WEISS GAMBIT EQ1-MKII Instructions d'exploitation télécharger pdf (Page 10)

OPERATING INSTRUCTIONS FOR GAMBIT EQUALIZER EQ1-LP OPERATION

Daniel Weiss Engineering Ltd., Florastr. 42, CH-8610 Uster Page 10 of 28

 +41 1 940 20 06  +41 1 940 22 14  http://www.weiss.ch  [email protected]

Linear Phase

The linear-phase feature is enabled by pressing

this key. If it is lit, the EQ1-LP is in linear-phase

mode, otherwise it is in minimum-phase mode.

This mode information is kept in the current

workspace, and can also be stored alongside all

the other workspace parameters in a snapshot.

Each snapshot is thus either linear-phase or a

minium-phase (this also counts for A-B work-

spaces: they can each have their own setting of

the mode switch).

Caution: Due to the nature of the DSP algo-

rithm, the linear-phase mode is not suitable

for notch filtering. Use the EQ1 in minimum-

phase mode for this application.

The following text describes basic signal processing

architectures for digital filter implementation, and

how exactly the EQ1-LP achieves the linear-phase

response. It can be skipped be the technically

uninterested person.

There are two basic kinds of frequency depend-

ent amplifiers (commonly known as filters): IIR

(infinite impulse response) and FIR (finite impulse

response). Both kinds can be implemented either

in digital or analogue domain, as discussed below.

IIR filters

Digital IIR filters can be derived from their ana-

logue IIR counterpart, thus transforming the ana-

logue filter parameters (Q, frequency, gain, etc.)

into the digital domain. This is the procedure for

digital EQ design favoured by Weiss Engineering

and most manufacturers, because it maintains the

well understood parameters from the analogue

equalizers. IIR filters have certain properties:

1. An infinite impulse response, hence the name

(in practice, the impulse response will even-

tually decay below the noise floor).

2. Some kind of phase response. Practical crite-

ria dictate a so called minimum-phase re-

sponse (which is linked to the amplitude re-

sponse), though this not necessarily need to

be so, in fact, virtually any arbitrary phase re-

sponse is possible.

Linear-phase IIR filters

Contrary to common belief, non-minimum phase

IIR systems are realizable, this usually involves a

minimum-phase section followed by an all-pass

section with any arbitrary phase response. Thus,

a linear-phase system would have a minimum-

phase system followed by an all-pass system de-

signed to have a phase response which, when

added to the minimum-phase, will result in a lin-

ear phase response. This can (theoretically) be

done in the analogue as well as the digital domain.

However, there are certain practical problems

which make this method difficult, if not impossi-

ble.

FIR filters

There is no direct way to transform analogue

equalizer parameters to digital FIR systems,

though there are algorithms that try to emulate a

specific frequency response on a FIR system.

These yield quite exact copies of the amplitude-

(and, if required, even of the phase-) response.

FIR filters also have certain properties:

1. A finite impulse response, resulting in a fixed

length output when excited by an input pulse.

2. Any arbitrary phase response.

3. A delay.

So linear phase-response is not a problem to

achieve with FIR filters. But IIR filters are favoured

over FIR filters in audio equalizers because of

several reasons:

4. The long delay of the FIR equalizer systems is

usually not acceptable for (online) audio proc-

essing.

5. The parameters of analogue equalizers are

easier maintained in IIR than FIR filters.

6. There is a considerably larger expense in-

volved for tuneable FIR audio equalizer than

for the IIR equivalent.

7. Digital signal processing for audio band FIR

filters is computationally more intensive than

for IIR filters.

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