It’s 2017, and because I’m such a dedicated techy committed to having the newest technology
at my fingertips, I just got back from the store with a fascinating piece of technology.
This video cassette recorder.
You probably think of the VCR as an uninteresting, outdated piece of tech.
But there’s some surprisingly interesting stuff inside here.
The VCR contains a device the solved what seemed to be an unsolvable problem.
Let’s have a look.
In the days of analog television, the actual signal that made up the images to be put on
the screen wasn’t a set of instructions on how to build an image using pixels.
It was a complicated, high frequency signal of continually varying intensity that contained
rudimentary triggers to help a television build a coherent image based upon the signal’s
instantaneous strength which corresponded to image brightness at a particular point
on the screen.
Anyway, an analog video signal is an insanely high frequency.
OK, it’s actually not that high by today’s standards, but bear with me.
The 5 megahertz signal of analogue television made for a perfectly acceptable picture, and
there weren’t any problems when it came to everyday use.
Except, that super high frequency meant that recording the video signal was impossible.
Let me explain.
Magnetic tape recording, already in use for recording audio signals from microphones,
has its frequency response, that is how high of a frequency it could reproduce, limited
by its speed.
Due to the structure of the tape itself, to record a high frequency signal it has to be
moving past the tape heads pretty fast.
This isn’t a problem for audio signals, as the highest frequency it needs to produce
is about 20 thousand hertz, or 20 kilohertz.
A speed of 7.5 inches per second is plenty for this purpose.
But video signals are about 5 megahertz, that’s 5 million hertz, much too high a signal to
put on normal tape.
Because of this limitation, TV shows tended to be live and not pre-recorded.
If a show was to be recorded before being broadcast, it was usually filmed with a conventional
motion picture camera, and then a device called a telecine would be used.
A telecine is a machine that can convert motion picture film into a television signal.
Aside from the necessary frame-rate conversion that was accomplished by the machine, you
can think of it as a glorified television camera pointing at a movie screen.
You could also use a kinescope, which was basically the opposite, like pointing a film
camera at a TV screen, the upshot of which was that frame-rate conversion wasn’t necessary
when playing it back due to an already matched framerate.
But using film wasn’t easy, and most importantly it wasn’t cheap.
It would be super convenient to put video signals right onto magnetic tape which was
cheaper and easier to use, not requiring film processing and also being reusable..
But again, speed was a problem.
In order to record video signals onto this tape, it would have to be traveling at many
feet per second.
With this 1,200 foot spool of tape, you could expect a recording time of about
72 seconds.
It would look a lot like this during normal operation:
But that didn’t stop progress.
At first, attempts were made to just make a really-fast tape recorder.
One such system, the Vision Electronic Recording Apparatus, or Vera, was developed in 1952
by the BBC, the project being led by (now here’s an awesome name) Dr. Peter Axon.
The VERA used massive 20 inch reels that contained 15 THOUSAND feet of tape.
That’s nearly 3 miles of continuous tape.
Yikes.
Even with that vast amount of tape, though, the recording time was only 15 minutes because
the tape traveled at 16.7 feet per second, or over 11 miles an hour.
If it wasn’t obvious that this was impractical, it should’ve been.
Other oddities exist in the linear-video-tape world, such as Toshiba’s LVR system that
used a loop of tape that moved very fast and a head that slowly moved along the tape from
top to bottom essentially making a spiral, but for the most part the idea was abandoned
because, let’s face it, this is just silly.
So what could be done to practically record video onto magnetic tape?
There’s no getting around the fact that the tape has to travel past the heads at at
least a dozen feet per second or so to get a reasonable picture.
To solve this problem, the American company Ampex, based in California, asked a brilliant
question: Why not have the heads move past the tape?
Ampex’s quadruplex system used a rotating drum containing four tape heads that sat perpendicular
to the traveling path of 2 inch wide magnetic tape.
The drum, rather than the tape, is what moved at a high speed.
Spinning at a rate of 3,600 rpm, the heads traveled past the tape very very fast, but
the tape only moved at a speed of 15 inches per second.
By slicing up the width of the tape into small parts of the video signal, the surface of
the tape could be used much more efficiently.
Electronics switched the output between the 4 heads allowing for a seamless video signal.
The Ampex machine was a hit, and it quickly became the standard format for television
studios nationwide.
But these machines were insanely expensive, with the 1956 price being 45,000 dollars,
equivalent to almost four hundred thousand dollars today.
Aside from costing more than a house, they were also huge, about the size of a large
chest freezer not including their many electronic components mounted on racks.
Not to mention, they were obviously very heavy, along with being decidedly not easy to use,
requiring training to operate them.
Fast forward to the mid 1970’s, and consumer video tape recorders are starting to appear.
Betamax and VHS were the two most common formats, with VHS eventually winning the drawn out
format war.
Both of these formats use similar technology to the original quadruplex system, so let’s
have a look at what’s on the inside.
This is a run-of-the-mill VHS cassette recorder from the early eighties.
All VCRs contain a video head system similar to the quadruplex system.
That's it there.
But before we get too involved in that, let's have a look at the cassette itself.
