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Old 7th July 2008, 08:16 AM   #2091
ecdesigns is offline ecdesigns  Netherlands
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Join Date: May 2006
Location: Holland
Hi omainik

Quote:
do You know if the current OS X does bit and sample exact playback?
This has little to do with the OS X version, but the way it's used together with applications like iTunes. Max OSX Panther, Tiger and Leopard can provide bit-perfect playback.

iTunes7 now has an integrated 24-bit digital processor for mixing, adding effects and volume control. This task was previously performed by core-audio (16-bit processor), and it could be disabled when no volume control or mixing was required.

In short it's (no longer) possible to get bit-perfect playback from mac OSX / iTunes7 when using the on-board audio interfaces (USB & mini-Toslink).

There is no easy way to solve this, well except for using an external Airport Express module.

This is caused by the fact that all audio data streamed to these on-board interfaces has to pass the 24-bit mixer. This mixer cannot be switched-off for the on-board digital audio interfaces. Setting volume control to maximum and disable other sound sources won't help either.

Processing means upsampling the original 44.1/16 digital audio data to 48 KHz, perform 24-bit processing (volume control / mixing / filtering), then downsample it to 44.1/16 again. The degradation in sound quality (even with 24 bit processing) is clearly audible.



The Apple Airport Express module (also supports iTunes for windows) simply receives a file over the (wireless) computer network including full error correction.

The file does not pass the 24-bit processor (check-box in iTunes allows for completely blocking the 24-bit processor). This also means that the volume control slider is grayed-out (disabled), and no effects or sources can be mixed. It's only possible to play the digital audio file in iTunes.

The AE module only handles (fixed) 44.1/16 format.
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Old 7th July 2008, 09:07 AM   #2092
omainik is offline omainik  Austria
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Join Date: Jul 2005
Location: vienna
Hi John,

do You have a replacement Toslink receiver PCB for the USB Receiver for Di16DAC?

Using a iTunes version pre 24bit integrated upsampler (iTunes 6) may be a solution too? Does someone try to install an older iTunes under Leopard?
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Old 7th July 2008, 12:42 PM   #2093
nicoch58 is offline nicoch58  Europe
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Join Date: Oct 2004
thanks ecdesigns as alway nice info...

diy resistor wow! my next step....

on smd side ? found this Z foil :

http://www.vishay.com/docs/63109/vfcd1505.pdf
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Old 7th July 2008, 06:37 PM   #2094
Wavelength is offline Wavelength  United States
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Join Date: Feb 2004
Location: Zinzinnati
ECDesign,

Quote:
iTunes7 now has an integrated 24-bit digital processor for mixing, adding effects and volume control. This task was previously performed by core-audio (16-bit processor), and it could be disabled when no volume control or mixing was required.
Were did you come up with this idea?

First off there is no Mixer in iTunes. Heck for that matter what in the world would it mix? It can only handle one task at a time.

Second all data inside iTunes, CoreAudio, and for that matter Vista and XP are handled in 32 bit floating point.

CoreAudio as in Vista does have a MIXER that is true. Not like the XP KMIXER which would manipulate the data but basically the application feeds the interface 32 bit floating point which is then converted to the playback size required for the device.

Quote:
Processing means upsampling the original 44.1/16 digital audio data to 48 KHz, perform 24-bit processing (volume control / mixing / filtering), then downsample it to 44.1/16 again. The degradation in sound quality (even with 24 bit processing) is clearly audible.
WHAT??????

Look the only time a unit is resampled is if the setting in Audio Midi for that device declares a rate other than the Fs rate of the song it's playing back. iTunes keeps a static variable when it loads to the rate declared in Audio Midi settings.

If that is 24/48 and it's plays a 44.1/16 bit track the data will be upsampled to 24/48.

If it is set in Audio Midi to 24/44.1 the sample would not be touched and the data would be padded with 8 zero's.

If the user changes the rate while iTunes is running then the problem occurs. iTunes would say upsample as in the first example to 24/48 and then CoreAudio would resample to the new set rate.

I have a USB analyzer hooked up too my USB dac development platform along with the Prism dScope III and serveral other testing devices so I can see exactly what data is sent from the computer at all times.

Thanks
Gordon
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Wavelength Audio, ltd.
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Old 7th July 2008, 07:18 PM   #2095
soundcheck is offline soundcheck  Germany
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Join Date: Mar 2005
Location: DUS
Gordon.

Good to see you over here .

Just to explain for the others around here: I opened up a thread at Audio Asylum
asking about the subject because I knew that Gordon is sneaking around.
In the past he posted solutions over there to get bit-perfect data from OSX.

http://www.audioasylum.com/cgi/t.mpl?f=pcaudio&m=33960

Now my 2 cents:

OK it is not a or "the" mixer. Perhaps it is the 32 bit processing, which is probably not done if you route the data to the Airport Express.
Or it might be the API which just uses the Core Audio mixer and can't get passed by. No idea how they're doing it.

Processing in 32bit float will for sure have a negative impact on the signal.
Of course this also depends on what you actually gonna do with the signal.
A conversion will for sure impact the signal. It won't be bit-perfect.

Running my Brutefir at 64bit float is quite an improvement over 32bit. 32bit DSP is an absolute NoGo for me.

Anyhow. You might want to let us know how to get a bit-perfect stream out of OSX.

I had a chance to listen to the difference at John's place. These were evident.



