[gerG]

Some new input on an old question. I had been under the impression that I could get BP output from itunes if I went through the Airport Express, which seems to bypass k-mixer (and pretty much everything else). I have a couple of CDs which exhibit audible digital clipping on the ipod (at any volume setting) and when played from the computer. I had assumed that the engineers had done this deliberately for effect. Then I noticed the clipping indicators on my outboard digital processor blinking red, indicating distortion on the digital input signal. I pulled up the headroom display and read consistent clip indication, again on the toslink signal coming straight out of the AE. Out of curiosity I pulled the itunes volume slider down slightly. The clipping indications ceased, as did the audible clipping! This indicates to me that at full volume itunes is applying some level of digital "boost". If so, the position of the volume control for "bit perfect" is some non-indicated point below max. Needless to say I am frightened by the implications. Anybody know how to set BP out of this software? fwiw this was on a windows machine. I think the Mac has the same issue, but I will try the experiment again tomorrow. Needless to say the EQ is off, as is sound enhancer. Quicktime panel has no output option when I am routing through the AE (PC has no internal sound card).


gerG

 

[awagner]

To get bit perfect output from iTunes application on a mac,

1. Turn off the equalizer, sound check, and cross fade.
2. Set the iTunes application volume slider to the maximum.

I don't have the equipment to test it yet, but I suspect you will be able to get bit perfect output by using Jack os x to route your audio directly from iTunes to your sound card, thus bypassing os x's mixer.

I posted the details of the experiments I used to prove this over at:

http://www.diyaudio.com/forums/showt...586#post647586

The path of the experiment was MATLAB -> wav file -> iTunes -> jackosx -> Quicktime Pro -> wav file -> MATLAB

Drew Wagner

 

[TwoFiveOne]

What are the recordings? Aren't some CDs recorded so hot that they clip right out of the box? (And at no extra charge!)

Also, do you have the box checked that says "Disable iTunes volume control for remote speakers"? I seem to remember that having something to do with it.

I'd like to hear other opinions on this as well.

 

[Jon L]

itunes can be bit-perfect if used with MAC.

But for PC, nobody has been prove to me iTunes is bit-perfect. There 'may' be outboard cards such as M-audio Transit, that plays i-Tunes bit-perfect, but my recent listening tends not to agree with that either.

If you are playing iTunes via good on-board soundcard, for example, on a PC, same music file played on iTunes vs. Foobar/ASIO sound SO different, it's not even funny.

 

[TwoFiveOne]

On a PC, iTunes audio gets mixed with all other sound events going on in the PC, even when sent via USB. You can play iTunes, surf web pages with sound, and receive audible incoming email notifications, all pumped through your cans at the same time. Aargh, Windows is the non-audiophile mix-master, folks...

But, still on a PC, if you disable the above iTunes options and send to an Airport Express, it will arrive at the AE lossless and bit-perfect with Windows in the act only to assure perfect data integrity for the network transmission. In fact, with the AE, Windows doesn't even know it's audio, as iTunes does an end-around on the entire Windows sound system. At this point you have the choice of Airport Express doing D/A or going optical out to your DAC of choice.

Please correct me if I'm wrong but also tell me why. Hope this helps. Thanks.

 

[gerG]

hmmm, I have not tried the disable volume control option. I will give it a shot tonight, thanks.

Output to the AE bypasses kmixer. I was using the optical output from the AE into a digital signal processor, which has various level check options. As I mentioned, I had assumed clipping in the mix, but that should not have given me red lights on the processor. Also the audible distortion would not have gone away when I turned down the digital volume if it was just in the mix. My conclusion is still that itunes is applying digital boost above 0 db. I hope the speaker level trick fixes it. I haven't gotten around to trying it with the mac yet.

btw, when I output from itunes through a usb box, I get a 44.1 khz signal. I think that quicktime is bypassing kmix somehow.


gerG

 

[Wodgy]

iTunes is definitely capable of bit-perfect output.

iTunes on the Mac is bit-perfect out of the box, assuming no equalizer and no "sound check". I've tested this myself.

iTunes on Windows is bit-perfect assuming you have a sound card that bypasses kMixer. I've also tested this myself.

Other people have tested the Airport Express and found it to be bit-perfect as well.

Quote:

Originally Posted by gerG As I mentioned, I had assumed clipping in the mix, but that should not have given me red lights on the processor.

