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Old 13th September 2008, 06:20 PM   #1
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Default "sound card" vs external DACs

okapi writes:

"Hi Fast Eddy,

... i am setting up a data acquisition system using a sound card on my computer. the software i am using for this system currently only supports 16- and 32-bit files preventing from obtaining the better S/N ratios available at 24 bit resolution.

in short, are 32 bit sound cards in development or is it seen as an unnecessary step? if they are in development i will wait, if not i may need to look into alternative methods of controlling whatever sound card i use. i am reluctant to make the latter choice because i have years of well developed code.

thanks ..."

=====

okapi: ...

In My Opinion:

"Sound cards" are best of the external variety ... so they can be tweaked to remove the host system power supply noise, etc. Examples from M-Audio, Roland Edirol, EchoAudio.com and many others, connected by FireWire or USB cables. It is extremely difficult to "mod" a PCI connected sound card, but it can be quite easy to tweak and "mod" external peripheral "sound cards" or external DAC devices. Mods: adding Ferrite lugs to the shielded digital cables is an example ... adding plastic capacitors to external power supplies is another ... all in the name of removing the noise of the host computers' switching power supplies.

The music professionals in the studio use FireWire connected DAC / ADC devices almost exclusively, because of the greater bandwidth and dramatically reduced "jitter" and other questions. FireWire can handle Dolby 7.1 (6 or 7 channels or eight stereo feeds or more) easily, USB can not handle better than Dolby 5.1 (4 or 5 channels X one feed only) in one direction only. FireWire is completely bi-directional and can record and playback, rip and roar, input and output, capture and feedback, USB = not without complications and degradation of quality. The reasons are quite technical, suffice to say that USB has handshaking problems that are all but negligible with FireWire. ("Jitter" is but one question.)

Bit rates in audio are somewhat confusing, CD "quality" is 16 bits / 44.1K and the best is the Red Book standard. DVD-Audio quality can be as high as 24 bit / 192K (stereo only), but generally is 24 bit / 98K (Dolby 5.1). DVD-Video (DV movies) sound tracks are generally 24 bit / 48K, but may be an "up 'verted" conversions of CD "quality" 16 bit. A trip through Wikipedia.org tells most of the story. (Note the bit rate chart.)

As for 32 bits? The digital bus rates of virtually all computer host connected devices, audio or whatever, are already 32 bit data bus streams. Whether PCI, USB, FireWire, SATA / eSATA, iSCSI, FiberChannel, modern EtherNet ... all share a similar data transfer scenario that is 32 bits. The digital audio and video information (8 bit, 12 bit, 16 bit, 20 bit or 24 bit) is "encapsulated" within the 32 bit digital data bit frame (or 32 bit data packet) and is done by the DAC / ADC converter chips, which are already 32 bit processors.

Why not make 32 bit audio data converters? This is possible, but not very cost effective. A better understanding of why this is not necessary is to understand decibel headroom, sound pressure levels and bandwidth. CD "quality" is at best 80 db in bandwidth, some would argue 85 db. DVD-A can be as high as 120 db but in practical application seldom exceeds 105 db.

Approximate sound pressure level references: The hush of a pine forest = +10 db to +25 db, your quiet living room might be +30 db to +55 db, the loudest live rock concert with crowd noise can exceed +100 db in the middle of the crowd [+115 db next to the PA speakers], a jet engine from 100 meters can exceed +120 db ... so unless you are planning to reproduce jet engine noise in your living room, dynamic range need not exceed 105 db, the practical bandwidth of FireWire and a few other esoteric digital scenarios. (Theoretically, + 0 db might be reproducible in outer space [no air], but not on Earth, because of Earth tremors and Brownian Motion of air molecules.) ... SO, there is no practical need to digital convert analog music to digital using 32 bit ADC converters, if they even exist.

It would be nice if someone else corrected my errors above ... I'm certainly not the absolute source for this ...

Mercenary announcement: I work for an online parts catalog company. We were first with USB online (1996) and first with FireWire online (1997). Imagine my amazement and joy when the first USB audio DACs appeared ! Imagine my disappointment to discover that my Canon DV camera audio ADC was only 12 bit ...

Is anyone making SATA connected 24 bit DACs ? They could easily fit into those cute little hard drive containers ...
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Old 13th September 2008, 07:33 PM   #2
SunRa is offline SunRa  Romania
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AK4397 is a 32bit DAC: HERE
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Old 13th September 2008, 09:33 PM   #3
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" AK4397 is a 32bit DAC: HERE ..."

" ... The AK4397 is a high performance premium 32bit DAC for the 192kHz sampling mode of DVD-Audio including a 32bit digital filter. ... Sampling Rate: 30kHz ~ 216kHz ... DR, S/N: 120dB ... Applications: DVD/DVD-Audio/SACD Player, Digital Mixer, Pro Audio and AV Receiver ... "

Glory-Asaki ... how do you do!

Nice catch, SunRa. Wanna help me put one on a board? I still thing that the hard drive enclosures should be put to good audio use ...

Also, I believe these are the guys that astonished the world with those "blue white" LEDs ... impossible, some said ...
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Old 13th September 2008, 10:00 PM   #4
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That part might be 32bit internally, but in terms of S/N, it's the same as a 24-bit DAC. The state of the art in the consumer space is approaching 130dB, e.g. PCM1794, AD1955.

