Playback Only Dynamic Noise Reduction Systems: Better Than Dolby Noise Reduction?

Are playback only cassette tape noise reduction systems like DNR offer better subjective performance than Dolby’s noise reduction systems?

By: Ringo Bones

Playback only noise reduction systems for use in cassette tape playback and reducing hiss when listening to FM radio broadcasts became widely available during the early 1980s. One of the most popular of these was the one based on the LM1894 IC used on what is referred to as a dynamic noise reducer or DNR. DNR units based on these types of ICs were praised for providing better subjective noise reduction results – sound quality wise – when compared to the then ubiquitous Dolby B noise reduction system used in entry level hi-fi cassette tape decks. But first here’s a discussion on how the DNR works.

According to its National semiconductor datasheet, the LM1894 is a 14-pin dual inline package stereo noise reduction IC for use with audio playback systems. This dynamic noise reduction – or DNR – system is non-complementary, meaning it does not require encoded source material. A noise reduction circuit using this IC is compatible with virtually all prerecorded tapes and FM broadcasts. Psychoacoustic masking and an adaptive bandwidth scheme allow the LM1894 IC based DNR to achieve 10dB of noise reduction. A DNR circuit based on the LM1894 IC can save circuit board space and cost because of the few additional passive components required. Described as a non complementary and single-ended noise reduction system that is compatible with all prerecorded tapes and FM broadcasts, has a 10dB CCIR / ARM weighed effective noise reduction, has a 4.5 volts to 18 volts power supply range that’s compatible for installation in automotive applications and a 1 volt RMS input overload.

The dynamic noise reduction system is a low pass filter that has a variable bandwidth of 1,000 Hz to 30,000 Hz, depending on music spectrum. The DNR system operates on three principles of psychoacoustics. The first of which is that white noise can mask pure tones. The total noise energy required to mask a pure tone must equal the energy of the tone itself. Within certain limits, the wider the bandwidth of the masking noise about the tone, the lower the noise amplitude need be. As long as the total energy of the noise is equal to or greater than the energy of the tone, the tone will be inaudible. This principle may be turned around; when music is present, it is capable of masking the noise in the same bandwidth. Next, the ear cannot detect distortion lasting less than 1 millisecond. On a transient basis, if distortion occurs that is less than 1 millisecond in duration, the ear acts as an integrator and is unable to detect it. Because of this, signals of sufficient energy to mask the noise allows the LM1894 DNR IC to open its operating bandwidth to 90-percent of its value in less than 1milliseconds. Reducing the bandwidth to within 10-percent of its minimum value is done in about 60-milliseconds: long enough to allow the ambience of the music to pass through, but not so long as to allow the noise floor to become audible. And lastly, reducing the audio bandwidth reduces the audibility of the noise. Audibility of noise is dependent on the noise spectrum or how the noise energy is distributed with frequency. Depending on the tape and the recorder equalization, tape noise spectrum may be slightly rolled off with frequency. Depending on the tape and the recorder equalization, tape noise spectrum may be slightly rolled off with frequency on a per octave basis. The ear’s sensitivity, on the other hand, greatly increases between 2,000 Hz and 10,000 Hz. Noise in this region is extremely audible. The DNR system’s low pass filters makes the noise less audible by filtering steady-state hiss in this frequency. The low frequency spectrum of the music will not appreciably open the DNR’s bandwidth, thus hiss in the 2,000 Hz to 10,000 Hz part of the audio spectrum is not heard.

The LM1894 IC is often used in conjunction with the 18-pin dual inline package LM3915 LED bar-graph driver array driver IC as a bandwidth indicator. The resulting set-up cosmetically resembles the dbx 563X Hiss Reducer that was sold during the late 1980s and early 1990s. Sadly, by around 1986, the LM1894 DNR IC was becoming hard to find in most DIY electronic stores – especially if you’ll going to buy just one IC instead of “a minimum quantity of 1,000 chips” as often advertized in popular electronics magazines at the time. While building my own DNR unit back in 1987, I was forced to replicate the transconductance amplifier portion of the LM1894 IC using discrete FETs.

