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Old 9th June 2012, 03:45 PM   #1
ycy is offline ycy
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Join Date: Jun 2012
Question Beaglebone as digital source?

Hello,

Recently, I brought a DAC(Yulong sabre D18), using PC sound card SPDIF output as digital source, and now seeking an alternative digital source. As an engineer, I think build a digital source myself is not a bad idea.

Basiclly what I want is find a suitable board that accept audio data and convert to SPDIF signal then feed to DAC. And I find beaglebone(it has already been discussed in some threads), which come with USB, ethernet and McASP interface. Audio data can be stored in HDD which connected to USB, or transfer via ethernet and McASP can be used to generate SPDIF signal. And finally I think I will get a mini audio server which running Linux and mpd.

Before turn a beaglebone board into a mini audio server, there are some problems I must sovle, both in hardware and software. The main problem in hardware is that I don't know whether McASP in AM355x is suitable for such a task(I don't have beaglebone board, I want to get more information before buy the board), the best I think is using a external clock as ACLKX for SPDIF output. The software problem is about alsa driver, but I think I can study and write the code myself. So, can anyone here that have experience with beaglebone or McASP give me any advice on the first problem?

Any way, I lack the skill of design PCB, so the best choice for me is using Proto Cape Kit(but I can not find anyone in China sell it, which is also a problem). And I am not familiar with analog circuit(I know FPGA and verilog), so can anyone here suggest a conversion circuit which generate coaxial signal from 3.3V LVTTL?

Thank you.
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Old 11th July 2012, 01:56 PM   #2
ycy is offline ycy
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Join Date: Jun 2012
After about one month, I finally make the beaglebone work as I want. The hardware is fairly simple: A 22.5792M crystal connect to AHCLK of McASP0, AXR0 of McASP0 connect to a 74LVC2G125 buffer, then divide 3.3V to 1V Vpp with some resistors, and connect to PE-65612NL pulse transformer and at last to a RCA connector.

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Old 26th August 2012, 05:06 PM   #3
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Nice job, how is it behaving?
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Old 27th August 2012, 02:12 PM   #4
ycy is offline ycy
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Join Date: Jun 2012
Only thing I can say is I can not hear significant artifact(I don't do the compare). The hardware can work at 24bit, 176.4KHz and Yulong SABRE D18 can lock the signal. But the AM335x CPU is too weak for resample the audio stream, so I use 16bit 44.1kHz as default parameter.
Another thing I have to say is, when use usb wifi(hardware is Tenda w311mi, software is wicd), there is some distortion every several minutes and signal might even unlocked(though this is rare). Change to ethernet will avoid this problem.

Finally, the kernel source code for anyone who interest:
https://github.com/yuchangyuan/linux-am33x
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Old 3rd December 2012, 10:56 AM   #5
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Default digital audio

hello
nice job, i'm going to try this , can you post a schematic.
Can the player output automatically switch to play at different rate, ex: if the source is 24/96khz it output at 24/96, and when at 16/44,1 switch to it without resampling ?

regards
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