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

  • From: 0e60wq102@xxxxxxxxxxxxxx
  • To: 24hoursupport@xxxxxxxxxxxxx
  • Date: Wed, 09 Feb 2005 12:22:39 -0600

I picked up a couple of discounted movie CDs at walmart for a 
dollar each. One I'm not sure about, but the other for sure had 
about a one-second mismatch between video and audio all the way 
through. I know, because at one point, someone was making a 
repeated noise, and it really became obvious when I watched the 
person 'make' the noise again, and counted off at least a whole 
second before the noise came. After that, I pretended I was 
watching something in a foreign language and dubbed into English, 
like an old Italian Roman gladiator movie, and manage to mostly 
ignore it.

chizotz-at-mchsi.com |24hoursupport@xxxxxxxxxxxxx| wrote:
> Dale,
> 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.
> Ron
>>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 
>>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 
>>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 
>>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 
>>the record.  Record a video tape at one speed for a few minutes, or even a 
>>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 
>>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) 
>>play or record the tape?
>>                            Dale
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