DAC IV stages
Reply #19 – 2013-02-21 17:44:30
Take a look at published specifications for D/A converters, DVD players, sound cards, iPod, etc. Many only have an 80 to 90 dB SNR. This means that the noise will be audible in an A/B/X test if the peak music levels exceed 80 to 90 dB SPL (easily achieved). That is far from being a universal truth. The actual answer depends on the test. I agree, but the test conditions may be closer to typical applications than you describe below:The answer given above probably presumes that we set up the system to play music at a desired high SPL, and then mute the music player... If we back off from a very high peak SPL to a more typical and comfortable SPL that is not the one we use with visiting firemen ... probably gets us below 100 dB ... As a practical matter, the vast majority of people find players and systems with 90 dB dynamic range to be highly satisfying, to say the very least. Using your numbers: 100 dB SPL (slow) 90 dB SNR D/A converter And adding the following assumptions: 10 to 20 dB music crest factor 30 to 40 dB SPL ambient (quiet room) 10 to 20 dB digital attenuation prior to D/A conversion system output noise = (average SPL) + (crest factor) - ((D/A SNR) - (digital attenuation)) Worst case given the above assumptions: (100 + 20) - (90 - 20) = 50 dB SPL Worst case, the converter noise is 10 to 20 dB louder than the room noise. This should easily be noticeable between tracks, and may be audible in quiet passages of music, even if the noise is white. Best case given the above assumptions: (100 + 10) - (90 - 10) = 30 dB SPL Best case, the converter noise is equal to the room noise, or 10 dB quiter than the room noise. It may be very hard to hear this noise between tracks IF the noise is white. But, please notice the "IF" The converter SNR may be limited by a single spurious tone and not by white noise. If so, the noise will be much more noticeable. Our ears have the ability to hear a 3 kHz tone 30 dB lower in amplitude than the surrounding noise (I encourage the skeptical reader to try this test for themselves). The 30 dB at 3 kHz number can also be derived from masking theory. If one of the converters under test has a noise floor that is limited by a single tone (AC hum, power supply switching frequencies, or crosstalk from other parts of the system), then this noise will be much more noticable (and much more objectionable than the above calculations would suggest). Bottom line, it is quite reasonable to expect noticeable differences in a converter's noise floor in a moderately demanding application. It is unreasonable to assert that all converters are good enough to be indistinguishable.