There's something in the digital audio world known as "birdies". It's when modulation artifacts (usually from "oversampling" or "noise shaping" DAC circuits) are in the audio band due to "closeness" to a desired output between available output values. For example, if you had 100 possible output levels, 1-100, and you wanted to produce the value 50.5 on the output, a very simple modulator may alternate the output values of 50 and 51, thus producing an average of 50.5. if these alternating values happen fast enough, then are low pass filtered, the average output will be 50.5. However, if you wanted to produce an average output value of 50.1, you'd need to output the value 50 for 9/10 of the time, and 51 for only 1/10 the time. So as the desired output gets closer to an available output value, the less frequent the "other value" occurs. Now imagine the desired output is 50.0001! Then, the value of 51 would only be used 1/10000th of the time, and even at a sample rate of 1MHz, there will be a "birdie" at 100Hz! I just wanted to explain what birdies are up front. Modern audio systems do a LOT to avoid birdies, such as higher order modulation techniques. Before I get too deep in modulation theory, let's get to the point of this thread....
I was testing a system's room response and decided to run through frequencies with a continuous tone from the REW generator function. Cherry MEGA MONO MK2s were driving the speakers (JTR), and a Cherry DAC DAC was driving the amps. This is a DC coupled system, so it can drive the speakers down to 0Hz (!). The purpose of playing these tones was to sniff out room rattles, so they were played at high volumes from 200Hz down. The frequency was changed slowly to allow resonances (windows, items on tables, door knocker, etc.) to rattle in an easily audible fashion. Anyone who's played loud music in a room with rattles will easily recognize the sound.
So, at 1Hz output, I heard "birdies"! After some investigation, I narrowed it down to the REW's generator
! There's a selector that allows 16/18/20/24 bits. No birdies at 16 bits, but they get worse with more bits. This is counter-intuitive, but there must be a logical explanation.
More investigation showed that I can make these "birdies" easily audible at 1kHz -40dB with the volume cranked on my laptop (listening with headphones). Same goes for the tablet PC I'm using to run room sweeps with REW. Has anyone else noticed this issue with REW?