Any cassette tape is really nothing more than Magnetic Tape stored inside of a plastic shell
so it can be handled more easily and so the machine can interact with it automatically.
The audio cassette used a relatively simple system of three access holes, located on the
bottom edge of the cassette, that the erase head, play head and pinch roller could fit
inside of.
This works fine for the relatively uncomplicated process of moving audio tape past two stationary
heads.
But for a video format, the tape has to go in many places and most importantly it has
to wrap around the video Head drum.
Earlier machines had to be threaded manually, but that isn’t exactly consumer-friendly.
To accomplish threading automatically, the machine actually removes some of the tape
from the cassette and pulls it through the path of the heads.
Cout-outs in the bottom of the cassette allow for two spindles to stick up behind the tape.
As the cassette is lowered into machine, the hinged lid which keeps the tape away from
grubby little hands is opened, and once play is selected the spindles move toward the rear
of the machine which pulls the tape past all the necessary components.
Let’s look at some of the components inside without a cassette in place.
The path the tape makes is in the shape of an M. First it travels past this erase head.
When recording, the erase head is energized which removes any signals currently on the
tape.
It’s next stop is the video head drum.
After being wrapped around, it exits the drum and goes past two more stationary heads.
These heads record the audio and a tracking signal.
These two signals are recorded along the edges of the tape.
Obviously the audio track contains sound, meanwhile the tracking signal contains reference
pulses to match each frame of video.
The recorder uses the pulses to maintain the correct tape speed when playing a tape back.
It also allows for the machine to compensate for slight differences between tapes recorded
on different machines, enabling video tracking.
Later machines used the tracking pulses as a sort of timecode, counting each pulse to
determine how much time had elapsed.
The last main component is the capstan and pinch roller.
These components work together to squeeze the tape between themselves.
The capstan spins at a very precise speed, and that’s how the tape is actually pulled
through the mechanism.
Cogged spindles engage with the spools that hold the tape, but they only actually pull
the tape with force when fast forwarding or rewinding.
Otherwise they simply serve to spool the tape and keep it taut, with the capstan doing the
real work.
So let’s go back to the most exciting part, the video head drum.
Here lies the heart of the machine.
If you remember the days of VCR’s, you probably remember this sound.
That’s the sound of the head drum starting to spin.
For machines in the US, it spins at about 1,800 rpm.
One complete revolution makes one frame of video, and since the framerate of US television
is 29.97 frames per second, it spins nearly 30 times in a second.
But where are the heads themselves?
The heads are really tiny and hard to see.
They are tucked away in the slit that separates the two halves of the drum.
All VHS recorders have at least 2 heads.
Because NTSC video is interlaced, each head records half of the video frame at one time.
Upmarket VCRs would have 4 or even six heads, with the extra heads helping to improve image
quality by recording a more precise signal tailored to the specific tape speed.
You might have noticed that the head drum doesn’t sit level with the rest of the machine.
In fact it looks sorta like it was just tossed in there and let to stay where it landed.
But in fact the wonky angle is deliberate.
If you look closely, you’ll see that the head travels diagonally down the surface of
the tape.
One pass of the head, and thus one half frame or field of the video signal, is recorded
on this long distance of tape.
This is called helical scanning.
By running the heads along the tape in this fashion, the tape didn’t have to be nearly
as wide as the 2 inch tape tape found in the quadruplex system, and it also meant that
one pass of the head contained an entire field of video.
See, because the quadruplex system broke up the fields into multiple sweeps, requiring
16 head passes per full frame of video, it wouldn’t produce any sort of intelligible
picture unless playing at the appropriate speed.
This meant fast forwarding or rewinding was done blind, and freeze-framing
wasn’t possible.
Using one sweep for one complete field eliminated those problems.
If you could see the information on the tape, it would look like this.
Two linear tracks are present at the edges, and a bunch of long, diagonal lines fill the
middle.
Each of these lines is one half of one frame of video, called a field.
Each field fills in the whole screen, but only every other line of the image.
Every sweep of the heads along the helical path of the tape made half of the image, with
the other head sweeping by to create the other half.
The VCR would automatically switch what it showed on the television back and forth between
the heads, creating an apparently seamless image.
Now here’s an interesting question.
How is the machine able to actually read the tape?
If the heads are spinning around, they can’t have wires attached to them like these stationary
heads do or they’d tangle.
Early video tape recorders used what’s called a slip ring pickup, essentially a set of thin
wires that brushed against a spinning ring which was electrically connected to the heads
with wires.
These proved problematic, however, as corrosion and wear would introduce noise to the signal.
By the time VHS was invented, rotary transformers were used to provide a wireless coupling between
the top and bottom halves of the head drum.
The actual slices of tape that made up the signal are very thin.
The tape traveled at only 1.313 inches per second, and with 60 slices of video fields
squeezed in the space, you’re looking at a slice width of about .022 inches or just
slightly more than half a millimeter.
To help avoid interference between the tiny tracks on the tape, the heads were assembled
with different azimuths, that is different angles between the tape and the head.
Rather than hitting it straight on, one head would hit the tape at plus seven degrees,
and the other at minus seven.
