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Why USB?

151.197.211.154
Posted on October 18, 2004 at 16:39:51
ketchup
Audiophile

Posts: 622
Location: Pennsylvania
Joined: February 29, 2004
 I'm just wondering why everyone seems to be gravitating toward USB output -> S/PDIF. There can't really be any less jitter because you're still converting to S/PDIF anyway. why not just come directly out of the computer with the S/PDIF? Is there any evidence that it is better to use a usb device as opposed to S/PDIF straight from a decent soundcard?

Just throwing the question out there to hopefully stimulate some thinking and educate mself and others. I'm not looking down upon anyone doing the USB thing!

 

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Two reasons for me....., posted on October 18, 2004 at 17:00:55
doodlebug
 First, outboard power supplies for the DAC function. USB allows me to get the digital stream out of the PC without using any soundcard that relies on the PC's power supply. These were never designed with audio in mind. Also, the cost of the connections are low given its availability on most any new digital device.

Second, USB is a syncronized data stream. Unlike SPDIF, it uses an internal clocking mechanism. There was an attempt years ago by Audio Alchemy to get the industry to use I2S (I squared S), which employed a separate clock signal to keep the SPDIF data stream locked. Didn't really happen.

I'm sure there may proponents and detractors here but that's my logic and I like the results.

Cheers,

David

 

To quote Gordon Rankin:, posted on October 19, 2004 at 04:36:41
jusbe
Audiophile

Posts: 5950
Location: North Island
Joined: April 4, 2000
 "USB unlike SPDIF is bidirectional and therefore has error correction and buffering on both sides. This happens automatically so the data on the disk is identical to what is going out all the time. Also since this interface is asynchronous the clocking problems associated with SPDIF go away.What happens is... On power up of the computer the 2 devices negotiate services. In this case the Cosecant [or any other USB DAC - detail added] tells the computer it can do 16 bit audio at 32K, 44.1K and 48K. Since the USB receiver only has to handle these 3 frequencies, the clocking to the separate DAC IC has almost no jitter. SPDIF actually has to be synched to the exact frequency of the transport (i.e. if the transport is working at say 44.0896K instead of 44.1K the dac has to sync to that frequency). Therefore the jitter problems of SPDIF almost go away using USB. So using USB we have a zero error protocol to link the computer to the DAC and very low jitter what else"

I was going to try and paraphrase this myself, but Gordon has already done a good job, so credit where credit is due. One thing I would add is that USB2.0 seems to sound nicer than 1.0, and I don't know why. There is enough bandwidth on 1.0 to cope with digital audio (CD data) so why the extra bandwidth should make a difference is a mystery. In fact, I'm not even sure its the badnwidth that does make the differece - perhaps there is something else different about the 2.0 protocol that is having a positive effect?

I'm currently playing with a TwinDACPlus! and am enjoying very much. *Much* more than I would have expected, even for a NONOS DAC, and much more than the CD7 or the SCD-1 that I used to own.

Big J.

 

J thanks, more info on 2.0, posted on October 19, 2004 at 07:25:22
Gordon Rankin
Manufacturer

Posts: 2928
Joined: June 9, 2000
 J.

Thanks for the nice comments... I don't know how the Twindacplus works but there are 3 USB Audio 1.1 formats (well six but most of those include eq and other stuff like vol control). Only one of them has error correction. The others work similar to SPDIF in that the output is time corrolated.

It maybe that the reason the 2.0 is better is because it has much better throughput do to buffering and this maybe why it sounds better on your system.

I use type 3 Audio format with error correction and have not found much difference in using an older iBook 500 with 1.1 controllers and the 2.0 controller on my newer iBook using the Cosecant.

Thanks
Gordon
J. Gordon Rankin

 

Re: Why USB?, posted on October 19, 2004 at 10:24:33
audioengr
Manufacturer

Posts: 6017
Location: Oregon
Joined: April 12, 2001
 Here are the benefits to USB->S/PDIF converters:

1) Galvanically isolates PC from stereo system
2) Eliminates one S/PDIF interface (transformer,capacitors etc..)
3) The S/PDIF cable or wiring if internal can be very short and impedance controlled
4) The USB cable can be much longer than an S/PDIF cable without any audible degradation
5) Reclocking in the converter can be low-jitter, such as using a Superclock2 etc.. - this can also be done in a USB to I2S converter
6) can have a separate power supply or even battery powered

 

You're welcome., posted on October 19, 2004 at 13:08:09
jusbe
Audiophile

Posts: 5950
Location: North Island
Joined: April 4, 2000
 Just happy to have access to information like this, which seems well ahead of the curve.

Tell me, weren't you in communication with Apple about the direction and feature-list of their new digital wares? Any progress?

Lastly, do you know of any decent USB cables or is this too 'old hat' in the PC age? Are they all pre-formed and moulded?

Big J.

 

I2S to USB, posted on October 19, 2004 at 13:11:30
jusbe
Audiophile

Posts: 5950
Location: North Island
Joined: April 4, 2000
 a'e',

Is it possible to connect a cd/dvd transport with I2S (such as a Pioneer deck) to a USB-equipped DAC? How ugly would it be, sonically, or are they just not compatible?

If so, it could prove a really useful bridge for curious audiophiles to make the transition to more solid-state digital replay in the future (assuming the same or similar audio benefits are available with such a theoretical configuration).

