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TOPIC: i2s loopback on the onboard SPDIF input?

i2s loopback on the onboard SPDIF input? 6 years 9 months ago #10593

  • OllBoll
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Hello there,

Since there is currently a hard limit of 2048 taps / channel I got the idea of looping through the other pair of inputs so I can cascade the FIR and by extension get more taps to play with.

I guess connecting the SPDIF TX to the SPDIF RX should work by default but my question is if it would work to connect say I2S_DATA_OUT3&4 to I2S_DATA_IN3&4 instead? ( or whatever I2S input the non Digi-FP SPDIF input is ).
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i2s loopback on the onboard SPDIF input? 6 years 4 months ago #12919

  • Thunderstruck
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Good idea but the problem is that in every I2S (inter integrated circuit stereo sound) exchange, one of the devices must be the master and the other the slave. The miniSHARC is configured to be the Master of anything attached to it's outputs (for example the currymanDACs). The miniSHARC is also the Master of it's inputs such as AN-FP or DI-FP. So, if you route the one of miniSHARC's outputs to its own input, the clocking gets screwed up because they both want to be da boss.

There are threads about miniDSP making the miniSHARC so that we can set it to be the master or the slave but I don't think it is ready yet. Even then, just how all the clocking works is still unknown to me for certain.
It seams to me that a device (miniSHARC) could simultaneously be the master of a device connected to it's output and the slave of a device connected to it's input. However, all that I read implies that a device is either a "master" or "slave" as if there are only 2 devices in the chain. But, there could be many devices connected in series and if each one must be ALL-MASTER or ALL-SLAVE then every other device in the chain is an ALL-MASTER and the others are all ALL-SLAVE. If the device could be configured to be a slave of it's input and a master of it's output then every device in the chain would be told it's timing and then in turn tell it's output what time it is.
If my logic is wrong about this, would you please correct me.
William Edwards
Marietta, GA USA
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i2s loopback on the onboard SPDIF input? 6 years 4 months ago #12921

  • curryman
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William,

just take a look to the latest miniSHARC user manual. It already includes the description how to set I2S input and/or output as master/slave. The corresponding firmware is about to be released soon ;)

The trick is actually the ASRC (asynchronous sample rate converter) which is already implemented in the DSP hardware but has not yet been used by the firmware. For example if miniSHARCs input is slave you'll need to provide BCK, LRCK and data to the input and miniSHARC will resample incoming data to its own clock. Similar for the output. So it is indeed possible to have input as slave and output as master ;)

The new firmware will enable to connect asynchronous USB to I2S adapters like USBstreamer, WaveIO or Amanero (which are always master!) to the SHARC: very convenient :woohoo:

kind regards, Daniel
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