Compact Portable MP3 Audio Signal Generator

June 08, 2014

An MP3 player (or smart phone), in combination with pure sine wave audio tracks, can be used as a very convenient portable audio test-signal source for troubleshooting audio gear. Audio pure sine wave tracks at full digital amplitude can be easily created using various programs and stored as MP3 audio files of some convenient length such as 2 to 5 minutes. For typical MP3 encoding quality, memory required is ~ 1Mb/minute. (Some better smartphones also support CD quality 16bit/44.1 kHz audio tracks without MP3 compression). The Creative MuVo N200 can be used for this purpose for testing stereo audio gear with input impedance of ~ 50 ohm or higher since the output of this player is intended to drive Creative earbuds of 32 ohm impedance. Signal level at 1 kHz is reduced by only about 5 percent under 33 ohm load compared to a high impedance loaded output. This player is extremely small and requires a single AAA cell (also rechargeable cells work):



The output level is adjusted using the player playback adjustment. For MP3 sine wave tracks with full digital amplitude, the useful range of output voltage level is 10mV to 650mV for the level range from 20 to 40 as shown below:



The scope trace below shows the output level for an MP3 track at f=1kHz at a player level of 35 (Vp~ 235mV):



The harmonic distortion is low enough for many useful audio measurements as shown for a 1kHz track below: