How
The Mellotron Works
Very
simplistically, the sound generation technology of the Mellotron
is very similar to what youll find in your hi-fis cassette
deck (assuming you still have a tape deck in these days of CD!).
Or, as somebody once put it, "rust being moved past a magnet".
So
to understand the Mellotron a little better, lets consider
a cassette deck analogy for a minute. For all you technology freaks
out there, to whom this is old hat, please bear with me as I wish
to start at first principles to ensure that anybody reading this
can understand the Mellotron.
The
basic principle of cassette replay is to encode a magnetic representation
of sound onto magnetic tape and then, to recreate the original sound,
pass the same tape over a magnetically sensitive replay head to
induce a current into some electronic circuitry. The current induced
into the head by the moving tape will be analogous to the original
sound recorded.
Within
your cassette deck, the basic transport mechanism and sound path
is:
- A
tape feed spool;
- A
tape head, and an associated sprung felt pad (within the cassette
itself - go on, have a look) to press the tape against the head;
- A
rotating metal spindle, called a capstan, and a rubber wheel called
a pinch roller;
- A
tape take up spool.
Playback
of the recorded sound is brought into effect when you press the
play button on the cassette deck by the mechanism of the deck:
- Moving
the playback head up from its rest position to press it against
the tape;
- Pressing
the pinch roller (and thus the tape) against the rotating capstan;
- Revolving
the take up tape spool (and on some decks the feed spool is also
motorised).
A common
misconception with tape decks is that the take-up spool drags the
tape past the head. Actually, its the capstan and pinch roller
that perform this, which is why theyre close to the actual
heads. The take up spool is turning to take up the tape that has
passed the head, as without this youll end up with a bit of
a tangle. Finally,
a cassette tape has four tracks upon it, with two tracks for stereo
playback in the forward direction and the remaining two for the
other side. Thus a replay head actually needs to sense the recording
on two tracks for stereo reproduction.
So,
how does this cassette talk map over to the Mellotron?
Im going to talk about a particular Mellotron model for this
description: The Model 400 or the most successful version. There
are some differences between this and earlier models, but more on
this later.
If you imagine a Mellotron keyboard, we actually have a very similar
transport in some respects to the cassette analogy, but its
also quite different.
First, under each key youll find a piece of tape with three
tracks, each with a sound recording on it. There are 35 tapes; one
for each key. Usually the pitch of the sounds on each tape are associated
with the key that it is under, but this dose not have to be the
case. Under
each key is a tape head. Each key also has a felt pad attached to
it, and also has its own pinch roller.
All
the Mellotron tape heads are only capable of mono playback, so why
the three tracks on the tape? The tape heads are mounted on a sliding
assembly, which allows the them to be laterally moved across their
respective tapes to align with one of the tracks. This is controlled
by a selector switch on the control panel which allows the user
to select one of the three sounds recorded on the tape. Think of
it as a patch change facility. You could also set the tape heads
so they were straddling two tracks, and this would give you a blend
of sound
The
first real difference we find between the Mellotron and our cassette
deck example is a continuously rotating capstan, which runs the
whole length of the keyboard and which is driven by a high torque
electric motor. Yes, were talking about a two foot feat of
precision engineering. Think about how easy it is for a "tiny"
dirty capstan to chew your cassette tapes, and then think about
how "true" the Mellotron capstan has to be to ensure that
it denst do the same on 36 tapes, especially for the tapes
furthest away from the capstan motor.

Click
Here for a diagram
Click
Here for a full size view
Click here
for the TIFF image
On
a cassette deck we have the feed and take up spools, whereas on
the Mellotron all the tape ends are fixed within a metal frame.
The frame itself is removable, so you can change frames and thus
change to another set of sounds. If youre really good you
can do this within a few minutes, and some bands used to perform
such changes in the middle of a live set. One common misconception
on the Mellotron is that it has tape loops. It denst; the
tapes are actually single sections, each six foot long. More on
this later.
When
a tape is at rest, it is threaded through a vertical pulley system,
which "folds" the tape about four times to reduce its
depth, before being threaded through the tape head/pressure pad
and the capstan and pinch roller. The pulley mechanism is sprung.
Click
Here for a diagram
Click
Here for a full size view
Click here
for the TIFF image
When
a key on the keyboard is pressed, the key brings its felt pad and
pinch roller, and thus its tape, into contact with its tape head
and pinch roller, and the tape is drawn past the head to recreate
the sound on the tape.
The
tape itself once past the head is collected into a collection bin.
