On 8/5/15 2:41 PM, John Haley wrote:
> Hi, Tom. Thanks for that great "real world" description of scrape
> flutter. Definitely something I am familiar with hearing, but it's great
> to understand it better. The "scraping" is literally from the tape
> scraping against the reel on playback (or recording), right?
No.
> Does this
> also happen from the tape scraping a part in the tape machine?
Yes. The scrape flutter we're talking about is caused by the tape
scraping across the heads and non-rotating guides.
Tape under tension does stretch. It's like a very stiff spring. A
section of tape stretched between adjacent fixed surfaces will resonate
at a fairly high frequency. In most professional recorders, this
resonance is somewhere between 1 and 5 kHz. The resonance is excited by
stick-slip friction on the fixed surfaces.
The common analogy is a violin string. However, the vibration of the
violin string is transverse (perpendicular to the length of the string),
and tape vibration is longitudinal (along the length of the tape).
Longitudinal vibration of the tape is, of course, a speed variation.
When this happens at a very high frequency, it has the deleterious
effects which Tom has described.
Here's a good survey of flutter problems from one of the recognized
experts on this subject:
http://www.manquen.net/audio/index.php?page=17
Scrape flutter occurs during both recording and playback. The
professional machines now used for most transfers will have low scrape
flutter *if* the scrape flutter idler(s) in the tape path are properly
adjusted and in good condition. Unfortunately, the correct operation of
the scrape flutter idler(s) is often not frequently (or ever) verified.
Playback on a machine with bad scrape flutter idler bearings usually
sounds worse than the same machine with no scrape flutter idler. So,
lack of maintenance can change the scrape flutter idler from an asset to
a liability.
Early professional recorders (and most consumer and many semi-pro
machines) did not employ any of the mechanical techniques now used to
reduce scrape flutter, so recordings made on those machines contain
"baked-in" scrape flutter.
-- John Chester
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