T-44/48 Series Operation Manual
15
T-44/48 Series | rev.
firm. 1.258 and above
2.5 LIR Linear Phase Crossover Filtering
The
T-44/48 Series
also includes a new type of crossover filtering “
Linea Impulse
Response
” (
LIR
) crossover filtering, which gives a Linear Phase crossover that has a constant
delay regardless of frequency (unlike other types of crossover which delay different frequencies
to a different extent, thus smearing the arrival time). The LIR crossover can thus be described
as having a flat Group Delay response, and thus entirely free of Group Delay Distortion.
The shape of the LIR crossover filter is quite similar to a 4
th
order or 24dB/Oct Linkwitz-Riley
filter, and maintains zero phase difference between the adjacent bands across the crossover
region to keep the polar response rock steady. See Figure 09.
100
1
.
10
3
1
.
10
4
36
30
24
18
12
6
0
6
24dB/Oct
LIR (Linear Phase)
Frequency, Hz
M
agni
tude,
dB
Figure 09: LIR Filter
2.6 FIR Linear Phase Equalisation
The Input High-Shelf Equalisers use
Finite Impulse Response
(FIR) filtering to produce
Linear Phase equalisation; that is all frequencies are delayed by the same amount, perfectly
preserving the transient response. This can also be important in applications where different
amounts of EQ are applied to different parts of a speaker cluster, such as to add 'Throw' EQ
boost so that parts of cluster which are throwing further can have HF absorption correction
added. If this EQ is not linear phase, then the zones where the speakers combine may suffer
frequency response anomalies.