With the length of tape and the speed of the mechanism, you have
about eight seconds worth of sound before the tape is stalled and
the sound dies. So you then have to release the key and allow the
time for the sprung pulley system to pull the tape back.

Click
Here for a diagram
Click
Here for a full size view
Click here
for the TIFF image
As
you can imagine this means that the Mellotron imposes a certain
playing technique: basically avoid those long drone notes, and your
chord progressions need to be planned to have totally different
notes in successive chords (forget all of your chord inversion tricks
for smooth progressions). Tonal
clusters are also best avoided (even two handed chords of more than
eight notes), because the torque of the capstan motor on some Mellotrons
couldnt handle that many pinch rollers being simultaneously
pressed against the capstan, which resulted in a lovely pitch drop
as the capstan slowed down!
So
there you have the basics of how it works, now lets think about
what made the Mellotron so attractive in its day, and even now.
The
immediate attraction must surely have been that it was a polyphonic
keyboard capable of playing back "natural" sounds. Remember,
the Mellotron was introduced in the early sixties and not even mono
synths were around then let alone poly synths, which didnt
see the light of day until the late seventies (remember the Polymoog?).
Until
the Mellotron, keyboard players were stuck with organs and pianos.
So imagine the excitement when they found they could have an instrument
that could play back "real" sounds, without the hassle
of hiring in a real string section. Of course, playing back string
sounds on a Mellotron that were recorded by "real" players
incurred the wrath of the Musicians Union (same old story
then it was nothing new in the seventies with the "synthesiser
threat" or the eighties with the "sampler threat")!
As
you can probably guess, you could have any sound you wanted on those
tapes. As well as an extensive library of stock Mellotron tapes
sold by the manufacturer, some Mellotrons were used for sound effect
"spotting" (think early Doctor Who and the BBC Radiophonic
workshop); some artists even had custom tapes made to their own
requirements. Paul McCartney had a Mellotron loaded with bagpipe
sounds so that his band could play "Mull of Kintyre" live,
and Steve Hackett had a tape frame with his own voice recorded on
it.
One
of the big things about the Mellotron was the fact that it used
tape strips and not loops. But why? Surely, if the system had used
tape loops you wouldnt have "run out of sound" whilst
playing a key? True,
but as we now know, the human ear/brain combination is a strange
device: give it an audio bandwidth full of information and it ignores
most of it! MP3 compression uses this fact to good effect by stripping
out the parts of the spectrum we ignore. Of importance to us (and
our tape loop question), is the fact that our brains take the start
of a sound and then sort of stop listening to it, as its already
decided what were listening to. So as long as you accurately
reproduce the startup transient of a sound, youre 90% there
on the authenticity trail.
The
Mellotron gave the recorded sound in its entirety, including the
all important start up transient and then eight seconds of the sounds
natural response. For example, a guitar sound would startup and
decay as expected. You couldnt do that with a tape loop. But
we still havent considered the character of a Mellotron. For
all that technology based upon original recordings of real instruments,
it had an ethereal quality and rich sound character all of its own,
and never really gave a totally faithful reproduction of the source
material. This
was partly due to the fact that each tape was a separate recording
in its own right. Think of a string player in the recording sessions
for the tapes: he played a note for eight seconds; stopped; played
the next note for the same length of time; and so on. All done with
probably no reference to the previous notes. Thus, each recorded
note had its own unique undulations in pitch, timbre and volume.
Put those different notes together in a chord and youll end
up with a very rich and naturally varying phasing of pitch, timbre
and volume between the notes akin to a natural string section.
Then,
the bandwidth of the Mellotron tape system was only around 8KHz,
so that made for a more mellow sound than typically produced by
the source instruments. Finally,
the playback mechanism itself imparted something on the sound. Think
of the changes in torque on the capstan as notes are pressed and
released affecting the pitch of the notes being played back, along
with the general "wow and flutter" characteristics of
the entire mechanism, of which the capstan is the only common component.
Just to add to the fun, each key had individually adjustments for
the pressure (felt) pad and the pinch roller, which probably had
at least a small effect on the playback characteristics for each
note.
As
I mentioned earlier, I deliberately picked the Model 400 for the
description. Out of interest some earlier Mellotrons had a slightly
different tape mechanism. Instead of the ends of the tape being
fixed in a frame, they were wound onto feed and take up spools.
The tapes were actually made up of six splices, with each splice
having its own set of three sounds, giving eighteen sounds per tape.
A "bank" was selected by the mechanism spooling the tape
to the start of a particular splice. Apparently this system wasnt
reliable enough and was renowned for mangling tapes during change
over, and was dropped on later systems in the quest for more reliability
and to save weight and cost.