Big J.

 

Re: I2S to USB, posted on October 19, 2004 at 14:01:15
audioengr
Manufacturer

Posts: 6017
Location: Oregon
Joined: April 12, 2001
 I suppose it is possible, but I dont think you would want to do this. The USB protocol is vastly different than the I2S. The I2S is close to what the DAC chip needs already. A direct I2S interface, such as that in the P-3A DAC makes more sense.

 

Re: Why USB?, posted on October 19, 2004 at 14:19:24
ketchup
Audiophile

Posts: 622
Location: Pennsylvania
Joined: February 29, 2004
 Is it safe to assume that there is no real advantage of A over B (other than the ability to use a very long USB cable)?

A) USB>external soundcard>s/pdif>DAC

B) s/pdif from soundcard>DAC

 

Re: Why USB?, posted on October 19, 2004 at 16:04:35
Lynn
Audiophile

Posts: 1493
Location: Southwest
Joined: June 24, 2003
 I think there is a definite advantage to the PC-->USB-->DAC, but if you introduce SPDIF with PC-->USB-->SPDIF-->DAC there is diminished benefit.

By eliminating SPDIF entirely from the chain and replacing it with USB, and use drivers that take advantage of the error checking/correction features of the USB bus, you can get a bit perfect transfer straight to the DAC.

With a hybrid USB/SPDIF approach, you can get the SPDIF source out of the PC so that the electrical noise from the PC (including power supply noise) is eliminated. Secondly, you can have a longer run of USB cable to the external USB-->SPDIF conversion device, allowing you to move the PC away from the stereo to minimize RFI, while keeping the SPDIF cable at an ideal low jitter length (I am told about 1.5-2 meters).

 

Re: I2S to USB, posted on October 20, 2004 at 01:08:14
jusbe
Audiophile

Posts: 5950
Location: North Island
Joined: April 4, 2000
 Putting concern aside for a moment, how could it be done? Is it as simple as buying some kind of adadpter, or would one ned toget 'messy' and do some circuit board surgery?

Many thanks for your knowledge on this.

Big J.

 

Really messy, posted on October 20, 2004 at 09:12:40
audioengr
Manufacturer

Posts: 6017
Location: Oregon
Joined: April 12, 2001
 You would need to design a protocol converter. You cannot buy this. And then prototype it on a perf-board. Very expensive...

What is the reason the you want this?

 

Yes, however, posted on October 20, 2004 at 09:19:43
audioengr
Manufacturer

Posts: 6017
Location: Oregon
Joined: April 12, 2001
 If you had an option C or D:

C) USB->S/PDIF external converter to DAC
D) USB->S/PDIF converter inside the DAC

Then the S/PDIF cable would all but be eliminated (short S/PDIF cable permanently attached to the converter) and the power supply for the converter could be external, not the PC power. Could even be battery. The PC would also be galvanically isolated from the audio system, so no ground loops, no hum. The converter could also use a stable clock, such as the Superclock2.

Both C and D will be superior to an internal PCI card.

 

I2S is real, posted on October 20, 2004 at 09:22:24
audioengr
Manufacturer

Posts: 6017
Location: Oregon
Joined: April 12, 2001
 Actually, there are some transports and DAC's that use I2S:

Some Pioneer transports (I've heard), an Alchemy Transport and the Perpetual Technologies P-3A DAC.

 

Looking for simple solutions where they don't exist!, posted on October 20, 2004 at 09:34:19
jusbe
Audiophile

Posts: 5950
Location: North Island
Joined: April 4, 2000
 Just thought it might be interesting to be able to use a Pioneer multi-format player with I2S straight into a USB DAC. But its clear that IDKWTFITA, if you catch my drift. Nice idea, and it would have given me an excuse to fit some other bits and pieces I have knocking about, for extra flexibility.

Thanks anyway.

Big J.

 

Re: You're welcome., posted on October 20, 2004 at 10:35:22
Gordon Rankin
Manufacturer

Posts: 2928
Joined: June 9, 2000
 J.

Apple has their hands full right now and none of my ideas are getting past DTS. So all that is a stall right now...

I send good quality 2.0 cables with my stuff. It really should not matter as much because the specification is soo much better than SPDIF.

Thanks
Gordon
J. Gordon Rankin

 

PC gnd is now connected to audio system, unless..., posted on October 20, 2004 at 10:41:00
Gordon Rankin
Manufacturer

Posts: 2928
Joined: June 9, 2000
 Audioengr,

The ground of the USB is now common with the audio system.

Unless you do what I did and totally isolate it. I have a 2 tier setup in my Cosecant and new Ultimate that isolates the gnd in the D2A section and also the output via a transformer.

PC's grounds because of DC-DC convertors can be pretty noisey.

Thanks
Gordon
J. Gordon Rankin

 

Re: Yes, however, posted on October 20, 2004 at 15:15:49
Lynn
Audiophile

Posts: 1493
Location: Southwest
Joined: June 24, 2003
 I am tempted to try the internal. I have heard that a shorter SPDIF cable (less than 1.5-2 meters), caused problems due to some type of reflection. Is this an issue for a shorter cable length in an internal converter?

 

Re: J thanks, more info on 2.0, posted on October 28, 2004 at 02:41:38
THE_COW_IS_OK


 
Gordon,

how do you specify which type of protocol you want to choose on the USB 1.1?

 

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