Low frame rate is the simplest algorithm for subsampling. For time $i$ the acoustic feature vector $f_i$ and the two prior vectors $f_{i-1}$ and $f_{i-2}$ are stacked (concatenated). This makes a new vector which is three times longer, following which $f_{i-1}$ and $f_{i-2}$ are deleted. Therefore, we end. up with a higher-dimensional vector with a shorter sequence length $n = \frac{t}{3}$. For example, instead of a 40-dimensional acoustic feature vector every 10 ms, we would get a longer vector (say 120-dimensional) every 30 ms.