iTunes and Audio Midi Settings

[catastrofe]

Chris,

 

Why is it necessary to close/reopen iTunes when changing audio midi settings?

 

My DAC displays input frequency, and when I change midi settings on the fly, they are immediately recognized by the DAC. My limited understanding is that midi adjusts the selected output device on the Mac, and is "downstream" of iTunes. Doesn't iTunes ouput a file in its native format "upstream" of the output device? If so, why is iTunes affected by the midi settings?

 

Thanks,

 

Bob

 

[audioengr]

It's because there are two audio stacks, iTunes and OSX. They each have mixers and upsamplers I believe. You want to envoke iTunes upsampler I believe, which is better.

 

Steve N.

 

[catastrofe]

Thanks Steve.

 

It's still confusing. . .I'm not upsampling, I'm playing the file in native resolution and I want the output device settings to match that resolution. So, if I change the midi settings to match the file, why do I need to do anything with iTunes?

 

[The Computer Audiophile]

Hi Bob - My DAC also displays the sample rate and changing audio midi certainly does change the sample rate of OS X and the display on the DAC. However, iTunes looks at the audio midi sample rate at the time the program is opened and locks on that rate. It will output that sample rate until it is closed and reopened.

 

Does this clear it up? Let me know.

 

[catastrofe]

Chris,

 

It makes perfect sense now. I didn't realize there was interaction between itunes and midi.

 

Thanks,

 

Bob

 

[Dave Clark]

If I launch iTunes with the Midi set to 16/44.1 and play a file that is true 24/96, my DAC shows it is receiving 16/44.1 input, but if I then change the Midi setting to 24/96 (while the 24/96 track is still playing) the DAC then changes to show that it is now receiving a 24/96 input. So is it actually sending the 24/96 file in its native resolution or not?

 

 

[The Computer Audiophile]

Hi Dave - iTunes is playing the file at 16/44.1 then audio midi is up sampling it to 24/96 which is why the DAC displays 24/96. It is not native 24/96 until iTunes is closed and reopened.

 

[Dave Clark]

Got it... so at first it is downsampling the 24/96 to 16/44.1, then upsampling the 16/44.1 to 24/96... and I guess if you change anything else after that it can become rather convoluted... would be nice if someone would fix this... iTunes 10 anyone?

 

[HiFiInsider]

On my new '09 Mac Mini, I noticed there a setting "encoded digital audio" in Audio Midi of OS X. If I mainly play 2 channel 16/44.1 files thought my PS Audio DLIII, would it be ideal to set it there or use the new "encoded digital audio"?

 

[AV-OCD]

I believe that the option you are referring to is used when you want to output a dolby digital or DTS surround signal. So the 16/44.1 would be the one to use for stereo output.

 

[HiFiInsider]

Thanks for the quick reply. What you said makes sense. However, I also use the Mini as a movie server (rarely) but I do. Does "encoded audio" allow what ever iTunes music files are ripped as and make changes as needed?

 

[AV-OCD]

DJ -

 

I would think that you would need to change the output type to match the audio file type, but the only way to know for sure is to set it to encoded audio and try playing a file in iTunes. My guess is that you will get no sound.

 

[HiFiInsider]

Well, I tried the "encoded audio" setting and when iTunes play a song, it automatically changed back to 16/44.1 since the source was ripped from 16/44.1. I have not tried any Dolby Digital audio yet because I don't have it set up to my HT system.

 

[dsbourque]

What happens when the Midi is set for 24/96 and I play a lower resolution file? My DAC continues to show 96kHz. Is iTunes actually upsampling?

 

[flatmap]

What happens when the Midi is set for 24/96 and I play a lower resolution file? My DAC continues to show 96kHz. Is iTunes actually upsampling?

 

Case 1: Midi is set to 24/96 before iTunes is opened. In this case iTunes believes that the output device wants 24/96 and therefore upsamples to same.

 

Case 2: Midi is set to 24/96 after iTunes is launched. In this case iTunes believes the output device wants whatever Audio Midi was set to at the time of launch, let's say 24/88.2. If iTunes were playing a 24/44.1 file, it would upsample to 88.2 and then Audio Midi would upsample to 96.

 

And you can see there are other undesirable cases as well based on what file is getting played, what the Midi settings are at the time of iTunes launch, and what Midi is set to currently.

 

I think that's how it works. :-)

 

[Cynic]

So if I understand correctly (assuming no Amarra or PureVinyl):

 

1. The iTunes sample rate converter is better than Core Audio's (OS X's), and you always want the computer to use it.

2. The only time the computer will use Core Audio's converter is when you (a) already have iTunes open, and (b) change the output setting in Audio Midi, and © fail to restart iTunes afterward.

