Note: Descriptions are shown in the official language in which they were submitted.
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FILTER SIESTA
This invention relates to filter networks for signal pro-
cussing systems.
One application is in a loudspeaker soys em using two loud-
speakers wherein there is often a difference in the time taken for sound from each loudspeaker to arrive at the ears of a
listener. The special separation between the drivers in a
loudspeaker system affect the radiation pattern over the ire-
quench range where more than one driver contributes to the
10 total acoustic output.
BACKGROUND OF THE INVENTION
A proposal has been made to provide additional active
delay networks to compensate for offsets in the acoustical
planes from which the individual drivers radiate. A proposal
15 has also been made to provide cross-over networks and equalizers
incorporating delay compensation for the distance traveled
by sound from each loudspeaker. These proposals are particular-
lye concerned with the acoustic section of the system and go
in after the position of the loudspeakers has been determined.
The present invention is not concerned with the delays
introduced due to the positioning of the loudspeakers in the
system but is concerned with the delays which are introduced
by the circuit components when a plurality of outputs are pro-
voided in view of the number of loudspeakers used. It has been
25 discovered that inequalities may be introduced in the phase
as well as variations in the amplitude output whereby the rest
posse of the filter system did not yield a perfectly flat am-
plotted response (within + or - 0.1 dub).
The above-mentioned inequalities are particularly pronounced
30 when more than a 2 way cross-over network is utilized.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an
improved filter system in which the above-mentioned inequalities
are obviated or substantially reduced,
According to the present invention there is provided a
filter system comprising at least three channels, each channel
being adapted to pass a selected range of frequencies, a first
I
delay means connected in one of said channels and a second
delay means connected in another of said channels, the delay
characteristics of said first and second delay means being
selected whereby the output of each channel is in phase with
the output of each other channel, the sum of all said outputs
being substantially unitary in magnitude.
DESCRIPTION OF TEE DRAWINGS
One embodiment of the invention will now be described
by way of example, with reference to the accompanying drawings
in which:
Figure 1 is a block schematic representation of an
embodiment of the invention which provides four outputs, and
Figure 2 is a more detailed diagrammatic representation
of the embodiment of Figure 1.
DESCRIPTION OF ONE EMBODIMENT
.. .. . ... _
Referring to Figure 1, two input lines 2 and 4 are
connected through Radio Frequency Interference (R.F.I.) filter
unit 6 to the input lines of an input signal conditioner
unit 8.
pa The output of the signal conditioner unit 8 is fed to
a delay equalizer unit 10 as well as to a delay equalizer
unit 12. A further delay equalizer unit 16 has its input
connected to the output of unit 12 whilst the output of unit
10 is connected to one terminal 38 of a three-terminal jumper
unit 62. The second terminal 40 provides a non-delayed line
from the output of the unit 8 whilst the third terminal 42 is
shown as being "jumped" to terminal 38. Terminal 42 is connect
ted to the input of the delay equalizer unit 14 in a first
channel and is also connected to a high pass filter unit 20
in a second channel. The function of the jumpers is to pro-
vise two, three or four frequency channels (bands). In Figure
1, the jumper units are shown connected for four-way operation
with all the delay units utilized.
The first channel also includes a delay equalizer unit
14, a jumper unit I and a low pass filter unit 18 having an
output connected to the first output channel terminal 30.
~2;~3~
The second channel includes the further high pass
filter unit 20, and a low pass filter unit I The output of
filter 22 is connected to a first terminal of a jumper unit 66,
a second terminal facilitating a direct connection to the
output of high pass filter unit 20 and a third terminal 54
providing an output connection to the second channel's output
terminal 32.
The output of delay equalizer unit 12 is connected
to the input of a high pass filter unit 24 in a third channel
and the delay equalizer unit 16 in a fourth channel.
Ire output of high pass filter unit 24 passes through
a low pass filter unit 26, to a first terminal 56 of a jumper
unit 68 having a second terminal 58 facilitating a direct
connection to the output of high pass filter 24. A third
lo terminal 60 of jumper unit 61 is connected to the third
channel output terminal 34.