In use, playback only single-ended DNR type noise reduction systems provide subjectively better sound quality results in comparison to Dolby noise reduction systems like Dolby B and Dolby C because most DNRs operate their cutoff frequency in the in the 1,000 Hz to 2,000 Hz part of the audio system, while Dolby B operates in the 500 Hz zone, thus making DNRs sound less “dull” than an over-aggressive Dolby B. But in actual use, Dolby encoded prerecorded tapes – in either Dolby B or Dolby C encoding – tends to perform well in a typical DNR unit in comparison to tapes recorded without any Dolby noise reduction because a Dolby encoded cassette tape has a substantial treble boost than ones recorded without Dolby. 

Headphones: Less Than Ideal Way To Listen To Cassette Tapes?

Even though it was deemed an indispensible component when the Sony Walkman became king back in the 1980s, are headphones, until now, has always been a less than ideal way to listen to cassette tapes?

By: Ringo Bones

Dubbed by conservatives as the most antisocial way to listen to music back when Sony’s Walkman was new, the shortcomings of the ubiquitous headphones has recently been brought back into the spotlight after the shortcomings of a flagship Apple product – i.e. the Apple AirPods wireless headphones and their Mainland Chinese made knockoffs – as early users complained of excessive background hiss, not to mention early users old enough to remember 1980s era cassette tapes saying that the hiss levels are worse than that of tape-to-tape dubbing a copy of a copy of a prerecorded cassette tape album. But are headphones really the weakest link of all the sound producing transducers in our high fidelity audio hobby – especially when it comes to listening to prerecorded cassette tape albums?

Despite its shortcomings, audio enthusiasts old enough to have taken sound quality seriously back in the 1980s had surprisingly managed to make due of listening to music via headphones. Weird between-the-ears stereo soundstage when listening to plain vanilla 2-channel stereo cassette recordings and producing more background hiss in comparison to conventional loudspeakers notwithstanding. But let us explore more on the headphone’s hiss issue given that, especially when listening to prerecorded cassette tape albums as your primary music source, is already inherently hissy in the first place.

Hiss is more noticeable through headphones than speakers for several reasons. Since headphones lie closer to the ears of the listener in comparison to loudspeakers, any hiss that the headphones’ produce is readily transferred to the eardrums. Headphones are more sensitive than loudspeakers – i.e. headphone sensitivity are typically measured in decibels per milliwatt as opposed to decibels per watt when measuring loudspeaker sensitivity – so headphones are more likely to pick-up the faint residual hiss produced by your audio rig, like that in the preamplifier stage and / or the Dolby noise reduction stage. And headphone models that isolate your ears from outside sounds can also prevent such sounds from masking hiss, denying the user from using Mother Nature’s naturally generated dither noise.

Unconventional Cassette Tape Deck Alignment Hacks For Extreme Cassette Sounds?

Even though proper alignment of tape heads are a necessity for playback compatibility when playing a tape from one deck to another but is it possible that a bit “unconventional” alignment results in a superior sound quality from cassette?

By: Ringo Bones

For those old enough to remember how “unconventional” cassette tape head azimuth alignment tweaks back in the late 1970s to the 1990s managed to transform their humble walkman or a car stereo like the famed Pioneer KP-500 to rival that of a full-sized open-reel tape deck or an audiophile CD player using various everyday items like strategically placed match-books, paper clips and even bobby pins knows how rewarding this bit of DIY can be. Though it can be taken to an extreme when it causes cassette tape skewing problems and an “eaten tape”, within a certain envelope, these tweaks can offer rewards in terms of much improved sound quality from prerecorded cassette tapes.

Unconventional cassette tape head azimuth alignment head tweaks gained notoriety back in the late 1970s due to neophytes and jumpy electronic tinkerers upon hearing a tape deck cough, they immediately get hold of a screw driver and give the azimuth alignment screw of the tape head a couple of tweaks. The first thing to remember most of all is that the heads need adjustment only when they are being replaced in the case of worn-out tape heads. To find out if the tape heads do need adjustment – or for those buying a used cassette tape deck at a keen price at a garage sale – check if the tape head’s wear pattern is rectangular. If it is trapezoidal or varies much from head to head, they need to be adjusted. On the other hand, never adjust heads that have actually been worn out unless you want some tape travel and skewing problems. Problems about worn-out cassette tape heads are solved by complete head replacement.

