SB3 volume control with 20-bit external DAC

[wshields]

Okay, I didn't realize Transporter changed volume in digital when outputting thru analog outs.

Now, I run the transporter thru an integrated amp. So I am concluding I should adjust volume using the amps volume controls. One question I still have is how should I decide if I should use the Transporter's jumpers to attenuate the volume before it reaches the amps volume control. Any one have any suggestions?

Thanks again.

Walt

[seanadams]

I think the easiest way to understand this is to forget about numbers, decibels, and bits per sample for a minute, and just think about what's coming out of the DAC.

To oversimply only slightly: there are two things always coming from the DAC. 1) signal and 2) noise.

The level of the noise output stays the same no matter what signal level is being produced. That is really important to understand!

When the DAC is making a loud signal, there is a lot of signal and a little noise. That's a high SNR, which is good.

However, when the DAC is making a quiet signal, you have a little signal and a little noise. If we now consider the noise level in relation to the signal level, the noise is now louder. The noise level hasn't gone up in absolute terms (eg volts), but relative to the signal it has, so you now have a bad SNR.

Now consider a simple resistor attenuator being fed by a loud (good SNR) signal from the DAC. When the voltage passes through the resistor divider, everything gets attenuated - the signal and noise together. You have the same* SNR coming out of the divider as you had going in, i.e., the DAC's optimal SNR is preserved.

OK, now back to bits per sample. As you can see, the above effects really don't have much at all to do with bits per sample. We could send a million bits per sample, and it would still be the same. So why does bit depth matter? What is the significance of 16 vs 24 bit?

What matters is that we send enough bits per sample that the DAC's full dynamic range is utilized. It is important to realize that the DAC's dynamic range is finite, and is less than its input word size - more like 20 bits, since it is limited by its output noise level.

By "expanding" a 16 bit signal to 24 bit, all we are doing is saying "these 16 bits go in the most significant slots of the 24 bit word". We haven't improved the SNR of the signal, any more than you can "enhance" a digital photo the way they do on CSI.

If we attenuate the 16 bit signal, yes, the zeroes and ones will migrate down into the least significant bits of the 24 bit word, and yes, if we still "have all the bits" we could then mathematically go in reverse and get back to the same data. But that is not what the DAC does with the signal! The bits represent a smaller signal now than they did before. We still have exactly the same decreasing SNR effect. Sending 24 bits into the DAC just means we aren't making it any worse than it already is. We haven't "bought more headroom"... it does NOT mean that those first 8 bits of attenuation are "free".

To prove this, you could play a sine wave through the DAC and measure the SNR at each volume step. We would expect to see the SNR decrease as the volume is decreased. If there were anything special about the point where we start "losing bits", or if we were really getting "extra headroom", then the plot would decrease slowly (or not at all) until it reaches that point, and then there would be an inflection.

However, that is not what you'll see. The SNR will simply decrease with the signal level, all the way down.

I hope this helps... for extra credit maybe someone will try testing this?

* Actally, there are a number of secondary effects which reduce the SNR by the time it gets through the amplifier, but these are vanishingly small in comparison.

[seanadams]

Sean,
So, if I set up my inernal jumpers on my Benchmark Dac (or Transporter for that matter) so if the SB3 volume reads 70 and my decibel meter in room reads 70 db then am I set up and getting maximum SNR, resolution and performance? Would I still need a preamp performance related to matching or volume?
Thanks and I think I finally understand,
Bill

No - you are making this FAR more complicated than it is. Forget about decibels!

All you do is set the jumpers so that when transporter is playing at full volume, you are at the maximum listening level you'd want to use. Just start at -30 and move them up towards 0, until you reach your max listening level. That is really all there is to it.

If you are already using a preamp, or if your amplifier has gain controls, then you should use those controls and don't touch the jumpers.

What you are trying to avoid is a situation where you are at your maximum listening level with the volume bar only half way up. This is what might happen if you connected transporter directly to an amp. That's when you need to change the jumpers.

