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I've posted this a few times to different questions. The hardware noted may be different, but the information within still applies:
With a sample track at approximately 123BPM, a 4 beat loop is measured in audio editing software at 1950.2494331ms (86,006 samples @ 44.1k).
The CDJ-2000nexus is capable of 1ms when using a quantized loop, or 6.6666667ms (1/2 frame @ 75frames/sec) with a non-quantized loop. This means if the loop is quantized, there is a loss of 0.2494ms per loop as the accuracy of 1ms means a total length of 1950ms. If the loop is NOT quantized, the resolution is lower, resulting in loop lengths of either 1946.6667ms or 1953.3333ms, a difference of -3.58ms or +3.08ms per loop – and that’s if you manage to get the loop as close to perfect as possible doing it without the quantization. Both of these also assume your beat grid is 100% accurate as the looping, quantized or auto-loop, is based on the beat grid. If you were to loop manually, you will be even less accurate than the timings noted. After just 4 cycles of this un-quantized loop, it will be out by approximately 12ms in either direction. The quantized loop however, would be out by a mere 1ms.
But that’s only 4 beats looped 4 times; 1 phrase is a mere 7.8s. Chances are you’ll wind up looping a bit longer, especially if you’re into any electronic music – let’s assume you loop it for 4 phrases (64 beats) – that’s 31.2s. If you had used a quantized loop, you’re now out of sync by 4ms and for that unquantized loop, by about 48ms.
These numbers will vary as the length of a 4 beat loop will differ based on the tempo. Likewise, if you set a 16 beat loop, it will take 4x longer to drift. And again, all of this assumes that your two tracks are 100% in sync to start with beat grids that are bang-on.
At what point do you start hearing that the loop is no longer accurate and out of sync with another track? Somewhere around 8-10ms for a trained ear (although some DJs may notice even around 5ms), 12-15ms for most people and it would most certainly audible to everyone at 20-25ms.
...and all that was to do with the CDJ-2000nexus. You're using the XDJ-RX, which, while it can read the grid from rekordbox, doesn't have 1ms accuracy, so you're limited to the 1-frame accuracy for auto-loops. The quantized loops may have a better accuracy (I'm not sure on the details for that with the RX), but the point remains that rekordbox will be 100% accurate for loops, but the RX has limitations.
