What are the minimum system requirements to run AuTester?

To work with the AuTester program, you need the Windows 10 operating system. Minimum hardware requirements: dual-core processor, 4 GB of RAM. When using a large number of program modules at the same time and using the maximum resolution settings of the program, you will need to use a more modern and productive processor. 

Which sound cards are suitable for working with AuTester?

The AuTester program can use any audio devices for input and output of the audio signal that function under the Windows 10 operating system. To implement all the capabilities of the AuTester program, two audio inputs are required. This can be a stereo input of the audio card or two mono inputs of two different audio devices. But it is necessary to remember that microphone inputs often have a hardware limitation of the frequency bandwidth significantly narrower than standard linear inputs. AuTester software can also be used on a laptop with a microphone input and an external USB sound card.

What are the features and advantages of the Stepped-Sine (Windowed Burst) technique compared to well-known sweep-tone techniques?

The Sweep vs. The Burst: A Comparative Analysis of AFC Measurement Techniques

In the pursuit of high-fidelity audio, the Amplitude-Frequency Characteristic (AFC) graph is a cornerstone of performance evaluation. It is our "map" of how a device treats different frequencies, showing us what it boosts, what it cuts, and what it leaves untouched. However, the map we get depends entirely on how we draw it. The two most common automated methods—the logarithmic sweep-tone (chirp) and the stepped-sine burst—operate on different principles and, consequently, can yield different results. Understanding these differences is crucial for correctly interpreting audio measurements and for choosing the right tool for the task.

Method 1: The Sweep-Tone (Log-Chirp)

This is the method famously employed by tools like some examples of well-known software for acoustics research. It consists of a single, continuous sine wave that glides across the frequency spectrum (e.g., 20 Hz to 20 kHz). The system's output is recorded and compared to the input.

How it Works: Deconvolution The "magic" of the sweep-tone method lies in a mathematical process called deconvolution. By comparing the output sweep to the input sweep, the system calculates the Impulse Response (IR) of the device. The impulse response is the (theoretical) output the device would produce if given a single, perfect, infinitely short "click."

Once the IR is obtained, a Fast Fourier Transform (FFT) is applied to it. This FFT is what generates the final AFC and PFC graphs.

Strengths:

• Separation of Linear and Non-Linear Response: This is the sweep's greatest strength. Because the fundamental frequency is always moving, any harmonic distortion (at 2f, 3f, etc.) moves at twice or three times the speed. Deconvolution effectively "filters out" these harmonics, which do not align with the original impulse response. The resulting AFC graph represents an almost pure linear response, free from contamination by harmonic distortion.

• Speed and Resolution: The entire spectrum is captured in a single pass (typically 1-10 seconds). The resulting FFT can provide extremely high-resolution data (e.g., 1/48th octave).

Weaknesses:

• Low Energy Density: The signal's energy is spread across the entire duration of the sweep. At any single frequency, the energy is relatively low, which can make the measurement more susceptible to background noise.

• Steady-State Measurement: The slow glide of the sine wave is a steady-state signal. It does not effectively test the device's transient response—its ability to react to the sudden attack of a sound.

Method 2: The Stepped-Sine (Windowed Burst)

This is the method employed by iTester in its "Testing sequence" mode. It is a discrete process:

1. A spectrally pure tone burst of a single frequency (e.g., 1000 Hz) is generated. This burst is shaped by a windowing function (like Nuttall, Gabor, or a simple sine half-period) to ensure it starts and ends smoothly, concentrating its energy at 1000 Hz.

2. The burst is played, and the system records the peak amplitude of the reference signal (Ch 1) and the peak amplitude of the signal from the device under test (Ch 2).

3. The ratio of these peaks forms a single point on the AFC graph.

4. The process repeats for the next frequency (e.g., 1050 Hz), stepping through the entire range.

Strengths:

• High Signal-to-Noise Ratio (SNR): All the signal's energy is concentrated into a short, powerful burst at a single frequency. This makes the peak measurement extremely robust against background hiss or hum, which is vital for accurate phase and low-level measurements.

