B U F F E R S A N D S A M P L E R A T E
A P P E N D I X A
The time slice T that the DSP buffer represents, often called the FFT size, equals the number of
samples it can hold (the Buffer size N) divided by the
Sample Rate
f
s
, e.g. if the Buffer size equals
1024 and the sample rate 96kHz, the DSP Buffer represents a time slice of 1024/96kHz = 10.67ms.
If a signal suddenly appears, we will need to wait until the Buffer is filled, at which time it can
be processed by the FFT. Waiting for the buffer to fill limits our resolution in the time domain
(we have to wait T seconds). This temporal resolution is also known as latency: a high temporal
resolution equals a low latency. To decrease our latency (increase our temporal resolution), we
therefore need to keep T
small
by either reducing the Buffer size or increasing the Sample
Rate, or both.
However, we have already seen that the frequency resolution equals 1/T Hz (ignoring effects
due to windowing). Therefore to increase frequency resolution, we need to increase T as much
as possible by using a
large
Buffer Size and a low Sample Rate. It is this frequency resolution
that determines the possible steepness and narrowness of our filters. A higher resolution
enables steeper and narrower filters.
Clearly we cannot have both a high temporal and a high frequency resolution. We must make a
trade-off by setting the temporal resolution (latency) to an acceptable level and accepting the
resulting frequency resolution (minimum filter width).
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