But the good – or bad – news is that within a certain “latitude”, some minimal tape head azimuth tweaks via strategically placed match-books, paper clips and bobby pins – especially in entry level walkmans – can result in treble clarity that can make your humble walkman, car stereo or boom-box rival that of an open-reel tape deck and audiophile grade CD players in the 2,000 to 5,000 US dollar price point. If you have old Hair Metal prerecorded tapes from the 1980s, sometimes, the result can be as eerie as akin to being less than 20 feet away from a well-struck crash cymbal. I remember now that this is how we did cassette tape high frequency clarification tweaks before “affordable” decks by Yamaha and NAD with the “Play Trim” feature became widely available during the 1990s.

Can You Use Dolby B Noise Reduction On Dolby C and Dolby S Encoded Cassette Tapes?

Even though your tapes won’t be damaged if you do this, are there any benefits of using Dolby B noise reduction on Dolby C and Dolby S encoded cassette tape audio recordings?

By: Ringo Bones

Fortunately, doing so will not result in a damaged cassette tape or tape deck, but the practice gained popularity back in the late 1980s and early 1990s – or whenever time Dolby C and Dolby S encoded prerecorded cassette tapes became widely available. I mean I even heard of one audiophile back in 1993 praising the sound quality of Gen. H. Norman Schwarzkopf’s spoken word album after playing a Dolby S encoded cassette copy of on a car stereo that’s only equipped with a Dolby B noise reduction system. But first, here’s a brief review of the Dolby C and Dolby S noise reduction systems.

Dolby B reduces tape noise at frequencies starting at 500-Hz and achieves a noise reduction of 8 to 10 dB while Dolby C starts to work at 150-Hz and above and achieves a noise reduction of 18 to 20 dB, But Dolby S is a simplified domestic use version of the Dolby SR – a professional signal processing scheme and not just a mere noise reduction strategy. Dolby S consists of a number of circuits operating together to reduce tape noise right across the 20-Hz to 20K-Hz audio band, not just in the high frequencies where Dolby B and Dolby C operate – and Dolby S will do this with minimum possible audible intrusion.

But given that doing so – fortunately will not damage both tape and deck – why is it then that this scheme results in an inexplicably improved subjective sound quality? Well, it could be the resulting “treble lift” due to the different playback curve of Dolby B compared to Dolby C and Dolby S noise reduction systems and the resulting treble lift compensated for the inherently noisy automotive environment. And the different playback curves of the various Dolby noise reduction systems also resulted in lifting the mid-range portion of the audio spectrum that resulted in a vacuum-tube like warmth in tonality of a solid state car tape deck. Which, unfortunately, resulted in car stereos – especially those models made by Alpine – target number one for car break ins. Using Dolby B noise reduction on Dolby C and Dolby S encoded cassettes also benefitted Walkman / portable cassette tape players during the late 1980s and early 1990s because most affordable Walkman models at the time are only Dolby B noise reduction capable - all of this proving Philip's claims that the Dolby C and Dolby S noise reduction systems are "backwards compatible". 

What Are The Most Important Specifications of a Cassette Tape Deck?

Whether a fist time buyer or a seasoned cassette tape archivist – what are the most important specifications of a cassette tape deck?

By: Ringo Bones

Given that the “lowly” cassette was the only egalitarian music medium for the budding audiophile back in the mid 1980s, it is easy to say that most of us have his or her own view when it comes to what specifications make a great cassette tape deck. But if you ask me, what I will rank from the most to least important goes as: (1) frequency response, (2) wow and flutter, (3) signal-to-noise ratio, (4) speed accuracy and (5) distortion – although some audiophiles may rank distortion higher as the most important specification.

Frequency response that’s flat- i.e. within ±3dB - and extends from 30-Hz to 18,000-Hz – could be considered impressive. But you should pay attention on how flat the response is, a frequency response specification of ±6dB is not impressive. However, if the specifications were ±3dB and applied to a range of 30-Hz to 16,000-Hz would be considered acceptable. Better still would be a much flatter range of ±2dB over such a frequency range. A high frequency range that extends to 19,000-Hz instead of just 16,000-Hz will not make an important difference to most listeners – especially “older” audiophiles. 

The wow and flutter should measure no higher than 0.05% peak. Although, people who are more sensitive than others when it comes to wow and flutter might require a tape deck with better specification in this department– i.e. 0.025% or lower.

The signal-to-noise ratio should be at least 75dB or better when using Dolby C noise reduction. And the deck should offer better S/N ratio results when a more advanced noise reduction system is used – like Dolby S.

