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Old 15th June 2005, 08:20 PM   #1
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Default 24 bit I2S source to 16 bit DAC?

Since I managed to get a free Revolution 7.1 sound card, I have been entertaining the idea of tapping the I2S directly out of the Envy24HT IC. I have found the datasheet for it and the right pins, but as the EEPROM is set at the moment it is configured for an AKM DAC and outputs 24 bit left justified I2S.

This would not be a problem except as a quick and cheap experiment I wanted to go right into the dreaded TDA1543, which of course is 16 bit. The datasheet for TDA1543 says it "accepts input serial data formats in two's complement with any bit length". However, the only timing diagram example shows a 16 bit input. I would be using it for only 16 bit pcm so I would think the Envy24 pads the rest of the word with zeros. Somehow though, I don't see how this could work. Is there any way to basically trim the zeros without editing the EEPROM to set the output to 16 bit to start? I guess you could demultiplex the serial data and use some logic but if that is the case I might as well just go DF1704 - PCM1704 to start.

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Old 16th June 2005, 10:46 AM   #2
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I think tda1543 only accept EIAJ input so you need to convert i2s to eiaj first. The additional bits shouldn't cause any trouble because (if I remember well) i2s containt 64 bits data
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Old 16th June 2005, 12:30 PM   #3
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I think only the "A" chips are EIAJ? At least the datasheet says I2S.
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Old 16th June 2005, 12:46 PM   #4
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TDA1543 : I2S
Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease
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Old 16th June 2005, 05:09 PM   #5
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Just read the I2S bus datasheet.. should have thought to do this before posting .
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Old 18th June 2005, 05:24 PM   #6
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it will work just fine, use software player, that can dither/noiseshape to 16bit if you'll be using volume control, like foobar2000 if you don't use it already.. if the bitclock is 64Fs, each channel carries up to 32bits of data, if it's just 32Fs, then only 16bits per channel.. you have 64Fs running on the soundcard, I hope the TDA can take it, I see no reason why it shouldn't..
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