Post by Adrian Caspersz<snip>
Post by NYI always wondered how the later VHS decks were able to maintain a tape
library, so that when you inserted a given tape, the deck would display
all the programmes that were recorded on that tape. I realise that the
list was maintained, per tape, in non-volatile memory on the deck, and
was not stored on the tape itself. But how did the deck manage to
distinguish between one tape and another as soon as the tape was
inserted, so as to present the correct tape-contents list? Did all VHS
tapes contain a unique RFID tag, or was there some other way that a
unique tape ID was recorded all the way along the control track?
Dunno. First I heard of it :)
Panasonic NV-FJ760 Tape Library
http://youtu.be/TNgaUd2CIfY
https://www.manualslib.com/manual/117527/Panasonic-Nv-Fj710-Series.html?page=26
Control track apparently.
https://en.wikipedia.org/wiki/Control_track
Yes that makes sense. If you put in a brand new tape it presumably allocates
a tape ID based on time/date (so it is guaranteed to be unique) and then
uses that ID for the whole tape.
As long as you insert a half used (partially virgin) the tape at a point
where there is a recording, it will pick up the ID and use for subsequent
recordings.
The only problem would come if you inserted the tape at an unrecorded point
because the VCR would not pick up the existing ID and so subsequent
recordings would get a new ID rather than continuing the earlier one. I
never tried that. The difficulty now is finding a VHS tape anywhere that has
a virgin section to test that "split personality" tape logical flaw. I
imagine every tape I possess has been recorded end-to-end so doesn't have a
long enough virgin section to see what would happen.
I never knew the control track contained so much information: I imagined it
to be a dumb "electronic sprocket hole", with maybe a blip to denote the
start of each new recording, but not to have space to accommodate a few
bytes for an ID.
On VCRs from (I'm guessing) the mid/late 80s onwards, the control track also
operated the real-time counter, with pulses still being readable even when
the tape was rewound or fast-forwarded at high speed. That made *such* a
difference over the arbitrary 4-digit mechanical "mileometer" counters which
were non-linear (a difference of 10 units meant different things at
different ends of the tape!) and was not at all reproducible. When I was in
the sixth form at school in 1980-1, my prefect duty was in the AV room. We
had a couple of these beasts
Loading Image...
and the counters
were terrible. We got into the habit of setting the counter to zero at the
beginning of the tape and then noting the position on the tape index info on
the cassette box for each subsequent recording. That meant that a teacher
who wanted to use a given programme in a lesson would wind the tape to the
beginning (if it hadn't been left in that state), zero the counter and then
wind to position 1275 (for example) and know he was at the start of
Tomorrow's World which was the third programme on the tape and started at
the counter number. Fine, except that the counter could be several minutes
out either way if you repeated the operation several times :-(
But it got you in the right locality: you hoped that if you didn't hit the
exact changeover, you could at least identify whether you were near the end
of the previous programme or a few minutes after the beginning of the
programme you wanted. We tended to deliberately *not* record all episodes of
a series consecutively on the same tape, but to deliberately make dissimilar
programmes follow each other: this had the disadvantage that various
episodes were dotted over a variety of tapes, but it made it easier to
distinguish between "end of wrong programme" and "start of desired
programme" ;-)
Of course a wise teacher came up and prepared the tape and got it to exactly
the right place, before the lesson, rather than doing it on the fly with a
tape that he'd never seen before.
The VCRs that we had had one thing that the one in the photo doesn't appear
to have: a *wired* remote control which allowed the tape to be played at
various speeds: something like freeze frame, 1/15 .. normal speed, 2x speed.
The 2x speed still played with sound (IIRC) though it sounded like Pinky and
Perky. Just about intelligible, even if it made the boys crack up with
laughter.
Nowadays with software players like VLC you can play at any speed you like,
with sound that preserves the correct pitch, so speech is intelligible over
a fairly wide range and even music sounds OK from about 0.75x to 1.25x. I
presume it takes short bursts of consecutive sound samples which it plays at
the correct speed (to maintain pitch) but ditches a variable number of
samples in between depending on playback speed. Technology which we'd have
killed for in the early 1980s. ;-)
One thing about those early VCRs: they unlaced the tape every time you went
from playback to rew/ff, so there was always a few seconds unlacing delay at
the start of the fast movement and a few seconds re-lacing delay at the end.
Subsequent VCRs (maybe all those which used the control track for the
counter) kept the tape permanently laced both for play and for rew/ff so
there was only a delay when you first inserted the tape or ejected it.
The top-loader mechanism on those early VCRs was a PITA. You had to be
*very* careful to press down very gently and evenly on both sides of the lid
as you inserted it, otherwise the lid mechanism would jam. I have very fond
(blush) memories of demonstrating this (using that sort of wording) to a
stunningly attractive female teacher who was probably only a few years older
than me, and her responding "Oooooooo, Matronnnnnnnnnn!" in her best Kenneth
Williams voice, which cracked us both up so I couldn't continue for several
minutes. Happy times...
