Tape Notes · Tape knowledge
Dolby noise reduction on cassettes: B, C and S explained
15 October 2026 · 7 min read
Dolby noise reduction cuts the hiss you hear on cassettes. When you record, it turns up the quiet, high-pitched parts of the music; when you play the tape back, it turns them down again by the same amount, and the tape's hiss goes down with them. The catch is that a tape has to be played back with the same Dolby setting it was recorded with. The short answer: record a mixtape you're giving away with Dolby off, and use Dolby B or C for tapes you'll play on your own deck.
Why cassettes hiss
Cassette tape is narrow and slow. Four tracks (two stereo sides) share a ribbon 3.81 mm wide, and it moves past the head at just 4.76 cm a second. The magnetic particles on the tape make a steady background noise, mostly in the treble. Loud music covers it up, but you hear it in quiet passages, fade-outs and the gaps between songs. Better tape helps (chrome and metal hiss less than normal tape; see our guide to tape types), and so does recording at a healthy level. Dolby goes further.
How Dolby works
Dolby is a compander: it compresses on the way in and expands on the way out. While recording, it boosts quiet treble by up to 10 dB (Dolby B). Loud sounds pass through untouched, because they already drown out the hiss. On playback the decoder does the exact opposite, so the music comes back to its original level, and the hiss, which was added by the tape after the boost, is pushed 10 dB further down.
Dolby B only works on the upper frequencies, from roughly 1 kHz upwards, where hiss is most audible, and its working band slides up and down with the music so that it doesn't make the background noise pump up and down (“breathing”).
Dolby B, C and S compared
| System | Arrived | Hiss cut | Where you'll find it | Played back without it |
|---|---|---|---|---|
| Dolby B | 1968; on cassette decks from the early seventies | About 9–10 dB, in the treble | Almost every deck with Dolby, many personal stereos and car players, and most pre-recorded albums from the mid-seventies on | Brighter and a bit hissy, but fine. Turn the treble down a touch. |
| Dolby C | 1980 | About 15 dB | Mid-price and better decks from the early eighties, marked “Dolby B-C NR” | Harsh and squashed. Playing it back with Dolby B on is a passable compromise. |
| Dolby S | 1989 | Up to 24 dB in the treble and 10 dB in the bass | A few high-end decks from the nineties. It's a home version of Dolby's studio system, SR. | Designed to sound reasonable on a Dolby B deck. |
Decks with Dolby C almost always have Dolby B too, so a B-C deck can play nearly anything correctly. Dolby S arrived as CDs were taking over, so S decks are rare and S tapes rarer still.
HX Pro isn't noise reduction
Plenty of decks also carry a Dolby HX Pro badge. HX Pro (“headroom extension”, developed by Bang & Olufsen and licensed through Dolby) works only while you record: it trims the bias when the music has lots of treble, so the tape takes loud high notes without choking. Tapes made with it play back normally on any machine, so there's no setting to match. If your deck has a switch for it, leave it on.
Was this tape recorded with Dolby?
- Shop-bought albums from the mid-seventies onwards were nearly all made with Dolby B. Look for the Dolby mark or the words “Dolby System” or “Dolby NR” on the J-card or shell.
- Home recordings: check the J-card. Many blank tapes came with a box to tick for noise reduction, and careful tapers wrote “NR B” or “Dolby C” on the label. Labels can be wrong, though, so trust your ears.
- Listen. Play a quiet passage with Dolby off, then with B. If “off” sounds bright and hissy with spitty S sounds, and B sounds natural, it was recorded with Dolby B. If B makes it dull and muffled, it was recorded without. A Dolby C tape played without C sounds harsh, with treble that jumps up in quiet parts.
When Dolby sounds wrong
Dolby depends on the playback deck hearing the tape at the same level the recording deck put it down. If the heads are dirty or out of line, the tape-type setting is wrong, or the tape has lost some treble with age, the decoder takes off too much. The sound goes dull, and the hiss can rise and fall with the music. This is called mistracking. Things to try:
- Clean the heads (our guide to looking after cassettes shows how).
- Check the tape-type setting matches the tape: normal, chrome or metal.
- Try another deck, ideally the one the tape was made on.
- As a last resort, switch Dolby off and turn the treble down a little. On a worn personal stereo that's often the best you can do.
Should you use it?
| You're making… | Setting | Why |
|---|---|---|
| A mixtape to give away | Off | You don't know what they'll play it on, and many players today have no Dolby at all. An “off” tape sounds the same everywhere. |
| Tapes for your own deck | B, or C | If the deck is in good order and you'll play the tape on it, C gives the quietest background. |
| Tapes for the car or a personal stereo | B at most | Most portables and car players had B only, and cheaper ones none. |
| A band or label release | Usually off | So it sounds the same on every player. Ask your duplicator; see our guide for bands and labels. |
| Digital copies of old tapes | Match the tape | Decode on a deck with the right Dolby, then record into the computer. Software decoders are rare and hit-and-miss. |
Tip: Dolby can't rescue a tape recorded too quietly. Set the level first, peaking around 0 on the meters (our mixtape guide goes through it), then decide on Dolby.
Dolby on today's tape machines
Most new cassette decks and players on sale today have no Dolby. Some have a simpler hiss filter of their own that only works on playback, and reviewers report that Dolby no longer licenses the system for new cassette machines. So if you want to record or decode with Dolby, you'll need a vintage deck, ideally one that's been serviced. That's one more reason to leave Dolby off on a tape you're sending out into the world.
Write it on the J-card
Whatever you choose, note it on the insert so whoever plays the tape gets the setting right. On jcard.studio you can rename the side headings (“Side A · Dolby B”) or add it to the credits on the inside panel. The Chrome looks such as Studio Black already print a hi-fi spec table, which suits a carefully recorded tape.
Making a tape? Design a J-card that fits the case.
Try the Spectrum EQ look →Sources
- Dolby noise-reduction system — Wikipedia
- Noise reduction — Richard L. Hess, audio tape restoration notes
- What is Dolby noise reduction? — Spencertified
- Old vs new cassette deck: which is better? — Galaxus, December 2021
- Audio tape specifications — Wikipedia
More Tape Notes
- How to look after cassettes: storage, cleaning and fixing a chewed tape
- J-card design through the decades: from blank-tape stripes to rave tape packs
- Reading a blank tape's J-card: MOL, SOL, bias, EQ and “high position”
- How to make a mixtape: choosing, timing and recording a cassette
- C-46, C-60 or C-90? How much music fits on a cassette
- Printing J-cards at home: paper, settings, cutting and folding
- Cassettes for bands and labels: a short-run release guide
- How to make a cassette J-card: sizes, printing and folding
- Cassette tape types explained: Type I, II, III and IV
- A short history of the cassette tape