[24hoursupport] Re: The Myth Of The 100-Year CD-Rom

  • From: chizotz@xxxxxxxxx
  • To: 24hoursupport@xxxxxxxxxxxxx
  • Date: Wed, 09 Feb 2005 15:21:19 +0000


Perhaps audio recorders do not record timing ticks. Audio was never my 
specialty. It was my understanding that they did, but I could be mistaken.
Video recorders, on the other hand, do record timing ticks, or they did circa 
1982. Perhaps technology has improved since then to get rid of the timing 
ticks, but I don't believe so.

I can't claim decades of experience in tape recording technology. In fact, my 
experience with tape technology is fairly limited. I did write, produce, 
direct, and edit a one-season TV series for the local St. Louis market some 
years ago. During one editing session, I kept getting bad edits. The picture 
would jump and there was distortion. The engineer explained that the editing 
machine I was using was malfunctioning by not properly synchronizing the 
timing ticks at the edit point. This mis-match of the timing ticks resulted in 
the "glitch" at the edit point.

That sounded logical and reasonable, but I wasn't sure if it was the truth or 
just a line the engineer was handing me. So, as I always do when confronted 
with something I want to understand but don't, I self-educated myself. The 
engineer was telling me just exactly how it worked. I don't recall the names 
of the books I consulted, but I got essentially the same information from them 
that the engineer gave me, plus a lot more.

Just to be sure I'm not totally mis-remembering things, I found this article 
at How Stuff Works http://electronics.howstuffworks.com/vcr.htm It also 
explains that there is a control track on VCR tapes, which is what I am 
referring to. Basically, you can think of the control track as containing 
timing ticks. The VCR reads the control track and synchronizes itself with the 
time ticks. That sets the speed that the tape was recorded at. If at some 
point on the tape the timing of the ticks change, there is a glitch at that 
point while the VCR re-synchs itself to the tape. That's the effect I was 
getting all those years ago when the editing deck I was using was not synching 
the timing ticks between edits.

As you described, the effect can be easily duplicated with modern VCRs by 
recording at one speed, stopping the recording, changing speeds, and recording 
some more, then playing the tape. You're absolutely correct that in those 
conditions the VCR will change speeds at the point when you changed recording 
speed, and there will be a glitch (what you refer to, correctly, as tracking) 
at that point too. It all is based on the difference in spacing of the timing 
ticks on the control track at that point changing.


> In a message dated 2/8/2005 1:29:55 P.M. Eastern Standard Time, 
> chizotz@xxxxxxxxx writes:
> Magnetic tape has (this is basic so don't hold me to specifics) a stripe down 
> one edge that contains timing ticks, and the rest of the tape width is used 
> to store the actual analog data, usually sound information. Stereo recorders 
> broke that area down into two stripes, one for each channel.
>     I've never heard of these timing ticks and I've been dealing with 
> recording tape for a good 35 to 40 years now.
>     And, I don't recall video tape having them either.  I've been dealing 
> with that stuff for at least 33 years now, back when it was still only 
> available 
> on open reels.  The video record head spins around at a very fast pace and 
> records the picture onto the tape at an angle like this: \\\\\\\\\\\\\\\\\\\\ 
> Each of those lines is equivalent to one pass of the video head.  There are 
> actually two heads on the cylinder that spins around.  Each of those lines is 

> probably also equal to one frame, but I'm not absolutely sure about that.  As 
> far 
> as I know, the modern video tape machine doesn't use ticks at all, it just 
> very quickly adjusts itself to the correct speed that the tape was recorded 
> at.  
> That's why the machine has to spend a few seconds tracking when you first pop 
> a tape into it.  It spends about 1 second adjusting to the correct speed and 
> the next few fine tuning to that speed so that the playback will match up 
> with 
> the record.  Record a video tape at one speed for a few minutes, or even a 
> few 
> seconds, and then stop tape and set it to record at a different speed.  When 
> you play the tape back from the start, when it get's to the point where you 
> switched speeds, you'll hear the sound speed up or slow down dramatically 
> within 
> about half a second.  That's because the machine is adjusting itself to the 
> correct speed.  It's also going to have to track again to adjust to the new 
> playback speed of the tape.
>     With audio tape the speed is adjusted by the person operating the 
> machine.  What good are ticks when you can fine tune the speed (the pitch) 
> while 
> you 
> play or record the tape?
>                             Dale
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