Noise reduction, technique and usage

Dolby's Noise Reduction technology dominated the tape media market. But what exactly does that noise reduction do? What are the advantages and also the disadvantages of this? And are there other suppression techniques besides Dolby?

Noise is what you often hear in the background, noise comes from the tape itself and can therefore be heard on any tape media, it differs a lot per type of tape. Noise is more pronounced if the tape runs slower and has less recording surface (narrower tape). Let it be the case that the tape on 'compact' cassettes is one of the slowest and smallest types of tape used for music. Partly because of this, noise reduction was very popular with the cassette, but also tape recorders, 8-track and Elcaset and even my Microcassette deck has it!

Over Dolby en zijn technieken

Dolby Laboratories is an American sound engineering company founded in 1965, with specialties in noise reduction and video and audio processing.
Many know Dolby from the well-known 'Double D' logo that carries the brand.

Below we discuss the noise reduction techniques that Dolby has released over the years for all kinds of tape media.

Dolby A

Dolby A was the first type of noise reduction they released in 1965. Intended for professional recording studios. Dolby A focuses on the high peaks in sound and reduces them to an acceptable level. With Dolby A it was possible to apply noise reduction of up to 10 to 15 dB.

Dolby B

Dolby B is a noise reduction released in 1968, intended for the commercial home market and is essentially a simple version of Dolby A. It is also specifically aimed at the cassette market. Its range is limited to 9 dB. But has the advantage that tapes recorded with Dolby B can also be played on devices without noise reduction. Music would sound clearly more clear with B because there is less noise present. Because Dolby B was so common in the 1970s, it is also labeled Dolby NR. Which is actually the all-encompassing name, very confusing actually...

Dolby C

Dolby C came on the market in 1980 and is in fact a double version of Dolby B. Dolby C works on both high and low frequencies. The disadvantage of this is that tapes with Dolby C cannot be listened to on a device that does not support it. Yet C has a range of 15 dB.

Dolby S

In 1989 Dolby S came on the market, this improvement on Dolby C aimed to become the standard as Dolby B was. With 10 dB in the low and a whopping 25 dB in the highs of noise reduction, Dolby S was clearly better than its predecessors. But because Dolby S came out when the Compact Disc was on the rise, it has never been used on a large scale.
By the way, Dolby S sounds terrible if you don't use it when playing a Dolby S recording!

Dolby SR

Dolby SR came out in 1986 (before Dolby S). But was aimed as a successor to Dolby A and, like its predecessor, was aimed at the professional market. With suppression up to 25 dB. Dolby SR uses a fairly aggressive technique where it tries to keep the recording level as high as possible continuously. Dolby SR can also be played on devices with Dolby A.

Dolby HX/HX-Pro

Finally, there is Dolby HX or rather HX-Pro (HX stands for Headroom eXpansion). Although HX-Pro is often seen as a noise reduction, this claim is not entirely correct.

HX (not HX-Pro) was developed in 1979 by Dolby itself, but would have been rejected by manufacturers who had to work with it, there would be flaws in the concept.
(unfortunately I can't find more information about the failure of the original concept).

Nevertheless, HX was continued, not by Dolby but by Bang & Olufsen. After which it was released in 1981 under the name HX-Pro. B&O had applied for a patent on HX-Pro that same year, allowing it to be used only on their own machines for the following 7 years. After this, HX-Pro became a standard for use on cassette decks.

The difference between HX-Pro and noise reduction lies in its operation, because HX-Pro does not suppress noise like other Dolby systems do, but (dynamically) plays with the bias control of the recording and raises the threshold between sound and noise.

In short, with HX-Pro you can record at a higher volume. This avoids the low sound threshold where noise is common (also explains the name 'Headroom'). This makes HX-Pro not so much a noise reduction technique but more a noise avoidance process.

The other difference between HX-Pro and noise reduction is that HX-Pro is only applied in recording and not used in playback. In fact, decks appeared on the market where this function is on by default and sometimes can not even be turned off.

An extensive report on HX-Pro can be read in this link to an article from August 1984.

How does it work?

Dolby Noise Reduction analyzes, compresses and expands the sound during recording and playback by playing with your volume and dynamic range to keep the noise out.

Maybe I should explain it a bit more clearly...

While recording, Dolby increases the volume, but only in the areas where noise can be heard.

In fact, Dolby works the same as an Equalizer, as can be seen here below the 8-track recorder. It allows you to adjust specific frequencies of your music.

Since Dolby increases the tones when recording, they also have to be brought back again. This then happens during playback. Wherever these are brought back, this also shifts noise further into the background and is beyond the range we can hear.

Now Dolby does come with a manual, for best use the tape must be both recorded with Dolby and played with Dolby. Otherwise, the music may sound very bad. This avoids the misunderstanding that Dolby would make music dull. This is what you get if sound is recorded without it, but is used during playback.

When recording music, you should note that your dynamic range is less when using noise reduction. To be able to correct this, Analog and digital meters often have a 0-limit with the Dolby symbol.

For best recording quality, it is recommended not to go far below or above this limit. Small peaks can't hurt, but if you go outside these limits for too long, you run the risk of your music being cut off and sounding bad when played back.

Decks with a bit more luxury sometimes have a small light that turns red when you exceed this limit. In the photo you can see one sitting in the middle.

The pros and cons:

As you can see, Dolby does quite a few tricks to prevent noise.

This means you have significantly less noise when playing. Otherwise, it is important to keep a close eye on your meters when recording, so as not to go too long and too far outside the limits. Dolby can straighten a lot but can't juggle.

Using noise reduction also comes with potential drawbacks.
Any form of compression (including those applied by Dolby) is accompanied by loss of quality. The compression technique determines how much is saved. Sound can sound clear at points where a lot of noise would otherwise come through, but parts where there would be no noise are also included in the suppression.