Cheers
\Klaus
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Old 7th July 2008, 08:30 PM   #2096
Wavelength is offline Wavelength  United States
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SC,

Quote:
OK it is not a or "the" mixer. Perhaps it is the 32 bit processing, which is probably not done if you route the data to the Airport Express.
Remember the AE is kind of different than all other Audio devices. This is the only device that iTunes actually talks to. Remember all outputs will be 16/44.1 for the AE no matter what. Therefore the AE does not fall into the same relm as other devices which have setting in Audio/Midi.

In iTunes it will always convert to 32 bit floating point. It will then go to the DSP part of the application if the EQ is on or Sound Enhancer. If the vol is set to 100% (and SoundCheck is off) it multiplies 1xsample = sample. It then goes to the output which forks to either CoreAudio or AE output.

Thanks
Gordon
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Wavelength Audio, ltd.
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Old 7th July 2008, 08:33 PM   #2097
agent.5 is offline agent.5  United States
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How does AE compare to Slimserver Duet?
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Old 7th July 2008, 08:47 PM   #2098
Wavelength is offline Wavelength  United States
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Agent5,

Got me I have not looked at the SQB code in a long time. If as it was before simply uses the computer as a library it would not matter. If you can select it in System Preferences for Audio output then it would just the same as any other audio device and would show up in Audio/Midi setup.

Thanks
Gordon
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Wavelength Audio, ltd.
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Old 8th July 2008, 10:18 AM   #2099
ecdesigns is offline ecdesigns  Netherlands
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Hi Gordon,

Good to see you on this thread


Quote:
First off there is no Mixer in iTunes. Heck for that matter what in the world would it mix? It can only handle one task at a time.
Try this, play a CD track in itunes, and open some Flash content in the safari browser, or play a movie. The sounds sources will be mixed (you hear both sources simultaneously).

This doesn't happen when using the AE module, and disabling volume control (check box in iTunes). The AE module provides consistent better sound quality when compared to my iMac on-board USB and mini-Toslink interfaces. It also matches the sound quality from my reference CD player with Toslink output.

The connected DI4T uses an advanced jitter blocking system, the Toslink twin-VCXO receiver. I added a block diagram of this system. When testing USB sources I use a USB to Toslink converter, based on a PCM2706.

There are no audible differences when connecting sources with different jitter amplitude and spectrum. It makes no difference if I use a 1 meter interlink or a 10 meter clear plastic interlink. Sound quality doesn't even change when I add an optical repeater and extend the clear-plastic interlink to 20 meters.


Quote:
WHAT??????

Look the only time a unit is resampled is if the setting in Audio Midi for that device declares a rate other than the Fs rate of the song it's playing back. iTunes keeps a static variable when it loads to the rate declared in Audio Midi settings.
Just a quote from an article in stereophile, discussing iTunes7

quote "Also, the end user should not hesitate to use the volume control in iTunes 7.x, as it is very well designed and operates at 24-bits for audio devices that support 24-bit operation."

The link is here:

http://www.stereophile.com/news/121707lucky/


and this:

quote "2) OSX still does not have the ability to follow sample-rate changes. We consider this a nuisance for most users and a show-stopper for users who want to play a mixed list of 44.1kHz and 96kHz material. An OSX sample-rate mismatch will invoke the very-poor-quality sample-rate conversion that is built into OSX. The iTunes 7.5 bug seems to be locking on this sample-rate conversion at all sample rates other than 96kHz. Until this is fixed, OSX is not the operating system of choice for audio playback.
[...] After extensive testing and communicating directly with the engineering team at Apple, some of these initial observations have been explained. We now know the reason for the poor performance observed in the initial tests, and we have conclusive information about the operation of iTunes 7.x on Mac OS X.

[...] If the user changes CoreAudio's sample-rate in AudioMIDI Setup to something different than what iTunes is locked to, CoreAudio will convert the sample rate of the audio that it is receiving from iTunes. In this case, the audio may be undergoing two levels of sample-rate conversion (once by iTunes and once by CoreAudio). (The SRC in iTunes is of very high quality (virtually inaudible), but the SRC in CoreAudio is horrible and will cause significant distortion.) If the user wants to change the sample rate of CoreAudio, iTunes should be restarted so that it can lock to the correct sample rate.

—Elias Gwinn, Benchmark Audio

The link is here:

http://lists.apple.com/archives/core.../msg00272.html

It specifically states the possibility of two levels of sample rate conversion, once in iTunes and once in core audio. It also mentions a SRC integrated in iTunes.


Quote:
I have a USB analyzer hooked up too my USB dac development platform along with the Prism dScope III and serveral other testing devices so I can see exactly what data is sent from the computer at all times.
Ok, then it would be interesting to compare the output of 44.1/16 material between the AE output and iMac on-board audio interfaces.

AE output (that gives by far best sound quality on my set) is verified to be bit-perfect:

Quote from a Stereophile article,

quote "Some audiophiles have dissed the AirPort Express on the grounds that its digital output is not bit-accurate. However, I found that this was not the case, that the data appearing on the AE's digital output were identical in the original file. To check this, I compared a WAV file with a duplicate that I had captured on my PC from the AirPort Express's S/PDIF output. I used iTunes on my PowerBook playing a version of the file encoded with Apple Lossless Compression to feed data to the AE. The files were bit-for-bit identical, proving that the AirPort Express is transparent to the music data (as is ALC, for that matter). "

Link:

http://www.stereophile.com/digitalprocessors/505apple/
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Old 8th July 2008, 12:07 PM   #2100
holey is offline holey  United States
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I wish you guys would hurry up and make my mind up for me, both of you are making what seems like valid points, too bad i am not smart enough to figure out who is right.
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