3. Otherwise, all sample rate changes (both upsampling and downsampling) will be done by iTunes. Assuming you did NOT do 2. above, then if Audio Midi is set to 44.1 and you are playing a 96 file, iTunes will downsample it. If Audio Midi is set to 96 and you are playing a 44.1 file, iTunes will upsample it. Core Audio will not get involved.

 

Does anyone know if that is correct?

 

 

 

 

[Purite Audio]

I would play each file in it's native format ie 44.1 as 44.1, players like Amarra ( PV too I believe ) will automatically sense and change differing sample rates, otherwise you have to quit itunes open audi midi, change rate reopen itunes.

Keith.

 

[lazycatken]

Just had a glance at Apple sample code "PlayFile".

Here's the section where AudioUnitGraph is setup

////////////////////////////////////////////////////////////////////////

// output node

cd.componentType = kAudioUnitType_Output;

cd.componentSubType = kAudioUnitSubType_DefaultOutput;

cd.componentManufacturer = kAudioUnitManufacturer_Apple;

 

AUNode outputNode;

XThrowIfError (AUGraphAddNode (theGraph, &cd, &outputNode), "AUGraphAddNode");

 

// file AU node

AUNode fileNode;

cd.componentType = kAudioUnitType_Generator;

cd.componentSubType = kAudioUnitSubType_AudioFilePlayer;

 

XThrowIfError (AUGraphAddNode (theGraph, &cd, &fileNode), "AUGraphAddNode");

///////////////////////////////////////////////////////////////////////////

Default Output audio unit is invoked, but without any setting.

File Player audio unit is set as the format of the file.

Looks like, all the conversions are done by Default Output audio unit.

This a simple way to play a file. Does iTunes do this way?

 

Another way is to do sample rate conversion by application.

Audiotoolbox SDK provides a function to create an audio converter object.

///////////////////////////////////////////////////////////////////////

AudioConverterNew

Creates a new audio converter object based on specified audio formats.

 

OSStatus AudioConverterNew (

const AudioStreamBasicDescription *inSourceFormat,

const AudioStreamBasicDescription *inDestinationFormat,

AudioConverterRef *outAudioConverter

);

////////////////////////////////////////////////////////////////////////

The property of a converter can be set, including these two:

///////////////////////////////////////////////////////////////////////

Sample Rate Conversion Quality Identifiers

Specifiers for sample rate conversion quality, used for the kAudioConverterSampleRateConverterQuality property.

 

enum {

kAudioConverterQuality_Max = 0x7F,

kAudioConverterQuality_High = 0x60,

kAudioConverterQuality_Medium = 0x40,

kAudioConverterQuality_Low = 0x20,

kAudioConverterQuality_Min = 0

};

 

Sample Rate Conversion Complexity Identifiers

Specifiers for the sample rate conversion algorithm, used for the kAudioConverterSampleRateConverterComplexity property.

 

enum {

kAudioConverterSampleRateConverterComplexity_Linear = 'line',

kAudioConverterSampleRateConverterComplexity_Normal = 'norm',

kAudioConverterSampleRateConverterComplexity_Mastering = 'bats',

};

/////////////////////////////////////////////////////////////////////////

So, questions:

If iTunes do sample rate conversion, what level of quality it uses?

If iTunes doesn't do sample rate conversion, this means, it's done by Default Output audio unit, , what level of quality that Default Output audio unit uses?

 

[Bob Stern]

As Chris said, iTunes always outputs the sample rate that was specified for the default output device in Audio MIDI Setup at the time iTunes was launched. If iTunes plays a file with any other sample rate, iTunes performs SRC to convert it to that initially specified sample rate. If the user or some other application changes the sample rate of the default audio device in Audio MIDI Setup without relaunching iTunes, Core Audio will perform a second SRC operation to convert the output of iTunes to the sample rate of the default output device.

 

An Apple engineer on the Core Audio discussion list refused to reveal which of the above quality settings iTunes uses, except that he did reveal:

(1) The quality depends on CPU load. In other words, iTunes will lower the SRC quality if your CPU cannot keep up with a higher quality.

(2) The "complexity" parameter is never the best setting, i.e., "mastering" or "bats".

 

The Apple engineer acknowledged that it is a bug that iTunes latches onto an output sample rate when it is launched, but he said the iTunes programmers had other priorities, so he couldn't predict when they would fix the bug.

 

[Cynic]

Gordon Rankin at Wavelength tells me that Apple says it is not a bug that iTunes locks onto the sample rate when it launches. Even if it is not a bug, it is a nuisance.

 

[HiFiInsider]

iTunes seems to sound better when AudioMidi is set to 24/88.2 and then open iTunes and play 16/44.1 files. Am I imagining?