Re: Recordings Stuttering


jdow
 

Unfortunately I do not have a pointer to it. The short version with regards to USB 3 with attached USB 2 devices (or USB 2 with attached USB 1 or 1.1 devices) involves time rather than data rate issues.

Every time a lower rated USB device tries to transfer data on a higher rated hub (port) the whole bus slows down to the lower rate for the duration of the transfer. This consumes time on the bus. So a 1.1 device may consume maybe 40 times as much of the nominal bandwidth on the port. Furthermore for perceived response on your screen mice send very small messages with fairly large overhead with a payload value of 1 mouse step. It's a bit difficult to get a mouse to actually send a payload of say 42 mouse steps. So what might be a (swag alert) 3 byte transfer turns into a (swag) 15 or 20 byte handshake plus transfer all at 10 MB/S or so. That's 50 to 90 megabytes/second carved away from the nominal 400 megabytes/second on the bus. So slow devices eat your hub's capability FAST. That is why AirSpys have trouble at 10 Msps on a USB2 bus that should "easily" handle the bit packed transfer rate of an AirSpy. The full USB2 bus is seldom really there. And if  you hook up to a USB 3 hub you are still stuck with the AirSpy eating 3/4 of the bus bandwidth give or take a little. Anything else on that bus, say an external drive or an internal DVD drive can cause extreme stuttering or outright failure.

(All numbers are Scientific Wild Assed Guesses rather than correct to more than 1 decimal place. The concept is what is of concern in the numbers above. If my memory serves the numbers are "close" if not exact.)

{^_^}

On 20210703 16:08:57, James Niven wrote:

Thanks Max, I hope Joanne can point me to her excellent post, would love to read this and learn more.

 

From: main@SDR-Radio.groups.io <main@SDR-Radio.groups.io> On Behalf Of Max
Sent: Saturday, July 3, 2021 5:45 PM
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] Recordings Stuttering

 

On Sat, Jul 3, 2021 at 10:17 PM, James Niven wrote:

I am trying to work out why my recordings are stuttering on playback when accessing them from an external hard drive.

There seems to be no issue from what I can see when playing them back when I record to c: drive.

Hi James. I guess one would need to know what is exact external drive make/model and how is it connected to the PC? Many PCs still have a mix of USB 2.0 and USB 3.0 ports. It's very easy to plug a USB 3.0 drive into a USB 2.0 port. I don't know if it's a universal standard, but often the USB 2.0 ports are black inside (well, the plastic "tongue" inside the socket anyway) and the USB 3.0 ports are marked blue. Needless to say USB 3.0 drive in USB 2.0 port is choked right from the start.

There is generally nothing inherently slow about external drives, and I have USB 3.0 drives that can transfer files at 100 Mb/s (single file performance, not multiple). As Joanne says, it's just hardware bottlenecks that are the problem, i.e. drive physical write/read speed, and connection speed.

Joanne wrote an excellent post about USB ports a few months back, but not sure where it is now....... Joanne? But essentially, to repeat what she already says, don't mix USB 2.0 and 3.0 devices on the same bus at the same time. The USB 2.0 will drag down the whole transfer rate.

Max


Join main@SDR-Radio.groups.io to automatically receive all group messages.