JustCurious
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USB speed on Pi?

Tue Sep 09, 2014 5:07 pm

What is the effective total USB speed the Pi can handle? Do the SD and Ethernet go over the USB circuitry?

I am asking this because I want to connect a 192khz/24bit DAC via USB.

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rpdom
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Re: USB speed on Pi?

Tue Sep 09, 2014 5:21 pm

The Pi supports USB2.0 speed 480Mb/s, but this is shared across all USB ports and the Ethernet. Also, the CPU isn't very powerful and might not keep up with the full USB speed.

The SD card does not use USB.

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redhawk
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Re: USB speed on Pi?

Tue Sep 09, 2014 5:27 pm

The Pi also has problems recording from USB audio capture devices with certain sample rates http://www.raspberrypi.org/forums/viewt ... 63&t=79402
I have no idea how reliable 24bit 192kHz capture would be - as a guess probably not very well.
If you serious need to record in this format then you're better off using a Wolfson audio card instead.

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JustCurious
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Re: USB speed on Pi?

Tue Sep 09, 2014 5:53 pm

redhawk wrote:The Pi also has problems recording from USB audio capture devices with certain sample rates http://www.raspberrypi.org/forums/viewt ... 63&t=79402
I have no idea how reliable 24bit 192kHz capture would be - as a guess probably not very well.
If you serious need to record in this format then you're better off using a Wolfson audio card instead.

Richard S.
I think there is a misunderstanding; I don't want to capture audio with the Pi, but play audio with the Pi ;)

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Re: USB speed on Pi?

Tue Sep 09, 2014 6:46 pm

JustCurious wrote:
redhawk wrote:The Pi also has problems recording from USB audio capture devices with certain sample rates http://www.raspberrypi.org/forums/viewt ... 63&t=79402
I have no idea how reliable 24bit 192kHz capture would be - as a guess probably not very well.
If you serious need to record in this format then you're better off using a Wolfson audio card instead.

Richard S.
I think there is a misunderstanding; I don't want to capture audio with the Pi, but play audio with the Pi ;)
Better use HDMI audio for this. If your Monitor/Receiver supports it, it will be played without too much stress for the CPU.
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JustCurious
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Re: USB speed on Pi?

Tue Sep 09, 2014 7:28 pm

When I roughly calculate 192khz/24bit stereo load, in (WiFi dongle), and out (DAC), then I come to approximately 20 to 30 Mbit/sec, which is largely below the theoretical limit of 480 Mbit/s. I estimate this shouldn't be a very big load for the Pi, or am I mistaking?
Last edited by JustCurious on Tue Sep 09, 2014 7:48 pm, edited 1 time in total.

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Re: USB speed on Pi?

Tue Sep 09, 2014 7:48 pm

192Ksps * 2 channels * 24 bits = 192ksps * 2 * 3 bytes = 1152k bytes per second, approx. 1.152Mbytes/sec, approx. 11.2Mbits/sec. Double that with USB overhead etc., and you get about 25Mbps so you are likely right.
JustCurious wrote:When I roughly calculate 192khz/24bit stereo load, in (WiFi dongle), and out (DAC), then I come to approximately 20 to 30 Mbit/sec, which is largely below the theoretical limit of 480 Mbit/s. I estimate this wouldn't be a very big load for the Pi, or am I mistaking?
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Re: USB speed on Pi?

Tue Sep 09, 2014 9:29 pm

It depends if your device is a USB full-speed or high-speed device.

192kHz/24bit exceeds the bandwidth capacity of the full-speed bus. If your device is receiving uncompressed audio then it must be a USB2.0 high-speed device. High-speed isochronous devices put far less strain on the host software than full-speed, but even so a high resolution audio DAC is not usually a problem when running at full-speed.
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JustCurious
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Re: USB speed on Pi?

Wed Sep 10, 2014 7:59 am

jdb wrote:It depends if your device is a USB full-speed or high-speed device.

192kHz/24bit exceeds the bandwidth capacity of the full-speed bus. If your device is receiving uncompressed audio then it must be a USB2.0 high-speed device. High-speed isochronous devices put far less strain on the host software than full-speed, but even so a high resolution audio DAC is not usually a problem when running at full-speed.
It is a HS async DAC. :)

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