Some words about digital volume control …
The benefit of digital volume control at first glance is, that there will be no more scratching,
there will be no channel mismatch, there will be no crosstalk issues any more.
Digital volume control can be made with up-down buttons or
incrementals or real potentiometers – like Violectric does.
In that case a linear tapper is used, because the volume control itself is made dB-linear inside the D/A converter.
As this mirrors a “real life” feeling only imperfect when turning the potentiometer,
we added some resistors to “bend” the responding law from the potentiometer to have a nearly perfect analog feeling.
A simple DC voltage is attenuated by the pot. The result is fed to a A/D converter, here a digital control signal
is made to attenuate the digital audio signal inside the D/A converter BEFORE converting it to analog.
A digital 24 bit signal represents a dynamic range of 144 dB – much more than can be found in real life !!
People who are doing real world recordings can tell, that it is nearly impossible to record more than 60 dB dynamic range
with a microphone - although microphone makers claim dynamic ranges from their mics to be more that 130 dB.
This may be true when recording a cricket near a starting F-14 Tomcat. But - who needs that.
Also, sitting in you living room, it is hard to follow dynamic ranges of more that 20 – 30 dB unattanuated without
having trouble with your neighbourhood afterwards.
Today´s pop music´s dynamic range is reduced during recording to 2 – 3 dB …
Please also note that harmonic distortion inside the signal cannot be smaller than the dynamic range.
It is not possible to have 100 dB THD (0,001%) with 90 dB dynamic range,
but it is possible to have 110 dB THD (0,0003%) with 120 dB dynamic range !
The CD format offers 16 bit which means a dynamic range of 96 dB and distortions which cannot be lower than 0,0016%.
A 24 bit signal offers a dynamic range of 144 dB with theoretical minimum distortions at 0,00001%.
This is not possible to achieve in real life.
The best today AD converters offer dynamic ranges from 120 dB with distortion figures about –110 dB THD.
Lots of losses have to be faced during recording, editing, mixing …
Digital attenuation is done by shifting the signal from MSB (Most-Significant-Bit) to LSB (Least-Significant-Bit).
Shifting a complete bit in LSB direction (and replacing it with a 0) means 6 dB attenuation.
When a 16 bit CD signal is input to a 24 bit DA converter, this signal may be attenuated
by 6dB x 8 Bit = 48 dB = factor 200:1 WITHOUT changing anything from the original data.
We learned from the above that also a real 24 bit signal carries a maximum of
20 “senseful” bits - in practice there are no more than 18 bits.
So, also a 24 bit signal may be attenuated by a minimum of 6dB x 4 Bit = 24 dB = factor 35:1 WITHOUT doing any harm to the original data.
So – for our opinion - digital attenuation is the best what can happen to a signal (except not being attenuated).
Of coarse provisions should be made to adapt different working levels in the audio chain.
It makes no sense to have a DA converter which offers its technical data only when dramatic amounts of output voltage is present on its outputs.
And because you need only 1 or 2 of these volts you are forced to always digitally attenuate the signal in advance.
The maximum output level from a DA converter should be adjusted in the analog make-up circuitry between
DA chip and output sockets without changing output impedances and without using special measuring equipment.
Afterwards, attenuating the signal digitally will not be an issue at all.
