Music to the ears: Effect simulation, emulation

Article By : Mark Stansberry

Audio simulation and emulation with LTspice and a simple digital audio system, such as Windows Multimedia Player, gives you a design advantage. But simulation and emulation have their own pitfalls.

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The audio files preamplfier_common_source_jfet_lsk489.wav and hard_clipping_jfet_lsk489_15volt.wav are the audio from the output waveforms in Figures 2 and 3. They can be listened to on a DAW or if you’re on a budget on your Windows Media Player which comes preinstalled on almost all PCs.  
With the LTspice schematic (Figure 1) it is easy to hear how component changes will affect the audio output. Simply change a component rerun the simulation and then listen to the new wav file.  You can also easily hear how different sampling rates will affect the audio quality.  For example if you want studio recording quality changing the sampling rate parameter of the wave. statement from 16000 to 192000 samples per second (sps) will do the job. But you don’t have to stop there. If you want to hear how your own music will sound you can reconfigure the input source in the LTspice schematic so that it will input your own wav file recordings.

Emulation concerns

Audio simulation and emulation with LTspice and a simple digital audio system such as Windows Multimedia Players gives pedal designers a tremendous design advantage. Analog emulation eliminates the expense and time associated with hardware prototyping. Analog emulation also opens the door to automated design flows. SPICE scripts can be written that cycle through component values analyse output and report on favourable harmonic outputs. Analog emulation also facilitates an intuitive design approach that can override the time-consuming process of analog circuit analysis.
Caution however must be exercised. Digital emulation of audio signals may lose subtle harmonics. However this loss can be overcome if ultra-high digital sampling rates are used. LTspice offers ultra-high sampling rates (Gsps) but for the real world a 192 Ksps with a 24-bit resolution studio quality will do. Other considerations are the quality of the amplifiers in an audio workstation and the headphones used. The sound circuit in the audio workstation should be able to play 192 Ksps music.
Care must also be exercised in the circuit design. Analog circuit simulators often don’t report violations of basic circuit design rules that can cause a circuit to fail. For example the gate-to-source voltage of a N-channel JFET should always be less than zero. Large positive excursions above 0 V result in forward biasing the gate-to-source diode junction. An excessive forward bias voltage will eventually result in an inoperable JFET. Most analog circuit simulators do not report a design error if you exceed the maximum drain-to-source voltage.

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Mark Stansberry participates in the technology and education industry as a writer, publisher, engineer, educator and technical research and market analyst.

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