A
Comparison between the Mellotron and Digital Sampling.
Hopefully,
by the end of this "How it works" section, youll
now be amazed at what the Mellotron was capable of in its day, and
is still capable of now. To emphasise this in a totally different
manner, lets compare the storage capacity available in a Mellotron
to that of both general computers and digital samplers, as this
provides a very interesting comparison of the growth of digital
storage capacity over the years compared to the Mellotrons
tape based storage capacity.
So
to start off, consider the Mellotron and the amount of sound it
stored. For this comparison, lets consider the Model 400 Mellotron
as this was the most popular model.
The
basic tape system of the Mellotron allowed for eight seconds of
sound, over 35 keys thats 280 seconds worth of sound per track.
If you consider the three sounds stored on each tape, thats
actually 840 seconds worth of sound stored within a Mellotron.
For
a MK2 or FX series Mellotron, with six sound banks and a dual manual
keyboard, the total storage capacity was over 10,000 seconds of
sound. Now to consider the digital equivalent of a Mellotrons
storage capacity, take the Mellotrons 8KHz bandwidth and double
it prior to sampling (remember, you need to sample at double your
signals bandwidth for accurate reproduction). Also, remember
the Mellotron was effectively a multi-sample playback device, with
a sample for each key, so we wont try and save space by playing
the same sample over several octaves. Now lets assume we are
using eight bit sampling. Double the capacity figures given below
for the sixteen bit sampling of Mellotron sounds. The amount of
memory needed to store a single sound of eight seconds on a single
key at 16KHz is 128,000 bytes of memory.
For
the three sounds, on each tape strip, thats equivalent to
384,000 bytes of memory.
For
35 tape strips, thats equivalent to 13.4M bytes of memory
needed to storage the digital equivalent of all the sound stored
within a Mellotron. For a MKII or FX series Mellotron, the equivalent
is 160M bytes of memory!
Now
lets contrast that to the storage capacity found within various
computers through the ages. As there were no home computers in the
sixties, Ive picked a high end mainframe that was about when
the Mellotron was first released.
Time
Line |
System |
Storage
Capacity Bytes |
1962
|
Mellotron |
13,440,000 |
1962 |
Ferranti
Atlas Computer |
49,192 |
1980 |
Sinclair
ZX80 |
1,024 |
1984 |
Sinclair
Spectrum 16K |
16,384 |
1987 |
Commodore
Amiga |
512,288 |
1990 |
386
Based PC |
4,194,304 |
1995 |
First
Pentium Based PCs |
16,777,216 |
2000 |
Modern
PC |
67,108,664 |
Thus,
you can see, the Mellotron had a thirty five year lead over the
typical storage capacity that we have in a standard desktop PC.
Compare that now to commercially available digital samplers. For
the purpose of this exercise, lets consider the stock memory
capacity of the sampler, with no additional RAM installed.
Time
Line |
System |
Storage
Capacity Bytes |
1962 |
Mellotron |
13,440,000 |
1976 |
Prototype
Fairlight |
32,768 |
1981 |
Emulator
I |
131,072 |
1984 |
Emulator
II |
524,288 |
1988 |
Emulator
III |
4,194,304 |
1994 |
Emulator
IV |
8,388,608 |
1998 |
Emulator
ESI 4000 |
4,194,304 |
As
you can see, even now the base memory capacity of samplers, has
yet to exceed the "base" storage capacity of the Mellotron.
If
we now move back to the EX series synthesisers, the stock memory
is 1,048,576 bytes of memory (or 524,288 words as its sampling is
16 bits). If you take a fully loaded EX with 1MB base memory, 64MB
DRAM and 16MB FLASH memory, thats 97MB of memory or 48.5MB
worth of 16 bit sound samples. But remember if we are sampling at
the higher rate on the EX of 44KHz, which gives us around 1,000
seconds worth of sound. Thus, for a fully loaded EX, sampling at
16 bit resolution and at 44KHz, we can get the equivalent of one
Mellotron into its sample memory (remember, the total sound storage
of a Mellotron is 840 seconds). When thinking about that, just remember
that the Mellotron predates the EX by about 40 years (or 55 years
if you also count its predecessor, the Chamberlin).
So
to round this technical tour off...
some
people see the Mellotron as a mechanical behemoth, that long outlived
its day. I hope Ive persuaded you to see things differently,
and that you actually now see the Mellotron from a different perspective.
Namely that it was ground breaking long before synthesisers were
even commercially successful, and that to have a sample playback
device at least thirty years before digital sampling could affordably
match the Mellotrons sound storage capacity was simply awesome!
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