Her er hvordan PS Audio løser digital volumkontroll i deres Directstream DAC:
Sitat Distinctive: ‘If my understanding is correct one still lose 1bit for every time the volume is reduced by 6dB, however the clever thing about the DS is that you may lose a lot of bits before you start to compromise the 24bit resolution of the music signal. If you start with e.g. 32 bit you would start losing resolution very early on.’
Svar fra designer Ted Smith: ‘Not quite – the volume control in the DS doesn’t loose bits at lower levels: it takes all input bits and multiplies them with a 20 bit volume, keeping all resultant bits. These bits go into the sigma delta modulator to be converted into single bit output. At any volume > 0, any bit changing in the input will cause the pattern of 1’s and 0’s in the output stream to change appropriately.
The confounding thing is that there’s also a noise floor. When FFTs are averaged or use more bins the measured noise floor is lower. With the DS and enough bins and averages you can see the effects of any single bit difference in a repeating pattern no matter what volume.
The digital noise from the sigma delta modulator and the analog noise from the DS hardware present a practical noise floor that is reasonably close to the dynamic range of the ear so with an appropriately level matched preamp or amp, all bits that you could hear will be there at any volume.
In the real world it’s sometimes hard to match the output levels of the DS with the existing preamp and/or amp and speakers that well and then you can loose some dynamic range if volume 100 on the DS isn’t just a little above TOO LOUD.
To be fair the analog noise floor of the whole system is the practical limit for DSD or PCM with more than, say 22 bits. But in the DS if you use enough averaging you’ll see that all of the bits are there at any volume and you’ll be loosing bits off the bottom end with most PCM systems.’
