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PCM1792 NOS DAC on Aliexpress
PCM1792 NOS DAC on Aliexpress
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Old 22nd November 2020, 08:51 PM   #1
terranigma is offline terranigma
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Default PCM1792 NOS DAC on Aliexpress

There is an interesting dual mono PCM1792 DAC on Aliexpress which claimed as NOS, PCM-384, DSD-512 capable by seller.

I know that PCM1972 is maximum 192kHz capable.. but when run in external filter mode, you can feed it with oversampled data. Btw, to run these dacs in external filter mode, you have to use some glue logic in order to separate left and right signals from digital data.

As you can see, there is only one logic chip on PCB which is a 74HC157. This chip generally used for switching between two different I2S sources. There is no other logic chip on bottom (seller sent the photo of the bottom side) or upper side of the PCB. If this design is NOS capable, data must be separated in somewhere so I want to ask you that might that be the Atmega88PA AVR chip on the upper left corner?

Other thing is, existence of 74HC157 is in question since there is only one I2S input/source on PCB. So, there is something going-on on AVR side, right?
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Last edited by terranigma; 22nd November 2020 at 08:57 PM.
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Old 23rd November 2020, 08:21 AM   #2
abraxalito is offline abraxalito  United Kingdom
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From a quick read of the 1792 DS, seems your sentence 'you have to use some glue logic in order to separate left and right signals' isn't correct. Where there are two chips, each can run in 'Mono' mode and receive both channels multiplexed in the normal way via I2S. There's a bit in one of the registers to tell the chip which one of the two (L/R) channels to convert.

The AVR is almost certainly being used to set up the control regs, either via I2C or SPI. It has no I2S peripheral so wouldn't be a good choice for re-purposing the I2S stream in any case unless the programmer/designer was a bit-banging wizard.
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Old 23rd November 2020, 10:18 AM   #3
terranigma is offline terranigma
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Quote:
Originally Posted by abraxalito View Post
From a quick read of the 1792 DS, seems your sentence 'you have to use some glue logic in order to separate left and right signals' isn't correct. Where there are two chips, each can run in 'Mono' mode and receive both channels multiplexed in the normal way via I2S. There's a bit in one of the registers to tell the chip which one of the two (L/R) channels to convert.

The AVR is almost certainly being used to set up the control regs, either via I2C or SPI. It has no I2S peripheral so wouldn't be a good choice for re-purposing the I2S stream in any case unless the programmer/designer was a bit-banging wizard.
You're right. I thought PCM1792 behaves same as PCM1794 in external filter mode. According to datasheet, in external filter mode, separation of Left and Right channel only required when PCM1792 is in "Stereo" mode..this is a bit strange.
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