limitations of using multiple RTL-SDR dongles simultaneously

[Müller, Marcus (CEL)]

Hi Olivier,

at a sampling rate of 250 kHz, and with the usual 8 bit complex
(meaning 8bit I + 8bit Q = 16 bit per sample), a single dongle will
send 4 Mbit/s, not 500 kbit/s. Are you perhaps confusing bytes with
bits?

Anyway, you're right, the hypothetical transfer rate of USB2 HiSpeed
would be 480 Mbit/s, so plenty enough for more than 4 dongles.

You say the fifth RTL dongle doesn't even start. Is there a specific
error? 
That very much doesn't sound like a data rate problem, but more like a
hardware issue. 

Can you see (without using them) more than four dongles on your USB
bus? What operating system are you using? Does it work under e.g.
Linux¹? (I see you're using bash, but I'm not sure this implies this is
Linux.)

Best regards,
Marcus

¹ you could download a USB image and try on your own. Trying in a VM
doesn't make sense, so really write the image to a USB dongle or DVD:

http://inpha.se/GNU_Radio-Live-Hamradio-binary.iso

It's half a year old already, but I built it to include rtl_sdr tooling

On Mon, 2018-01-08 at 22:37 +0100, Olivier SCHMITT wrote:
> Hello dear RTL-SDR users,
> 
> I whant to use multiples dongles as array for decode wspr.
> My problem is that I can not use more than 4 USB dongle
> simultaneously.
> 
> I check with usbmon, each dongle uses 500kbits / s around. So I am
> far 
> from the maximum bandwidth allowed by usb 2.
> I tried using a USB switch with external power supply but it does
> not 
> work better.
> 
> The default test performed is:
> 
> #!/bin/bash
> for ((i=0;i<=4;i++))
> do
>          rtl_test -d $i -s 250000 &
> done
> 
> First 4  is starting but last 5 doesent. And i whant to use more...
> 
> Is this a limitation of the RTL-SDR library or the kernel driver?
> Do we have a solution to get around the problem?
> 
> Thank you in advance for your valuable feedback.