Audio Asylum Thread Printer

Get a view of an entire thread on one page

Computer Audio Asylum

Guys remember what the cable does and why they make a difference

216.196.208.221

In Reply to: RE: Synergistic Research USB Cable posted by Mercman on February 17, 2008 at 09:42:27

Gang,

Look there are 2 cables inside a USB cable. There is the DATA differential pair that must be designed for traffic up to at least 12Mhz and the POWER cable which is VBUS 5V's and Ground.

I have like tons of cables now and some do sound different. But remember it will be device dependent.

All of the PCM27xx devices require VBUS to determine the computer is there. This means there is current running from the computer to the USB Device via the POWER side of the cable.

Other devices like the TAS1020 look for signal on the DATA portion of the cable only. Therefore the VBUS is not used and only the Ground connection on the POWER side is used.

On the DATA side termination on the Device side or endpoint will have an effect on transmissions. I usually get the 5M cables put them on a couple of computers with my USB analyzer on the DAC side and check for errors.

It's not suggested to use a 5M with streaming audio. These cables were meant for low speed devices. 2M and under for audio will make all the difference in the world.

With the POWER side being used it can bleed noise from the computer into the dac.

It would be nice if you could switch the VBUS signal on the computer side from VBUS to Ground if your device doesn't use it.

Anyways another thing about the POWER side of the cable is that noise from the computer can end up on the device. The cable should do all it can to make sure that doesn't happen.

It's best like all aspects of this hobby to try this stuff before you buy it.

Remember using these expensive cables on a hard drive is worthless they are in Block mode not Streaming and will not be effected by the use of costly cables.

Thanks
Gordon
J. Gordon Rankin

Follow Ups:

This may have already been asked somewhere, BUUUT.... - TopPop 19:09:05 02/18/08 (1)
Are there any cables out there that run the power and ground cables externally (i.e. on a - preferably shielded - line, separate from the line that transfers the signal)?

I'd imagine that this would do wonders for the audio quality...

"Music is God's gift to man, the only art of Heaven given to earth, the only art of earth we take to Heaven."
-Walter Savage Landor
- RE: This may have already been asked somewhere, BUUUT.... - leica_man@hotmail.com 05:48:46 02/19/08 (0)
  "Posted by TopPop (A) on February 18, 2008 at 19:09:05
  Are there any cables out there that run the power and ground cables externally (i.e. on a - preferably shielded - line, separate from the line that transfers the signal)?
  
  I'd imagine that this would do wonders for the audio quality..".
  
  The Synergistic Research Tricon USB cable "looks" like what you describe. It has two rather stiff (for a usb cable) geometries running in parallel that terminate into the usb connectors. I have no idea however if this means the signal / power are separated or if they ride in each cable. This could however be part of the huge improvement these cables deliver over the Kimber and the Belkin Gold / Pro.
RE: Guys remember what the cable does and why they make a difference - Adal 16:01:58 02/18/08 (4)
One question.

I seem to remember reading that all USB packets, including the synchronous ones, have a header before the payload. Also, even if the bandwidth is specifically reserved for them by the host, they are still multiplexed with other packets if other devices are attached (say a keyboard).

But even if the audio device is the only one on the bus, how does the presence of the headers and of the temporal gaps between consecutive packets affect the audio quality? Does the DAC stream the data in real time as it arrives? No buffering whatsoever (not even 100 bits)? This would require extraordinary precission from the host device since it would have to put the packets on the wire at exactly the right time. Am I missing something?
- A little insight - Gordon Rankin 09:54:57 02/19/08 (0)
  Adal,
  
  All devices on the USB link are sent packets. Each device specifies what is called MaxPacketSize for each endpoint.
  
  In audio using ISO packeting the amount of data sent is specified by the SOF packet. You can specify this from I think 1ms to 32ms. But in audio we do as most do and set this to the maximum which is 1ms.
  
  For 16 bits you need 4 bytes per sample (stereo) and 24 bits you need 6 bytes.
  
  If we are doing 44.1/16 at 1ms increments we need 176.4 bytes per SOF packet. Since this cannot be done all OS's send 176 bytes 9 times and then 180 bytes ounce for a total of 1764 bytes over 10ms.
  
  If we run 24/96 then we need 6*96= 576 bytes per 1ms SOF.
  
  The PCM27xx series controllers have fixed buffer sizes of 2x48*4=384.
  
  With the TAS1020/TUSB3200 you can specify the buffer size to what ever you want and control the DMA actually everything. So you can make the buffer allot larger and assure the lack of pops and clicks if there are errors in the bus.
  
  The USB controllers only look at what their USB_ID is. They disregard all other communication seen on the link. With many computers and hubs the device will only see what it is suppose too.
  
  Thanks
  Gordon
  J. Gordon Rankin
- RE: Guys remember what the cable does and why they make a difference - audioengr 18:10:18 02/18/08 (2)
  You got it. There is very little buffering in the typical USB interface. Gordons has a bit more than typical because he does a hand-shake and presumably gets it in bursts.
  - RE: Guys remember what the cable does and why they make a difference - Adal 06:59:13 02/19/08 (0)
    The PLL is locked on all the bits on the USB wire or only on the payload information? If only on the payload, it is stopped in the interval between consecutive packets?
  - In bursts? - Slider 22:00:48 02/18/08 (0)
    I believe he still uses isochronous, only with async endpoint to adjust source rate, so PLL still needed to lock on bus. Plenty of ways for jitter from the PLL clocks to get to the audio clock circuits, even when the sample clock is derived from a local oscillator. No huge surprise there. Still looks like a nice, simple design though.
RE: Guys remember what the cable does and why they make a difference - Adal 16:03:57 02/18/08 (0)
One question.

I seem to remember reading that all USB packets, including the synchronous ones, have a header before the payload. Also, even if the bandwidth is specifically reserved for them by the host, they are still multiplexed with other packets if other devices are attached (say a keyboard).

But even if the audio device is the only one on the bus, how does the presence of the headers and of the temporal gaps between consecutive packets affect the audio quality? Does the DAC stream the data in real time as it arrives? No buffering whatsoever (not even 100 bits)? This would require extraordinary precission from the host device since it would have to put the packets on the wire at exactly the right time. Am I missing something?
RE: Guys remember what the cable does and why they make a difference - audioengr 12:48:35 02/18/08 (0)
Gordon - Why does the USB cable seem to make a difference with your Async USB protocol DAC's? This should isolate pretty well I would think.

Post a Followup:

FAQ

Post a Message!

Forgot Password?

Moniker (Username):
Password (Optional):
	Remember my Moniker & Password (What's this?) Eat Me
E-Mail (Optional):
Subject:

Message:	(Posts are subject to Content Rules)

Optional Link URL:
Optional Link Title:
Optional Image URL:
Upload Image:
E-mail Replies:	Automagically notify you when someone responds.

To view your new posting or follow-up, click on the RELOAD or REFRESH button on your browser.