24-bit DAC theoretical S/N: 20log(2^24) = 144.5 dB

32-bit DAC theoretical S/N: 20log(2^32) = 192.7 dB

So these days, even in a 24-bit DAC, they're fudging a couple of bits.
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Old 13th September 2008, 10:55 PM   #5
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" ... So these days, even in a 24-bit DAC, they're fudging a couple of bits. ..."

Still, that AK 4397 is a fascinating new chip ... I'm glad to see there is still an ongoing upgrade / innovative program in DAC chips ... New blood, new blood, I feel the need for new blood ...

... And you can always count on Analog Devices to be in the forefront of audio ... http://www.analog.com/static/importe...ets/AD1955.pdf ... distortion = -110 db ... Whenever I see any s/n or distortion number > 105 db, I'm impressed. AND power supply voltage, digital and analog, is a piece of cake to make clean = 0 / + 5.0 VDC =
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Old 13th September 2008, 11:08 PM   #6
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whenever I see a SNR figure greater than -110dB I'm suspicious. There's usually some specific test conditions or measurement filtering going on.

Achieving >100dB 'A' weighting (or equivalent bandwidth measurement) with normal power rails requires significant attention to detail.
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Old 14th September 2008, 01:26 AM   #7
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" ... Achieving >100dB 'A' weighting (or equivalent bandwidth measurement) with normal power rails requires significant attention to detail. ..."

Ditto that !! ... Checkout TI's use of very close coupled plastic caps, right across the chip power & ground pins ... the only way to design for maximum clean power rails to op-amps ...
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Old 14th September 2008, 09:34 PM   #8
2quad is offline 2quad  Norway
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FastEddie's nick initiated this reply:

Let's try a slightly different wiev of DAC's. The problem with DACs today is not the technology itself, it what we feed them. Everybody have heared of the "loudness war" but no-one seem to take action.

CD's that are mastered "to hot" distort on you at three different levels.

1. The audio is clipped. The sound is crap. Sadly, nothing can be done by that. Send the mastering engineer som hate mail.

2. The PCM audio on the CD are valid samples but when you apply oversampling or sample rate conversion or any other kind of DSP, your DSP creates clipping conditions. Why? I'ts because, in oversampling, the new samples you create between the nyquist samples are "over the top". It's the same for all other DSP.

3. If you (heaven forbid!) apply lossy compression, it's getting even worse. If you look at the waveform, the shape of your audio is totally different after lossy compression. The peak value of your audio may be 3 to 5 dB above what's on your CD. And -- of course -- it clips and sound nasty.

I believe there's a simple solution to this. Since audio in CD's is 16 bits and SPDIF and moderns DACs support "24" bit, theres a lot of "spare" bits. If all CD samples are shifted right one bit, i.e. attenuated 6.02 dBs, the clipping in DSP and lossy compression will disappear. In offset binary terms this would give 17 bit samples with a leading zero.

Of course you're still left with the clipping in the CD, but you might get far better sound from CD's this way.

To date, I still have not seen any CD/DVD-player or DAC manufaturer adress this problem. Maybe we DIY'ers should take the lead here?

More info on "hot CD's" etc. can be found on the excellent tech library at TC Electronic:

http://www.tcelectronic.com/TechLibrary.asp

cheers,

The Bear
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Old 15th September 2008, 12:54 AM   #9
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Quote:
Originally posted by 2quad
Of course you're still left with the clipping in the CD, but you might get far better sound from CD's this way.

I fear not. All you've done is turned the digital volume down which will most likely be compensated by the user turning the analog volume up, which by the way, raises the noise floor by an equal amount.

You say no one takes action. Well, I for one will not purchase a compressed CD. I recently bought a 25year anniversary gold digital remaster of Dark Side of the Moon. It was clipped in many places. The remastering engineer didn't even bother to check the statistics on his conversion. I sent it back for a $25 refund. It's passive action I know but as pee-on consumers it's all we can do,
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Old 15th September 2008, 01:22 AM   #10
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" ... You say no one takes action. Well, I for one will not purchase a compressed CD. I recently bought a 25year anniversary gold digital remaster of Dark Side of the Moon. ..."

Ditto and ditto, although I obtained a copy of "Dark Side ..." as SACD ... mmmmmm good. And how about "Supernatural", Carlos remixed this for 24 bit DVD-A and what a difference! You can actually hear the individual instruments, so's now you can tell Carlos and Eric apart and the rest of the sidemen come across as individuals too.

For a pleasant surprise and an eye opening about 16 bit vs. 24 bit, check out George Martin's double disc "Love" ... Play each disc, the CD and the DVD-A, on the same player and you definitely can hear the difference 'tween 24 and 16 bits. Plus it is the most fantastic redux, redo, retro update of Beatles music ... highly recommended. (Stick the CD in your car and keep the DVD-A for home stereo play ...

As for CD "quality", I don't buy it anymore. Nothing but SACD, DVD-A and DVD-Videos ... check out the two "Crossroads" DVD albums ... 24 bit / 48 K and "CR 2007" has the most amazing Steve Winwood and Eric Clapton duets ... much better than the original Blind Faith stuff from the studio (!), CD or vinyl .....
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