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In the Forum: Analog Playback
In the Thread: A longer turntable belt.
Post Subject: Belt CreepPosted by George on: 12/17/2006
Here are some quotes from AA that I found interesting:
Rubber transmits vibration poorly, that's why it's used. Unfortunately this also means that it creates speed variation, mostly due to belt creep (NB I do NOT mean belt slip).
The motor is also coupled to the TT through its mountings and attempting to reduce noise transmission along this path is fraught with perils.
Flywheels offer advantages but these are often offset by the increase in belt creep that the extra couplings entail. This is so application specific that generalisations are worse than useless.
In my opinion the "one true path" is to ensure that the motor is as quiet as possible (cogless DC) and then reduce compliance in the transmission path as much as possible. Everything else is putting make-up on a pig.
and
The problem is that belt drives must lose speed around the drive pulley to work - there can be no transmission of force unless the tension difference in the belt is transferred to the drive pulley. Since the belt is elastic the tension difference must create a length difference. In turn this means the belt creeps against the pulley so the speed at which the belt is "taken up" is greater than the speed at which the belt is "let out". This speed difference depends on the torque demand from the table so any torque modulation creates speed variation. The simplest model of torque variation is that the torque reflected to the drive belt from primary stylus drag diminishes as the torque arm shortens across the record. This leads to an easy test - measure the speed with the stylus off the platter then cue the stylus at the lead in groove. Modulation drag is harder to estimate but will be proportional to the variability caculated by this method so it remains valid.
Just to knock two things on the head; Yes such small changes are perceptible, no no amount of platter mass can "fix" the energy lost due to belt creep, it just slows down the rate at which the speed changes. This will help with modulation drag indiced pitch change but not with drag induced rhythm changes.
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I have not read about "belt creep" any where else. You can find more by searching "belt creep."
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