You aren't going to improve on turnaround time. You need to add more IRPs
so your math comes out to a better number :)
i.e.
I irps * B bytes/irp
-------------------- = sample rate (bytes/sec)
turnaround time (sec)
you tried 1 irp with 200 bytes, for:
1 irp * 200 b/irp
------------------- = 16.7k bytes/sec
.012 sec
now try 12 irps with 200 bytes, for:
12 irps * 200 b/irp
------------------- = 200k bytes/sec
.012 sec
You can of course use more IRPs, but (assuming USB 1.1) since you only get
1 isochronous IRP per frame, and each frame is 1ms, you aren't going to
get more than 1 IRP per ms ;-)
However queueing up IRPs is always the best thing to do. Try using maybe
20 or so. Keep in mind your driver is in userspace, so this is a
different world than kernel drivers (where you can handle data in hardware
interrupts, i.e. 0 latency).
On Tue, 5 Jul 2005, Charles Chen wrote:
>I am trying to retrieve data from a microphone that connected to iMic USB,
>and I found the time of submitting a usbIrp to an isochronous pipe is about
>12 ms (synd or async). iMic USB has three wMaxPacketSize settings, and the
>largest one is 200 bytes, that amounts to 8.3k samples/second (for a two
>byte sample). But the standard audio sample rate is 44.1k, so 8.3k is way
>too slow. Is there any way to enhance the speed? I noticed iMic USB works
>fine under Windows with 44.1 k sample rate. Any help will be appreciated.
>
>Charles Chen
>
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