audio quality, and ran at 1.875 ips (4.75 cm/s) to maximize recording time in the relatively small (compared to open-reel) tapes. This resulted in significant tape hiss. Combined with their limited width, which limits the dynamic range of the signals, the hiss tended to overwhelm any high frequencies in the signal, especially low-volume ones.
During the 1970s, several new types of magnetic recording films were introduced, notably “chrome” and “metal”, that used smaller particles and thereby pushed the tape hiss to much higher frequencies. During the same period, noise reduction systems like dbx and Dolby attempted to do the same using conventional media and actively addressing the tape noise through electronics.
dbx Type I and Type II are types of “companding noise reduction”. These systems work by first compressing the dynamic range of the signal into a range that can be safely recorded on the tape. This type of compression, dynamic range compression, mutes down loud sounds and amplifies soft ones, making the volume of the recording much more even. On playback, the dynamic range is expanded by the same amount, causing the low-volume sounds to become low-volume again and vice versa. The combination of compression and re-expansion gives rise to the name companding. Companding is useful even outside the field of noise reduction; a cassette might have 40 decibels of dynamic range before the media saturates, while the original signal might use 70 for, say, a live recording of a concert. In this case, companding at 2-to-1 will result in a signal with 35 decibels of range, which can be recorded without clipping.
The reason this technique works for noise reduction is that the tape hiss manifests itself as a constant low-volume signal. When the signal is recorded in its original form, without compression, the amount of hiss may be the same volume as softer sounds, masking them entirely. However, when the signal is compressed before recording, those soft sounds are recorded at a louder volume, so now even the soft sounds are louder than the noise. This improves the signal-to-noise ratio.
When the signal is re-expanded, the tape hiss is expanded along with it, making it louder as well. However, the ratio of the signal to noise remains (close to) constant through this process, so the resulting output retains this higher signal-to-noise ratio. Ultimately, it means that while tape hiss does get louder during “soft” portions of the recording, the recording itself is (hopefully) always greater in volume and renders the hiss much less noticeable
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