If you were born some time after, oh, let’s say 1988, then when you see this symbol, the
Dolby logo, you probably think of a movie theater or DVD with Dolby Digital 7.1 surround
sound, or perhaps Dolby’s newer technologies such as Atmos or Dolby Vision HDR.
So you might be a little surprised to see it on an old cassette tape.
What’s it doing there?
Well, Dolby didn’t start out in the movie business.
In fact, they started in the business of sound.
I suppose that’s not a surprise, after all that’s pretty much exactly what they do
today, but their first product had nothing to do with surround sound, movies, or anything
high-tech like that.
Their first product was Dolby Noise Reduction.
Ray Dolby started the company in 1965, and their first product was the Dolby 301 unit,
a machine designed to help eliminate tape noise.
With magnetic tape, the random structure of the particles that make up the tape create
an audible signal that we don’t want, a quiet but still noticeable noise that sounds
like a hiss.
This noise is always there, again due to the structure of the tape itself.
During loud sections of a recording, it’s not that noticeable because compared the the
signal, it’s barely perceptible.
But during quiet parts of a song, it becomes much more of a problem.
Through the magic of video editing, I’m going to take the audio from this video and
slap it on a cassette so you hear what this noise sounds like.
There.
That’s, better?
Well I suppose it’s actually worse, but there you go, that’s the problem.
This tape noise permeates all recordings, and particularly in quiet or silent portions,
it’s quite noticeable.
Ray Dolby wanted to figure out a way to get rid of it.
His solution is quite elegant.
First, let me turn on Dolby Noise Reduction so you can hear the difference.
There, that is better.
Now the noise is much less noticeable.
It’s still there, sure, but it’s definitely not as annoying.
But what’s most interesting is the fact that the actual signal we wanted is unaffected.
I don’t sound any different now that we’re using noise reduction, the only thing that
happened is now you can hear me better because the noise has gone away.
So how does this work?
Well, the first thing Dolby did was to figure out what the tape noise was.
By using spectral analysis of a blank tape, you can see what sound frequencies make up
the noise.
Therefore, you can get a noise profile.
Once Dolby had figured out the particular frequencies that made up the sound, it was
simply a matter of using an equalizer to tune out those frequencies.
You’ve probably used an equalizer before as they come built into many music playing
applications such as iTunes and the like.
The equalizer has a slider for particular ranges of sound frequencies, and you can either
boost each band or suppress it.
Dolby noise reduction is a lot like taking the frequencies that make up noise and tuning
them down a lot to make the noise quieter.
OK, so let’s do that.
(pause) Now the noise is a lot less noticeable, But we have a new problem.
Now the sound we want is dull, and my voice sounds muffled.
This isn’t really any better, it’s just a different problem.
So how come nothing changed when we used it the first time?
Let me turn that back off.
Nothing changed the first time because Dolby Noise Reduction is actually a two way process.
In order for it to work, the recording also has to be processed with Dolby NR.
During recording, the Dolby Chip inside the cassette deck actually does the opposite of
what it does during playback.
Instead of cutting the frequencies associated with noise, it boosts them.
Now I’ll turn on Dolby noise reduction at the time of recording, but I’ll play the
tape back without Dolby turned on.
(pause)
Now my voice sounds really harsh, with ess sounds being particularly annoying.
The ess sound is so satisfactorily nasty such that some sadistic, senseless sad sack might
succumb to the sickening task of overusing these sounds to facilitate the driving of
a point home.
(clears throat) Yes, but there is a reason for this.
By over-boosting these frequencies during recording, we eliminate the problem of a dull
recording when using noise reduction.
Let’s look at the equalizer again.
This is what happens when we use dolby during playback.
These frequencies are pulled down to kill the noise.
But by doing this, we also pull down any components of the signal , that is the sounds that we
want, that lie within that range.
But if you boost the frequencies that lie in the spectrum with the noise while recording,
you end up with a recording that has these particular frequencies recorded too loud.
