SB3 volume control with 20-bit external DAC

[jonhdahl]

I have a Entech 203.2 external DAC that I would like to use with my SB3. I also want to use the digital volume control on my SB3. Now I understand that the "safe" range with the digital volume control is about 40-100, because the Squeezebox uses 8 bits for volume information (from 24 bits).

But what happens when I send the signal to a 20-bit DAC? Will I lose any info from my music if I drop the volume down to 40? Is (say) 70-100 a safer range? Or does a 20-bit external DAC not affect things at all?

Apologies if this is a dumb question.

[Triggaaar]

You will be fine, the fact that the external DAC is 20 bit does not matter. The squeezebox does not use 8 bits for volume information, but it does start with 24 bits, which is 8 more than were on the CD you probably started with. You will lose bits when reducing the volume digitally, and some cleaver chap has worked out that you can digitally reduce the volume to 40 before reducing your signal below 16 bits. The fact that your external DAC is 20 bit, and the internal DAC is 24 bit, does not negatively change the principle.

[seanadams]

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

You MUST match your source to your amp using analog attenuation. You can use 1) the gain controls on your amp 2) a preamp or 3) a RCA in-line attenuator or 4) Transporter's built-in jumpers.

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

This has been discussed many times here, but I won't refer you to our archives as there have been far too many people still getting it wrong. Instead, here's a detailed and correct explanation from Lavry (login needed):

http://lavryengineering.com/lavry_forum/login.php?redirect=viewtopic.php&t=76

If you don't "believe" the theory (which many people don't for some reason) here is a simple test which one customer did which gives the expected results. Note that he is using older software which shows the volume range as 0-40. In the current software it is 0-100.

http://mysite.verizon.net/forumwebspace/RightMark/Test%20Reports/Volume.htm

[seanadams]

You will be fine, the fact that the external DAC is 20 bit does not matter. The squeezebox does not use 8 bits for volume information, but it does start with 24 bits, which is 8 more than were on the CD you probably started with. You will lose bits when reducing the volume digitally, and some cleaver chap has worked out that you can digitally reduce the volume to 40 before reducing your signal below 16 bits. The fact that your external DAC is 20 bit, and the internal DAC is 24 bit, does not negatively change the principle.

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

[Triggaaar]

Hi Sean. Is the info in that first link too vast to cut & paste here (to save everyone interested in this topic registering)?

Where I use squeezeboxes, I have my maximum volume set by pre/integrated amps, and I then use the squeezeboxes to adjust the volume down, and while I expected a reduced SNR, I thought this would be insignificant, as I was reducing the volume at the same time (maybe it's still significant, but I've never noticed it). I can see this would not be the same if you were already lowering the volume on the squeezebox to get to your maximum volume setting, where the reduced SNR would be more noticeable (eg, with the Entech DAC & no preamp).

I assume the details of losing dynamic range by digitally reducing the volume is in the link you posted, so I shall go study.

[wshields]

What are your recomendations

So if you send digital output to a dac I assume that you should use fixed digital out of either a SB or Transporter and then adjust volume after entering analog domain, right?

How about when using analog out of Trasporter to a preamp? Does using volume control on a preamp act differently than volume control on the transporter? Any general suggestions on where to adjust volume or how to figure the correct ranges on each piece of gear?

The link to the Lavry site doesn't seem to work right. I get sent to their general forum start and not to any specific topic.

I would like to suggest you should write a FAQ about this whole volume setting as there does seem to be so much conflicting information given.

Thanks

[mr_bill]

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

You MUST match your source to your amp using analog attenuation. You can use 1) the gain controls on your amp 2) a preamp or 3) a RCA in-line attenuator or 4) Transporter's built-in jumpers.

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

This has been discussed many times here, but I won't refer you to our archives as there have been far too many people still getting it wrong. Instead, here's a detailed and correct explanation from Lavry (login needed):

http://lavryengineering.com/lavry_forum/login.php?redirect=viewtopic.php&t=76

If you don't "believe" the theory (which many people don't for some reason) here is a simple test which one customer did which gives the expected results. Note that he is using older software which shows the volume range as 0-40. In the current software it is 0-100.

http://mysite.verizon.net/forumwebsp...rts/Volume.htm

Sean,

As an example, could you explain to us how to best use the jumpers in the Transporter and the volume range of 0-100 to properly match source to amp. That would be very helpful.

If you have an SB3 and Benchmark Dac1, you can use the Benchmark's internal jumpers to match the source to amp also (I think!!)
Thanks,
Bill

[seanadams]

Sean,

As an example, could you explain to us how to best use the jumpers in the Transporter and the volume range of 0-100 to properly match source to amp. That would be very helpful.

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.

If you have an SB3 and Benchmark Dac1, you can use the Benchmark's internal jumpers to match the source to amp also (I think!!)
Thanks,
Bill

Correct - exactly the same concept.

[Ben Diss]

That is one tough read. It doesn't help that our friend Dan is a bit of a prickly pear. If the link doesn't work, look for the thread titled "DA10 for the main pair in a 5.1 setup".

-Ben

[mr_bill]

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.



Correct - exactly the same concept.

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

[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.