Article Index

Sound Tools Extended (STx) is an acoustic speech and signal processing application for Windows. It provides tools to analyse, visualise, segment, and annotate wave files. It uses it's own annotation file format, but can import and export PRAAT TextGrid files. The software is actively being developed at the Acoustics Research Institute in Vienna, and is used in a variety of acoustic fields around the world. For non-commercial, scientific and educational purposes, we offer STx free for download.

The upcoming version STx 5.0 will include a compact mode specially designed for linguists.

Signal Processing

STx incorporates a sophisticated sound analysis and signal processing package:

  • frequency analysis based on Fast Fourier Transformations (FFT)
  • spectrum estimation methods, e.g. autoregressive (AR), Pseudo-Wigner distribution
  • fundamental frequency and formant frequency extraction, e.g. cepstrum, linear predictive coding (LPC)
  • digital filtering

Graphical Analysis

STx can display signal analysis in the following forms:

  • spectrogram/sonogram
  • waveform
  • waterfall plot
  • function plot

Programming Language

All functionality available in STx is also available in a dedicated programming language.  Additional applications can be easily programmed in this language (either by the end user, or the Institute). STx also includes a command line interpreter and a sophisticated debugger. Using script file programs, STx can be used in batch mode for:

  • automated sound analysis procedures
  • interactive experiments
  • test designs
  • automated signal and process control.

For a list of available functions and commands, along with the complete syntax of the scripting language see the STx Script Programmer's Handbook. STx comes with a large number of example script programs.

Interface Protocols

STx supports the following protocols from within the scripting language:

  • DDE
  • DCOM

An STx/DDE/DCOM demo application is currently available for Microsoft Excel and the statistics software package "R".



  • creates, imports and exports MS Windows WAVE files.
  • runs on Windows Vista, Windows 7, Windows 8, Windows 10 and under Wine.
  • stores its metadata in XML files.


STx metadata format (segment addresses, attributes and parameters) is compatible with archiving systems and requirements, since it is stored in an additional file alongside the original wave file.These files can easily be imported into an archiving database system.

STx provides tools for digitization of sound, single or multi (up to 128) channel signal recorder, 16/24 bit, sampling frequencies depending on audio hardware; analogue to digital transfer.

STx-Recorder | Real Time Frequency Analyser | Sound file Management | Sound file SegmentationVisualization | Frequency Analysis | Spectrograms | Quasi-continuous Wavelet-Transform | Wavelet-Phase-Diagram | Spectral Smoothing | Extraction of Formant Frequency Candidates (Speech) | Fundamental Frequency Analysis (Speech) | De-noising, Signal Enhancement | Auditory Perception Modeling | Simultaneous Masking | Order Analysis | STx Sequencer 

The STx Recorder

The STx Recorder is supplied with a peak programme meter. Digital recordings allow no overload at all. For that reason it is extremely important to adjust the signal input level to -12 dB in average, providing enough headroom for signal peaks.

Real Time Frequency Analyser (2-channel)

Sound file management

Sound file management, storage, archiving and migration of large sound files including metadata; DataSet management (xml-based)

Sound file segmentation and annotations (xml-based)

Visualization, labeling and annotation of sound data in the time domain, narrow segmentation (phonetic transcription, Unicode)

Frequency analysis (FFT, DFT,Wavelet, Filter Banks)

Spectrograms (FFT, Wavelet, Cepstrum, Linear Prediction Coding Formant Extraction) 

Visualization, labeling and annotation of time-varying signals by means of spectrograms (in the frequency domain)

Quasi-continuous Wavelet-Transform

and other time-frequency representations (Pseudo-Wigner-distribution).


Comparison of partial# 8 of 4 different music strings.

Spectral Smoothing

  1. Cepstrum Analysis: free selection of order (liftering)
  2. Linear Prediction Coding (LPC) Analysis: free selection of model order

Extraction of formant frequency candidates (in speech)

Fundamental Frequency Analysis (of speech signals)

De-noising, signal enhancement

spectral subtraction and frame averaging

Auditory perception modelling - (Computational Hearing)

Comparison of auditory model with LPC-model (for speech)

Simultaneous Masking - Over-Masking: spectral relevance

Order Analysis:

To identify gear orders in a multiple motor component environment a specific method for the generation of order spectrograms has been implemented. The method applies smoothing on the rpm-signal and uses re-sampling as well as the Discrete Fourier Transform (DFT) in combination with the anti aliasing filter to create order analysis spectrograms at reasonable computational cost.

STx Sequencer:

The spectrogram shows from left to right the real time filtering of white noise: Lowpass, Highpass, Bandpass and Bandstop

Basics in Sound Synthesis: STx-Sequencer.pdf

STx is currently free for non-commercial, scientific, and student use. If you use STx, then please cite one of these articles. To apply for a license for commercial use, please contact This email address is being protected from spambots. You need JavaScript enabled to view it..

The STx 5.0 release is planned for September 2019. This will include the workspace compact mode especially designed for linguists.

Do not install the rtanalyser and the freeware version in the same directory: the configuration files are incompatible.

If you use STx in your scientific work, please cite one of the following publications, if appropriate:

  • Balazs, P., Noll, A., Deutsch, W., Laback, B. (2000): Concept of the integrated signal analysis software system STx, in: Jahrestagung der Österreichischen Physikalischen Gesellschaf 2000, ÖPG 2000. , CD-ROM. (proceedings)
  • Balazs, P. (2003): Masking Filter, Phase Vocoder and STx - an introduction, in: SAMPTA03-Sampling Theory and Applications 2003 (extended abstracts)
  • Kasess, C., Noll, A., Majdak, P., Waubke, H. (2013): Effect of train type on annoyance and acoustic features of the rolling noise, in: Journal of the Acoustical Society of America 134 (2), 1071-1081.
  • Moosmüller, S., Brandstätter, J. (2014): Phonotactic information in the temporal organization of Standard Austrian German and the Viennese dialect, in: Language Sciences 46, 84-95. (article) DOI: 10.1016/j.langsci.2014.06.016
  • Zala, S. M., Reitschmidt, D., Noll, A., Balazs, P., Penn, D. (2017): Automatic mouse ultrasound detector (A-MUD): A new tool for processing rodent vocalizations, in: PLOS ONE 12(7), e0181200 DOI: 10.1371/journal.pone.0181200
This will help in securing the continued development of the software.

The STx documentation for the current version is here