USB PCM to PWM?
Just a thought,
Is it possible to mod a USB audio device, like M-Audio Transit, to output its PCM signal straight to the PCM->PWM chipset of some digital receiver, like Panasonic XR55? The purpose is to bypass the step of converting to S/PDIF. I guess the cable will have to be hardwired to both devices and be really short. Or is there already some device that takes USB as an input and amplifies it by PWM without internally converting it to S/PDIF?
Any comments/feedback/rants:) will be appreciated.
I would guess most USB sound cards will be implemented as monolithic IC's. Even if you had access to raw PCM output (say before an independant DAC) a 32 wire bus (for only two channels) would be a pain to mod on top another board.
You could try and make something with a chip that does USB to a FIFO/parallel type endpoint such at the FT245BQ, but then you still have to handle channel muxing etc etc yourself. Another option would be a micro with USB support.
My opinion would be it would just be easier to use S/PDIF.
The TI chips used in the Panny receivers take I2S as input, and there are a variety of USB solutions that can be hacked to pull the I2S signal out, including the Transit (I think)
The problem will be finding a way to patch the I2S input into the power stage inside the receivers. My suggestion is to look at one of two options:
- look for a used SA-XR10. These were the first of the Equibit receivers, and the I2S lines are nicely exposed on a ribbon cable.
- get the TI EVM board. The 5186 EVM is available for $400 (I think it's the 5186 anyway), and takes I2S in on headers. You have to provide a power supply, but a couple SLA batteries for 24V will do nicely. It needs a PC to control it via USB, but if you're looking for a USB solution this obviously isn't a problem.
The big-time bonus of the 5186 is that this most recent chip includes DSP features right in the modulator. You get 8 channels with 7 biquad stages per channel plus on-chip volume and channel balance. Very very interesting stuff - you can do a full active 3 way system with only the 5186 and a stereo digital input (unfortunately, it seems to be missing delay, which is a shame)
The other receivers can probably be modded as well, I just don't recall hearing about anyone that has done it successfully, so it's a bit more of an uncharted adventure.
You can tap I2S on the M-Audio transit without any problems.
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