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21489 FIR Accelerator Interpolation Delay

I have the FIR accelerator running for a 4x interpolation. I'm getting the correct results but the output is delayed by one sample (the first output sample is zero). This isn't really a problem but I would like to understand why and see if I can eliminate it.

I have the following parameters.
FIR taps = 64

Input block size = 32

Input buffer length = 48

One additional question: Can I avoid reloading the FIR coefficients if I'm applying the same filter to two channels?

Thanks in advance for any guidance.