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Old 28th October 2012, 11:39 PM   #81
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Problem is, for this to ever be useful, it needs to be written as an ALSA driver, and use DMA. For now, it's an interesting experiment, but will keep you posted.
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Old 29th October 2012, 03:32 AM   #82
MrSlim is offline MrSlim  Canada
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I'd like to know if the Rev 2 board has corrected the issues with the USB. I think audio out via USB is going to be, while not the simplest, a quicker route to driving a high quality DAC (that is if the issues have been resolved that showed up at higher sampling rates)
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Old 29th October 2012, 05:39 PM   #83
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Have you tried the latest Raspbian? I believe there were some improvements to usb functionality made in the drivers (although I've not done anything with USB DACs).
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Old 29th October 2012, 06:19 PM   #84
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Quote:
Originally Posted by MrSlim View Post
I'd like to know if the Rev 2 board has corrected the issues with the USB. I think audio out via USB is going to be, while not the simplest, a quicker route to driving a high quality DAC (that is if the issues have been resolved that showed up at higher sampling rates)
I tend to agree.

while the i2s stuff on the 'pc' sounds interesting, I would rather trust a well designed usb UAC2 dac or usb/spidif or even usb/i2s converter.

my guess (just a guess) is that i2s won't buy you anything in audio that you can't already get (better) via uac2.

but it would be fine to be proven wrong whatever gets us decent quality i2s or spdif is great for us all.

I'm still waiting for a breakthru on usb stability on the pi, though. until this is fixed, the platform is mostly useless (I got tired of sd card corruption and suspended my use of this platform for now)
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Old 29th October 2012, 07:44 PM   #85
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I need to play stereo CD content, so I want to feed my PMD100 and TDA1541A with I2S. There's no point converting to USB or SPDIF, just to convert back to I2S to feed a DAC chip - if you can help it. Think of all the jitter!
I suppose, if you have lots of 7.1 content, then you may have different requirements.
(please, no arguments!!!!)

However, like yourself, I'm also at the point that it's not a solution until it's working with DMA in kernel space. I can't use what I have with MPD for instance. But, hey! This is DIY after all.

Anyway, Raspberry Pi • View topic - I2S: Anyone got it running?, shows the code changes I made.
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Old 29th October 2012, 08:09 PM   #86
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I'd rather have an spdif carried data stream than a poorly done i2s one.

my understanding is that unless you do i2s VERY well, its going to be suboptimal compared to spdif.

with reclocking dacs, I don't see the urgency in i2s like it was 10 or more years ago. reclocking and rebuffering is what makes the spdif quality mostly irrelevant. as long as no bits get lost, the receiver is now responsible for timing. the pc is not the critical path anymore, its just pushing data (again, talking uac2, here).

for example, would I trust the audio widget to clock out data at very low jitter rates? or would I trust a $35 rasp pi that was NEVER thought of as a high end audio transport?

I have some appreciation for the design and work that went into the audio widget. I don't think that kind of care went into the soc or the pi. it wasn't its focus, afterall, it was more a video player source and the target audience is not expecting very expensive sounding audio from it, especially not over i2s.

sometimes, vendors add features just to say they have them. I'm thinking i2s is such a thing on this soc device.
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Old 29th October 2012, 08:18 PM   #87
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yes there is very much a reason to use USB, the i2s you get out of this thing is never going to be great. i2s for i2s sake is a fool's errand, thinking that less is always more in this situation is not taking in the whole picture. i2s can easily be worse than anything else out there including optical, in fact in this case with unknown power supply quality/noise and clock jitter, even optical is almost guaranteed to be better and LOWER jitter given a proper spdif implementation. add in using a NOS dac and you have a recipe for awful quality.

i2s is the least resilient of the digital audio formats, clock jitter, noise, ground bounce etc are all passed through, no error correction, all single ended...we can bet that the i2s on the Pi is not going to be properly terminated either

if thats your motivation for using i2s, I dont think youve really thought this through, usb will be leagues ahead, i2s was never meant to be an interconnection scheme between different modules.

give me USB with UAC2 anyday for this purpose, you need all your ducks in a row for i2s to be great, and it has to be local; then it is the way to go

haha I see linuxworks has said pretty much the same thing while I was making coffee, oh well it stays, just to reinforce the point.

Last edited by qusp; 29th October 2012 at 08:22 PM.
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Old 29th October 2012, 08:49 PM   #88
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lol. yeah, we see things similarly, in this regard.

I'm not an spdif hater. it seems hating spdif is trendy, these days

I see it as a data transport, and if buffering and reclocking can be done well (any disagreement, here, anyone?) then why care about getting a crappy i2s to work?

before uac2, yeah, it would be worth getting into host i2s. but the uac2 was a game changer. the burden of setting timing is no longer on the host. that's a much better way to go.
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Old 29th October 2012, 09:16 PM   #89
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Whatever you say guys, whatever you say.
I'm very happy for you.
(as likewise, I'm sure you are for me )
Anyway, back to getting a raspberry pi to do stuff...
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Old 29th October 2012, 09:16 PM   #90
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P.S. And I don't normally use NOS! Let's get THAT straight ;-)
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