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Thread: Why do pre-recorded movies on cassettes play on their own?

  1. #1 Why do pre-recorded movies on cassettes play on their own? 
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    The commercial versions of movies on video home systems automatically play when inserted into cassette recorders. What enables that?


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    Moderator Moderator Janus's Avatar
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    A video cassette designed to be used for recording has a "recording tab". Is it basically a piece of plastic that covers a hole in the case. As long as this tab is there, you can record on the tape. If you break that tab off, you can't record. (this is so you can protect something you recorded from being recorded over.) When you load the tape, the machine checks for that tab. With commercial prerecorded tapes, this tab was never added (so you can't accidentally record over the movie*) A machine can be also designed to automatically start playing the tape if it notes that the tab is missing.

    *You can record over a prerecorded or a tape which has the tab removed by placing a piece of tape over the hole in the case.


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    Thank you, Janus. I have always wondered about this. Wouldn't a video cassette recorder tear plastic adhesive, especially since it feels for a tab? How could the player not know the difference between a hard, thick part and a thin, delicate piece of tape?
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    Moderator Moderator Janus's Avatar
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    Quote Originally Posted by Theresa View Post
    Thank you, Janus. I have always wondered about this. Wouldn't a video cassette recorder tear plastic adhesive, especially since it feels for a tab? How could the player not know the difference between a hard, thick part and a thin, delicate piece of tape?
    The "sensor" that checks for the tab doesn't need to put a lot a pressure on it. It is just checking for an unobstructed "hole". Remember, the tab itself needs to be able to easily be broken off by the consumer if he want to protect what he recorded. This means that the sensor can't push too hard on it, or repeated use of the tape could result in an unintended breaking off of the tab.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    Oh, it's a sensor. I have taken it as a part that manually works with the tab. Thank you.
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    Quote Originally Posted by Theresa View Post
    Oh, it's a sensor. I have taken it as a part that manually works with the tab. Thank you.
    The two aren’t mutually exclusive. The sensor here is usually nothing more than a switch. You can design a switch that needs only a small force to operate, not the break-a-tab-poke-through-adhesive-tape mauling you are assuming. If you search for “micro switch” you’ll see what I mean. Switches come in all sorts of shapes, sizes, and activation forces. Think more of remote control buttons, rather than light switches.
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