AirPlay 2 - digital volumeregulation resolution 16 bit or more ?

[Tangband]

I am seeking some information about this but havent found clear evidence that the volume regulation resolution for airplay2 is ”only” 16 or 24 bits.

I like airplay because its so user friendly

AirPlay - Wikipedia

en.wikipedia.org

”The protocol supports metadata packets that determine the final output volume on the receiving end. This makes it possible to always send audio data unprocessed at its original full volume, preventing sound quality deterioration due to reduction in bit depth and thus sound quality which would otherwise occur if changes in volume were made to the source stream before transmitting.”

Reading this I wonder how good the volume regulation quality is ? How does it work ?

 

[Tangband]

Noone knows more about this ?

 

[HappyPantherFan]

It’s up to the receiver to decide. I’d bet those with motorized pot would rotate in response. Some DAC-pre combos have resistor networks and they can actuate those.

As for the scale-volume correlation, should this protocol resemble UAC, Core Audio and others, it would still be mostly device dependent. In Core Audio the control is an abstract 0-1 number, and different devices can assign -48 to 0 dB, -63 to 0 dB, -48 to +6, -99 to 0, etc. to this scale.

 

[DVDdoug]

I don't know anything about AirPlay but I wouldn't lose sleep over it either way...

When you reduce the signal with an analog volume control you also tend to lower the signal-to-noise ratio (if the noise comes-in after the volume control).

You also lose digital resolution during quiet passages & fades-outs. But you generally don't hear any loss of resolution unless you re-amplify.

If you hear a loss of resolution it's heard as quantization noise. Quantization noise is similar to analog noise in-that the signal-to noise ratio is worse with quiet sounds. But unlike regular noise, it goes-away completely with "digital silence". If you want to hear what it sounds like you can make an 8-bit file with Audacity.*

When a 16-bit file "fades" to -48dB you are down to 8-bits of resolution. But the quantization noise remains below -90dB so it's still inaudible, unless you amplify.



* If you try Audacity, turn OFF dither. Dither is added noise that's supposed to sound better than quantization noise.

 

[mdsimon2]

This is an interesting question that I never really thought about. When using Airplay I try to use my DAC for volume control but sometimes I'll use my phone to adjust if my DAC remote isn't handy.

I ran some tests starting from a MacBook pro airplaying to an AppleTV 3. The first interesting thing I found was that I could only attenuate to -30 dB, after that I just get a mute. Second is that attenuation is done after transmission as I see no degradation on the digital output after attenuation. I made measurements at 0 dB, -20 dB and -29.96 dB. All show -100 dB THD+N.

You can see this two ways, first measurements plotted on a dBr scale, here the relative noise floor stays the same.

Second is measurements on a dBFS scale, here you can see the noise floor shifts down with signal level.

Pretty cool IMO.

Michael

 

[mdsimon2]

One more comparison as a sanity check.

Here is Airplay attenuation at -20 dB and REW signal level at 0 dB. -100 dB THD+N.

Here is Airplay attenuation at 0 dB and REW signal level at -20 dB. -80 dB THD+N

Michael

 

[palm]

With Airplay 1 and an AirPort Express you could guess that the bitstream transmitted was unaffected by the volume.
A stream would take a second or so to start, a buffering you also experienced when changing tracks.
But adjusting the volume was immediate

 

[Tangband]

One more comparison as a sanity check.

Here is Airplay attenuation at -20 dB and REW signal level at 0 dB. -100 dB THD+N.
View attachment 309933


Here is Airplay attenuation at 0 dB and REW signal level at -20 dB. -80 dB THD+N
View attachment 309934

Michael

Does this mean that the AirPlay volume regulation in this case has a greater resolution than 16 bit ? And is the resolution entirely dependent on the receiver , not the transmitter ?

 

[Tangband]

This is an interesting question that I never really thought about. When using Airplay I try to use my DAC for volume control but sometimes I'll use my phone to adjust if my DAC remote isn't handy.

I ran some tests starting from a MacBook pro airplaying to an AppleTV 3. The first interesting thing I found was that I could only attenuate to -30 dB, after that I just get a mute. Second is that attenuation is done after transmission as I see no degradation on the digital output after attenuation. I made measurements at 0 dB, -20 dB and -29.96 dB. All show -100 dB THD+N.

You can see this two ways, first measurements plotted on a dBr scale, here the relative noise floor stays the same.

View attachment 309931

Second is measurements on a dBFS scale, here you can see the noise floor shifts down with signal level.

View attachment 309932

Pretty cool IMO.

Michael

Thanks for the measurements - AirPlay seems to be really good

 

[mdsimon2]

Does this mean that the AirPlay volume regulation in this case has a greater resolution than 16 bit ?

Yes. Difficult to say with the -30 dB attenuation limit but would guess it as least 24 bit. Certainly looks like any attenuation applied is completely transparent for 16 bit audio.

And is the resolution entirely dependent on the receiver , not the transmitter ?

That would be my guess based on what you posted and these measurements. It appears that the volume control is implemented in the receiver.

Michael

 

[Snarfie]

But is this preferable above cables (if no quality loss) because it could be a weapon against the WAF no cables any more.