Using Video-DACs for audio - diyAudio
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Old 18th December 2003, 12:57 AM   #1
ojg is offline ojg
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Lightbulb Using Video-DACs for audio

Here is an idea that's been floating around in my head for a while, and I would like to get your comments on it.

The idea is to use one of the many video-DACs that runs at more than 100MHz with resolution from 8 to 16 bits. The 24-bit audio would be oversampled by a very large number, say 1024, from 48kHx to 49.152MHz. Then dither would be added, and the result truncated to the DACs width of say 8 bits. The dither would need to be noiseshaped, but since the oversampling-ratio is so high, the noiseshaping function could be first-order. Now all you need is a first-order analog filter after the DAC to remove the noise above the audioband.

This approach is different from a multi-bit delta-sigma DAC (PCM1792,AD1955), since these DACs often use very high-order noise-shaping.

Pros:
-Video-DACs have high output current, 20mA differential, ensures good SNR.
-Video-DACs are designed to be used with a simple resistor as I/V converter, no need for fancy op-amps.
-Video-DACs have an externally adjustable reference current, this can be used for volume control.

Cons:
-The extreme oversampling would require an FPGA which is expensive and not DIY friendly.

See here for an example of a video-DAC.

I used a 20Hz sinewave sampled at 48kHz to illustrate what a 20kHz sinewave would look like when sampled at 48MHz. A sinewave was created at -100dB with 32-bit resolution, then converted to 8-bits with 1-bit TPD dither with noiseshaping-function A. The result is over 130dB dynamic range in the audioband. This would improve even more if a 12-bit DAC was used.

What I would like to hear is: Do you know if somebody is already doing something like this? Have I overlooked something that would make this not work?

What I don't want to hear is: "Why don't you just use a PCM1738,TDA1541,<insert favorite chip here> instead?"
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Old 19th December 2003, 02:28 AM   #2
ojg is offline ojg
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No takers? Or are you all still ROTFLYAO?
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Old 19th December 2003, 11:07 AM   #3
Werner is offline Werner  Europe
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Actually the more interesting idea is to use 10-12 bit flash or pipelined video ADCs in a recording system that is much less oversampled/noisehaped than your average audio ADC, running at 176kHz, no anti-aliasing filters, and then show the recording industry (and the SACD-sect) a thing or two.
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Old 19th December 2003, 02:04 PM   #4
Previously: Kuei Yang Wang
 
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Konnichiwa,

Quote:
Originally posted by ojg
No takers? Or are you all still ROTFLYAO?
I have been thinking about something somewhat similar, but rather using tripple 12-Bit DAC's and "stacking" them with added interleaving to average out any errors. But I think a semi-discrete R2R DAC is more worth spending time on.

Sayonara
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Old 19th December 2003, 06:20 PM   #5
ojg is offline ojg
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Quote:
Originally posted by Werner
Actually the more interesting idea is to use 10-12 bit flash or pipelined video ADCs in a recording system that is much less oversampled/noisehaped than your average audio ADC, running at 176kHz, no anti-aliasing filters, and then show the recording industry (and the SACD-sect) a thing or two.
I completely agree, but unfortunately they don't give us DIY'ers access to the recording sessions. I wonder why... So all we are left with is trying to improve the decoding end.

Quote:
Originally posted by Kuei Yang Wang
I have been thinking about something somewhat similar, but rather using tripple 12-Bit DAC's and "stacking" them with added interleaving to average out any errors. But I think a semi-discrete R2R DAC is more worth spending time on.
Yes I have been thinking about a discrete R-2R DAC as well, and you could ofcourse substitute the video-DAC with a discrete version. The interleaving is more tricky though. I assume this would be similar to how the dCS ring-dac works? Do you have any detailed information on it?
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Old 21st December 2003, 04:35 AM   #6
andrei is offline andrei  Canada
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I have been toying with the idea of building a DAC around video DAC chips (AD768 in particular) for a while too... Not a first order Delta-Sigma though, but a straightforward non-oversampled design (which are in fashion nowadays). That needs some glue logic to do serial to parallel conversion and data retiming, which I was planning to stuff into a single CPLD, if I ever got around to it. Using FPGA will give the design flexibility to switch upsampling algorithms at will, with the downside that somebody will need to write the software.

It might be a fun thing to do . Maybe I should get one of those FPGA evaluation boards and read up on VHDL...
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Old 21st December 2003, 05:21 AM   #7
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One big problem is that the audio dac's are designed to run from the timming and data format that provided by the current crop of digital filters. So that means you would have to add a lot of logic to this dac to get to work.

So even if you could find one with the same low level resolution and linearity the data format would be different. Also, what the point the we have no requirements for 1 Mhz sampling .
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Old 21st December 2003, 07:40 AM   #8
Dave is offline Dave  New Zealand
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Jim,

It is pretty easy to reformat the data to pretty much anything with a small CPLD.
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Old 21st December 2003, 03:27 PM   #9
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The AD768 and the AD9752 are NOT video dacs. Video dacs accept data in specified formats with wordlengths between 8 and 10 bits, though the internal wordlengths are often longer for processing,and none of them that bear any relation to the format of digital audio. They are also often loaded with video specific add-ons and not all of them can be disabled.
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Old 21st December 2003, 03:51 PM   #10
peufeu is offline peufeu  France
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This is actually what dCs does in their Elgar series. The digital signal is oversampled a lot, noise-shaped, and fed to a 8 bits or so DAC which is (I think) implemented in discrete parts.

I have never heard these products though.

I have thought of similar things, using a high-frequency DAC (most of which, but not all, are video dacs) and oversampling. But you'd need to use something incredibly fast to pump out these samples, and the only thing which comes to mind is a FPGA. It would have potential, though.

However I'd advise against using 1LSB dither, rather use dither over several LSB's ; you'll get more noise (won't matter anyway) but more spreading of the LSB nonlinearities in the DAC (which is better).

"Multibit" sigma delta DACs have so few bits they still seem suspicious to me...
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