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DACs + SPDIF Out for a DAP (i.e. "mp3 player")
DACs + SPDIF Out for a DAP (i.e. "mp3 player")
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Old 1st July 2007, 04:52 PM   #1
threpointone is offline threpointone  United States
diyAudio Member
Join Date: Jun 2006
Default DACs + SPDIF Out for a DAP (i.e. "mp3 player")

Basically, I'm working on an audiophile DAP, and I need some help on the digital audio section. I've been mostly working with analog audio amplifiers, so I don't have much experience with DACs and I'm not entirely sure where to start.

The microcontroller I'm working with, the scf5250, has built-in SPDIF-out. I'm not sure if this output is up to audiophile standards, and I'm not sure how to improve it. If necessary, the SCF5250 also has a number of I2S outputs, so I might be able to use a separate SPDIF transceiver chip if necessary.

Then the DAC--there are simply so many selections for the DAC that I'm not sure which one to go with. I'd really like to just go all-out and make the best DAC there is available, but the selections don't seem to be as clear-cut as say, op-amps. Preferably, there should also be a balanced output available (either directly from the DAC or done somewhere in the I/V stage if I understand correctly)

There's also the issue of clocking which I'm not very familiar with--if I want to reduce jitter and improve the quality of the output, will I need to put some sort of high quality clock for the microcontroller, or do I isolate the DACs with a good clock, or what?

And sample rate--I can do up to 24/192 if I want, but will that improve the audio? Since I'm designing the source, it should be eventually possible to play back higher-sample-rate recordings without any up/downsampling. Of course, most recordings don't come at those high bitrates/samplerates

Sorry if I'm asking something obvious--this is my first time working with DACs & audio (I've always worked analog audio only or digital microcontrollers only, but I've never quite gotten to "bridging" them until now).

btw, threads for this DIY DAP design are available here:
http://www.head-fi.org/forums/showthread.php?p=3080077 (DAP)
http://www.head-fi.org/forums/showthread.php?t=248321 (DAC/SPDIF design portion @ head-fi, which unfortunately doesn't seem to have much interest)
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Old 3rd July 2007, 11:15 PM   #2
threpointone is offline threpointone  United States
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Join Date: Jun 2006
*bump* I would have edited my message, but it wouldn't let me. Anyway, you can ignore all of the digital newbishness up there. As I understand, the main audio concern in DACs/SPDIF is a low jitter clock. The specific SPDIF module used isn't as important as the clock quality the whole system is based on, if I understand correctly.

The only problem is that in this application, I need a pretty specific clock: for the microcontroller, I'll probably need a low jitter clock ~20-30Mhz clock; most of the low jitter clock ICs I've seen are for much higher speeds. Reclocking is also a possibility, but does anyone have any recommended design for reclocking I2S (not SPDIF)?

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