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A continuous-time speech signal $x_a(t)$ is sampled at a rate of $8\:kHz$ and the samples are subsequently grouped in blocks, each of size $N$. The DFT of each block is to be computed in real time using the radix-$2$ decimation-in-frequency FFT algorithm. If the processor performs all operations sequentially, and takes $20\:\mu s$ for computing each complex multiplication (including multiplications by $1$ and $-1$) and the time required for addition/subtraction is negligible, then the maximum value of $N$ is _________