Interface : USB or NOT

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Imagine you are designing the ultimate DAC in the box.

Everything is here: finest converters, all the connectors you need (SPDIF, glass, plastic XLR...), clocks from Guido :D and all and all.

Yeah ! But there is a question you're unable to answer: do you need (or not) an USB in ? :xeye:

You're thinking USB is a not required because :
- USB is by definition poor quality : who need the ultimate DAC with such a poor quality source ?
- you need to add something in you design that means less quality package, a sort of compromise to add the interface you're not a great fan

But in other hand you know many people use their computers like a drive with thousands records on their HDD.

So what to do ? USBing or not ?
 
renaudagnes said:
USB is by definition poor quality


Are you considering a commercial development? If i was in the market for a ready made dac i wouldn't consider anything without USB support. Maybe because i have no hi-rez sources and all i'm interesed in is 44/16.

The out-of-the-box USB solutions are really not that great and i doubt the addition of a single 270x chip would do USB much justice, unless you reclock.

Apparently async USB is a completely different animal and not "by definition poor quality"
 
Thanks analog for your response.

I am considering the commercial development of a little product.

I want explain to the technical team WE NEED USB ! But the technical team considers computer is devil and USB are the daemons :D

I use my computer since many years to store a lot of records, and the more I look, the more I see people dematerializing their records.

Today, I've seen B&W and Peter Gabriel are starting a new service with the ability to download records (err, one per month) in looseless format (APE I think) here :

http://www.bowers-wilkins.com/display.aspx?infid=777

And so many radios from the web, etc...

I would appreciate positive returns for USB, it will serve the cause!
 
I have a Glow Audio One (tube) amp with a USB input and an internal DAC. It sounds great, and I really appreciate having that feature in the amp. I know, French engineers are perfectionists - I've been working with them for 25 years. Your guys probably call themselves scientists instead of engineers like ours always have. Don't rely on them for marketing decisions. ;-)

The result of the USB input may not be the best, but a lot of people will appreciate its convenience. "Don't let the best be the enemy of the good"!

Best wishes in your venture!
 
Implementing PCM2704

analog_sa said:
The out-of-the-box USB solutions are really not that great and i doubt the addition of a single 270x chip would do USB much justice, unless you reclock.

Apparently async USB is a completely different animal and not "by definition poor quality"
There is a discussion on implementing PCM2704 here : USB to SPDIF converter

Here is some specification from datasheets
SLES081E-JUNE 2003-REVISED NOVEMBER 2007

The PCM2704/5/6/7 accept 16 bits data audio at 32, 44.1 et 48 kHz sampling frequency.

USB interface
All data to/from the PCM2704/5/6/7 are transferred at full speed.

The PCM2704/5/6/7 employs SpAct™ architecture, TI's unique system that recovers the audio clock from USB packet data. On-chip analog PLLs with SpAct enable playback with low clock jitter.

Page 26
When receiving the audio data, the PCM2704/5/6/7 stores the first audio packet, which contains 1 ms of audio data, into the internal storage buffer. The PCM2704/5/6/7 starts playing the audio data after detecting the next subsequent start-of-frame (SOF) packet.


The PC send data to PCM2704 so PLL is needed. There is no ASRC (Asynchronous Sample Rate Converter) in the device. Keeping 3 ms of audio data permit to keep jitter lower than continuous tracking. PCM170X is not sync device nor real async device!

Eric
 
Re: Implementing PCM2704

Eric Juaneda said:

The PC send data to PCM2704 so PLL is needed. There is no ASRC (Asynchronous Sample Rate Converter) in the device. Keeping 3 ms of audio data permit to keep jitter lower than continuous tracking. PCM170X is not sync device nor real async device!

Eric

I didn't quite understand your post. The 270x works in isochronous mode with a PLL. It's intolerably fussy towards everything and invites $300 usb cables to work well. I only like it if followed by an ASRC, otherwise the jitter is unacceptable.

Sadly, due to diy difficulties and laziness i have no direct experience with chips working in async mode. Hopefully, positive results are more easily achievable with those.
 
more about PCM2704

Hi analog_sa,

I never implement PCM2704 too, I decrypts datasheets and confuse post previously linked.

