6.7.12 Synthesize

Outline of the node

This node converts signals of frequency domain into waveforms of time domain.

Necessary files

No files are required.

Usage

This node is used to convert signals of frequency domain into waveforms of time domain.

When to use

\includegraphics[width=\linewidth ]{fig/modules/Synthesize}
Figure 6.95: Example of a connection of Synthesize 

Input-output and properties of the node

Table 6.69: Parameters of Synthesize 

Parameter name

Type

Default value

Unit

Description

LENGTH

int 

512

[pt]

FFT length

ADVANCE

int 

160

[pt]

Shift length

SAMPLING_RATE

int 

16000

[Hz]

Sampling rate

MIN_FREQUENCY

int 

125

[Hz]

Minimum frequency

MAX_FREQUENCY

int 

7900

[Hz]

Maximum frequency

WINDOW

string 

HAMMING

 

Window function

OUTPUT_GAIN

float 

1.0

 

Output gain

Input

INPUT

Map<int, ObjectRef> type. ObjectRef is Vector<complex<float> > .

Output

OUTPUT

Map<int, ObjectRef> type. ObjectRef is Vector<float> .

Parameter

LENGTH

FFT length; must be equal to the other node (MultiFFT ).

ADVANCE

Shift length; must be equal to the other node (MultiFFT ).

SAMPLING_RATE

Sampling rate; must be equal to the other nodes.

MIN_FREQUENCY

The minimum frequency used for waveform generation

MAX_FREQUENCY

The maximum frequency used for waveform generation

WINDOW

Window function. Select HAMMING, RECTANGLE or CONJ

OUTPUT_GAIN

Output gain

Details of the node

Inverse FFT is performed on four low bands of the input signals of the frequency domain by substituting 0 for frequency bins over $\omega _ s/2-100$ [Hz]. A designated window is adopted for overlap-add processing, a method of reducing the influence of a window by performing inverse transformation for every frame, adding the signals for which the time domains are shifted back while shifting them. For details, see the web pages in References. Finally, the temporal waveforms are multiplied by the output gain and output. Further, subsequent frames must be read for overlap-add processing; therefore, this node delays the entire processor. The length of the delay can be calculated as

  \begin{equation}  delay = \left\{ \begin{array}{@{\, }ll} \left| \mathrm{LENGTH}/\mathrm{ADVANCE} \right|- 1,&  \mbox{if~ ~ } \mathrm{LENGTH} \, mod\,  \mathrm{ADVANCE} \undefined 0,\\ \mathrm{LENGTH}/\mathrm{ADVANCE}, &  \mbox{otherwise}. \end{array} \right. \end{equation}   (134)

Since the default values for HARK are LENGTH = 512 and ADVANCE = 160, the delay is three frames, which resulting in a 30 [ms] delay for the entire system.

References

(1) http://en.wikipedia.org/wiki/Overlap-add