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Issue is System Architecture, not FW vs. USB

Posted by Tony Lauck (A) on June 28, 2011 at 14:28:43

In Reply to: See my most recent posting above. posted by Norm on June 28, 2011 at 06:37:06:

". I am really saying what many pros have told me, FW does not have the liabilities that USB does, name only occasional access to the processor."

The issue is not Firewire vs. USB. It's the entire system architecture.

Most of the required access by a DAC (FW or USB) is to system memory (RAM) so I will focus on that.

Both Firewire and USB are shared buses. As such, only one device can use them at a time. However, because they are fast this is generally not a problem for many applications, since there is generally sufficient buffering that a small wait in access won't matter. However, for audio this may not be the case, since any delay impacts the exact timing of the packets and this can affect the sound (at least by second order effects using John S's terminology).

If there is only one device on a bus then no problem. Hence if one has two devices (e.g. DAC and disk drive) one can avoid possible conflicts by using a dedicated bus for each device. Since it is possible to have multiple controllers of each type one can accomplish this goal three ways: have two USB controllers, have two Firewire controllers, or have one controller of each type. Given that many computer systems come out of the box with multiple USB controllers this can often be accomplished without adding any extra hardware without using Firewire. However, since a USB controller is dedicated to a specific set of ports it will be necessary to understand which specific USB sockets are wired to which controller. This can be done by various ways as others have described. (One such way, albeit inefficient is trial and error.)

If one runs two controllers of either type and dedicates one to the disk and the other to the DAC there may still be conflicts in the computer system because there are shared resources that are accessed by both the disk and the DAC. (The most obvious of these is the RAM memory.) These resources are connected through various complex logic that juggles all the conflicting activity in such a way to maximum system performance while minimizing manufacturing cost, and this logic may be unique to each model of motherboard. Suffice it to say that the problem here is similar for both Firewire and USB. Certain brands of computer have a reputation for giving poor audio performance. This probably relates to this portion of the motherboard.

In order to understand these issues completely one needs a lot of knowledge in computer architecture and performance analysis, a subject that few audio professionals have. (I do not consider myself such an expert, but I have worked with these people and so I am familiar with many of the issues involved. My department designed high speed computer network interfaces and ran into many similar issues. )


Tony Lauck

"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar

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