An off board strobe such as the KAB Keystrobe and Disc to show 50hz(100 flashes per second) or 60hz(120 flashes per second), as it is powered by a built in power supply designed to offer a correct amount of flashes for each frequency, it will be a very accurate method to check for rotation speed consistency/inconsistencies.
Additionally it will not be effected by fluctuation in a Hz reading that can occur in a Power Line.
The flashes on a TT's built in strobe will be effected by the syncing to the Power Line Hz frequency being received by the TT, and the built in strobe will usually offer a visual where the demarcated increments appear to be stationary, but this reading can prove to be insufficient to offer the most accurate RPM reading.
I'm confident that in a past investigation there has been information discovered that has claimed certain Strobe Bulbs or LED only function at their correct per second rate when the mains Hz is very accurate at the TT.
Additionally it will not be effected by fluctuation in a Hz reading that can occur in a Power Line.
The flashes on a TT's built in strobe will be effected by the syncing to the Power Line Hz frequency being received by the TT, and the built in strobe will usually offer a visual where the demarcated increments appear to be stationary, but this reading can prove to be insufficient to offer the most accurate RPM reading.
I'm confident that in a past investigation there has been information discovered that has claimed certain Strobe Bulbs or LED only function at their correct per second rate when the mains Hz is very accurate at the TT.