Email Address:

Lost your password?

This is the legacy website; please use the new website.

Digital Audio Signal Generator, Part 1

This Digital Audio Signal Generator has TOSLINK and coax (S/PDIF) digital outputs, as well as two analog audio outputs. If a digital output is used, the harmonic distortion from a high quality DAC is extremely low. Alternatively, if you use the analog outputs the harmonic distortion of the sinewave signal is typically still very low at less than .06%.

By Nicholas Vinen

S/PDIF Audio Generator: Main Features

• Five waveform types supported: sine, square, triangle and two sawtooth

• Frequency range: 1Hz - 24kHz in 1Hz steps at 48kHz sampling rate or 1Hz - 48kHz at 96kHz sampling rate (see text)

• Five waveform generation modes and four output modes (see Tables 1 & 2)

• Runs off a plugpack (9-10V DC) or a battery (4 × AA or AAA cells).

• Built-in battery voltage monitor with settable low battery voltage warning

• Status display for pulse and sweep modes, to show amplitude and frequency

• Sweep can be manually triggered or paused/resumed/restarted

• Digital output can be switched between “consumer” (S/PDIF, 20-bit data) and “professional” (AES/EBU, 24-bit data) modes

• Can enable pre-emphasis bit on digital output if desired

• 10 setting banks for storing modes and configuration

• Digital LCD contrast and backlight brightness control


• RMS and music power testing for power amplifiers

• Speaker placement optimisation

• Sub-woofer or speaker crossover optimisation

• Finding faults in audio equipment

• Audio quality testing for analog or digital audio equipment with appropriate measurement equipment (THD, SNR, channel separation, intermodulation distortion, frequency response, etc)

• Analog circuit prototyping and development

• Testing DACs or other equipment that accept a digital audio signal

• Whenever you need an adjustable audio-frequency signal source.

Click for larger image
Fig.1: this block diagram shows the main circuit functions of the Digital Audio Signal Generator. It's based on a dsPIC33FJ64GP802 microcontroller (IC4) and features both analog and digital outputs.

As well as sinewave outputs with low distortion, this Digital Audio Signal Generator produces a range of other waveforms which you would normally obtain from a high-quality function generator. These waveforms include square, triangle and sawtooth etc, as well as advanced functions that include waveform mixing, pulse and sweep modes.

If you connect the SPDIF digital output to our high-quality Stereo Digital-to-Analog Converter (DAC), (SILICON CHIP, September-November 2009), you get a sinewave output with very low distortion in the audio band. We measured around 0.0006% THD+N (20Hz-22kHz bandwidth) for a 1kHz full-scale sinewave with a sampling rate of 48kHz and less than 0.001% THD+N for any frequency between 20Hz and 2kHz.

The distortion is less than 0.006% up to 20kHz (or 0.005% with a sampling rate of 96kHz). That is lower distortion than from any commercial audio generator that we know of.

There is one important proviso. Using a DAC for signal generation means that there will be high-frequency switching noise in the output. This is true whether you use an external DAC or the internal one which drives the analog outputs.

Usually, this will not be an issue, however it is important to keep it in mind. If you use the signal as part of a noise or distortion test, the measuring equipment will need to be able to ignore residuals above 20kHz.

Share this Article: 

Privacy Policy  |  Advertise  |  Contact Us

Copyright © 1996-2021 Silicon Chip Publications Pty Ltd All Rights Reserved