Then, when you pull those frequencies down during playback, you not only pull the noise
down and make it quieter, but you also bring the boosted parts of the signal down back
to where they should be.
In essence, the boosting of sounds we want during recording is canceled out bringing
them back to normal during playback.
Because any components of the signal that lie within the noise profile are boosted to
compensate for the noise reduction, the effect is that only the noise is reduced.
The signal remains intact.
This is a process known as companding.
The dynamic range of the recording, that is the difference between loud and soft sounds,
it compressed during recording because some frequencies are recorded louder than they
should be.
Then during playback, the dynamic range is expanded by lowering those frequencies, and
thus the noise floor.
There is a catch, though.
Because some sounds are boosted by the dolby circuitry when recording, the record levels
that you use must be reduced a little bit to ensure they don’t get distorted.
That’s why the Dolby symbol usually appears on the level meter of a tape deck--this is
the point on the meter that should not be exceeded when using noise reduction.
When setting the level for recording, you’d want the meter to just barely touch it at
the loudest sections.
The Dolby system used on consumer machines like this wasn’t the best Dolby had to offer.
Dolby A noise reduction is what Ray Dolby first invented, and was what was found in
the professional recording studio.
But Dolby-B, what’s used on the vast majority of cassettes, worked pretty well, and it was
a lot simpler and thus cheaper to implement.
Later consumer systems, such as Dolby C and Dolby S, further improved on the noise reduction
process.
Dolby C came about in 1980, and it worked very well, about 60 % better than B. The problem
with Dolby C, however, was that on non-compatible equipment, the tape sounds really weird.
Have a listen.
Here’s a clip of some music from the YouTube Audio Library.
Now here’s what it sounds like on a Dolby-C encoded tape, played back without Dolby circuitry.
That’s messed up.
Turning on Dolby B helped a little, but not much.
Dolby C used a much more complicated process to reduce noise, and that’s why recordings
made with it are less compatible.
It also meant that pre-recorded tapes didn’t shift to Dolby C, since no one wants to buy
music that doesn’t sound any good, and even fewer people were willing to put down the
money for a new cassette deck.
The next consumer development, Dolby S, was supposedly so good that with a decent quality
Chromium tape, you couldn’t tell the difference between a cassette encoded with Dolby S and
a CD.
It also was more compatible with Dolby B equipment, not suffering from the distortion of dolby
C. The problem with Dolby S, though, was that it came about far too late, in 1989, with
the CD already having found a prominent place in the market.
The best noise-reduction technologies, such as those used in the professional Dolby SR
or the competing DBX system, used very aggressive companding.
The idea is to keep the signal recorded on the tape loud relative to the noise at all
times.
Remember, in loud sections of the recording, the noise isn’t noticeable compared to the
signal.
So the best possible system would make the recording loud all the time, and during playback
could tell which parts were intended to be quiet and thus lower the volume of the whole
signal back to normal, therefore making the noise practically imperceptible.
Though this isn’t quite how companding is accomplished in the best systems, the effect
is similar.
But the problem with using this technique is that listening to a DBX encoded tape without
a DBX capable decoder is very unpleasant.
The same goes for Dolby SR.
And that’s precisely why the better systems never took off in the consumer space.
Dolby B was simple and still pretty effective, but best of all a Dolby-B encoded tape could
be played back on a standard cassette deck with reasonable results, again with the only
difference being an over-emphasis of the treble sounds.
And with most stereos having a bass and treble adjustment, just slightly turning down the
treble would bring the sound back to more or less normal while still providing some
noise reduction, of course the noise reduction you’d get would be less precise and effective.
We don’t use noise reduction any more because, well, noise isn’t a problem any more.
Now that we use digital sound in virtually all applications, there’s no need to add
this sort of complex circuitry.
But I’m still impressed by the elegant solution to the problem.
Automatic pre-emphasis of sounds within the noise profile when recording and then de-emphasis
when playing it back is so delightfully simple, effective, and logical, that I can’t help
but admire it.
Thanks for watching.
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