Volume control at WiiM Ultra vs DAC

[Agentcooper1979]

Hello! I’m considering using the sub output of the WiiM Ultra to control, well, my sub! Then using the USB output to my DAC. In this instance, volume would need to be controlled at the WiiM vs my DAC/pre.

So, the question is, as a result of managing volume at the WiiM, is bit rate reduced going to the DAC? Is this a problematic setup I’m suggesting?

 

[Mr Ee]

Hello! I’m considering using the sub output of the WiiM Ultra to control, well, my sub! Then using the USB output to my DAC. In this instance, volume would need to be controlled at the WiiM vs my DAC/pre.

So, the question is, as a result of managing volume at the WiiM, is bit rate reduced going to the DAC? Is this a problematic setup I’m suggesting?

Welcome.

The bit rate shouldn't be affected, but the bits will necessarily be modified to alter the volume.
Many would argue that this is the only/best way to integrate a sub so that its volume changes along with the main speakers when using the ultra for volume control.
Many would then argue for hours over where volume control should occur, but imo no, this is not a problematic suggestion.
You have the choice, so try all ways and see which works best for you, for both sound and convenience, thus overall enjoyment.

I don't think usb and sub out has quite been released yet? Not got a sub, so haven't been paying too much attention! Expected within the next week or so, iirc.

 

[hgo58]

@WiiM Team How is the volume control implemented in the WiiM on the USB output?

1. **WiiM Volume Control**: This adjusts the digital audio signal within the WiiM before it is sent to the DAC.

2. **USB Protocol Volume Control**: This sends volume control commands from the WiiM to the DAC via the USB Control API. The DAC then adjusts the digital signal's amplitude based on these commands.

This would be great to know when setting up the DAC.

 

[Mr Ee]

I'm betting on the former - why would all the other signal changes, including volume limit and volume control, be done within the wiim for all other outputs yet volume control for usb output not?

 

[harkpabst]

2. **USB Protocol Volume Control**: This sends volume control commands from the WiiM to the DAC via the USB Control API. The DAC then adjusts the digital signal's amplitude based on these commands.

My understanding is that seamless nesting of "control" transfers (for volume control) and isochronous transfers (for the actual audio data) is not possible as this would go against the idea of isochronous transfers (guaranteed bandwidth). This would mean an interruption of audio transfer for volume control.

I won't pretend to know , but because of that I'm betting on the former, too.

 

[hgo58]

My understanding is that seamless nesting of "control" transfers (for volume control) and isochronous transfers (for the actual audio data) is not possible as this would go against the idea of isochronous transfers (guaranteed bandwidth). This would mean an interruption of audio transfer for volume control.

I won't pretend to know , but because of that I'm betting on the former, too.

No. The control transfer don't interrupt the data. This is part of the USB audio protocol.

 

[harkpabst]

No. The control transfer don't interrupt the data. This is part of the USB audio protocol.

View attachment 16207

I know that diagram but I don't understand how it proofs the point. Are IN and OUT transfers happening at the same time as well?

 

[bherren]

I know that diagram but I don't understand how it proofs the point. Are IN and OUT transfers happening at the same time as well?

USB has bandwith for both. Isochronous endpoints get a fixed allocation of bandwidth and bulk ones (which is what the control channel uses) gets what's left (to simplify)

 

[hgo58]

I know that diagram but I don't understand how it proofs the point. Are IN and OUT transfers happening at the same time as well?

Yes. In and out can happen at the same time. WiiM audio output only uses isochronous, interrupt and control transfers.

There is a good explanation here.

https://www.edn.com/fundamentals-of-usb-audio/

My question to @WiiM Team is just how the volume control is implemented.

 

[hgo58]

USB has bandwith for both. Isochronous endpoints get a fixed allocation of bandwidth and bulk ones (which is what the control channel uses) gets what's left (to simplify)

Yes. Plenty of bandwidth in high speed USB.

The isochronous endpoints run at a rate of one transfer per 125 us; or 8,000 transfers per second.

A single isochronous transfer can carry 1024 bytes, and can carry at most 256 samples (at 24/32 bits).

This means that a single isochronous endpoint can transfer 42 channels at 48 kHz, or 10 channels at 192 kHz.

 

[pieterv1]

Interesting read so far! I wanted to know this as well in case I ever invest in an external DAC/digital preamp.

If I choose to control the volume still through the WiiM (to have the benefit HDMI CEC, app volume control and the WiiM remote), will the audio information sent to the USB DAC be encoded (to lower volumes) or is it unaffected and are volume control signals sent to the DAC - which at its turn controls the volume internally?

