"Birdies" from REW Tone Generator

AmpDesigner4444

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While using REW to generate tones for sniffing out room resonances, I've noticed "birdies" from the REW generator while generating low (subsonic) frequencies.

The original thread about this is on Audio Circle, and includes an explanation of "birdies" for those not familiar with this effect. However, when I try to post the link in this message, the editor won't let me!

I've found this problem independent of the playback system used since this happens on various computers with various DACs (external and internal).

My reference system is DC coupled end-to-end, and very high performance. I'm using a Cherry USB into a Cherry 130dB+ DAC DAC 2 HSV into a pair of Cherry MEGA MK2 MONOs into either JTR 210RT or Magnepan 1.7i.

With the generator set to 1Hz and -40dB, these birdies are easily heard on multiple systems. With the generator set to a much higher levels (-3dB), and the DAC set to attenuate, there are no birdies, and the woofers of the JTRs move slowly in and out with no noise at all from the tweeters, even with my ear in the horn!!

See the original Audio Circle thread for details about changing the output resolution (has a significant effect on this issue). Any idea why this is happening?
 

AmpDesigner4444

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Here's the link mentioned above, but you need to remove the spaces for it to work:
https:// www. audiocircle.com/index.php? topic=165490.0
 

John Mulcahy

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The selector you referred to on that thread is the bit depth of dither added to the signal to prevent quantisation distortion, as explained in the help. If you were using the Java drivers on a Windows system the data path is 16 bits, so the dither should be set to 16 bits to match. The additional dither levels are provided to match systems operating at the corresponding word lengths (ASIO will typically use 24-bit data, for example). The addition of dither prevents quantisation distortion. If you reduce the dither amplitude below the lsb level on a 16 bit data path by selecting a greater word length the dither will have no effect and you will hear the quantisation distortion. That is a characteristic of digital data, not a shortcoming of the REW generator.
 

AmpDesigner4444

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John,

Interesting. Does this mean that REW running in Windows 10 can't output more than 16 bit resolution? Does the Java version matter? Thanks for your help.

Best Regards,
Tommy O
Founder
Digital Amp Co
CherryAmp.com
 

John Mulcahy

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No, it doesn't. The currently available Java runtimes on Windows only support 16-bit data, but ASIO drivers (which can be selected form the soundcard preferences) support any format the device supports. In the beta versions the current format is shown at the bottom left of the main REW window, next to the sample rate.
 
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