In order to beatmatch two loops, you need first to sample 2 loops that match. If you're doing this with vinyl it's pretty easy. Just make sure you're sampling the same number of bars, and it'd help if the tempo is similar.
Once you have them both perfectly cut to perfect-enough loops, calculate their tempos by entering the number of bars in each loop's sample edit menu, and turn Looping ON. The MPC will calculate the loop's BPM for you automatically, and you can adjust this resultant number indirectly by adjusting the pitch or tone of the sample. The BPM number will change accordingly.
So set each of the loops' BPMs in proximity to one another, and then set the TEMPO and BARS of the sequence in question to match the loops'. Set them to play at the beginning using step editing, and play with the duration parameter until the loops terminate at the proper time, creating a smooth transition from end to beginning.
So basically, this does work the same as your old sampler, just the MPC does the calculation for you.[/color]
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