This created destructive interference between the tracks, thus ensuring each head picked
up only what it was supposed to.
This became even more important as the Long Play, LP, and then Super Long Play, or SLP
recording speeds were introduced.
LP halved the normal tape speed and thus halved with width of the track, with SLP only being
a third the standard speed.
This doubled and tripled the amount of time you could record on the tape, but it reduced
the quality of the image noticeably, particularly when recording at the SLP speed as now the
width of the tracks created by the heads was less than 2 10ths of a millimeter.
The tape-to-head speed of consumer formats wasn’t quite fast enough to reproduce broadcast
quality images.
VHS had a bandwidth of only 3 megahertz, compared to broadcast bandwidth of 5.
This meant that the quality of the signal coming from the tape wasn’t quite as good
as live tv.
Are you ready?
This is VHS quality.
No joke, this has been recorded onto a VHS tape, fed through a capture device, and then
back into this video.
If you’re not watching full-screen, you should be.
It’s awful.
But keep in mind that back in the day we weren’t using massive TVs with 4k displays, let alone
even 720 p.
On an old tube-set, this quality was perfectly adequate.
Let’s talk about those different recording speeds, shall we?
This is what you could expect from a recording made at the standard play speed.
This is the best picture you’re going to get, and it goes downhill from here.
Now I’ll switch to LP. LP’s not terrible, but sound quality
got noticeably worse.
Now here’s SLP.
However, the later development of VHS-HiFi, which stored FM stereo audio within the video
signal of the tape using a second set of heads on the video drum, meant that sound quality
was constant even with an EP tape, and it’s really good, too.
VHS HiFi has a full 20 to 20 kilohertz frequency response, excellent signal-to-noise ratio
in addition to dynamic range, and excellent stereo channel separation as well.
It was very close to CD quality, with many people (myself included) not being able to
tell the difference.
Thus the best way to play 8 hours of music nonstop in 1985 was to record your favorites
on a T-160 tape running at SLP speed on a hi-fi equipped VCR.
The only caveat to VHS-HiFi was that because the audio was recorded along the helical scanned
portions of the tape, the source of the sound had to switch back and forth with the heads.
Our ears are actually far more sensitive to gaps in information than our eyes, so if the
tape was damaged or the heads didn’t line up quite right, you would hear a low 60hz
buzz.
Ordinarily, though, this was a fairly rare occurrence.
As a side note, for reasons I don’t really understand, the LP speed disappeared from
many VCRs.
Though all but the very earliest VHS recorders can play back a tape made at the LP speed,
few made after the 1980’s could record at LP.
I remember as a kid in the nineties being bummed when our new VCR couldn’t record
LP, as I found it a nice compromise between recording time and picture quality.
Oh well, I guess only nineties kids remember...
The longer recording time available on VHS was the main reason that Beta didn’t win
the format war.
Sony’s decision to use a smaller cassette with less actual tape inside meant that the
longest tapes typically held was four and a half hours, compared to the eight + possible
with VHS.
VHS was continually improved during its life, with the most noticeable improvement being
the S-VHS standard released in 1987.
S-VHS, short for super VHS could record a 5.4 megahertz signal with improved tape formulation
and recording techniques, actually providing a better picture than broadcast television.
This was only half-true, though, because while the luminance bandwidth was very good, S-VHS
did not improve the color rendering of standard VHS.
See on VHS and beta as well, the bandwidth required to make the image was dedicated mostly
to luminance, or a black-and-white signal.
VHS used a “color-under” encoding method whereby color data, recorded after the luminance
signal, was essentially drawn on top of a black and white image.
This was done to prioritize image sharpness with the limited bandwidth available.
The resolution of this coloring was by contrast quite poor, on the order of just 12% that
of the black and white detail.
So while the picture of S-VHS was sharper than broadcast tv, the color rendering left
a lot to be desired.
Partly because of this, and along with the significantly higher price tag of S-VHS machines
and media, S-VHS never really went anywhere.
Standard VHS was good enough.
You might be surprised to learn that toward the end of VHS’s life, D-VHS was introduced,
with the D standing for Digital.
It is certainly possible to record digital data onto magnetic tape, and the D-VHS equivalent
of a standard T-120 tape could hold 25 gigabytes, the same as a single layer bluray disc.
In fact, the D-VHS standard included support for 1080i video.
You should definitely check out these video clips from one of my favorite YouTubers Techmoan,
where he shows a demo tape with HD scenes of New York from 1994.
It actually kind of jarring to see street footage of that age in that clarity.
As I close this video out, I hope that you can admire the ingenuity in these old machines.
These intricate mechanisms and the out-of-the-box thinking that led to their creation are just
more fun.
The video head drum solved an unsolvable problem in an ingenious fashion, and the intricacies
involved in the machine just to get the tape to wrap around it are far more interesting
to me than a laser diode and optical pickup reading data off a spinning disc.
We’ll be exploring VHS and Beta as well in more detail, and I’ll also be discussing
the format war between them in later episodes.
Thanks so much for watching, I hope you enjoyed the video!
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I’m doing my best to keep video like this coming your way.
I’ll see you next time!