Digital audio ouput - why no volume control??

How would you suggest they make the digital sound quieter? Make the 1s and 0s smaller?

Think about it - its a stream of digital information, not an analogue wave form that you can make bigger or smaller with more or less current.

It is up the receiving device which is decoding the digital information and using an amplifier to produce an analogue waveform out to speakers to set the volume...

If you are talking about airtunes through airport express - the volume control is being passed on to the amplifier in the airport express...

This item probably won't be addressed. Its been there since the inception of OSX. The third party device is expected to control the volume. Digital control is not like firewire or usb, both of these can be controlled through software programming. If you ever find a way then by all means patent the idea and share your millions with us.

Could you please specify why do you use optical out? What device it is connected to etc.? Oh also please, by all means define a single product on the market carrying digital out and volume control for it.

Well I connect my iMac to my surround sound system using an optical cable, which makes use of the digital-out signal, and as Paul says, the volume control in iTunes does indeed alter the volume being played, so the volume must be being carried along with the digital signal. I can see no reason why a system-wide volume control could not work.

nitramluap wrote:
It IS possible, actually and its omission makes no sense.

Yes, it is possible but it isn't desirable. Here's why:

For any linear digital audio encoding process, the bit depth determines the resolution of the volume levels the format can reproduce. For example, for 16 bit audio there are 65536 possible volume levels between dead silent & the absolute maximum possible level (digital clipping). For 24 bit audio there are about 16.7 million levels.

So let's say that you want to turn down the volume at the digital output. The only way to do this is by throwing away all of those levels louder than the desired volume by scaling the output to what is effectively a lower bit depth. One half as loud means the most significant bit ("MSB") will never be a one; half that loud & the next MSB won't be used, & so on.

At first, this doesn't seem so bad but the problem is the human ear doesn't interpret "one half as loud" as you might think: hearing is logarithmic (which is why we usually use the logarithmic unit "dB" to measure it), so a sound that is 1/2 the intensity of maximum is only slightly quieter to the ear. Depending on the maximum level the digital input device can produce, it could easily be necessary to scale the digital output to one ten thousandth or less of the maximum output just to achieve a comfortable level, & to scale it much smaller still for background music levels.

So, for an effective volume control with a usable range from "very quiet" to "full tilt" you would really be going from "miserably inadequate resolution" to "high quality resolution."

If you want to hear just how miserable sounding the quiet settings would be, & you have any sound editing software that can change digital volume levels of samples, try this:

Select a short sample & reduce it by 40 dB. Save it & then renormalize that saved sample to full scale. Listen to the result & compare it to the original. Not too pleasant, is it?

Froglet wrote:
... as Paul says, the volume control in iTunes does indeed alter the volume being played, so the volume must be being carried along with the digital signal.

Most digital audio formats do not support encoding volume level as metadata, so there is no way to implement a standardized, global digital audio volume control for a standard S/PDIF output, period. There are only four bits available for this use in the standard & they are rarely used for metadata, especially in consumer gear.

The audio can be scaled digitally, but as previously described, this is a low fidelity approach not suitable for general use.

Therefore I suggest purchase of Salling Clicker, I use it daily and ... it has the volume control for Optical

Maybe if they just allowed Front Row to have a pre-output volume control - that would solve the only problem I have.

If you are referring to a volume control that varies the volume of the digital S/PDIF output, you are again asking for something that can't be done without a huge hit on fidelity. S/PDIF is capable of high fidelity because it transmits audio at 20 bit resolution. This means there are about one million (2 to the 20th power) possible audio levels each sample can have, 16 times better than CD quality resolution.

So (one again), if you want to reduce the maximum volume by 1/2, you can transmit numbers no greater than about 500,000 on the S/PDIF output. In effect, that reduces the resolution by one bit. The same is true for greater volume reductions: 1/4 of max uses only the lower 18 bits, 1/8 of max uses only the lower 17 bits, & so on. And because the ear responds to the log of signal intensity, you will need to provide reductions of at least several thousand times to create a useful volume control. It should be obvious that very quickly, the effective resolution drops below CD quality & keeps going down the quieter you want the volume control to be capable of.

The way around this is to transmit the digital signal at full resolution, decode it, & apply an analog volume control to the output ... exactly what the iPod HiFi does when you use the remote to control its volume. Your problem, of course, is that the remote does not have the ability to control more than one Apple device at a time, so you need two remotes, a learning remote, or some similar solution.

As far as I can tell, the ability to control the digital output volume level is neither an unreasonable ask nor is it impossible to achieve without impact on resolution etc.

If you use the DVD Player and output audio via a digital interface you can .... change the volume level using the DVD Player's volume control!

The issue here is consistency.

This may sound odd, but I'm trying to achieve something similar, in that I only want to use the Apple Remote for day-to-day access of TV (don't get me started on the lack of DTT Controller integration opportunities with Front Row), iTunes and DVD Player.

I want the audio from EyeTV to come from my Mac (analog) and everything else to go via an EDIROL UA1D USB digital audio interface into my hi-fi system.

Infuriatingly, there appears to be no way to differentially set the DVD or iTunes outputs in Front Row to the digital out, and further, the Apple Remote's volume controls don't operate the DVD Player's volume control, lazily defaulting to the system output.

The more I read these posts and try to explain what I'm trying to achieve, the more I realise how much of a tangle this has become.

Here's the rub:

- I would like to see a standard means of selecting the default Audio Output should exist for each application that makes any sort of noise (including Front Row!) just like when selecting a Printer, for example.

- I'd then like to see the Apple Remote volume honor that selection when operated.