[wshields]

Sean,

Thanks so much for clearing these questions up. How many other CEOs are on the company forums on Christmas helping their customers? Outstanding service for us.

Thanks again.

Walt Shields

[seanadams]

How many other CEOs are on the company forums on Christmas helping their customers?

Well, actually I'm not anymore. Title updated.

[mr_bill]

No - you are making this FAR more complicated than it is. Forget about decibels!

All you do is set the jumpers so that when transporter is playing at full volume, you are at the maximum listening level you'd want to use. Just start at -30 and move them up towards 0, until you reach your max listening level. That is really all there is to it.

If you are already using a preamp, or if your amplifier has gain controls, then you should use those controls and don't touch the jumpers.

What you are trying to avoid is a situation where you are at your maximum listening level with the volume bar only half way up. This is what might happen if you connected transporter directly to an amp. That's when you need to change the jumpers.

Thanks Sean,
That's a simple way of setting and understanding!
Bill

[Patrick Dixon]

This is completely, 100% wrong. It doesn't matter how many people say this, it is still wrong.

Yes, you only maintain all the resolution of the original 16 bits by using all 24 bits - so truncating to 20 will loose you some information.

You _ALWAYS_ lose SNR and dynamic range. The "you still have all the bits" concept is completely flawed.

Well not completely flawed ;-) Any digital signal which doesn't swing the whole numerical range available to it, will loose some SNR (compared to the maximum available), at the DAC. However, this 'loss' of SNR may or may not be better than any SNR loss in a subsequent buffer and volume control circuit.

Use the digital volume for day-to-day adjustment within your normal listening range, not to compensate for a badly matched system.

Absolutely.

The new 0 to 100 scale is, I believe, true decibels. (In the past when we used only 40 steps, it was a more complicated curve designed to stretch out the low end of the range). So -10 corresponds to 90, -20 to 80, and -30 to 70.

Well it's your product (nice title BTW), but AFAIK each step corresponds to 0.5dB. So 90 is -5dB, 80 is -10dB and 70 is -15dB.

The previous 0-40 range gave steps of 1.25dB each.

[seanadams]

AFAIK each step corresponds to 0.5dB. So 90 is -5dB, 80 is -10dB and 70 is -15dB.

You're correct, my bad.

[Triggaaar]

From another volume thread (2 months old)

Slim Server 6.5: If I have the volume control set at 80 instead of 100, does this take out information from the digital signal?

No - it just gives you a slight reduction in the actual signal to noise ratio at the DAC.

and now, in response to Sean's comments "This is completely, 100% wrong. It doesn't matter how many people say this, it is still wrong":

Yes, you only maintain all the resolution of the original 16 bits by using all 24 bits - so truncating to 20 will loose you some information.

I'm now more confused than I started.

So Patrick is saying that reducing the digital volume will NOT take out information from the original signal, just slightly reduce the SNR.
And Patrick is saying that even if you leave the volume at 100, you will lose original resolution of the 16 bits if you have a 20 bit DAC (as you are sending it a 24 bit signal).

I apologise if I have miss interpreted everything, but it seems that Sean and Patrick are saying completely different things.

The one bit that does seem clear, is that digitally reducing the volume, prior to converting to anologue, reduces the SNR.

[Patrick Dixon]

I'm now more confused than I started.

Sorry about that.

So Patrick is saying that reducing the digital volume will NOT take out information from the original signal, just slightly reduce the SNR.

Yes - providing the input signal is 16 bit audio and you don't go lower than (IIRC) -35dB.

And Patrick is saying that even if you leave the volume at 100, you will lose original resolution of the 16 bits if you have a 20 bit DAC (as you are sending it a 24 bit signal).

No, I'm not saying that! At 100, the digits will be passed through untouched, it's only if you reduce volume you can loose resolution.

The one bit that does seem clear, is that digitally reducing the volume, prior to converting to anologue, reduces the SNR.

Almost everything you do to an analogue signal reduces its SNR too, so what I'm trying to say is that digital volume control vs analogue volume control is something of a trade-off.