USB PCM streaming to USB soundcard

[Gauss]

I came to this idea because I couldn't find any other DAP than the iRiver H100 series in production. Why these ones? Well, they have digital out and support Rockbox! That means I can use a small portable device with Hard Disk with Rockbox and send the digital signal to a portable DAC/Amp to listen through good headphones.

Lurking the web I found the Lyre project. My attention was grabbed by the Mini2440 and its Rockbox port. My idea was: why not to code an interface that sends the final output from Rockbox to the USB port? Of course it would involve more work, maybe to write the protocol to send audio in PCM through the USB port, recognize a valid USB soundcard and its many chips (or at least the one in my iBasso D12 hehe), etc.

How feasible would it be to include such a feature? Would it be hard?

Why this? Well, a portable solution is missing on the market with digital output. An inexpensive solution! There are DAPs that have digital out but cost around $400 USD or more, DAC and AMP are included, not better as in a good portable dedicated solution for just the amplification to quality phones but acceptable. I know this market is too small, but many people who uses Rockbox is the market that actually like to do stuff by themselves, are perfectionist (Rockbox target is quality audio am I right?), and don't exactly pay big bucks, so the market would not make something like that commercial just like that.

Other solution I found was to get a cheap tablet from here: http://www.eletroworld.cn but is well known that cheap stuff become more expensive as they could fail and I would have to get something else later. These tablets can run Ubuntu so connecting the external soundcard to an USB host port shouldn't be a big deal. Disadvantages? Maybe not as reliable, short battery life, bulky device... well, maybe just as big as my cell phone. Who knows!! At least I know that such a feature wouldn't be needed in Rockbox when its running on an OS that already deals sending the PCM audio signal through the USB port.

[gevaerts]

http://forums.rockbox.org/index.php?topic=24422.0

[Gauss]

http://forums.rockbox.org/index.php?topic=24422.0

Well, that's a different story. They are proposing to use a rockbox device as an USB soundcard which means to use the USB port of the rockbox device as "B type", that would make the rockbox device act as a device while connected to a PC which will be the host. The PC would send the PCM stream data to the rockbox device and use the DAC on the rockbox device.

Here I'm proposing something totally different. The rockbox device would be the host and use one of its USB ports as "A type" so an external soundcard could be connected to the rockbox device. Then the rockbox device should send the PCM data stream to the external soundcard to use an external DAC. Why this feature? There are external DACs that much more better than the one in a rockbox device and with USB input! This would allow to reproduce sound for people minded in sound quality or anyone else wanting to use all the potential of the audio system in house, as it is not the same to send the analog signal to any  RCA input on your home amplifier.

And as I already told why. It would be an alternative for devices that doesn't have a digital output but just an USB host port. There were devices such as the iRiver H100 series that had digital output and does run rockbox! But it's a thing of the past.

[saratoga]

That might be neat.  I have no idea which players actually have the required hardware to act a USB master device though.

[Gauss]

I was thinking in writing the interface for the port made by Lyre Project on the Mini2440, but that device is not as portable as I would like. However that project is going and going so hopefully something better would come later.

I found the iAudio X5 to have USB OTG, there's also a list of devices supported by its host mode. This device has Rockbox firmware done. It would be interesting to add PCM streaming support for external DAC chips through it's USB OTG port. It's a shame is out of production.

How's the Linux's Rockbox version? As Linux has drivers to support external USB soundcards shouldn't be hard to make this happen on a tablet running Linux. Although that would be not as portable as a simple DAP nor exactly cheap. For that I'd rather get a Cowon Q5W, but then we wouldn't be talking on the unexpensive-portable-Rockbox alternative.

A device that I'd like to have Rockbox and that have USB host port is the Archos 28 internet tablet for just $100!!!  Or why not the 32 or 43 models? Those also have USB host port. On the Archos page  this can be read:
ARCHOS has opened up this device, thus allowing creative minds to program their own tablet, or create what could be the tablet of the future.
Just look on the section titled "Pocket Computer - Possibilities beyond Android™ with the ‘Dual OS’" for more.

Other interesting devices are the SmartDevices tablets. I've already mentioned about it on the first post. While there's the project to make Rockbox to run as an app, it would be interesting to see Rockbox as a port on some of these devices which are having USB host port as a common.

[Llorean]

Step 1 would be programming USB host architecture and drivers for a USB sound card into Rockbox in the first place. Neither of these exist, so rather than focusing on all the potential targets that could make use of it, why not pick an existing target with the necessary hardware and get to work?

As for Rockbox as an App, wouldn't it be the responsibility of the OS to decide which sound device the audio is sent to? So that's less a Rockbox project if it's going to exist as a hosted app on something with digital out.

[Gauss]

Yes and yes Llorean, thanks for your reply.

I found interesting the possibility of step 1. Although it's an early stage as there are not many ports for such mentioned devices as those devices are just starting to grow on the market.

Rockbox as an app maybe would be the one to stay on such market. Either way Rockbox FTW! Any input it's still welcome, I may add the following:

Any external USB soundcard should not be powered directly by the Rockbox device unless it has the specs to give enough power. Either the soundcard should has its dedicated power source or be powered by an USB powered hub (sometimes with a "Y" USB cable).

[pixelma]

Just for your interest: while the Iriver H300 and the Iaudio X5 have the USB host hardware, they don't have host drivers (and capabilities) under Rockbox. Development is probably going to be very hard, especially on the X5, because there's no datasheet available. Maybe it would be better to focus on targets that use software USB?

[gevaerts]

Just for your interest: while the Iriver H300 and the Iaudio X5 have the USB host hardware, they don't have host drivers (and capabilities) under Rockbox. Development is probably going to be very hard, especially on the X5, because there's no datasheet available. Maybe it would be better to focus on targets that use software USB?

"Software USB" on the device side isn't really relevant. All devices that support USB host do so with a software stack.

The rockbox-running devices I'm aware of that can do host are Gigabeat F, H300 (except the ones sold in the US I believe) , X5, Gigabeat S, and I think some of the Samsungs (the PortalPlayer-based ones). Of those, the Gigabeat F is regular OHCI, the H300 claims to be OHCI but is something else (it's documented though), X5 is undocumented (and therefore unlikely to ever be supported), Gigabeat S and the Samsungs are EHCI (or a close derivative).

If I would happen to be the person working on this, I'd concentrate on the Gigabeat F first, because the specification is available and clear, because the dock has a full-size A connector on it (which makes testing easy without having to look for the right cable all the time), and because this happens to be the player I use most.