DK Digital CDB-600 Specifikace stáhnout pdf (Strana 44)

Chapter 10 FFT Spectrum Analyser. PRELIMINARY Release MSD Software Manual.

loudspeakers etc. With Write-Protect Off the SETMEM function will save the

reference curve in the MSD’s FLASH memory saving the reference curve even

after re-powering the unit.

The Predefined reference curve [I+PRE] is a pre-emphasis curve 'lifting' the

data readout with +3dB/Octave, resulting in a visual appearance of a 1/3

Octave analyser. This lift provides a closer relationship between the aural and

the visual judgement of the musical audio spectrum.

Both reference curves can be enabled at the same time.

10.6 FFT Window Functions.  

Since the spectrum analyser is based on the Fast Fourier Transform principle,

the sampled audio data needs 'windowing' (An in-depth mathematical

explanation is not possible within the scope of this manual, we would therefore

recommend that a detailed explanation of windowing be retrieved from other

technical literature. However, a basic explanation of the windowing concept is

as follows):

After the Audio is sampled and entered in a digital buffer with 1024 stages the

data is processed by the FFT algorithm and finally the results are displayed.

While FFT processing the 1024 samples, a new set of data are recorded. No

data is lost. Since the 1024 samples used for these calculations are a window

of continuous audio samples, the beginning and the end of the 1024 samples

need some 'smoothing' in order not to develop erroneous results. This data

smoothing is known as 'windowing'.

The MSD system is implemented with four different windowing functions listed below:

Rectangular Window.

Samples are multiplied by a factor of one (i.e. by themselves). This window

function is merely a method of disabling windowing of the data. The

Rectangular Window provides the highest selectivity, but the lowest dynamic

range.

Hanning Window.

The Hanning Window is useful in most applications and should be selected as

the default. The Hanning Window provides high dynamic range, but low

selectivity.

Hamming Window.

The Hamming Window is ideal for applications where the requirement for the

dynamic range is known. The Hamming Window compromises between

selectivity and dynamic range.

Nuttall Window.

The Nuttall Window is especially useful for distortion measurements. The

Nuttall Window provides the highest dynamic range and compromises the

selectivity.

The recommended window function for most applications is the

HANNING window.

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