DIY Discrete R-2R Ladder DAC: Serial Data Demultiplexing

b1o2r3i4s5 · 8th February 2012, 01:30 PM

Hello,
This is my first thread and I have a problem with demultiplexing the digital PCM audio data stream.

Recently I designed a discrete R-2R DAC using information from the I2S spec sheet and wiki,
but when i first tested it, the output wave is split in 2: upper part and lower part.

I later found out that the PCM stream is coded as a signed integer using the 2's compliment method
but the shift-register can only decode a unsigned integer.

My question is if theres anyway I could change the signed PCM stream into unsigned?
OR
Is there another way of decoding this PCM Stream?

Thanks,
Boris

oshifis · 8th February 2012, 02:02 PM

You need to invert the MSB.

b1o2r3i4s5 · 8th February 2012, 02:09 PM

Thanks for your Advice
But i have tried that and it seems that the mid-point is out of place
is there a way to fix it?
Another question: the output wave seems to have some of its voltage levels shifted vertically, is this caused by the resistor errors in the R-2R ladder?
I will try to get a photo from my oscilloscope ASAP.
Thanks,
Boris

SoNic_real_one · 04:59 PM

Invert (and add one) will give the "inverse" voltage:
http://en.wikipedia.org/wiki/Two's_c...27s_complement

b1o2r3i4s5 · 8th February 2012, 05:37 PM

but how exactly do I "Add 1" to a digital stream...

DF96 · 8th February 2012, 06:41 PM

With an adder? You can get a chip which will do this, and deal with carry in and out. Known as a 'full adder'.

b1o2r3i4s5 · 8th February 2012, 06:53 PM

The main problem is that the data is in serial format

oshifis · 8th February 2012, 06:55 PM

Quote:

Originally Posted by b1o2r3i4s5

Thanks for your Advice
But i have tried that and it seems that the mid-point is out of place
is there a way to fix it?
Another question: the output wave seems to have some of its voltage levels shifted vertically, is this caused by the resistor errors in the R-2R ladder?
I will try to get a photo from my oscilloscope ASAP.
Thanks,
Boris

What do you mean by "mid-point is out of place"? Digital zero of twos complement (2C) signal is 0000. Maximum positive value is 7FFF, maximum negative value is 8000. If you invert the MSB, these numbers will be 8000 (analog zero), FFFF (maximum positive signal), and 0000 (maximum negative signal), respectively. Exactly the 0000 to FFFF offset binary (OB) span.

b1o2r3i4s5 · 8th February 2012, 07:06 PM

Quote:

Originally Posted by oshifis

What do you mean by "mid-point is out of place"? Digital zero of twos complement (2C) signal is 0000. Maximum positive value is 7FFF, maximum negative value is 8000. If you invert the MSB, these numbers will be 8000 (analog zero), FFFF (maximum positive signal), and 0000 (maximum negative signal), respectively. Exactly the 0000 to FFFF offset binary (OB) span.

By "Out of place" I meant the analogue signal, every time the wave crosses the analogue mid-point a positive spike appears.

oshifis · 8th February 2012, 07:14 PM

That is called glitch, caused by imperfect switches. Do you have a sample-and-hold circuit after the DAC? That should hold the previous analog level during the glitch. I have built a similar R-2R discrete DAC, but I could reach 10-bit resolution with stock 0,1% resistors. The remaining 6 bits had to be trimmed.

8th February 2012, 01:30 PM	#1
b1o2r3i4s5 diyAudio Member Join Date: Feb 2012 Location: HK	DIY Discrete R-2R Ladder DAC: Serial Data Demultiplexing Hello, This is my first thread and I have a problem with demultiplexing the digital PCM audio data stream. Recently I designed a discrete R-2R DAC using information from the I2S spec sheet and wiki, but when i first tested it, the output wave is split in 2: upper part and lower part. I later found out that the PCM stream is coded as a signed integer using the 2's compliment method but the shift-register can only decode a unsigned integer. My question is if theres anyway I could change the signed PCM stream into unsigned? OR Is there another way of decoding this PCM Stream? Thanks, Boris

8th February 2012, 04:54 PM	#4
SoNic_real_one diyAudio Member Join Date: Jan 2008 Location: Virginia	Invert (and add one) will give the "inverse" voltage: http://en.wikipedia.org/wiki/Two's_c...27s_complement Last edited by SoNic_real_one; 8th February 2012 at 04:59 PM.

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8th February 2012, 02:02 PM	#2
oshifis diyAudio Member Join Date: Mar 2004 Location: Budapest, Hungary	You need to invert the MSB.

8th February 2012, 02:09 PM	#3
b1o2r3i4s5 diyAudio Member Join Date: Feb 2012 Location: HK	Thanks for your Advice But i have tried that and it seems that the mid-point is out of place is there a way to fix it? Another question: the output wave seems to have some of its voltage levels shifted vertically, is this caused by the resistor errors in the R-2R ladder? I will try to get a photo from my oscilloscope ASAP. Thanks, Boris

8th February 2012, 05:37 PM	#5
b1o2r3i4s5 diyAudio Member Join Date: Feb 2012 Location: HK	but how exactly do I "Add 1" to a digital stream...

8th February 2012, 06:41 PM	#6
DF96 diyAudio Member Join Date: May 2007	With an adder? You can get a chip which will do this, and deal with carry in and out. Known as a 'full adder'.

8th February 2012, 06:53 PM	#7
b1o2r3i4s5 diyAudio Member Join Date: Feb 2012 Location: HK	The main problem is that the data is in serial format

8th February 2012, 07:14 PM	#10
oshifis diyAudio Member Join Date: Mar 2004 Location: Budapest, Hungary	That is called glitch, caused by imperfect switches. Do you have a sample-and-hold circuit after the DAC? That should hold the previous analog level during the glitch. I have built a similar R-2R discrete DAC, but I could reach 10-bit resolution with stock 0,1% resistors. The remaining 6 bits had to be trimmed.

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