Huron & SNAP Documentation

HURON TECHNICAL REFERENCE
files that are scaled down too low will produce output from the
FIR filter that is well below full scale, resulting in poor signalto-noise
performance in the D/A converter.
The diagram below illustrates the way filter coefficient scaling
effects the Convolver’s output level:
Figure 98 — Scaling of filter output due to filter coefficients
The amplitude of the filter coefficients (h(k)) determine the
overall gain of the filter. More specifically, the filter scale (S) is
defined as:
N−1
1
S = ∑ h(
k )
2097152
k=
0
As a reference for comparison, the Huron Convolution Tools
are supplied with an example file called
c:\Huron32\sim\unity.sim. This file specifies an all-pass,
unity gain filter. It contains one impulse in the first sample of
the vector, and all zeros following this. The magnitude of the
impulse is 2097152, which results in a gain of S=1.
For most real filter responses, the overall gain S of the filter is
mostly useful in defining the average gain of the filter, for white
noise audio input. In reality, typical audio signals have most
energy in a narrower band (from about 200Hz to 8000Hz). For
this reason, it can be more meaningful to compute the gain S
based on a band-limited portion of h(k).
In order to achieve optimum data scaling in the Huron system, it
is advised that all SIM files be scaled up to achieve a gain of
S=1. This can be done by the following procedure :
• Determine the gain (S) of a SIM file (or a collection of SIM
files that are to be used together). Determine the gain for a
band-limited portion of the SIM file.
• Use the MultVec utility to scale the data up or down by a
factor of 1/S. This means that the newly scaled SIM file
should now have an average gain of S=1.
• If a collection of SIM files are to be used together, then the
relative amplitude difference between their gains must be
preserved, so the files should all be scaled up by the same
amount. The Lake Technology SimScale tool described
below uses the formula 1/max(Sn) to achieve this result.
HURON TECHNICAL MANUAL PAGE 292
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