The output of delay equalizer unit 16 in the fourth
channel is connected through high pass filter unit 28 to the
fourth channel's output terminal 36.
A summing amplifier unit 70 is connected to the
output of each of the four channels, as shown in Figure 1 to
verify the flatness of the summed response at test point 72.
In Figure 1, the delay equalizer units are identified
by the word DELAY whilst the low pass and high pass filter
US are identified by the word LOW or HIGH respectively.
Letters A, B, and C have been applied to the units
of Figure 1. By a suitable combination of the units, one can
achieve equality of the phase response of all channels, for
example:
For four-way r there should always be a unit A, a
unit B, and a unit C in each channel;
For three-way, there should be a unit A and a unit B;
For five-way, A B + C + a further unit.
The different delay v. frequency characteristics of
units A, B, and C are identical, whether they arise from a
delay unit low pass filter or a high pass filter. The total
delay of any one channel is the sum of A B + C, all channels
being identical.
~Z293~3
In Figure 2, the units of Figure l are shown in
greater detail.
The same reference numerals are used in Figure l as
have been applied to the corresponding units in Figure 2.
Referring to Figures 1 and 2, in use, an input signal
is applied across lines 2 and 4 and after passing through the
R.F.I. filter 6, it is applied to the input signal conditioner
unit 8. The signal is then applied to a delay equalizer unit
10 in the first channel as well as to a delay equalizer unit 12
for the third and fourth channels. These delay equalizer units
are of flat magnitude response and constitute a phase filter
which is an all-pass network. The unit lo in the first channel
is followed by another unit 14 constituting a delay equalizer
of flat magnitude response and forming a second phase filter
in the first channel
These are followed in the first channel by the low-
pass filter unit 18.
The low-pass filter unit 18 constitutes an amplitude
filter which is an all-pass type, as a result of the cascading
of two end order Butter worth filters to yield a 24db per okay
; low-pass filter.
In combination with a second order all-pass delay
equalizer having identical phase response to that of the
cascaded second order Butter worth filters one achieves the
advantages of the described embodiment of the invention.
s will be seen, delay equalizer unit lo is common to
the second channel, as well as the first channel, whilst the
second channel also includes high-pass filter unit 20 followed
by low-pass filter unit 22, which latter is similar to the
low-pass filter unit 18. The high-pass filter unit 20 also
constitutes an amplitude (magnitude) filter of an all-pass
type which is achieved by the cascading of two end order
Butter worth filters to yield a 24db per octave high-pass filter.
This is followed by the low-pass filter unit 22.
The phase filter comprising the delay equalizer unit 12
is common to the third and fourth channels and, in the third
~22~3~,~i9
channel is followed by a high-pass filter unit and a low-pass
filter unit, as shown in the Figures.
In the fourth channel a phase filter, constituting
delay equalizer unit 16 follows the delay equalizer unit 12
5 and is succeeded by an amplitude (magnitude) filter comprising
high-pass filter unit 28.
It will be seen that the illustrated circuit is a multi-
way crossover filter network and it does have a unique property
in that all outputs, at terminals 30, 32, 34 and 36 are sub-
10 staunchly in phase at all frequencies, whether the circuit misarranged as a 2-way, 3-way or 4-way crossover network, or even
if more terminal outputs are provided by circuit extension to
5-way or more, whilst at the same time maintaining a high
degree of out of band attenuation of 24db per octave. This
15 "equality" of phase is accomplished concurrently with a well
defined amplitude response whereby a direct sum of all the out-
puts of the multi-way filter system yields a perfectly flat
amplitude response (within + or - 0.ldb).
As shown, the circuit consists of two basic elements
which are the amplitude (or magnitude) filter and the phase
filter. The linking of these two filters in a complimentary
fashion appears to give rise to the uniqueness of the illustrated
circuit and the advantages thereof.
In previous circuits the cascading of low-pass and high-
25 pass sections to form band pass sections required for more thin crossovers gives a band pass output which is no longer
in phase with the low-pass or high-pass outputs, since these
are only made up of single section (i.e. frequency filters.
