Its already possible to get music out of the BBB via I2S, I have had it running using the latest Debian Wheezy OS using FFMPEG. Also had music using Volumio tho I did have a problem with distortion on one channel. However there are issues with resampling to 48khz (and possibly other issues) which need addressing before its a fully working solution. As above a few people looking into it on different forums. Rune Audio also looking to get a new relase with I2S support but I don't think they will have the 48khz issues sorted. I wish I could contribute but I am hitting the limits of my knowledge!
I also have a Raspberry Pi running fine via I2S but really want to get the BBB going - better platform or our needs.
Puting my hopes on Russ to get this one sorted
It seems that way. Also the processor only has two integer coefficients for clock division, so there is no way to generate the 44.1K clocks from the on-board clocks.
A cape with two clocks, clean power supplies and connectors for I2S would be a good starting point... (and the drivers that would take advantage of these clocks)
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It might play at the closest frequency to 44100 it can generate, e.g.:
25000000 / 567 = 44091.71Hz
It is definite that BBB cannot support 44.1KHz and family. See page 75 of the System manual: https://github.com/CircuitCo/BeagleBone-Black/blob/master/BBB_SRM.pdf?raw=true
There is a 24.576 MHz connected to the Audio subsystem in order to generate the audio frequencies.
Fortunately, this is also connected to an expansion pin (GPIO3_21) and the oscillator can be disabled in s/w. For now the only way for bit perfect output is through USB.
There is a 24.576 MHz connected to the Audio subsystem in order to generate the audio frequencies.
Fortunately, this is also connected to an expansion pin (GPIO3_21) and the oscillator can be disabled in s/w. For now the only way for bit perfect output is through USB.
I took a quick look at the RPI specs.
- I2S is generated by the Broadcom chip, a 19.2MHz clock feeds the chip
- The spec says:
"The General Purpose clocks can be output to GPIO pins. They run from the peripherals clock sources and use clock generators with noise-shaping MASH dividers. These allow the GPIO clocks to be used to drive audio devices.
The fractional divider operates by periodically dropping source clock pulses, therefore the output frequency will periodically switch between:
source-freq/DIV1 and source-freq/DIV1+1
- I2S is generated by the Broadcom chip, a 19.2MHz clock feeds the chip
- The spec says:
"The General Purpose clocks can be output to GPIO pins. They run from the peripherals clock sources and use clock generators with noise-shaping MASH dividers. These allow the GPIO clocks to be used to drive audio devices.
The fractional divider operates by periodically dropping source clock pulses, therefore the output frequency will periodically switch between:
source-freq/DIV1 and source-freq/DIV1+1
Ah crap, pretty much all my music is CD ripped to .flac
I knew it sounded too good to me true ...
Give it some time. In the mean time, use a USB interface with dedicated clocks...
I had read here MPD is not offering any audio output : Beaglebone Black - Page 3 • RuneAudio Forum that the Pi is bit perfect or are you referring to the fact that the I2S signal is out of the HDMI chip with the Pi rather than prior with the BBB (and thus likely more jitter on the Pi)?
I am running a Pi via I2S to an ES9023 - sounds fine to me but that is purely subjective.
If resampling is used as you suggested is the end result likely to be audibly different to bit perfect via USB?
If this is what is actually happening, then for 44.1K you will get and output frequency fluctuating between these two values:
19200000/435=44,138
19200000/436=44,037
For HDMI Audio, 48KHz, you get:
19200000/400=48,000 which is perfect.
Back to the BBB:
24576000/557=44,122
24576000/558=44,043
You could argue that for fixed divider, you get a better approximate frequency with BBB
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