 

[harkpabst]

Any form of volume control always alters the signal. Attenuating a signal by definition means to change it, no matter if it's done in the digital or analogue domain.

WiiM have confirmed in the past that they are applying their digital volume control using an 8 bit volume table, with input signals being restricted to 24 bit and internal processing being performed at 32 bit.

Post in thread 'Quality basics: please improve digital volume control'

Apr 6, 2024

Dear WiiM-Team,

can you improve the quality of the digital volume control implementation? (Do you use e.g 64bit/dithering algorithms?)

The current algorithm seems to be detrimental to sound quality, apart from the numerical bit-reduction. This general improvement would be a gain for all WiiM devices and in the future.

Some users will not recognise, but many (current) users ask for 1:1 bit-perfect, which means that they are not satisfied with the current implementation. This user group may appreciate the WiiM technical quality level.

Perhaps it is possible to add a ISO226 adapted...

• WiiM Support

• 
• 

That leaves plenty of headroom for digital volume control without any loss in resolution using really tiny steps.

It could be possible that they are doing it differently with USB out but we're still waiting for an answer. I would be surprised if they did. They had to rely on the DAC even exposing volume change capabilities.

 

[pieterv1]

It could be possible that they are doing it differently with USB out but we're still waiting for an answer.

Cool!

 

[darth]

I have connected WiimU to both a USB DAC (SMSL DL100) and to a USB DDC (Quloos QU02) and in both cases volume is controlled at the Wiim and not at the attached device. I kept the attached USB devices at max volume aka 0dB. The QU02 doesn't even allow adjustment anyways to keep the signal transparent and this is my current setup. Volume control on WiimU is my preferred way anyway as someone pointed out to take advantage of HDMI CEC.

 

[onlyoneme]

It's easy to be tested using both volume reduction and a PEQ filter with the positive gain.

 

[hgo58]

It's easy to be tested using both volume reduction and a PEQ filter with the positive gain.

I don't see how. Preprocessing is for sure done by WiiM but final volume calculations may be done by WiiM or by DAC from control data forwarded by WiiM.

I myself think all is done in the WiiM but would be good to know for sure.

 

[onlyoneme]

I don't see how. Preprocessing is for sure done by WiiM but final volume calculations may be done by WiiM or by DAC from control data forwarded by WiiM.

I myself think all is done in the WiiM but would be good to know for sure.

If volume calculation is done after the eq processing then absence of the headroom will affect results. If volume calculation is done by the WiiM, it happens before the eq processing to provide the necessary headroom.

 

[bherren]

That leaves plenty of headroom for digital volume control without any loss in resolution using really tiny steps.

I think (I may be wrong) that this reasoning only applies when using the internal DAC. If you have a 24b input stream, halve the volume and send it to an external DAC at 24b, then you *will* have lost a bit of vertical resolution.

When using the internal DAC you can have the DAC itself perform the volume adjustment by first ramping to 32b and then applying the correction, or maybe they feed the internal DAC with a 32b stream.

But with an external DAC, unless you send 32b out you don't have that option and thus do lose dynamic range.

Now using the USB volume control capability of the DAC instead would allow this processing to be done entirely within the external DAC who can then implement a similar 32b scaling or even do analog volume control, it becomes a feature of a given DAC.

The other option would be for WiiM to output a 32b upscaled digital stream if at all possible

 

[bherren]

I think (I may be wrong) that this reasoning only applies when using the internal DAC. If you have a 24b input stream, halve the volume and send it to an external DAC at 24b, then you *will* have lost a bit of vertical resolution.

When using the internal DAC you can have the DAC itself perform the volume adjustment by first ramping to 32b and then applying the correction, or maybe they feed the internal DAC with a 32b stream.

But with an external DAC, unless you send 32b out you don't have that option and thus do lose dynamic range.

Now using the USB volume control capability of the DAC instead would allow this processing to be done entirely within the external DAC who can then implement a similar 32b scaling or even do analog volume control, it becomes a feature of a given DAC.

The other option would be for WiiM to output a 32b upscaled digital stream if at all possible

Note: This is of course mostly an issue on paper as it's quite debatable that anything higher than 16b makes any audible difference

 

[bherren]

Oh and continuing on this line of reasoning.... If your source is 16b and you reduce the volume to 1/4, the WiiM will have no choice but lose 2 bits when sending that 16b stream to an external DAC, so you are down to 14b of resolution.

That might mean that when listening at low volume it could be better to use the fixed output resolution option to force 24b out, assuming (hopefully) that WoiM applies the volume correction*after* the upscaling to 24b.

Only @WiiM Team can tell us what really happens here however