Speed accuracy that that will deviate no more than 0.5% from correct speed. While some engineers consider such deviation in pitch is inaudible to most people, vinyl LPs are mastered on a device whose pitch deviation is under 0.2%.

Low distortion is of vital importance, especially if you exclusively play recorded acoustic instruments because it is much easier to hear distortion creeping in with a violin recording – than with a Gibson Les Paul played through an overdriven Marshall stack.

Why Cassette Tapes Aren’t Magnetically Shielded?

Given their propensity to go dull whenever they are accidentally placed near magnetic objects – even magnetic screwdrivers - why aren’t cassette tapes provided with inherent magnetic shielding?

By: Ringo Bones

Unlike your Barclay-Crocker prerecorded Open Reel tapes that came in mu-metal cans as standard, prerecorded cassette tapes back in the 1980s and even well into the 21^st Century doesn’t come with protection against erasure when accidentally placed against a strong magnet like refrigerator magnets and electric guitar pickups and electric bass guitar pickups. And if you Google search cassette tapes that are magnetically shielded, you are directed into Google’s lists of invented patents stating that the tiny nickel plated piece of steel behind the pressure pad functions as a “magnetic shield” for the cassette tape. Given that this concern was largely ignored by the leading cassette tape manufacturers during the format’s lifetime, is it even possible to magnetically shield cassette tapes against erasure and loss of high frequencies when accidentally placed closed to magnetic screwdrivers, fridge magnets, and electric guitar and electric bass guitar pickups?

Back in 1979, the world’s leading cassette tape manufacturer TDK released their MA-R series of cassettes and they were the top of the line of the company’s Type –IV metal particle cassettes. The tape formulation used is based on TDK’s patented Finavix particle. It’s very heavy and its metal shell construction – later revealed to be an aluminum-zinc alloy – has the potential to be made with an actual mu-metal alloy capable of actually provide magnetic shielding for the cassette housing instead of just a vibration reduction measure. Sadly, given that these were top of the line in the TDK’s range of cassettes, they were also very expensive. From the 1980s through to the 1990s, TDK’s MA-R series of metal particle cassette often carry a retail price of around a dollar and a half to two dollars less than a price of a compact disc - which for all intents and purposes just too high a price to pay for a blank cassette tape. This largely explains why I totally abandoned prerecorded cassette tapes in the mid 1990s when the “relatively affordable” Marantz CD-63 Ken Ishiwata Signature model entered the market. 

Avilyn Magnetic Particles: The Greatest advancement In Cassette Tape Development?

Though now largely forgotten, did the Avilyn magnetic particles represent the absolute zenith of cassette tape advancement?

By: Ringo Bones

This substance has now virtually become an “alien technology” for those born way after the heyday of the cassette tape. But back then, this represented the value-for-money in cassette tape technology terms when it comes to sound quality. Besides the more expensive metal particle tapes, cassette tapes that use Avilyn magnetic particles are probably the closest yet to sounding like professional open-reel tapes. Stranger still, Avilyn magnetic particle technology has its origins way before cassette tapes become a runaway commercial success in the 1980s.

Developed as a high coercive magnetic particle developed for sound and video recording back in 1974, Avilyn is an acicular magnetic iron oxide on whose surface certain cobalt compounds have been adsorbed. The coercive force of the particles is determined by the amount of cobalt and other preparing conditions, and is controllable between 450 and 800 Oe (Oersteds). The particles display much improved thermal stability with respect to remanence and coercive force. The new video tape having a coercive force of 550 Oe is compatible with chromium dioxide video tape in recording characteristic according to measurements on a conventional video tape recorder. The head wear rate of the new Avilyn tape is about one-fifth that of chromium dioxide tape and is almost the same as for conventional iron oxide tape. Tape demagnetization is not as serious as is in the case of cobalt-doped gamma ferric oxide tapes.

During the 1980s, Avilyn was mostly used in TDK’s high-bias Type-II cassette tapes and VHS video tapes. TDK also produces Avilyn open-reel tapes for professional studio recording use at the time when 16-bit digital audio recording technology was still in its infancy. Sadly, during the early part of the 21sr Century as the prices of recordable compact discs and associated equipment came tumbling down and availability increased, everyone’s love affair with the cassette tape slowly waned.