The described embodiment in Figures 1 and 2, overcomes this
30 limitation by ensuring that all filter bands in a multi-way
crossover system have the same total delay (and thus phase)
as a function of frequency, whether a single band uses one,
two or more filter sections in cascade, by adding delay
equalizers which have identical delay vs. frequency kirk-
35 teristics to those of filter sections of a given frequency not included in any one band. As an illustration, a 3-way
crossover system would use a single low-pass section of
~.2~3~
6 --
frequency Fly for the first band, and then a cascade of high-
pass section of frequency Fly and a low-pass section of
frequency F2 for the end band, and finally a high-pass section
of frequency F2 for the last band. The illustrated circuit
5 adds a delay equalizer to the first band so as to compensate
for the inherent added delay of the low-pass filter section of
frequency F2 of the end band, and thus brings the outputs of
both the sty and end bands (channels in phase at all frequent
ales. The expansion to the next band (channel 3) and to larger
lo systems is then apparent.
A mathematical proof of the phase "equality" and flat
summed magnitude response of the illustrated circuit is as
follows for a 3-way crossover system. The proofs are of a
similar form for other multi-way systems or for other filter
15 response types.
~2~3~
_
Best c Cowan on .
Band 1: dot a (F2) *l opus (Fly )
Eland 2: h i gh--pass t F 1 ) *1 opus ( F2 )
Eland 3: dot a (Fly ) phi gh--pass (F2)
Where F1 F2 art thief crossover frequencies.
Residency unctions ox the bus c blocks.
s= j w wow
Dælay(F1) S - I S
H ( s )
5
Delphi) Ho -- So I
Lopez tFl )
S r
So S J
Low-pass OF ) I,
Ho r _ AL .
us L Sly
Hi gh--pass ( F 1
H I) = I 5
S + Lo
High-pass(F2) 2 - Jo
30H~s) -- r 1 s
L 7LS We
I
Phase responses
Del a OF 1 )
Lopez F 1 )
r
L
- Hi gh--pass t F 1 )
lo L I. -
Delphi) , L 3
Lopez ~F2)
Hi gh--p~ss (F2)
pa It may be observed that the delay sections have the
aye phase responses as thy f i 1 ton sect Ought of the sap
frequency tF1 or F7), whether the filters be high or Lopez.
This i 5 fundamental to the circuit Boeing described.
Lowry tune response 5
Delay fly ) Del a ~F2)
G AYE ) _ I
Low-pass (Fly ) I,
f
Pi I Z
Lopez ~F2)
5 t I) =
2 ",~ to So
.,
I
g
Hi gh--pas~; fly )
of , 3
Hi gyps tF2) Y
6 (pa) = ,
Comb nod resDclnses
ED
Band 1 H < S ) =
I AL S + I LO 2~LS t' )
Eland 2 S So
sty fly )LS~L~2~J
Band I y Jo
H US) = S ( S
(S ~rL~S~ 4~,L)(S~ )z.S~z)
I Cobb noted phase rosins
End 1 ] lo}
land 2 AL AL
Andy 3 Lo AL aye
e v L
The combined phase responses art found to be identical
for all bands and thus the cruiser filter stem Avis
toe stated characteristic of phase 'ec~ualit~, of all outputs
(bands ) .
2 9 3 I
-- 10 --
Summed r en con so
( s ) _
( 5 w, S Lo 2 So
f ~"~ r y Y
) f Jo Y AL
A summed magnitude response, GO is obtained which
for any realistic set of frequencies has an amplitude flatness
of + or - 0.ldb.
In the illustrated embodiment each channel may include
a four pole filter and each delay means may be a two pole
delay means.
It Jill also be understood that the invention is equally
applicable to higher numbers of channels (bands), for example
five, six, or ten channels, by appropriate extension of the
basic configuration.
It will be seen from the above discussion that a
specific set of properties, i.e. phase and magnitude response,
may be obtained using the descried circuit. One obvious use
for the circuit is as a loudspeaker active cross-over network,
but it will be understood that the invention is not restricted
thereto and the circuit could readily find use in other signal
processing applications.
It will be readily apparent to a person swilled in the
art that a number of variations and modifications can be made
without departing from the true spirit of the invention which
will now be pointed out in the appended claims.
US