This means that not all music and types of bands go well with noise reduction.
For example, it is highly recommended for Ferro bands, where it may not be necessary for 'low noise' Chrome bands. So if you theoretically record music that uses a wide spectrum of the frequency range, there is a chance that you will lose sound quality.

To keep it simple, it is best to record rough music without Dolby. And also on a Low noise Ferro band, because Ferro has a wider range than Chrome and with Low noise a natural noise reduction. At least that's what I would say with this information. It is of course up to the user to decide what sounds good and what doesn't.

What's next to Dolby?

Although Dolby is by far the best known and largest in the field of noise reduction, there are also other techniques with the same effect.

DNL (Dynamic Noise Limiter)

DNL is a technology for noise reduction designed by Philips in 1971, it differs from Dolby because you only apply it for playback and not for recording. This allows you to use DNL with radio broadcasts and any other media played on a device that supports DNL. You can also apply DNL to tape that has been pre-processed with Dolby B. I just have no experience with this application so can't comment on the quality of the sound.

DNR (Dynamic Noise Reduction)

DNL was later developed further and released in 1981 under the name DNR, this suppression technique was widely used to improve telephone conversations as well as when using a microphone. Also DNR is only applied during playback. DNR can often be found on devices from Philips itself. For example, I have already worked with tape recorders that use this technique. Both DNL and DNR can suppress up to 10 dB of noise.

DBX Type I & II

Then there's DBX (Decibel eXpension), which compresses the sound by a factor of two when recorded in anticipation of background noise in the high and low ranges. During playback, DBX mutes the sound back to its original form. Just like Dolby, DBX also had its drawbacks, one of the drawbacks is that actively directing the sound can be heard at quiet moments in the music, because more noise can then be heard and the noise suppression of DBX tries to block it.

DBX in general never became popular partly because it couldn't be played on devices that don't support DBX. Yet DBX Type I was still fairly popular in the semi-professional world. DBX has also been used in vinyl, tape recorders, television sets and films.

See also this video on Youtube where DBX is explained in detail.

MPX Filter

Finally, we will discuss a bit of radio noise suppression. The best known is the MPX Filter. This function is specifically intended for recording radio signals (FM) on analog media such as tape.

Radio broadcasts do not only send sound over the bandwidth, but also information that the device uses to, for example, display information about the radio station and music itself. These signals (especially in the 19kHz region) can be heard on cassettes and their ilk. MPX filters these disturbing signals out of your recording so that they can no longer be heard during playback.

By the way, the filter only works when recording. It is not needed during playback and does nothing if it is turned on. The filter can often only be used in combination with a Dolby noise reduction, so it is not possible to activate the MPX filter without using noise reduction at the same time.

It is also strongly recommended not to use this function if you are recording from other sound sources. Some devices do not have a manual MPX function, but have it built-in. This is then automatically activated when you start recording from the radio, again in combination with noise reduction.

My experience and judgment.

Now that we have covered the many techniques that can improve the sound quality of your recording, I dare to throw my own judgment on it.

I often use Dolby B, because most of the devices I have come from the era when it was widely available on the market. The difference in quality is clearly audible, so I almost always use it when recording. Especially with tapes that are known to have a lot of background noise. But (as mentioned earlier) Dolby does not always have a positive effect. Songs with lots of highs and lows are hard to keep within range without sacrificing quality.

For example, while writing this blog, I recorded a tape with fairly rough music without noise reduction. I opted for a high quality chrome strap. Noise is naturally suppressed here already. the result is also worth talking about! Noise is almost unnoticeable (even at low moments in the music), the high peaks remain unaffected.

Still, I remain a supporter of Dolby, especially with Type I tapes it has a strong positive effect on the sound quality. The loss in sound quality can be noticed but is often minimal.

Finally, please note that Dolby (as I'm writing this) no longer applies noise reduction to newly released cassette decks. Keep this in mind when purchasing a new cassette deck. If a deck has Dolby, the 'Double D' logo is proudly displayed on the front of the unit.

24-04-2022
Time for an update on my experience and judgment. Judgments change sometimes...

So also my opinion about the use of Dolby, I still use it but at the same time I notice that I think some tapes sound better when I play them without Dolby (while I recorded them that way). The sound seems much duller when I record and play it with Dolby. On the other hand, the noise also becomes more noticeable in the foreground if I leave it off during playback, but if the music sounds a lot clearer then I'll be happy with this.

I still use dolby B the most for the same reason I already described in my previous review. I've also tried Dolby C and SR but only have one cassette deck that supports these types of noise reduction. If you play a tape with C or SR in a device that does not support it, the music is often not fun to listen to and the sound jumps in all directions.

Nowadays I make a decision before I record, when it comes to tapes of type I (ferrous) then Dolby is definitely worth using, especially when it comes to the cheaper tapes of this type. When using tapes of type II (chrome) or even better type IV (metal), I sometimes prefer to ignore the use of Dolby. Despite the type of tape, the music to be recorded also plays a role in this. Dolby seems to have less positive effect on rougher music than calmer music. This is mainly because noise is suppressed more if the music is loud enough. Only in quiet passages is the noise more audible.

And to emphasize the use of cassette tapes in these times. Secretly, the noise does have something, after all, you choose to play your music with cassette tapes for a special reason. Over time I have took noise more for granted and sometimes you are not even aware that it is there. Only if noise dominates over the music (which is often the case with ferrous tapes) Dolby is still certainly a welcome edition. 

But in the end it's up to the user himself, I'm just sharing my opinion and experience about noise reduction here, you may swear by Dolby or its related systems. Another praises the noise for the nostalgic value.

I myself am still happy with the possibility that Dolby gives, but I have become a bit more aware of its use. Not all music seems to sound better on it.