I link picture from page 26 PCM270X datasheets. It explain data transfer between PC and IC. PC don't send continuous audio data stream to IC. It send 1ms audio packet at max USB 1.0 speed.
Synchronizing PLL on continuous data stream could generate high and low frequency jitter. With 1 ms buffer you can smooth (or cancel) high frequency jitter. [Is dealing only with low frequency jitter inside audio band is better than high frequency jitter out the audio band?]
Jitter level can be 'dilute' by buffer size: big buffer large dilution, low buffer small (or no) dilution.

I agree with you, device quality is SpAct PLL quality design.

Eric
 

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Well,

I have used a lot of sound card.

For me, one of the best choice is the RME pci card solution. The sonic performance is excellent.

But big computers are noisy, not very environment friendly, use a lot of space etc...

That's why I have few portable computers, or netbooks at home now. And that's why I need and I try to use external USB sound cards to produce SPDIF signal from computers to my DACs.

I must explain I have few dacs at home : ooooold Wadias (1000, x32, x64) :
http://www.acec13.fr/tvc/Wadia/Wadia1000.html

old Micromega DAC (sorry no link)

and a recent and very expansive Audio Synthesis :
http://www.audiosynthesis.co.uk/dax_discrete.htm

Old Wadias are cools because they can lock spdif signal even if the signal is very bad. To see an example of bad signal, have a look on the one from a CEC expansive unit :

http://www.lampizator.eu/LAMPIZATOR/REFERENCES/CEC/CEC TL-1X.html

Of course the Audio Synthesis, because it's a modern unit, is waiting for a good signal. So, I can say that the USB SPDIF signal is so miserable that the AS cannot lock on it : I have to use the wadias... When it's working !

Well, let's talk about USB sound card. The one of the "best result" is produced by the terractec : it's very cheap and it's working (only with Wadia), but it's only Toslink

Terratec correct usb :
Terratec Aureon Dual USB Sound Card for PC and Notebook: Amazon.co.uk: Electronics & Photo

Then I tried usb with COAX spdif : on flybay you can find units from HK... It's not so bad, some "clicks" two times a minute with th Wadias, but at least you can heard something.

This is (not this one) this kind of gear :
SM29 MKII HIFI USB Sound Card+SPDIF&Toslink&I2S for DAC en vente sur eBay.fr (fin le 04-déc.-09 10:22:55 Paris)

But I need more, I want more, I want BNC or XLR spdif ! So here is the pop pulse :
CryoParts Pop Pulse USB to SPDIF Converter

This is the best unit in term of input output... But it does not work AT ALL ! No synchronization at all...

Of course I did not tried everything : the next step is the DacMagic from Cambridge. But for the moment I am not a great fan of the USB SPdif solutions...
 
Last edited:
Originally Posted by Eric Juaneda View Post
...PC don't send continuous audio data stream to IC. It send 1ms audio packet at max USB 1.0 speed.
Synchronizing PLL on data stream could generate high and low frequency jitter...


I'll have to look into this -- data is data -- hoe does it translate into anything audible?
Or does it?

And if not -- who cares?

Isn't jitter just a slight clocking error?

I have a PCM2702 and a couple of opamps I use as a preamp -- I love it.
 
Clock synchro between PC and PCM2704

Hi jmar,
I'll have to look into this -- data is data -- hoe does it translate into anything audible? Or does it?

And if not -- who cares?

Isn't jitter just a slight clocking error?
Yes data is data, and it is not important to have a continuous or discontinuous stream during data transfer. Problem occur when data is transmitted to S/PDIF (PCM2704 include S/PDIF output) or during digital to analog conversion (PCM2704 includes analog converters).

- fisrt problem : PCM2704 work with 12MHz crystal oscillator who is not a multiple of 44.1KHz.
- second problem : you MUST synchronize data output and data input. Else you can lose sample or overflow input buffer.

Synchronization between PC and PCM2704 is necessary and jitter problem start here. Jitter is a clock variation (noise, instability, variation...at high and low frequency).
 
But -- you can divide or "whatever" to get the clocking you need, generally.

Especially with all the math combinations available.

If something is set to decode PCM like the PCM series from TI, doesn't it automatically adapt to the incoming waveform?

Isn't all the PCM "setup data" part of the waveform itself?

I can't remember.
 
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