This is where you're mistaken. All the "digital clipping" light on the processor means is that the encoded digital signal reaches the loudest possible value (0dB digital). (Some processors may give you a clipping warning if the encoded digital signal comes within 2% of the maximum value.) Your processor is not able to distinguish between clipping in the mix and clipping from the source. If there is clipping in the mix, you will still see red lights on the processor.

The red lights went away when you reduced the digital volume because you eliminated the digital peaks (by reducing the volume), not because you made the system any more accurate.

Quote:

My conclusion is still that itunes is applying digital boost above 0 db.

No, it's not. Your methodology is flawed, and you can't draw any conclusions from your setup. It is easy enough to do a proper check to verify that your system is bit-perfect; you don't have to depend on clipping lights, which tell you no useful information.

 

[Wodgy]

Quote:

Originally Posted by awagner I don't have the equipment to test it yet, but I suspect you will be able to get bit perfect output by using Jack os x to route your audio directly from iTunes to your sound card, thus bypassing os x's mixer.

You don't need to do this on OS X. iTunes on OS X is bit-perfect out of the box, because the system mixer is bit-perfect out of the box. I've tested this. There is no need to bypass anything; it's not like Windows, where the system mixer (kMixer) genuinely munges the signal in many cases.

 

[tyrion]

I tested itunes using the AX for bit perfect play and it was. Using a pc, you must go into preferences and uncheck crossfade, sound enhancer, and sound check. You also have to check Disable itunes volume control.

 

[Jahn]

Quote:

Originally Posted by tyrion I tested itunes using the AX for bit perfect play and it was. Using a pc, you must go into preferences and uncheck crossfade, sound enhancer, and sound check. You also have to check Disable itunes volume control.

hmm iv'e done all of that except the itunes volume control thing. i'll go home and find out where that is - under preferences probably. the tunes sound good enough to me already anyhow!

 

[tyrion]

Quote:

Originally Posted by Jahn hmm iv'e done all of that except the itunes volume control thing. i'll go home and find out where that is - under preferences probably. the tunes sound good enough to me already anyhow!

In properties, in the Audio tab, at the very bottom, check the last box.

 

[Jahn]

Quote:

Originally Posted by tyrion In properties, in the Audio tab, at the very bottom, check the last box.

gonna do it then! thanks!

 

[gerG]

ok, more data, the solution of one question, and creation of another.

I tried to retrace my steps, but the CDs that I remember encountering the problem with were not on my main PC. I ripped them to lossless on the PC, hit play, and.... no clipping. What did I screw up before? Turns out that I had been listening off the Mac ti-book. That library is all AAC, mostly 256 kbps. I sent a stream to the AE from the Mac, and presto, clipping. I tried the volume control lock, but that only seemed to lock it at max, and I still had clipping. I backed down the itunes volume control slightly, and still had clipping indication. Eventually I could back it down to the point that there was headroom. Whether the processor is only sensing 0 db or a prescribed number of words at 0 db, it tells me the same thing, there is digital gain coming from itunes at this point. I killed the play, and tapped into the library on the Windows box with the Mac and played the same track from the lossless file with the same settings through the AE (still from the Mac). No more clipping.

So now it seems that the codec is triggering some extra gain somewhere. This would also explain my problem with the ipod. Is there a setting that I overlooked?


gerG

 

[awagner]

Quote:

Originally Posted by Wodgy You don't need to do this on OS X. iTunes on OS X is bit-perfect out of the box, because the system mixer is bit-perfect out of the box. I've tested this. There is no need to bypass anything; it's not like Windows, where the system mixer (kMixer) genuinely munges the signal in many cases.

What volume setting do you need to achieve this? If you don't use something like jack to catch the audio as it comes out of the application and bypass the system mixer, the signal has the opportunity to be altered by the system mixer, and will thus ~not be bit perfect in general.

Unless you have a few extra bits of resolution on your dac and the os volume control boosts the signal only by multiples of 2, I don't see how you are likely to get bit perfect output without a specific volume setting. Could you please be more specific on what you mean by "the system mixer is bit-perfect out of the box"?

BTW, know of any way to catch the audio coming out of the mixer so one could test this? It just occured to me that my work machine has both optical digital out and optical digital in, so if I buy a cable that would be one way for me to test it. I don't have an external DTS decoder, so the clever trick of trying to play a DTS encoded stream is out.

Thanks,
Drew