• Psychoacoustic Relevance (Attack): This method directly tests a device's response to a transient attack at a specific frequency. The measured peak is the total response of the system—the combination of its linear gain, its settling time ("ringing"), and any distortion products (harmonics) generated by the transient. This is arguably closer to how we perceive musical notes, which are themselves transient events.

Weaknesses:

• Speed: This method is inherently slower, as it requires many individual tests to build the full graph.

• Distortion is Included: This is the most significant difference. The peak measurement does not separate the fundamental frequency from the harmonics it creates. The measured peak is the crest of the total waveform.

When and Why the Results Will Differ

Understanding the fundamental difference—Sweep separates distortion, Burst includes it—allows us to predict when the two methods will yield different AFC graphs.

1. On High-Distortion Devices (e.g., Tube Amps, Guitar Pedals):

   • Sweep: Will show a "clean" and often relatively flat AFC, as all the (often intentional) 2nd and 3rd harmonics are filtered out by the analysis.

   • Burst (iTester): Will show a "bumpier" AFC. In regions where the device produces significant harmonics, those harmonics add to the fundamental waveform, increasing its peak height. The iTester graph will accurately show that at 1kHz, the total signal envelope is X dB louder than the reference, even if part of that energy is distortion.

2. On Devices with High-Q Resonances (e.g., Speakers, Filters):

   • Sweep: A fast sweep may "glide" over a sharp, narrow resonance (like a poorly-damped driver or crossover) without fully exciting it. The resulting AFC may show only a modest peak.

   • Burst (iTester): The test "parks" at the resonant frequency for several cycles. This fully excites the resonance, causing the system to "ring." This ringing builds on itself, dramatically increasing the measured peak amplitude. The iTester graph will show a much sharper and more severe peak, which often correlates better with audible "honking" or "ringing" at that frequency.

3. On Low-Distortion, Well-Damped Devices (e.g., Modern Solid-State Amps):

   • The AFC graphs from both methods will be virtually identical. With no significant distortion or ringing to contaminate the peak, the peak measurement of a burst becomes just as accurate as the linear analysis of a sweep.

Conclusion: Which Method is "Better"?

Neither method is "better"; they answer different questions.

• Use a Sweep-Tone (like some examples of well-known software for acoustics research) when your goal is system diagnostics and linear characterization. It is the superior tool for answering: "What is the pure, linear frequency response of this device, separate from its harmonic distortion?" It is ideal for speaker design, filter modeling, and generating traditional specification sheets.

• Use a Stepped-Sine Burst (like iTester) when your goal is psychoacoustically relevant performance analysis. It is the superior tool for answering: "What is the total peak envelope response of this device when hit with a musical-like transient at this frequency?" It excels at revealing transient-related issues, quantifying the audible effect of resonance, and providing high-SNR phase data that is critical for time-domain analysis.

Scientific References

The concepts discussed here are foundational to signal processing and audio engineering:

1. On Sweep Measurements and Deconvolution:

   • Farina, Angelo. (2000). "Simultaneous Measurement of Impulse Response and Distortion with a Swept-Sine Signal." Presented at the 108th AES Convention, Paris, France.

   • Müller, S., & Massarani, P. (2001). "Transfer-Function Measurement with Sweeps." Journal of the Audio Engineering Society.

2. On Transient Distortion (TIM):

   • Otala, Matti. (1970). "Transient Intermodulation Distortion in Transistorized Audio Power Amplifiers." IEEE Transactions on Audio and Electroacoustics.

   • Jung, W. G., & Stephens, M. L. (1979). "An overview of SID and TIM." Audio.

3. On Psychoacoustics of Transients:

   • Blauert, Jens. (1997). Spatial Hearing: The Psychophysics of Human Sound Localization.

   • Zwicker, E., & Fastl, H. (1999). Psychoacoustics: Facts and Models.

4. On the Audibility of Phase Distortion:

   • Lipshitz, S. P., Pocock, M., & Vanderkooy, J. (1982). "On the Audibility of Midrange Phase Distortion in Audio Systems." Journal of the Audio Engineering Society.

   • Hansen, V., & Møller, H. (1989). "Perception of phase distortion in electroacoustical systems." Presented at the 86th AES Convention, Hamburg.

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