DAC filters have been discussed in several threads now. I have decided to measure responses of filters in DacMagicPlus especially with respect to mirror images, that create beats at difference frequency and possible IMD tones in inevitably nonlinear components of the audio chain like speakers.
23 kHz sine sampled at Fs = 48 kHz was played through the DacMagicPlus and captured with 48 kHz bandwidth. Three filters of the DacMagicPlus were used, the Linear Phase filter, the Minimum Phase filter and the Steep filter. They are switchable by a button on the front panel. Then the frequency response to a white noise was measured to see the behavior in the passband and near Fs/2.
1. Linear Phase filter
See 25kHz mirror image about 42dB below the original 23kHz signal
Response to white noise see low attenuation above Fs/2 = 24kHz
2. Minimum Phase filter
Again we can see almost same mirror image
Response to white noise and again low attenuation above Fs/2
3. Steep filter
There is an attenuation of the 23kHz sine, however no mirror image
Response to white noise and good attenuation above Fs/2
Conclusion
The only filter that prevents mirror images is the steep filter. The decay below Fs/2 starts earlier than in the first two filters, however this would be inaudible. The benefit of good attenuation below Fs/2 is much higher than the small trade off in the passband.
23 kHz sine sampled at Fs = 48 kHz was played through the DacMagicPlus and captured with 48 kHz bandwidth. Three filters of the DacMagicPlus were used, the Linear Phase filter, the Minimum Phase filter and the Steep filter. They are switchable by a button on the front panel. Then the frequency response to a white noise was measured to see the behavior in the passband and near Fs/2.
1. Linear Phase filter
See 25kHz mirror image about 42dB below the original 23kHz signal
Response to white noise see low attenuation above Fs/2 = 24kHz
2. Minimum Phase filter
Again we can see almost same mirror image
Response to white noise and again low attenuation above Fs/2
3. Steep filter
There is an attenuation of the 23kHz sine, however no mirror image
Response to white noise and good attenuation above Fs/2
Conclusion
The only filter that prevents mirror images is the steep filter. The decay below Fs/2 starts earlier than in the first two filters, however this would be inaudible. The benefit of good attenuation below Fs/2 is much higher than the small trade off in the passband.
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